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Environmental factors controlling floral zonation and the distribution of burrowing and tube-dwelling… Swinbanks, David Donald 1979

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ENVIRONMENTAL FACTORS CONTROLLING FLORAL ZONATION AND THE DISTRIBUTION OF BURROWING AND TUBE-DWELLING ORGANISMS ON FRASER DELTA TIDAL FLATS, BRITISH COLUMBIA by DAVID DONALD SWINBANKS B.Sc , St. Andrews University, 1975 A THESIS SUBMITTED IN THE REQUIREMENTS DOCTOR OF PARTIAL FULFILLMENT OF FOR THE DEGREE OF PHILOSOPHY In THE FACULTY OF GRADUATE STUDIES (Department of Geological Sciences and I n s t i t u t e of Oceanography) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA March, 1979 {£) David Donald Swinbanks, 1979 In present ing t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree tha t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r re fe rence and s tudy . I f u r t h e r agree tha t permiss ion f o r ex tens i ve copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s en t a t i v e s . I t i s understood tha t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed wi thout my w r i t t e n pe rm iss i on . Department of Geological Sciences The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook P lace Vancouver, Canada V6T 1W5 Date March 19, 1979 )E -6 B P 75-51 1 E ABSTRACT The d i s t r i b u t i o n of various burrowing and tube-dwelling organisms, t h e i r biogenic sedimentary structures, and the rates at which they turn over sediment are Investigated on three d i f f e r e n t t i d a l f l a t environments of the Fraser Delta. The organisms studied include Callianassa c a l i f o r n i e n s i s and Upogebia pugettensls, both thalassinldean burrowing shrimps, the burrow-ing polychaete Abarenicola sp., the tube-dwelling polychaetes, P r a x i l l e l a sp. and Spio sp., the bivalve Mya arenaria and the gastropods B a t l l l a r i a  attramentaria and Nassarius mendicus. Thalassinidean shrimps are of most i n t e r e s t because they are widespread over the Delta, i n p a r t i c u l a r Callianassa and because t h e i r d i s t i n c t i v e burrows are well known i n the g e o l o g i c a l record. The 'marine' 5tidal.".flats of Boundary Bay on the i n a c t i v e southern flank of the Fraser Deltas^ are mantled with fi n e to very f i n e , w e l l to very well sorted sands. The i n t e r t i d a l region has f i v e f l o r a l / s e d i m e n t o l o g i c a l zones delimited p r i m a r i l y by elevation and exposure and characterized by d i s t i n c -t i v e macrofaunal assemblages. These are from the shoreline seawards, the saltmarsh, a l g a l mat, upper sand wave, eelgrass and lower sand wave zones. Topography of both small and large scale of biogenic or p h y s i c a l o r i g i n creates l a t e r a l heterogeneity within the b i o f a c i e s of each zone. g An estimated 4.25 x 10 Abarenicola on Boundary Bay t i d a l f l a t s annually 6 3 rework about 10 m of sand. The bioturbation of this worm may be a f a c t o r l i m i t i n g the extent of the a l g a l mat zone. By i r r i g a t i n g i t s burrow, Abarenicola can separate a sand/clay mixture by f l o a t i n g the clay out i n the head shaft i r r i g a t i o n current. Thalassinidean burrowing shrimps are most abundant on the 'marine' t i d a l f l a t s of southeastern Roberts Bank on the active Delta-front. These . t i d a l f l a t s i i i can divided- into four.' florai/sedH.^ntolo^i.cai^pnes: the saltniarsh", a l g a l mat, sandflat and eelgrass zones. Thalassinidean burrowing shrimps dominate the sandflat zone. Upogebia densities are p o s i t i v e l y correlated to mud con-tent of the sediment. Callianassa show no clear grain s i z e preference and are abundant i n sediments ranging from 5 to 50% i n mud content and from 2.6 to 4.0 0 i n median grain s i z e . At t h e i r peak density (446 burrow openings -2 m ) Callianassa rework the substrate they l i v e i n to a depth of 50 cm i n about f i v e months. On c e n t r a l Roberts Bank a major t r a n s i t i o n from a 'marine' to a brackish environment occurs. A brackish marsh zone extending to much lower i n t e r t i d a l l e v e l s than the saltmarsh l a t e r a l l y replaces the a l g a l mat zone and the upper h a l f of the sandflat zone. A sandflat/mudflat zone cross-cut by channels displaces the eelgrass zone and lower h a l f of the sandflat zone. The peak i n C a l l i a n a s s a d i s t r i b u t i o n moves to lower i n t e r t i d a l l e v e l s because of the presence of low s a l i n i t y water at higher t i d a l l e v e l s and because of the absence of eelgrass i n lower i n t e r t i d a l regions. Upogebia although physio-l o g i c a l l y better adapted to cope with reduced s a l i n i t y demonstrates lower tolerance of brackish water i n i t s d i s t r i b u t i o n than C a l l i a n a s s a , probably because the function of i t s mud-lined burrow as a conduit f or suspension feeding and r e s p i r a t i o n exposes Upogebia to low s a l i n i t y surface waters, while C a l l i a n a s s a , i n i t s unlined burrow used for deposit feeding, i s pro-tected from surface waters by high s a l i n i t y i n t e r s t i t i a l waters. The d i s - ..; t i n c t i o n between these two types of burrow i s considered to be very ' s i g n i f i -cant-: for:1 paleoenvironment'al reconstructions. A new system of subdividing the i n t e r t i d a l region i n t o exposure zones (the atmozone, amphizone and aquazone), based on c r i t i c a l t i d a l l e v e l s at which the maximum duration of continuous exposure or submergence 'jumps,' i s advocated. I t allows cross c o r r e l a t i o n between d i f f e r e n t t i d a l regions i v experiencing d i f f e r e n t types of astronomically c o n t r o l l e d tides and much of the i n t e r t i d a l zonation of Fraser Delta t i d a l f l a t s may be causally r e l a t e d to these exposure zones. [J V TABLE OF CONTENTS Page ABSTRACT 1 1 TABLE OF CONTENTS . v LIST OF TABLES x LIST OF FIGURES x i i ACKNOWLEDGEMENTS x x i i INTRODUCTION 1 REFERENCES 8 Part 1 — INTERTIDAL EXPOSURE ZONES: A NEW SCHEME FOR SUBDIVIDING THE INTERTIDAL REGION 9 Abstract 10 Inroduction 11 C r i t i c a l T i d a l Levels 12 Exposure Zones 18 I n t e r t i d a l Zonation 22 Acknowledgements 24 References 25 Part 2 — BIOSEDIMENTOLOGICAL ZONATION OF BOUNDARY BAY TIDAL FLATS, FRASER RIVER DELTA, BRITISH COLUMBIA .27 Abstract 28 Introduction . ' 30 Methods 33 Floral/Sedimentological Zonation of the T i d a l F l a t s 39 Description 39 Discussion of Sand Waves 42 Environmental Factors and Zonation 44 Grain Size of Surface Sediments 45 S a l i n i t y and Tur b i d i t y 49 Exposure Time 51 v i TABLE OF CONTENTS (Cont'd) Page Part 2 — Fl o r a Fauna and Their Biogenic Sedimentary Structures 60 (Cont'd) Saltmarsh Zone 60 A l g a l Mat Zone 61 B . a t i l l a r i a 61 Spio 65 Fl y Larvae 70 Upper Sand Wave Zone 70 Abarenicola 70 Mya 74 Callianassa 74 Eelgrass Zone 81 Upogebia 82 P r a x i l l e l a 85 Nassarius 88 Lower Sand Wave Zone 88 Discussion of Zonation 89 Summary 94 Alg a l Mat Zone 94 Upper Sand Wave Zone 97 Eelgrass Zone 97 Conclusions 98 Acknowledgements 99 References 101 Part 3 — SEDIMENT REWORKING AND THE ^ 10GENICjFORMATION OF-CLAY ' ''LAMINAE'- BY" ABARENICOLA PACIFIC^ -" 106 Abstract 107 Introduction 108 v i i TABLE OF CONTENTS (Cont'd) Page Part 3 — Methods ' 111 (Cont'd) Results 112 F i e l d Results 112 Sediment Reworking Rates 112 Budget of Sediment Turnover _ 112 Laboratory Results 117 Discussion 117 Acknowledgements 121 References 122 Part 4A - ENVIRONMENTAL. CONTROLS ON THE DISTRIBUTION OF THALASSINIDEAN BURROWING SHRIMPS ON FRASER DELTA TIDAL FLATS, BRITISH COLUMBIA: A Marine T i d a l F l a t Between Two Man-Made Causeways--on-iSoutheastem.'Roberts Bank 124 Abstract 125 Introduction 12 7 Methods 130 The Inter-Causeway T i d a l F l a t 134 Floral/Sedimentological Zones 134 Grain Size of Surface Sediments 144 S a l i n i t y and Tur b i d i t y 146 D i s t r i b u t i o n of Thalassinidean Shrimps 147 Influence of Grain Size on Thalassinidean Shrimp D i s t r i b u t i o n 152 Thalassinidean Shrimp I n t e r r e l a t i o n s h i p s 158 Biogenic Reworking of Sediment 158 Discussion 163 Upper Limits of Thalassinidean Shrimp D i s t r i b u t i o n 163 v i i i TABLE OF CONTENTS (Cont'd) Page Part 4A - Lower Limits of Thalassinidean Shrimp (Cont'd) D i s t r i b u t i o n 164 Factors Influencing Thalassinidean- Shrimp Density . 166 Acknowledgements ' •p. j— 170 . References Part 4B - ENVIRONMENTAL CONTROLS ON THE DISTRIBUTION OF THALASSI-NIDEAN BURROWING SHRIMPS ON FRASER DELTA TIDAL FLATS, BRITISH COLUMBIA: The Marine to Brackish T i d a l F l a t s of Central and Northern Roberts Bank . 173 Abstract 174 Introduction 176 Methods ' 179 Northern and Central Roberts Bank 183 Floral/Sedrmentological Zones 183 S a l i n i t y 191 Discussion of S a l i n i t y Regime 199 D i s t r i b u t i o n of Thalassinidean Shrimps 200 Description . 200 Relationship Between Shrimp Density and Substrate Parameters 203 Discussion of Thalassinidean Shrimp D i s t r i b u t i o n 208 Burrow Geometry 213 Review and Conclusions 221 Acknowledgements 226 References 227 SUMMARY AND CONCLUSION 230 APPENDIX 1 — SURVEY DATA FOR BOUNDARY BAY (Part 2) -235 . APPENDIX 2 — FAUNAL DENSITIES AND GRAIN SIZE DATA ON TRANSECTS A AND B, BOUNDARY BAY (Part 2) 244 ix TABLE OF CONTENTS (Cont'd) APPENDIX 3 — SURVEY DATA FOR STATIONS ON THE INTER-CAUSEWAY TIDAL FLAT (Part 4A) APPENDIX 4 — GRAIN SIZE AND THALASSINIDEAN SHRIMP DENSITY DATA FOR STATIONS ON THE INTER-CAUSEWAY TIDAL FLAT (Part 4A) APPENDIX 5 — SUPPLEMENTAL INFORMATION REGARDING SURFACE SUBSTRATE SALINITY AND SUBSTRATE SALINITY PROFILES ON INTER-CAUSEWAY TIDAL FLAT, ROBERTS BANK (Part 4A) APPENDIX 6 — SUPPLEMENTAL DATA ON CORRELATIONS BETWEEN THALASSINI-DEAN SHRIMP DENSITIES AND GRAIN SIZE ON THE INTER-CAUSEWAY TIDAL FLAT (Part 4A) APPENDIX 7 — CHARACTERISTICS OF THE STATIONS USED TO DETERMINE REWORKING RATES BY THALASSINIDEAN SHRIMPS APPENDIX 8 — SUPPLEMENTAL DATA ON SALINITY FOR NORTHERN AND CENTRAL ROBERTS BANK (Part 4B) APPENDIX 9 — SUPPLEMENTAL DATA ON CORRELATIONS BETWEEN CALLIANASSA DENSITY AND SUBSTRATE PARAMETERS FOR NORTHERN AND CENTRAL ROBERTS BANK (Part 4B) Page 248' 254 257 261 •265 '2661? '272, X LIST OF TABLES Part 2 Table Page I. Dates and Locations of F i e l d Observations 36 I I . E l e v a t i o n s of Zone Boundaries 40 I I I . A e r i a l photographs and s a t e l l i t e imagery i l l u s t r a t i n g the low t u r b i d i t y l e v e l s of Boundary Bay waters 50 IV. S a l i n i t y of Water i n T i d a l Pools at Low Tide 52 Part 3 I . Rates of Sediment Turnover by Abarenicola 113 I I . Rates of Sediment Turnover by Abar e n i c o l a (Cont'd) 114 I I I . Annual Budget of Sediment Turnover f o r Abar e n i c o l a i n Boundary Bay 116 Pa r t 4A I. Rates of b i o g e n i c reworking of sediment by C a l l i a n a s s a and Upogebia, as measured i n pr o t e c t e d metal enclosures on the surface of the su b s t r a t e 160 Pa r t 4B I . E l e v a t i o n ranges f o r zone boundaries on the 'marine' t i d a l f l a t s as determined from a topographic map. The accuracy of e l e v a t i o n s obtained from the map i s checked by comparing w i t h surveyed e l e v a t i o n data obtained i n the inter-causeway area (Part 4A) . 186 I I . E l e v a t i o n of the Lower L i m i t of the B r a c k i s h Marsh 188 I I I . Comparison of Thalassinidean Shrimp D e n s i t i e s at S t a t i o n s Sampled i n 1977 and Reoccupied i n 1978 202 IV. Comparison of C o r r e l a t i o n C o e f f i c i e n t s (r) Between C a l l i a n a s s a Burrow Opening Density and Percent Mud Using Pooled and Unpooled Percent Mud Data 206 V. R e l a t i o n s h i p Between Surface Substrate S a l i n i t y and C a l l i a n a s s a Burrow Opening Density at a Fixed T i d a l L e v e l at S i x S t a t i o n s Midway Between Canoe Pass and the Coalport Causeway ( F i g . 2) 207 x i LIST OF TABLES (Cont'd) SUMMARY AND CONCLUSION Table Page I . SUMMARY OF ENVIRONMENTAL FACTORS LIMITING THALASSINIDEAN SHRIMP DISTRIBUTION ON FRASER DELTA TIDAL FLATS 233 I I . SUMMARY OF EFFECTS OF VARIOUS ENVIRONMENTAL FACTORS ON THALASSI-NIDEAN SHRIMP DENSITY 233 I I I . Schematic Summary of A l l F l o r a l and Faunal D i s t r i b u t i o n a l L i m i t s on Fraser D e l t a T i d a l F l a t s Which L i e W i t h i n 15 cm Or Less of an Exposure Zone Boundary Or Other Extreme C r i t i c a l T i d a l L e v e l 234 x i i LIST OF FIGURES Part 1 Figure Page 1. (a) Schematic d a i l y t i d a l curves for the three main types of 14 t i d e . Shading indicates submergence. For mixed tides there are four c r i t i c a l t i d a l l e v e l s (dashed l i n e s ) at which the duration of exposure or submergence 'jumps' - and which define f i v e exposure l e v e l s within which exposure and submergence changes, continuously with respect to elevation. Higher high water defines the boundary between Exposure Levels 1 and 2, and i s a c r i t i c a l t i d a l l e v e l above which the duration of expo-sure doubles from les s than one lunar day to at .least nearly two. Lower high water defines the boundary - between Exposure Levels 2 and 3 and i s a l e v e l above which the duration of exposure doubles from les s than h a l f a lunar day to j u s t under one lunar day. Equivalent c r i t i c a l t i d a l l e v e l s l i e . at the heights of higher low water and lower low water but involve steps i n submergence duration. They define Exposure Levels 4 and 5. The same c r i t i c a l t i d a l l e v e l s and exposure l e v e l s can be recognized f or semi-diurnal t i d e s . Exposure Level 3 cannot be defined f o r d i u r n a l tides because they lack lower high water and higher low water stages. (b) The predicted d a i l y heights of lower low water at Point Atkinson, B.C. between August and November, 1977 are p l o t t e d as dots. The low waters are modulated into two neap tides and two spring tides per month. Successive spring tides are of d i f f e r e n t ranges. The spring tide of l e s s e r range defines a c r i t i c a l t i d a l l e v e l at which duration of continuous submer-gence jumps from about 10 to 20 days, and that of greater range a jump from about 24 to 45 days. (c) Annual c r i t i c a l l e v e l s are defined by the spring higher high waters of June and December for predicted tides i n 1977/78 at Point Atkinson, B.C. Dots indicate heights of spring and neap t i d e s . The spring high tides of June, 1978 and December, 1978 are so s i m i l a r i n height that these two c r i t i c a l t i d a l l e v e l s merge into one, inv o l v i n g a jump from about six.months to at l e a s t nearly two years of continuous exposure. The spring high tide of December, 1977 was higher and defines a c r i t i c a l t i d a l l e v e l above which the duration of exposure i s at l e a s t nearly three years. (d) The l e v e l of the lowest lower low water ( i . e . , extreme spring lower low water) for each year from 1967 to 1987 at Tsawwassen, B.C. i s graphed. This l e v e l has r i s e n gradually from about -3.1 m Geodetic Datum i n 1968 to about -2.6 m Geodetic Datum i n 1978 based on observed t i d a l records. Mean spring lower low water ( i . e . , the mean of twelve monthly spring lower low waters for each year) helps define the trend. The predicted trends over the next nine years are dashed i n . The l e v e l of the x i i i LIST OF FIGURES (Cont'd) Figure ;Page lowest extreme spring lower low water i n December, 1968.defines a c r i t i c a l t i d a l l e v e l below which the duration of continuous submergence jumps from about 18 years to at l e a s t nearly 36 years. 2. (a)y:"• Ccosaacofrelat-ionxof^e-xtreme^crltical^tldal^ levelsabetween the 19 i n t e r t i d a l regions of St. John,; New Brunswick and Tsawwassen, B.C. Based on predicted tides f o r 1978 for St. John's and obser-ved tides between June, 1977 and June, 1978 at Tsawwassen. The numbers above and below c r i t i c a l t i d a l l e v e l s i n d i c a t e the maxi-mum duration of continuous exposure i n days i n the case of nt.i atmozonal c r i t i c a l t i d a l l e v e l s , and the maximum duration of continuous submergence i n days i n the case of aquazonal c r i t i c a l t i d a l l e v e l s . For example at the upper l i m i t of the lower atmozone at St. John the jump i s from 10 to 20 days.and the aa : . . : . next c r i t i c a l t i d a l l e v e l up involves a jump from 26 to 72 days. In the case of atmozonal c r i t i c a l t i d a l l e v e l s only the lowest l e v e l attained by a p a r t i c u l a r c r i t i c a l t i d a l l e v e l i s indicated. In the case of aquazonal c r i t i c a l t i d a l l e v e l s only the highest l e v e l attained by a p a r t i c u l a r c r i t i c a l t i d a l l e v e l i s ind i c a t e d , i . e . , only extreme c r i t i c a l t i d a l l e v e l s are indicated. (b) Levels of exposure zone boundaries over the past ten years at Tsawwassen, B.C. based on observed t i d a l records. In the case of the boundary between the lower atmozone and the upper atmozone the numbers above and below c r i t i c a l t i d a l l e v e l s i n d i c a t e the maximum duration of exposure i n days. In the case of the boundary between the upper aquazone and the lower aquazone the numbers above and below c r i t i c a l t i d a l l e v e l s i n d i c a t e the maximum duration of submergence i n days. On.the extreme r i g h t hand side of the diagram h o r i z o n t a l bars indi c a t e the mean l e v e l s of exposure zone boundaries while the v e r t i c a l bars indi c a t e the standard deviation from the mean. Mean l e v e l s are not indicated i n the case of the boundaries of the lower aquazone because they undergo s i g n i f i c a n t modulation by the e f f e c t s of an 18.6 year s o l i - l u n a r d e c l i n a t i o n a l cycle. Part 2 1. Map of the Fraser Delta area, showing the l o c a t i o n of Boundary Bay t i d a l f l a t s (reproduced from K e l l e r h a l s and Murray, 1969). 32 2. Floral/sedimentological zonation of Boundary Bay with the loca-tions of transects A and B indicated. Between the t i d a l channels the waterline approximates to the -2.4 m (-8.0 f t ) contour (Geodetic Datum). The waterline i n the t i d a l channels does not follow a s p e c i f i c contour since these depressions remain water-f i l l e d during low t i d e , despite the fa c t that they are w e l l above sea l e v e l . 34 x i v LIST OF FIGURES (Cont'd) Figure Page 3. Refl e c t i o n interference patterns i n the upper sand wave zone 43-. i n the area of Beach Grove. Also indicated are the probable dir e c t i o n s of wave induced currents. 4. Variations i n mean grain s i z e , s o r t i n g and mud content on tran^ sect A. 46' 5. Variations i n mean grain s i z e , s o r t i n g and mud content on tran-sr.- sect B. 47 6. Mean d a i l y exposure with respect to elevation f o r Boundary Bay 54 tid e s . This was compiled from 16 representative d a i l y t i d a l curves (eight mean t i d e s , four spring tides and four neap tides) selected from 25 ava i l a b l e i n the data of Weir (1963) covering the period of June to September, 1959. Using more than 16 days of t i d a l records would probably reduce some of the standard deviations, but i t would i f anything increase the ranges of possible values. 7. (a) The f i v e 'exposure l e v e l s ' possible f o r mixed semi-diurnal 57 tid e s . ; (b) The ranges of duration of continuous exposure for each of the f i v e exposure l e v e l s . (c) The ranges of duration of continuous submergence f o r each of the f i v e exposure l e v e l s . (d) The monthly modulation of the f i v e exposure l e v e l s f o r the period June 21-July 31, 1959. T i d a l data from Weir (1963) 8. The requency of four of the f i v e exposure l e v e l s with respect to elevation f o r a period of 73 lunar days from June 21-Septem-ber 4, 1959 (source: Weir, 1963). L e f t hand scale i n days, right hand scale i n percent. Also p l o t t e d i s the maximum number of consecutive lunar days of exposure or submergence f or those elevations where exposure or submergence can exceed one lunar day. 59 9. (a) Densities of B a t i l l a r i a sp. on transect A;.; d i f f e r e n t i a t i n g 62 between dry s i t e s and under water s i t e s (shallow t i d a l pools). (b) Densities of B a t i l l a r i a sp. on transect B. 10. (a) B a t i l l a r i a attramentaria producing a grazing t r a i l . I ts 63 s h e l l i s about 3 cm long. (b) B a t i l l a r i a grazing t r a i l s and r e s t i n g traces ( p i t s ) . Note the t r a i l s leading to p i t s . Trowel head i s about 5 cm wide. (c) Resting trace p i t s produced by B a t i l l a r i a sp. Four B a t i l l a r i a sp. can be seen s t i l l occupying p i t s . Trowel head i s about 5 cm wide. XV LIST OF FIGURES (Cont'd) Figure Page 11. (a) B a t i l l a r i a sp. heading upstream producing grazing t r a i l s 66 p a r a l l e l i n g the current d i r e c t i o n (eelgrass at top of photo indicates current d i r e c t i o n ) . Ballpo Lnt'pen i s about 15 cm long. (b) Behaviour of B a t i l l a r i a sp. i n weak currents: the gastro-pod heads upstream grazing, and produces a t r a i l p a r a l l e l i n g the current. Occasionally the current causes the gastropod to r o l l . Once s t a b i l i z e d again the gastropod turns i n towards the current and reverts to grazing i n an upstream d i r e c t i o n . 12. (a) Mounds produced by the feeding a c t i v i t i e s of Spio sp. 67 Pen i s about 15 cm long. (b) Plan view of Spio sp. Two t e n t a c l e - l i k e palps draw food i n t o i t s tube and are also used to void sandy pseudo-fecal s t r i n g s i n a r a d i a l pattern. (c) Cross section of Spio sp. i n i t s dwelling tube. 13. (a) Densities of Spio sp. on transect A. 69 (b) Densities of Spio sp. on transect B. 14. (a) Morphology of an Abarenicola burrow ( a f t e r Hylleberg, 71 1975). Arrows indi c a t e d i r e c t i o n of r e s p i r a t i o n current. (b) Patchy d i s t r i b u t i o n of Abarenicola f e c a l casts. The highest densities of casts occur i n and around t i d a l pools. Trowel i s about 25 cm high. 15. (a) Densities of Abarenicola f e c a l casts on transect A. Upper 73 histogram distinguishes between wet and dry s i t e s , and i s based on four wet s i t e readings and four dry s i t e readings at each s t a t i o n with a 0.25 m quadrat. Stations A8 and A9 had no wet s i t e s which could be sampled. Stations A11-A17 had no dry s i t e s . Station A5 had dry s i t e s , but Abarenicola was absent from them. Lower histogram presents the average d e n s i t i e s , based on random quadrats. (b) Densities of Abarenicola f e c a l casts on transect B. 16. (a) Densities of My a sp'. on transect A. 75 (b) Spreite traces l e f t by Mya sp., and downwarping of lamina-tions caused by movement of the clam within i t s burrow due to i t s growth or changes i n the l e v e l of the sediment water i n t e r f a c e ( a f t e r Reineck, 1958). 17. (a) Densities of C a l l i a n a s s a and Upogebia burrow openings on 76 ' transect A. x v i LIST OF FIGURES (Cont'd) Figure Page 17. (b) Densities of Callianassa burrow openings on transect B. 76 18. (a) Plan view of a C a l l i a n a s s a burrow cast, showing bulbous 79 'turnarounds.' Cast i s about 60 cm i n plan view length. Metric r u l e r (1 m) with centimeter subdivisions provides scale. (b) Side view of a C a l l i a n a s s a burrow cast showing h o r i z o n t a l mine-like nature of burrow system. Burrow extends to about 30 cm depth. Metric r u l e r (1 m) with centimeter subdivisions provides scale. Overflow of r e s i n produced 'heads' on cast. (c) Plan view of a large Callianassa burrow cast which i s j u s t over 1 m long. Metric r u l e r (1 m) provides s c a l e . 19. (a) Cast of two Upogebia 'Y' shaped burrows joined by a 83 c o n s t r i c t e d neck. Cast i s j u s t over 50 cm i n depth. Metric r u l e r (1. m) with centimeter subdivisions provides scale. (b) Side view of cast i n (a.) showing shrimp entombed within the cast. . 20. (a) Densities of P r a x i l l e l a sp. and Nassarius sp. on transect 86 A. (b) Densities of P r a x i l l e l a sp. and Nassarius sp. on transect B. 21. V e r t i c a l agglutinated sand tube of P r a x i l l e l a sp. 87 22. Zonation of biogenic sedimentary structures i n three of the f l o r a l / s e d i m e n t o l o g i c a l zones of Boundary Bay t i d a l f l a t s , and the expected s t r a t i g r a p h i c succession of biogenic sedimentary structures and grain s i z e parameters, i f the t i d a l f l a t s are prograding seawards without subsidence. 95 23. L a t e r a l heterogeneity within zonal b i o f a c i e s caused by topo- 96 graphy of small and large scale. ' (a) A l g a l mat zone: L a t e r a l heterogeneity created by upraised a l g a l mat platforms. (b) Upper sand wave zone: L a t e r a l heterogeneity created by sand waves. (c) Eelgrass zone: L a t e r a l heterogeneity created by C a l l i a n a s s a mounds and burrows. x v i i LIST OF FIGURES (Cont'd) . :•- Part 3 Figure P a g e 1. Location of study area. Upper map shows the general l o c a - 109 tion of Boundary Bay on the Fraser Delta and the lower maps the f l o r a l / s e d i m e n t o l o g i c a l zones of the t i d a l f l a t s . The two transects A and B were set up i n 1976 (Swinbanks, 1979). 2. Grain s i z e s o r t i n g by Abarenicola: 118 (a) Start of the experiment. An i n d i v i d u a l Abarenicola • (-2 g) was placed i n a homogenized mixture of sand (>63 /'m) and montmorillonite (<63/im). (b) A f t e r 24 hours a thick, b i o g e n i c a l l y formed lamina of montmorillonite (white) has developed as a r e s u l t of the i r r i g a t i o n a c t i v i t i e s of the worm. (c) A f t e r three days the lamina has been buried by f e c a l casts and the lamina has been deformed and bioturbated by the feeding and i r r i g a t i o n a c t i v i t i e s of the worm. 3. Sketch of a comparable s i t u a t i o n to that i l l u s t r a t e d i n Figure 2b. The important features of the Abarenicola burrow are l a b e l l e d and the d i r e c t i o n of flow of the r e s p i r a t i o n current which i r r i g a t e s the burrow i s indicated. A clay lamina has developed as a r e s u l t of fine grained clay p a r t i -cles f l o a t i n g out i n suspension i n the head shaft i r r i g a t i o n current and then s e t t l i n g on the substrate. 119 Part 4A 1.. Location of the study area i s indicated by h o r i z o n t a l cross-hatching. T i d a l f l a t s are s t i p p l e d , land area of Recent alluvium i s blank, and older deposits diagonally cross-hatched (adapted from Luternauer,/, and Murray, 1973). 128 2. The fl o r a l / s e d i m e n t o l o g i c a l zones of the inter-causeway t i d a l f l a t with the locations of transects, stations and bench marks indicated. 131 3. The e l e v a t i o n a l l i m i t s of the four major floral/sedimentoV . l o g i c a l zones on the inter-causeway t i d a l f l a t with respect to Geodetic Datum and the average exposure zone l i m i t s f o r observed tides between 1968-78 at the Tsawwassen t i d a l gauge (source: Swinbanks, 1979). The dotted envelopes ind i c a t e one standard deviation from the mean l e v e l of the exposure zone boundary. The exposure zone l i m i t s f o r the lower aquazone are based on only two years of records (1976-78) because these boundaries are s i g n i f i c a n t l y modulated by. an 18.6 year d e c l i n a t i o n a l cycle i n the moon (Swinbanks, 1979). 136 x v i i i LIST OF FIGURES (Cont'd) Figure Page The elevation of the saltmarsh/algal mat zone boundary was determined at seven points between transects B and C (Fig. 2) 4. (a) Laminated s i l t s of the saltmarsh. The coarser laminae 138 stand out i n r e l i e f . Pen i s 15 cm long. (b) The weathered surface of the saltmarsh reveals that the saltmarsh deposits are r i d d l e d with r o o t l e t s , which have weathered out as holes here. Pen i s 15 cm long. 5. The mud contents of surface sediments. Numbers next to stations i n d i c a t e the percent mud at each s t a t i o n . The general trends of the contours between transects have been determined by q u a l i t a t i v e f i e l d observations. 140 6. (a) The a l g a l mat zone. The channel i n the foreground i s 141 about one meter wide. Taken on flo o d t i d e , j u s t as the channels are beginning to f i l l . Mudcracked plateaus l i e between the channels, and w a t e r - f i l l e d depressions are present on the plateaus. (b) Mudcracked surface of the a l g a l mat zone. The a l g a l mats b l i s t e r and c u r l under the e f f e c t s of desiccation, and cracking produces i s o l a t e d ' a l g a l mat cakes.' (c) Undersurface of an ' a l g a l mat cake,' r i d d l e d by crab burrows. (d) Laminated sediments of an a l g a l mat cake, cross-cut by a crab burrow (Hemigraspus oregortensis). 7. The median grain s i z e . (0) of,surface sediments.' The numbers next to-stations i n d i c a t e median' vgrain s i z e (0) at each s t a t i o n . 145 8. Surface substrate s a l i n i t i e s as measured on transects A, B and C August 13, 1977 at low tide between 11:30 and 14:25 i n the order indicated. Numbers next to stations i n d i c a t e s a l i n i t y . 148 9. T y p i c a l substrate s a l i n i t y p r o f i l e from the inter-causeway area. 149 10. D i s t r i b u t i o n of Callianassa burrow openings i n contoured map form. General trends of contours between transects deter-mined by q u a l i t a t i v e f i e l d observation. 150 11. D i s t r i b u t i o n of Upogebia burrow openings i n contoured map form. 151 12. (a) Density of Upogebia burrow openings vs median grain s i z e 153 (0), regardless of elevation. x i x LIST OF FIGURES (Cont'd) Figure 12. (b) Density of Upogebia burrow openings vs mud content (%), regardless of elevation. . 13. Relationship between mud content (%) and Upogebia burrow opening density, with data grouped i n t o 0.25 m elevation class i n t e r v a l s ; B e s t - f i t l i n e a r regression l i n e s are indicated, along with t h e i r c o r r e l a t i o n c o e f f i c i e n t s (r) and confidence l e v e l s (r_ t e s t ) . Elevation (Geodetic Datum) increases from A to J. 14. Relationship between mud content (%) and C a l l i a n a s s a burrow opening density, with data classed into 0.25 m elevation class i n t e r v a l s . B e s t - f i t l i n e a r regression l i n e s are indicated, along with t h e i r c o r r e l a t i o n c o e f f i -cients (r) and confidence l e v e l s (r t e s t ) . Elevation (Geodetic Datum) increases from A to L. 15. Relationship between Upogebia burrow opening density and Callianassa burrow opening density with the data classed into 0.25 m elevation class i n t e r v a l s . B e s t - f i t l i n e a r regression l i n e s are i n d i c a t e along with t h e i r c o r r e l a t i o n c o e f f i c i e n t s (r) and confidence l e v e l s (r_ t e s t ) . Elevation (Geodetic Datum) increases from A to J. 159 16. The near-surface stratigraphy of the t i d a l f l a t on transect A. Upogebia burrows extend down into a blue-grey clayey mud horizon (the v e r t i c a l dimensions of the burrows are drawn to s c a l e ) . This bioturbation may account f o r the anomalously high mud contents of the surface sediments. 162 Page 153 155 156 Part 4B 1. Location of study area. T i d a l f l a t s are s t i p p l e d , land area of Recent alluvium i s blank, and older deposits cross-hatched (adapted from Luternauer and Murray, 1973). 177 2. Locations of stations sampled by hovercraft,; h e l i c o p t e r and on foot i n 1977 and 1978. 180 3. Topographic map of Roberts Bank i n 1967. The datum for contours i n t h i s map i s -2.63 m Geodetic Datum. Contours i n meters (source: Swan Wooster, 1967). 181 4. Floral/sedimentological zones of Roberts Bank, prepared from a colour a e r i a l photograph of June, 1978 (A37597-146, N.A.P.L., Ottawa, Canada). 184 5. High l e v e l a e r i a l photograph of the Fraser Delta. The water-l i n e l i e s at about -0.12 m Geodetic Datum on a flooding t i d e , 190 XX LIST OF FIGURES (Cont'd) and l i e s above the lower l i m i t of the marsh at Westham Island (centre) i s approaching the a l g a l mat zone i n the inter-causeway area and l i e s at the upper l i m i t of the eelgrass zone i n Boundary Bay (refer to F i g . 1 for l o c a -tion) . Water i s beginning to flood into the d i s t r i b u t a r y channels of the Brunswick Point marsh from Canoe Pass. Percent mud i n surface sediments of northern and c e n t r a l Roberts Bank. Mechanical contouring employed. Discharge curves for the Fraser River in c l u d i n g the freshet portion of the runoff f o r 1948, a severe flo o d year i n the Fraser Valley (adapted from R. Thompson, unpublished data). Surface substrate s a l i n i t i e s on Roberts Bank at low tide on August 17, 1978. Mechanical contouring employed. (a) Surface-water s a l i n i t i e s at high tide on ebb between Canoe Pass and the f e r r y causeway on June 8, 1978. Stippled area indicates region where a h a l o c l i n e i s present i n the water column. Mechanical contouring employed. Three representative salinity/temperature' p r o f i l e s included. (b) Surface substrate and shallow water s a l i n i t i e s on approaching low tide on June 8, 1978. Mechanical contour-ing employed. Percent thickness of the s a l t wedge, for 17.5%. as the boundary between marine and brackish water masses. Mecha-n i c a l contouring. Numbers next to stations i n d i c a t e , percent thickness of s a l t wedge. D i s t r i b u t i o n of thalassinidean shrimp burrow openings on Roberts Bank based on data c o l l e c t e d i n 1977 and 1978. Where stations sampled i n 1977 were reoccupied i n 1978 the average density has been used. Data for the inter-causeway area are presented i n Part 4A. Mechanical contouring employed. Relationship between Callianassa burrow opening density and the s a l i n i t y of surface substrate waters. B e s t - f i t l i n e a r regression l i n e s are drawn along with t h e i r c o r r e l a t i o n c o e f f i c i e n t s (r) and s i g n i f i c a n c e l e v e l (r t e s t ) . (a) Relationship between Callianassa burrow opening density and surface substrate s a l i n i t y at a f i x e d t i d a l l e v e l (-1.55 m Geodetic Datum), September 10-11, 1977. (b) Substrate s a l i n i t y p r o f i l e s at Stations 1-6, September 10-11, 1977. xxi LIST OF FIGURES (Cont'd) Figure Page 14. Ty p i c a l geometry of Ca l l i a n a s s a and Upogebia burrows on the Fraser Delta. Dimensions for cross-section of Upogebia burrow l i n i n g obtained from Thompson (1972). 214 15. (a) Resin cast of a Ca l l i a n a s s a burrow coated i n sand with 216 a knobbly surface reminiscent of OphiomOrpha. Note that at top l e f t the sand l i n i n g has been worn away by the s t r i n g and the inner burrow l i n i n g i s smooth. (b) F o s s i l i z e d Callianassa burrow having the appearance of Thalassinoides. Sand has i n f i l l e d a burrow i n mud. 16. Cast of a Ca l l i a n a s s a burrow taken from an area of high 218 burrow density (446 burrow openings m ). Two c o n s t r i c t e d entrances meet as a bulbous chamber at about 10 cm depth, and a v e r t i c a l stem extends from t h i s to about 50 cm depth. Cryptomya c a l i f O r n i c a c l u s t e r around the bulbous chamber at the junction of the two e x i t s . About t h i r t y of these commensal bivalves are attached to the cast. The burrow i s occupied by one shrimp. A burrow system of smaller diameter branches off from t h i s system. I t i s joined to the main burrow system by a narrow c o n s t r i c t e d neck and i s occupied by a small j u v e n i l e shrimp. Scale i n centimeters. 17. Summary of the d i s t r i b u t i o n of thalassinidean burrow and 222 f l o r a l / s e d i m e n t o l o g i c a l zones on a l l the t i d a l f l a t s of the Fraser Delta south of Main Channel, i n the form a s t r a t i g r a p h i c succession, constructed by p r o j e c t i n g a l l the density data i n Figure 11 onto a v e r t i c a l plane pass-ing through points A, B and C i n Figure 11. The data on percent thickness of the s a l t wedge i n Figure 10 have also been included to demonstrate the r e l a t i o n s h i p between thalassinidean shrimp d i s t r i b u t i o n and s a l i n i t y regime. No data on the s a l t wedge are a v a i l a b l e NW of Canoe Pass. Data from Boundary Bay are based on Transect A alone (Swinbanks, 1979). Exposure zones allow cross c o r r e l a t i o n between Roberts Bank and Boundary Bay. Winter t i d a l data are not a v a i l a b l e f o r Boundary Bay and as a r e s u l t the upper l i m i t of the atmozone cannot be defined, but the :. spring t i d a l l e v e l s f o r June indicated allow cross corre-l a t i o n i n the uppermost i n t e r t i d a l regions. x x i i ACKNOWLEDGEMENTS I w o u l d l i k e t o s i n c e r e l y t hank D r . J . W . M u r r a y , my s u p e r v i s o r , f o r p r o v i d i n g c o n s t a n t m o r a l and f i n a n c i a l s u p p o r t , and f o r i n i t i a l l y d i r e c t i n g me i n t o t h i s f a s c i n a t i n g l i n e o f r e s e a r c h . I am g r a t e f u l t o D r . J . L . L u t e r n a u e r , G e o l o g i c a l S u r v e y o f Canada f o r a r r a n g i n g f i n a n c i a l s u p p o r t and t r a n s p o r t a t i o n f o r much o f t h e f i e l d w o r k , and f o r h i s c o n s t a n t e n t h u -s i a s m and i n t e r e s t . I am a l s o g r a t e f u l t o D r . C D . L e v i n g s , f P a c i f i c E n v i -ronment I n s t i t u t e , f o r m a k i n g a v a i l a b l e l a b o r a t o r y f a c i l i t i e s f o r t h e < J X ? C : > e x p e r i m e n t s on A b a r e n i c o l a and f o r h i s a d v i c e and c o n s t r u c t i v e c r i t i c i s m o f t h e b i o l o g i c a l c o n t e n t o f t h i s t h e s i s . I t h a n k D r . W . C . B a r n e s f o r c r i t i c a l l y r e a d i n g and r e - r e a d i n g t he m a n u s c r i p t . D r . M. P o m e r o y , P a c i f i c E n v i r o n m e n t I n s t i t u t e , D r . P . G . H a r r i s o n , U n i v e r s i t y o f B r i t i s h C o l u m b i a and D r . T . H . C a r e f o o t , U n i v e r s i t y o f B r i t i s h C o l u m b i a i d e n t i f i e d many o f t h e f l o r a and f a u n a . I am e s p e c i a l l y i n d e b t e d t o t h e o f f i c e r s and c rew o f t h e C a n a d i a n C o a s t Guard H o v e r c r a f t U n i t , V a n c o u v e r I n t e r n a t i o n a l A i r p o r t f o r p r o v i d i n g and o p e r a t i n g t h e i r c r a f t f o r f i e l d i n v e s t i g a t i o n s on R o b e r t s B a n k . M r s . M. M u h l e r t , M r . J . P . N a p o l e o n i , M r . G . Hodge and D r . J . P . S y v i t s k i a b l y a s s i s t e d i n t h e f i e l d . Gordon H o d g e , E l s p e t h A r m s t r o n g and B e r n i e von S p i n d l e r d r a f t e d many o f t h e d i a g r a m s . T h e i r p a t i e n c e and e x c e l l e n t work i s much a p p r e c i a t e d . L a s t b u t n o t l e a s t I must • thank'#Nof.$ko,tjinyggiM if friend,u.^holi:hegiped&int<tehef f i e l d g o n i n n u m e r a b l e o c c a s i o n s , d r a f t e d many o f t h e r o u g h d r a f t v e r s i o n s o f d i a g r a m s , t y p e d , r e - t y p e d and r e - r e - t y p e d t he s c r i p t and who had t o l i v e t h r o u g h t he c o m p i l a t i o n o f t h i s t h e s i s . 1 INTRODUCTION Geologists are becoming i n c r e a s i n g l y aware of the p o t e n t i a l of tra c e f o s s i l s as paleoenvironmental i n d i c a t o r s . Study of biogenic.sedimentary s t r u c t u r e s and animal-sediment r e l a t i o n s h i p s i n present day environments i s an expanding f i e l d of research, which i s a t t r a c t i n g the a t t e n t i o n of sedimen-t o l o g i s t s , p a l e o n t o l o g i s t s and marine b i o l o g i s t s . In the past research by g e o l o g i s t s i n modern environments has tended to be l a r g e l y d e s c r i p t i v e . I n -v e s t i g a t i o n of the f a c t o r s c o n t r o l l i n g the d i s t r i b u t i o n of organisms was the domain of the marine b i o l o g i s t . However t h i s s i t u a t i o n i s changing and geoUrofgI'sitfs are becoming i n v o l v e d i n i n c r e a s i n g numbers i n e c o l o g i c a l l y o r i e n t a t e d s t u d i e s of animal-sediment r e l a t i o n s h i p s . A wealth of e c o l o g i c a l data^already e x i s t s i n the b i o l o g i c a l l i t e r a t u r e , but o f t e n i t s relevance to sedimentology and paleoenvironmental s t u d i e s i s not immediately apparent. For example much of the c l a s s i c work on i n t e r t i d a l zonation by marine b i o l o -g i s t s has been c a r r i e d out on rocky i n t e r t i d a l s h o r e l i n e s , which i n themselves are of no p a r t i c u l a r i n t e r e s t to sedime n t o l o g i s t s . However, many o f ; t h e - p r i n -cip-Ies and f i n d i n g s on rocky i n t e r t i d a l s h o r e l i n e s can be a p p l i e d to t i d a l f l a t s , which are, of course, of great i n t e r e s t to se d i m e n t o l o g i s t s . I f trace f o s s i l s are to a t t a i n t h e i r f u l l p o t e n t i a l as paleoenvironmental i n d i c a t o r s f u r t h e r i n f o r m a t i o n on the e f f e c t s of p e r t i n e n t environmental f a c t o r s on the d i s t r i b u t i o n of tra c e making organisms must be obtained. I t was w i t h t h i s i n mind that the f o l l o w i n g study was c a r r i e d out. This study of the Fraser D e l t a i s of n e c e s s i t y i n t e r - d i s c i p l i n a r y i n nature i n v o l v i n g the d i s c i p l i n e s of sedimentology, b i o l o g y , oceanography and even some astronomy! The buzz word I n t e r - d i s c i p l i n a r y may be new, but the approach i s as o l d as science i t s e l f . This t h e s i s analyzes the d i s t r i b u t i o n of various t r a c e making organisms 2 on the t i d a l f l a t s of the Fraser Delta? The organisms investigated include the thalassinidean burrowing shrimps Callianassa c a l i f o r n i e n s i s and Upogebia  pugettensis, the burrowing polychaete Abarenicola p a c i f i c a and the tube-dwelling polychaetes P r a x i l l e l a a f f i n i s p a c i f i c a and Spio rsp., the gastropods B a t i l l a r i a attramentaria and Nassarius mendicus and the bivalve Mya arenaria. As an irit^igXiail-; part of th i s research the d i s t r i b u t i o n of various f l o r a l zones i n the i n t e r t i d a l region (e.g., saltmarsh, a l g a l mats, eelgrass) i s also analyzed. The burrowing shrimp Callianassa c a l i f O r n i e n s i s i s the organism of most i n t e r e s t because i t i s ubiquitous to a l l Fraser Delta t i d a l f l a t s , t o l e r a t i n g wide ranges i n substrate type and s a l i n i t y regime. Of almost equal i n t e r e s t i s Upogebia pugettensis, but t h i s shrimp i s more r e s t r i c t e d i n occurrence. Thalassinidean shrimp burrows are w e l l known i n the trace f o s s i l record (Thalassinoides and Ophiomorpha) extending at l e a s t as f a r back as the Cretaceous (Borradaile, 1903). The questions which t h i s thesis sets out to answer are: (1) What i s the d i s t r i b u t i o n of these organisms on the t i d a l f l a t s of the Fraser Delta and how do various environmental factors influence t h e i r d i s t r i b u t i o n ? (2) What i s the nature of the biogenic sedimentary structures the organisms produce? (3) At what rate do they rework the substrate by burrowing into and/or ingesting sediment? The environmental factors which have been considered include: a) Elevation; which determines the duration of exposure and submergence b) Grain s i z e of the substrate c) Environmental energy d) S a l i n i t y regime e) Bio-inter a c t i o n s . Bio-rinter..ae.tions£in£ludetfiheiiif fijj.ue.riGeoof f f l o r a l c o o v e r ((e«g. , saltmarsh, eelgrass or a l g a l mats) on faunal d i s t r i b u t i o n and vice versa, and also any i n t e r - f a u n a l i n t e r a c t i o n s — e.g., trophic group ammensalism (Rhoads and Young, 1970). The study area l i e s on the coast of . B r i t i s h Columbia i n temperate l a t i -tudes and experiences a west coast maritime climate, characterized by summers 3 which are cool, sunny and not very humid and by winters which are cloudy, mild and wet. The Fraser River i s the dominant source of terrigenous sediment to the S t r a i t of Georgia (Pharo and Barnes, 1976). The S t r a i t of Georgia l i e s on the western margin of the North American p l a t e . The basin which i t occupies began i t s formation about 150 m i l l i o n years ago as part of the extensive Georgia Depression. Mountain b u i l d i n g ceased about.two m i l l i o n years ago and the S t r a i t took i t s present form about a m i l l i o n years l a t e r . Since then, g l a c i a l scouring, downwarpirjg and erosion have continued to modify i t . Present evidence indicates that the Fraser Delta began to fan out from a gap i n the Pleistocene uplands at New Westminster, where the present Fraser River b i f u r c a t e s i n t o North Arm and Main Channel (Fig. 1), about 8,000 years ago. Since then an estimated 120-210^m of d e l t a i c sediments have been deposited (Mathews and Shepherd, 1962). The present d e l t a meets the sea along a perimeter about 35 km long. Of t h i s , a front approximately 20 km long faces west i n t o the S t r a i t of Georgia, while a now i n a c t i v e front about 15 km long faces south onto Boundary Bay. Between these two fronts l i e s a former i s l a n d , now the Point Roberts Peninsula. I n t e r t i d a l marshes and t i d a l f l a t s 4 to 8 km wide slope gently seaward from the edge of the cu l t i v a t e d lands of the de l t a , before the de l t a surface dips more steeply into the S t r a i t of Georgia. The mean t i d a l range i s about 3iamwi'th extreme spring t i d a l ranges of 5 m and neap t i d a l ranges of 1.5 m. Tides are of mixed semi-diurnal type i . e . , there are two high waters and two low waters each day but successive high and successive low waters are of d i f f e r e n t height. The Fraser River reaches i t s peak discharge during l a t e spring and early S u m - * merger The less dense r i v e r water spreads over the s a l i n e waters of the S t r a i t of Georgia as a v i s i b l e plume of muddy fresh to brackish water. Central S t r a i t of Georgia waters are thus s t r a t i f i e d into a brackish surface layer and an underlying s a l t wedge (Waldichuk, 1957). The s a l t wedge intrudes 4 Figure 1. General l o c a t i o n of Fraser D e l t a . T i d a l f l a t s are s t i p p l e d , land area of Recent a l l u v i u m i s blank, and o l d e r deposits cross-hatched (adapted from Luternauer and Murray, 1973). 5 the Fraser River on flood tides extending as f a r as 20 km upstream of the inner t i d a l f l a t s during winter, but no further than the inner edge of the f l a t s during the freshet (Ages and Woollard, 1976). The format of t h i s thesis i s a s e r i e s of papers — P a r t s 1 to 4. Part 4 i s s p l i t i nto two sections Part 4A and B. I t i s intended that shortened versions of each Part w i l l be published i n the near future. Each Part must therefore be able to stand on i t s own. Inevitably, as they a l l deal with the same study area, there i s some r e p e t i t i o n of facts and ideas between Parts but as f a r as possible t h i s has been kept to a minimum. Cross r e f e ^ rences are usually made as Swinbanks (1979), unless i t i s not obvious the Part referred to i n which case Part number i s included, e.g., Swinbanks (1979, Part 3). Part 1 provides a basis f o r describing l o c a t i o n i n the i n t e r t i d a l region which i s applied i n the t i d a l f l a t studies that follow. I t i s a new scheme fo r subdividing t h e i i n t e r t i d a l region based on c r i t i c a l t i d a l l e v e l s at which the maximum duration o"fjiconti>nuous^exposu-re or submergence increases abruptly i n a s t e p - l i k e manner. This scheme i s advocated because (1) i t allows meaningful cross c o r r e l a t i o n between i n t e r t i d a l regions experiencing d i f f e r e n t types of astronomically c o n t r o l l e d t i d e s , e.g., between a region experiencing mixed tides with one experiencing semi-diurnal (2) the c r i -t i c a l t i d a l l e v e l s on which the scheme i s based may be causally r e l a t e d to i n t e r t i d a l zonation. None of the schemes currently i n use, based on mean t i d a l l e v e l s or mean exposure l e v e l s , f u l f i l l s e i t h e r of these r o l e s . A mean t i d a l l e v e l c a r r i e s with i t no information on the duration of continuous exposure or continuous submergence at that l e v e l , nor does a mean exposure value, as i n the computation of.mean exposure no d i s t i n c t i o n i s drawn between continuous and discontinuous exposure or submergence. One might expect that organisms should respond to predictable recurrent extremes i n exposure or 6 submergence, but one can hardly expect them to have a concept of abstract averages. Part 2 i s a study of the zonation of Mora:.\, fauna and t h e i r biogenic sedimentary structures,, on the t i d a l f l a t s of Boundary Bay on the i n a c t i v e southern flank of the Delta. These t i d a l f l a t s are unusual i n that the grain s i z e of the substrate over much of the Bay varies l i t t l e , c o n s i s t i n g predominantly of f i n e to very f i n e w e l l to very well sorted sands. The influence of the Fraser River on 'thisgbay i s s l i g h t and the Bay waters are 'normal marine' f o r the southern S t r a i t of Georgia. As a r e s u l t a d i s t i n c t f l o r a l / f a u n a l zonation e x i s t s , delimited p r i m a r i l y by elevation, which i s comparable i n many respects to the precise e l e v a t i o n a l d e l i m i t a t i o n of zonation found on rocky i n t e r t i d a l shorelines experiencing stable s a l i n i t y regimes. The scheme of i n t e r t i d a l subdivision outlined i n Part 1 i s applied and developed f o r the s p e c i f i c case of Boundary Bay t i d e s , and i t i s demon-stra t e d that much of the zonation of Boundary Bay t i d a l f l a t s may be causally r e l a t e d to c r i t i c a l t i d a l l e v e l s . Part 3 supplements Part 2. I t i s a study of the sediment reworking and size s o r t i n g c a p a b i l i t i e s of Abarenicola p a c i f i c a a polychaete which i s abundant on the Boundary Bay t i d a l f l a t s . By constantly turning over the surface sediments t h i s organism may we l l influence zonation on the t i d a l f l a t , and i n p a r t i c u l a r the bioturbation of t h i s worm may be a factor l i m i t i n g the extent of the a l g a l mat zone, one of the f i v e major f l o r a l / s e d i m e n t o l o g i c a l zones of Boundary Bay t i d a l f l a t s . Part 4 moves onto the active t i d a l f l a t s of the Fraser Delta. I t i s a study of the d i s t r i b u t i o n of the thalassinidean burrowing shrimps, Callianassa. c a l i f o r n i e n s i s and Upogebia pugettensis on Roberts Bank. Roberts Bank divides n a t u r a l l y and abruptly into a 'marine' environment to the southeast and a brackish environment to the northwest. The t r a n s i t i o n occurs between the 7 Coalport causeway and Canoe Pass (Fig. 1). I t was therefore f e l t appropriate to s p l i t Part 4 into two sections, Part 4A dealing with an e x c l u s i v e l y 'marine' environment between the two man-made causeways on southeastern Roberts Bank and Part 4B dealing with the marine to brackish t r a n s i t i o n on northern and c e n t r a l Roberts Bank. The inter-causeway t i d a l f l a t studied i n Part 4A l i e s between the Tsawwassen fe r r y terminal causeway and the Coal-; r . port causeway (Fig. 1). I t has many of the c h a r a c t e r i s t i c s of Boundary Bay and a s i m i l a r f l o r a l / f a u n a l zonation i s developed, .but there are differences which probably r e s u l t from differences i n the s t y l e of t i d a l channel drainage i n the two areas, which i n turn i s a function of grain s i z e . There!is much greater v a r i a b i l i t y i n the grain s i z e of the substrate on t h i s t i d a l f l a t than i n Boundary Bay. Mud contents of the sediment are an order of magnitude higher and grainfsd-zer plays an important r o l e i n c o n t r o l l i n g the d i s t r i b u t i o n of thalassinidean shrimps. Thalassinidean shrimps a t t a i n t h e i r highest densities on the inter-cause-causeway t i d a l f l a t and the e f f e c t s of grain s i z e on shrimp d i s t r i b u t i o n and inter a c t i o n s between the two species of shrimp can r e a d i l y be analyzed i n an environment where s a l i n i t y can be considered non-variable. Armed with the information from t h i s t i d a l f l a t i n Part 4B the complexities of the environ-ment of northern and cen t r a l Roberts Bank ate' tackled where s a l i n i t y becomes an added v a r i a b l e among the factors i n f l u e n c i n g shrimp d i s t r i b u t i o n . On passing from the 'marine' to brackish environment on c e n t r a l Roberts Bank the flo r a l / s e d i m e n t o l o g i c a l zones of the t i d a l f l a t s are completely restructured and shrimp d i s t r i b u t i o n responds to the changes. Some discoveries are made regarding the apparent s a l i n i t y tolerance of these two species of shrimp which have important paleoenvironmental implications. 8: REFERENCES Ages, A. and Woollard, A., 1976, The tides i n the Fraser Estuary: Pac. Mar. S c i . Rept. 76-5, 100 p. Borradaile, L.A., 1903, On the c l a s s i f i c a t i o n of the Thalassinidea: Ann. Mag. Nat. H i s t . , s e r i e s 7, 12, p. 534-551. Luternauer, J.L. and Murray, J.W., 1973, Sedimentation on the Western Delta-front of the Fraser River, B r i t i s h Columbia: Can. Jour. Earth .Sci., -, v. 10, p. 1642-1663. Mathews, W.H. and Shephard, F.P,,,'Yl962, Sedimentation of Fraser River Delta, B r i t i s h Columbia: B u l l . iAmer .Assoc. Pet. G e o f C , " " v. 46, p.: 1416-1443. Pharo, CH. and Barnes, W.C., 197.S-', D i s t r i b u t i o n of s u r f i c i a l sediments of the c e n t r a l and southern S t r a i t of Georgia, B r i t i s h Columbia: Can. Jour. Earth Sci"., v. 13, p. 684-696. Rhoads, D.C. and Young, 1970, The influence of deposit-feeding benthos on bottom s t a b i l i t y and community trophic structure: Jour. Marine Res. v. 28, p. 150-178. Waldichuk, M., 1957, Physical oceanography of the S t r a i t of Georgia, B r i t i s h Columbia: Jour. F i s h . Res. Brd. Canada, v. 14, p. 321-486. Part 1 INTERTIDAL EXPOSURE ZONES: A NEW SCHEME FOR SUBDIVIDING THE INTERTIDAL REGION 10 ABSTRACT There are at l e a s t four orders of c r i t i c a l t i d a l l e v e l within the i n t e r t i d a l region, at which the duration of continuous exposure or submer-gence 'jumps' - d a i l y (1st order), monthly (2nd order), annual (3rd order) and 18.6 year (4th order) c r i t i c a l t i d a l l e v e l s . Daily c r i t i c a l t i d a l l e v e l s delimit three exposure zones which experience markedly d i f f e r e n t extremes i n exposure and submergence duration - the atmozone, the amphizone and the aquazone. Monthly c r i t i c a l t i d a l l e v e l s divide the atmozone and aquazone i n t o upper and lower subzones. Subdivision of the amphizone i s only possible for mixed t i d e s . This scheme allows cross c o r r e l a t i o n between i n t e r t i d a l regions experiencing d i f f e r e n t astronomically c o n t r o l l e d t i d e s , and may be causally r e l a t e d to i n t e r t i d a l zonation. 11 Introduction The study of i n t e r t i d a l zonation has attracted the attention of marine b i o l o g i s t s since the beginning of nineteenth century and a vast l i t e r a t u r e e x i s t s on the topic (Ricketts and Calvin, 1968,). Geologists have been studying the d i s t r i b u t i o n of organisms, both f l o r a l and faunal, on carbonate t i d a l f l a t s f o r several decades, and more recently the expanding study of animal-sediment r e l a t i o n s h i p s has included study of organism d i s t r i b u t i o n on c l a s t i c t i d a l f l a t s . A record of the presence and d i s t r i b u t i o n of i n t e r t i d a l organisms i s preserved i n ancient sediments i n the form of biogenic sedimentary structures. Despite this extensive research over many years no u n i v e r s a l l y accepted scheme for subdividing the i n t e r t i d a l region e x i s t s . Geologists have discussed the problem of defining i n t e r t i d a l l o c a t i o n l i t t l e , with the exception of Ginsburg et a l . (1970) who advocate the use of 'exposure index' (mean percent exposure). Among, b i o l o g i s t s the extent, of the role which:tides play i n i n t e r t i d a l zonation has been a matter of much controversy (Carefoot, 1977). At one extreme there are those who believe that tides are not causally r e l a t e d to zonation (Stephenson and Stephenson, 1949) and they advocate a scheme of i n t e r t i d a l subdivision based purely on b i o l o g i c a l grounds (e.g., the upper l i m i t of barnacles or laminarians, e t c . ) . At the other extreme are those who believe that s p e c i f i c t i d a l l e v e l s can be causally r e l a t e d to zone boundaries (Doty, 1957). The current consensus of opinion among b i o l o g i s t s appears to be that the i n t e r t i d a l zone should be subdivided on the basis of biology, while only loosely c o r r e l a t i n g this s u b d i v i s i o n to mean t i d a l l e v e l s or mean exposure l e v e l s (Ricketts and Calvin, 1968; Chapman, 1974). While adhering to this viewpoint Chapman and Chapman .believe, that "when more i s known -abo.ut the causal .relationships between t i d a l phenomena and. the major b e l t orga-• nisms: jtt would probably be more desirable to use t i d a l data" (1973, p. 353). But 12 there i s no reason to expect causal r e l a t i o n s h i p s between mean t i d a l l e v e l s or mean exposure l e v e l s and zone boundaries, and the continued use of mean t i d a l values to describe l o c a t i o n i n the i n t e r t i d a l region i n h i b i t s further understanding of causal r e l a t i o n s h i p s between t i d a l phenomena and zonation. Further, schemes based on mean t i d a l l e v e l s cannot j u s t i f i a b l y be used to cross correlate areas experiencing mixed tides with those experiencing semi-d i u r n a l , because there i s no reason to think that a semi-diurnal mean t i d a l l e v e l (.e.g., mean high water) i s c o r r e l a t i v e with the 'equivalent' mixed mean t i d a l l e v e l (mean higher high water) e i t h e r i n respect to the frequency or duration of continuous exposure or submergence. Nor can mean exposure schemes be used, because i n the c a l c u l a t i o n of mean exposure no d i s t i n c t i o n i s made between continuous and discontinuous exposure or submergence. C r i t i c a l T i d a l Levels The concept on which the scheme presented here i s based - c r i t i c a l t i d a l l e v e l s - was f i r s t described over t h i r t y years ago by Doty (1946), and b i o l o g i s t s were aware of the concept long before then (Doty, 1957). Doty, • however, does not advocated the use of c r i t i c a l t i d a l l e v e l s i n subdivision, of the" i n t e r t i d a l region, because he considers" such ~a system 'too complex to be •satis f a c t o r y 1 ^(T9'57',' p. 542).'" C r i t i c a l t i d a l l e v e l s are defined here,; as ....'4 l e v e l s at which the duration of continuous exposure or submergence changes abruptly i n a s t e p - l i k e manner at the height of a crest or trough i n a d a i l y , monthly, annual or longer term t i d a l cycle. This d e f i n i t i o n should not be confused with the ' c r i t i c a l t i d a l l e v e l s ' discussed by Underwood (1978) and other workers in.'.'Britain (Colman, 1933; Evans, 1947a, b, 1957; Lewis, 1964), which are defined by breaks i n slope of mean exposure curves and/or by the clumping of the upper and lower l i m i t s of organisms at p a r t i c u l a r t i d a l l e v e l s . 13 I t has not, u n t i l now, been pointed out that there are several orders of c r i t i c a l t i d a l l e v e l s which can be recognized, depending on the duration of the lunar, solar or earthly cycle responsible (Fig. 1). Daily (1st order) c r i t i c a l t i d a l l e v e l s are a r e s u l t of the earth's r o t a t i o n on i t s axis combined with the d e c l i n a t i o n of the moon. Of the three p r i n c i p a l types of da i l y t i d a l cycle - semi-diurnal, diurnal and mixed - the mixed t i d e can be considered to be the general case and semi-diurnal and diurnal tides to be s p e c i a l forms. For mixed tides there are two high waters and two low waters for each lunar day of 24 hours and 50 minutes, but successive high and low waters d i f f e r i n height due to the e f f e c t s of the moon's d e c l i n a t i o n . On any p a r t i c u l a r day an i n t e r t i d a l region experiencing mixed tides can be subdivided i n t o f i v e 'exposure l e v e l s , ' which are defined by the heights of high and low waters (Fig. l a ) . The duration of continuous exposure or sub-mergence i s at l e a s t halved or doubled on passing from one exposure l e v e l to the next. Figure l a i s somewhat o v e r - s i m p l i f i e d , as mixed tides have both a height asymmetry and a time asymmetry, and the times between successive high tides and successive low tides are not equal due to l a g e f f e c t s which are dependent on t i d a l range. For example, the time between higher high water and lower high water i s less than the time between lower high water and the next higher high water. As a r e s u l t , the maximum duration of continuous exposure or submergence i s not exactly halved or doubled on leaving Exposure Level 3. Nevertheless, there i s an abrupt step i n exposure or submergence duration of the order of magnitude ind i c a t e d . In areas experiencing semi-diurnal tides the moon's d e c l i n a t i o n has l i t t l e influence on the tides and thus they lack a pronounced d i u r n a l i n e q u a l i t y i n t i d a l heights. As a r e s u l t , Exposure Levels 2 and 4 are suppressed, spanning only a very narrow el e v a t i o n range. In regions experiencing d i u r n a l tides the moon's de c l i n a t i o n has such a pronounced influence on the tide that the lower 14 Figure 1. (a) Schematic d a i l y t i d a l curves for the three main types of ti d e . Shading indicates submergence. For mixed tides there are four c r i t i c a l t i d a l l e v e l s (dashed l i n e s ) at which the duration of exposure or submergence ' j.umps' and which define f i v e exposure l e v e l s within which exposure and submergence changes continuously with respect to elevation. Higher high water defines the boundary between Exposure Levels 1 and 2, and i s a c r i t i c a l t i d a l l e v e l above which the duration of exposure doubles from less than one lunar day to at least nearly two. Lower high water defines the boundary between Exposure Levels 2 and 3 and i s a level," above which the. duration'of exposure doubles from l e s s than h a l f a lunar day to just under one lunar day. Equivalent c r i t i c a l t i d a l l e v e l s l i e •• at. the heights of higher low water and lower low water but involve steps i n submergence duration. They define Exposure Levels 4 and 5. The same c r i t i c a l t i d a l l e v e l s and expo-sure l e v e l s can be recognized f o r semi-diurnal tides.' Exposure Level 3 cannot be defined for d i u r n a l tides because they lack lower high water and higher low water stages. (b) The predicted d a i l y heights of lower low water at Point Atkinson, B.C. between August and November, 1977 are p l o t t e d as dots. The low waters are modulated into two neap tides and two spring tides per month. Successive spring tides are of d i f f e r e n t ranges. The spring t i d e of l e s s e r range defines a c r i t i c a l t i d a l l e v e l at which duration of continuous submergence jumps from about 10 to 20 days, and that of greater range a jump from about 24 to 45 days. (c) Annual c r i t i c a l l e v e l s are defined by the spring higher high waters of June and December for predicted tides i n 1977/78 at Point Atkinson, B.C. Dots indi c a t e heights of spring and neap ti d e s . The spring high tides of June, 1978 and December, 1978 are so s i m i l a r i n height that these two c r i t i c a l t i d a l l e v e l s merge into one, inv o l v i n g a jump from about s i x months to at least nearly two years of continuous exposure. The spring high tide of December, 1977 was higher and defines a c r i t i c a l t i d a l l e v e l above which the duration of exposure i s at l e a s t nearly three years. (d) The l e v e l of the lowest;.low water ( i . e . , extreme spring lower low water) f o r each year from 1967 to .198.7-r at Tsawwassen, B.C. i s graphed. This l e v e l has r i s e n gradually from about -3.1 m Geodetic Datum i n 1968 to about -2.6 m Geodetic Datum i n 1978 based on observed t i d a l records. Mean spring lower low water ( i . e . , the mean of twelve monthly spring lower low waters for each year) helps define the trend. The predicted trends over the next nine years are dashed i n . The l e v e l of the lowest extreme spring lower low water i n December, 1968 defines a c r i t i c a l t i d a l l e v e l below which the duration of continuous submergence jumps from about 18 years to at l e a s t nearly 36 years. 15 A. DAILY CRITICAL TIDAL L E V E L S c 2 + D I U R N A L - - f l — S E M I - D I U R N A L 0 1 2 Lunar Days " A 1 JJj It 0 1 1 2 Lunar Days e + 2 o « a - ui B. MONTHLY CRITICAL TIDAL LEVELS '. K ft A- (\ ft / 5 - 2 ° H O ** | -I .5H -3.04 45 Aug. Sept. Oct . Nov. C. ANNUAL CRITICAL TIDAL LEVELS E _ *2.0n h-to <D > 0 - 2 0 .-: h-2.5 | I--3.0 2 •i.s-l w • 1.0-S3 y«ars i 2 years ——r r^--6 months HHW V V *• '• ; T N ' D ' J ' F ' M ' A ' M ' J ' J ' A D. FOURTH ORDER CRITICAL TIDAL LEVEL S 1 0 • 2.0 = > 0 • 1.5 UJ O (-•1.0 | o • o £ -2.25 c o 3 - 2 . K H 2 -2.754 • Tl O 0 O -3.00H •3.25 Masn Spring Lower Low Walar • : \ Extreme Spring * Lower Low Watar - - 18 years -- S 3 6 years • 1 — i — i — r 1 9 6 7 ~ i — i — i — i — r 1 9 7 2 1 r-1 9 7 7 ~ i — i — r -|—r 1 9 8 2 n — r -2.25 '2.50 E c o (0 > s -2.75 2 o i — r 1 9 8 7 -3.00 -3.25 •o o 0 16 high water and higher low water stages are eliminated due to extreme di u r n a l i n e q u a l i t y i n the tides . As a r e s u l t Exposure Level 3 i s absent. Despite these differences between t i d a l types the three types have two important features i n common. The step i n exposure duration i n going from Exposure Level 2 to Exposure Level 1, in v o l v i n g a jump from less than one lunar day of continuous exposure to at l e a s t nearly two lunar days, i s common to a l l three t i d a l types, as i s the jump i n submergence duration i n passing from Exposure Level 4 to Exposure Level 5, which s i m i l a r l y involves a jump from less than one lunar day of continuous submergence to at least nearly two lunar days of continuous submergence. These two c r i t i c a l t i d a l l e v e l s defined-by the fundamental cycle of the lunar day form the basis of the scheme of i n t e r -t i d a l s ubdivision to be outlined here. F i r s t order c r i t i c a l t i d a l l e v e l s are modulated'into monthly cycles of spring and neap t i d a l periods which define 2nd order (monthly) c r i t i c a l t i d a l l e v e l s ( Fig. l b ) . Depending on the t i d a l region the monthly cycle can be caused by the phases of the moon (period 29.5 days), the d e c l i n a t i o n a l cycle of the moon (period 27.2 days), or the apogee/perigee cycle i n earth-moon separation (period 27.5 days). These cycles are a l l of very s i m i l a r .duration as they are a l l governed by the period of.the moon's o r b i t around the earth. There are two neap and two spring t i d a l periods each month but successive spring t i d a l periods are of d i f f e r e n t range. Each spring t i d a l period defines a 2nd order c r i t i c a l t i d a l l e v e l f or both high and low waters. Figure lb i l l u s t r a t e s these c r i t i c a l t i d a l l e v e l s f o r lower low water at Pt. Atkinson, B.C. The f i r s t c r i t i c a l t i d a l l e v e l i s defined by the spring tide of le s s e r range and involves a jump from about 10 to 20 days of con-tinuous submergence, while the second c r i t i c a l t i d a l l e v e l i s defined by the spring tide of greater range and involves a step from about 24 to 45 days of continuous submergence. The s i t u a t i o n i s the same f o r higher high 17 water.(or high water) except that i t involves steps i n exposure rather than submergence. Second order c r i t i c a l t i d a l l e v e l s are i n turn modulated i n t o annual cycles which define 3rd order (annual) c r i t i c a l t i d a l l e v e l s ( F ig. l c ) . T i d a l ranges are maximized i n June and December at the time of the s o l s t i c e s when the sun i s over, the t r o p i c s , and minimized at the equinoxes i n March and September when the sun i s over the equator. T i d a l ranges are greater i n December than i n June because the earth i s near p e r i h e l i o n (nearest the sun) i n December while i t i s near aphelion i n June. For Pt. Atkinson t i d e s , 3rd order c r i t i c a l t i d a l l e v e l s are defined by the spring high tides of June and December (Fig. l c ) . That of June involves a jump i n the duration of continuous exposure from about s i x months to almost one year, while that i n December involves a jump from about one year to at le a s t nearly two years./ I n F i g u r e v i e these .two steps .are .top_.close.-„tp_resol-v.e. There"are^equivalent c r i t i c a l t i d a l l e v e l s f o r lower low water (or low water). Third order c r i t i c a l t i d a l l e v e l s are i n turn modulated by an 18.6 year s o l i - l u n a r cycle d e f i n i n g 4th order c r i t i c a l t i d a l l e v e l s (Fig. Id). Every 18.6 years, when the maximum d e c l i n a t i o n of the moon coincides with the maximum d e c l i n a t i o n of the sun, t i d a l ranges are maximized. This e f f e c t i s only detectable f o r spring lower low waters for tides i n the southern S t r a i t of Georgia (Fig. Id). The lowest l e v e l of lower low water reached around 1968/69 defines a c r i t i c a l t i d a l l e v e l at which the maximum duration of sub-mergence probably jumps from about 18 to 36 years, although i n s u f f i c i e n t records are ava i l a b l e to'confirm t h i s . There are, no doubt, higher order c r i t i c a l t i d a l l e v e l s beyond 4th order defined by very long term astronomical cycles, but records are not a v a i l a b l e to analyze them. A l l of the c r i t i c a l t i d a l l e v e l s defined above can be recog-nized for a l l astronomically c o n t r o l l e d tides the world over, with the 18 exception of the 1st order (daily) c r i t i c a l t i d a l l e v e l s defining Exposure Level 3, which cannot be defined for diurnal t i d e s . Thus, c r i t i c a l t i d a l l e v e l s o f f e r a means of cross c o r r e l a t i o n between d i f f e r e n t t i d a l regions regardless of the t i d a l types involved. Figure 2a i l l u s t r a t e s such a cross c o r r e l a t i o n . Here the d e c l i n a t i o n a l mixed tides of the southern S t r a i t of Georgia on the P a c i f i c coast of Canada (Tsawwassen, B.C.) are cross correlated with the anomalistic semi-diurnal tides of the Bay of Fundy on the A t l a n t i c coast of Canada (St. John'y~T"; New Brunswick) . using extreme- c r i t i c a l t i d a l l e v e l s . a t which the maximum duration of continuous, exposure or submergence' jumps.:: Despite the very d i f f e r e n t types of tide in>the. two areas, 24 extreme c r i t i c a l t i d a l l e v e l s at Tsawwassen can be cross correlated with 18 at St. John. • In several instances one c r i t i c a l t i d a l l e v e l at St. JohnV i s s p l i t i n t o two or three c l o s e l y spaced steps at Tsawwassen, and occasionally vice versa. Exposure Zones The fundamental, 1st order c r i t i c a l t i d a l l e v e l s caused by the earth's r o t a t i o n about i t s axis can be used to subdivide both i n t e r t i d a l regions i n Figure 2a into three exposure zones. The c r i t i c a l t i d a l l e v e l s used are those between Exposure Levels 1 and 2 where the duration of exposure drops from at l e a s t nearly two lunar days to less than one lunar day, and between Exposure Levels 4 and 5 where the duration of submergence jumps from les s than one to at l e a s t nearly two lunar days. The former c r i t i c a l t i d a l l e v e l reaches i t s lowest l e v e l , and the l a t t e r i t s highest l e v e l , i n March and September at the time of the equinoxes, and these extreme l e v e l s are i l l u s t -rated i n Figure 2a. The i n t e r t i d a l 'amphizone' l i e s between these two extreme c r i t i c a l t i d a l l e v e l s and thus forms the core to the i n t e r t i d a l 19 Figure 2. (a) Cross c o r r e l a t i o n of extreme c r i t i c a l t i d a l l e v e l s between the i n t e r -t i d a l regions of St. John>x\ ..-New Brunswick and Tsawwassen, B.C. Based on predicted tides f o r 1978 for St. John\_ and observed tides between June, 1977 and June, 1978 at Tsawwassen. The numbers above and below c r i t i c a l t i d a l l e v e l s i n d i c a t e the maximum duration of continuous exposure i n days i n the case of atmozonal c r i t i c a l t i d a l l e v e l s , and the maximum duration of continuous submergence i n days i n the case of aquazonal c r i t i c a l t i d a l l e v e l s . For example at the upper l i m i t of the lower atmozone at St. John^ the jump i s from 10 to 20 days and the next c r i t i c a l t i d a l l e v e l up involves a jump from 26 to 72 days. In the case of atmozonal c r i t i c a l t i d a l l e v e l s only the lowest l e v e l attained by a p a r t i c u l a r c r i t i c a l t i d a l l e v e l i s indicated. In the case of aquazonal c r i t i c a l t i d a l l e v e l s only the highest l e v e l attained by a p a r t i c u l a r c r i t i c a l t i d a l l e v e l i s i n d i ^ a cated, i . e . , only extreme c r i t i c a l t i d a l l e v e l s are indicated. (b) Levels of exposure zone boundaries over the past ten years at Tsawwassen, B.C. based on observed t i d a l records. In the case of the boundary between the lower atmozone and the upper atmozone the numbers above and below c r i t i c a l t i d a l l e v e l s i n d i c a t e the maximum duration of exposure i n days. In the case of the boundary between the upper aquazone and the lower aquazone the numbers above and below c r i t i c a l t i d a l l e v e l s i n d i c a t e the maximum duration of submer-J gence i n days. On the extreme righ t hand side of the diagram h o r i -zontal bars i n d i c a t e the mean l e v e l s of exposure zone boundaries while the v e r t i c a l bars indi c a t e the standard deviation from the mean. Mean l e v e l s are not indicated i n the case of the boundaries of the lower aquazone because they undergo s i g n i f i c a n t modulation by the ef f e c t s of an 18.6 year s o l i - l u n a r d e c l i n a t i o n a l cycle. 20 ST. JOHN , NEW BRUNSWICK Predicted Tides (1978) TSAWWASSEN, B. C. Observed Tides 1977-June,1978) 24 z O 0 UJ _i UJ - 1 - | o I— UJ a O 2 UJ O Mean Levels with Std. Dev. Upper Atmozone 19 23 2_? 20 2 3 2 0 2 0 22 T7 24 . 2 3 28 2 9 22 _..i:=-.** Q ^ 7 " - : i T > « - - ' < i : : : , . • !=«... 1 9..-.v. -14-. 2 2 20 - 22 2 3 . . -10 10 9 - - ' 9 9 8 8 1 1 1 2 1*4« 12-ro*" 1 4 v - ' 8 " ? : : : : : : « 1 — 7 — 1 4 8 5 Lower A t m o z o n e Upper A m p h i z o n e Lower A m p h i z o n e Upper Aquazone 10 13 «> „ „ 13..?. * 6 . ^ . 1 3 . . . ^ 1410.;4-> 21 T8 1 7 '22,'«---..,.1V1« 1 9 '•-•••:-1-6-"'8 2 4 2 2 22 31 24 21 ''•=' 21 19 28 8 2 2 Lower Aquazone h2 h + 1 E ho o r-• > UJ - I ' —1 U l o t-UJ o •2 o UJ O h-3 D J ' I ' " , 1 , 1 1 1 1 1 , 1 1 , , 1 1 1 , 1 1 1 1 1 1 1 1 1 . I , , D J D J D J D J ' D 0 J 0 J 1968 1969 1970 1971 1972 1973 1974 1975" 1976" 1977 J D J D J 0 (b) 21 region, experiencing both exposure and submergence every lunar day (hence amphizone from the Greek "amphi' meaning both). Above the amphizone l i e s the 'atmozone' where the maximum duration of exposure exceeds nearly two lunar days. The atmozone can be subdivided at the l e v e l of the lowest 2nd order c r i t i c a l t i d a l l e v e l which involves a jump i n exposure duration from about 10 to 20 days. Above this l e v e l are a whole series of cl o s e l y spaced 2nd and 3rd order c r i t i c a l t i d a l l e v e l s . The upper l i m i t of the atmozone l i e s at the l e v e l of the highest high tide of the year. Below the 'amphizone' l i e s the 'aquazone' within which the maximum duration of submergence exceeds nearly two lunar days. As i n the case of the atmozone the 'aquazone' can be subdivided into an upper and lower part at the l e v e l of the highest 2nd order c r i t i c a l t i d a l l e v e l . The lower l i m i t of the aquazone l i e s at the l e v e l of the lowest tide of the year. For mixed tides the amphizone can be sub-divided at the lowest l e v e l of the c r i t i c a l t i d a l l e v e l between Exposure Levels 2 and 3 (Fig. 2a). For semi-diurnal tides the amphizone cannot be subdivided because c r i t i c a l t i d a l l e v e l s only occur within i t s upper and lower fringes. The i n t e r t i d a l region should be considered 'open-ended' i n the sense that c r i t i c a l t i d a l l e v e l s of i n f i n i t e order and i n f i n i t e exposure or submergence duration define i t s upper and lower l i m i t s . But for most p r a c t i c a l purposes t h i s can probably be taken to l i e at the l e v e l s of the highest and lowest 4th order c r i t i c a l t i d a l l e v e l s occurring within an 18 year period. Under t h i s system there would be no such thing as the sup r a t i d a l zone. For those geologists who define s u p r a t i d a l as l y i n g above mean high water, s u p r a t i d a l i s roughly equivalent to upper atmozonal. For t h i s scheme to be f e a s i b l e the l e v e l s of the exposure zone and sub zone boundaries based on observed t i d a l records must l i e close to the same l e v e l each year, or, i f not, must follow a predictable trend. Figure 2b i l l u s t r a t e s the l e v e l of exposure zone and sub zone boundaries over the past 22 ten years at Tsawwassen, B.C. based on observed t i d a l records. With the exception of the l i m i t s of the lower aquazone a l l boundaries stay close to the same l e v e l and the standard deviations from mean l e v e l s are 14 cm or less (range 8-14 cm) i n a t i d a l range of 5 m. Occasionally, boundaries show e r r a t i c deviations (e.g., the l i m i t s of the lower amphizone i n 1970, F i g . 2b) but t h i s i s only to be expected as exposure zone boundaries are defined by extreme t i d a l l e v e l s and these may occasionally be the r e s u l t of unusual, unpredictable meteorological conditions rather than predictable astronomical events. Thus, this system probably could not be applied to areas where meteo-r o l o g i c a l e f f e c t s dominate over astronomical unless the meteorological e f f e c t s are predictable (e.g., seasonal). The upper and lower l i m i t s of the lower aquazone at Tsawwassen are s i g n i f i c a n t l y modulated by the 18.6 year s o l i -lunar cycle mentioned above and the upper l i m i t of the lower aquazone has r i s e n about 30 cm over the past nine years from a l e v e l of -1.8 m, Geodetic Datum i n 1968/70 to -1.5 m Geodetic Datum i n 1976/78. I t should return to i t s 1968/70 l e v e l by the mid to l a t e 1980's. I n t e r t i d a l Zonation Quite apart from th e i r use i n cross c o r r e l a t i o n between d i f f e r e n t t i d a l regions there i s j u s t i f i c a t i o n f o r thinking that the exposure zones and c r i t i c a l t i d a l l e v e l s described above may be causally r e l a t e d to the i n t e r -t i d a l zonation of f l o r a and fauna. Doty (1946), and Widdowson (1965) have attempted to test the tide factor hypothesis by comparing f l o r a l and faunal zone l i m i t s with c r i t i c a l t i d a l l e v e l s , but, i n the case of mixed tides the tide factor hypothesis w i l l never be proven or disproven using this approach, because, for areas experiencing mixed t i d e s , any point w i t h i n the i n t e r t i d a l region w i l l at some time i n the year l i e within a few centimeters of a 1st order c r i t i c a l t i d a l l e v e l , since high and low water stages span the whole i n t e r t i d a l region. In addition, within the upper atmozone and lower aquazone there i s a very high p r o b a b i l i t y that one or more 2nd or 3rd order c r i t i c a l t i d a l l e v e l s w i l l coincide with a zone boundary at any ele v a t i o n one chooses to s e l e c t . The question should not be "does the zone boundary coincide with a c r i t i c a l t i d a l l e v e l ? , " but "which c r i t i c a l t i d a l l e v e l ( s ) does the zone boundary coincide with and i s there any j u s t i f i c a t i o n for thinking they are causally related?." Swinbanks (1979) has demonstrated that Callianassa  c a l i f o r n i e n s i s , a thalassinidean burrowing shrimp, extends up to lower atmozonal elevations on the Fraser Delta t i d a l f l a t s , but does not extend beyond an elevation at'which the-maximum duration of continuous exposure'(rises abrup^lyf^rom 14^-to '^9^Jl'ays;,;: H because p h y s i o l o g i c a l studies (Thompson and Prit c h a r d , 1969) i n d i c a t e that Caboye.) t h i s point anoxia due to exposure would be l e t h a l to Cal l i a n a s s a. Upogebia pugettensis, another thalassinidean burrowing shrimp extends up to but not beyond the lower l i m i t of the upper amphizone. Within the upper amphizone the duration of anoxia during periods of Exposure Level 2 i s probably l e t h a l to postmolt Upogebia (Swinbanks, 1979). The saltmarshes of the Fraser Delta are r e s t r i c t e d to upper atmozonal eleva-ti o n s . This may be because saltmarsh plant s'eedlangs require the periods of exposure i n excess of 10 days which occur i n the upper atmozone i n the spring, i n order to germinate and root s u c c e s s f u l l y (Swinbanks, 1979). In contrast to mixed t i d e s , for semi-diurnal tides c r i t i c a l t i d a l l e v e l s do not occur within the amphizone except near i t s l i m i t s (Fig. 2a). S i g n i f i c a n t l y b i o l o - ; g i s t s have found that for semi-diurnal tides the m i d - i n t e r t i d a l (or mid-l i t t o r a l ) regions are devoid of f l o r a l or faunal zone boundaries (Stephenson and Stephenson, 1949), whereas f o r mixed t i d a l regions a major zone boundary occurs close to mean sea l e v e l (Ricketts and Calvin, 1968). I f exposure zones and c r i t i c a l t i d a l l e v e l s are causal i n i n t e r t i d a l zonation, t h i s observation might be anticipated _a p r i o r i . ACKNOWLEDGEMENTS I thank the Regional T i d a l Superintendent, W.J. Rapatz, and F. Stevenson of the Canadian Hydrographic Service, I n s t i t u t e of Ocean Sciences Sidney, B.C. for providing t i d a l records for Tsawwassen, B.C. and St. John} New Brunswick. Dr. J.W. Murray, Dr. W.C Barnes, Dr. CD. Levings and Dr. L.F.„Giovando c r i t i c a l l y read the manuscript. 25 REFERENCES Carefoot, T.C, ±917, P a c i f i c Seashores: a guide to I n t e r t i d a l Ecology: J . J . Douglas Ltd., Vancouver, 208 p. Chapman, V.J. (ed.), 1974, Saltmarshes and s a l t deserts of the world (2nd ed.): Leonard H i l l , London, 392 p. . and Chapman, D.J. (eds.) , 1973, The algae: MacMillan and Co. Ltd., London, University Press, Glasgow, 497 p. Colman, J . , 1933, The nature of the i n t e r t i d a l zonation of plants and animals: Jour. mar. b i o l . Ass. U.K.", v.- 18, p. 435-476. Doty, M.S., 1946, C r i t i c a l t i d e factors that are correlated with the v e r t i c a l d i s t r i b u t i o n of marine algae and other organisms along the P a c i f i c Coast: Ecology, v. 27, p. 315-328. , 1957, Rocky i n t e r t i d a l surfaces: Geol. Soc. Amer. Mem. 67,-. v. 1, p. 535-585. Evans, R.G., 1947a, The i n t e r t i d a l ecology of Cardigan Bay: Jour. Ecol., v. 34, p. 2 7 3 - 3 0 9 . . . ' ; . ' J. ".-~: ' ' ~<' '-• ; , 1947b, The i n t e r t i d a l ecology o f ( s e l e c t e d l o c a l i t i e s , i n the /Plymouth, neighbourhood: Jour., mar. b i o l . Ass. U.K., v. 27", "p. 173-218. , 1957, The i n t e r t i d a l ecology of some l o c a l i t i e s on the A t l a n t i c coast of France: Jour. Ecol.", v. 45, p. 245-271. Ginsburg, R.N., Bricker, O.P. , Wanless, H.R. and Garrett, P., 1970, Exposure index and sedimentary structures of a Bahama t i d a l f l a t ( a b s t r . ) : Geol. Soc. Amer. Abstr., v. 2, No. 7, p. 744-745. Lewis, J.R.., 1964, The ecology of rocky shores: English U n i v e r s i t i e s Press, London, 323 p. Ricketts, E.F. and Calvin, J . (eds.), 1968, Between P a c i f i c Tides (4th ed.): Stanford University Press, Stanford, C a l i f o r n i a , 614 p. Stephenson, T.A. and Stephenson, A., 1949, The universal features of zonation between t i d e marks on rocky coasts: Jour. E c o l . , v. 37, p. 289-305. Swinbanks, D.D., 1979, Environmental factors c o n t r o l l i n g f l o r a l zonation and the d i s t r i b u t i o n of burrowing and tube-dwelling organisms on Fraser Delta t i d a l f l a t s , B r i t i s h Columbia: unpub. Ph.D. t h e s i s , University of B r i t i s h Columbia, Vancouver, B.C. Thompson, R.K. and P r i t c h a r d , A.W., 1969, Respiratory adaptions of two burrowing crustaceans, Callianassa c a l i f o r n i e n s i s and Upogebia pugettensis (Decapoda, Thalassinidea): B i o l . B u l l . , v. 136, p. 274-287. 26 Underwood, A.J., 1978, A r e f u t a t i o n of c r i t i c a l t i d a l l e v e l s as determinants of the structure of i n t e r t i d a l communities on B r i t i s h shores: Jour. exp. mar. B i o l . E c o l . , v. 33, p.- 261-276. Widdowson, T.B., 1965, A survey of the d i s t r i b u t i o n of i n t e r t i d a l algae along a coast t r a n s i t i o n a l i n respect to s a l i n i t y and t i d a l f a c t o r s : Canada Fis h . Res. Board Jour., v. 22, p. 1425-1454. Part 2 BIOSEDIMENTOLOGICAL ZONATION OF BOUNDARY BAY TIDAL FLATS, FRASER RIVER DELTA, BRITISH COLUMBIA 28 ABSTRACT Boundary Bay t i d a l f l a t s l i e on the in a c t i v e southern flank of the Fraser Delta. The Bay waters are cl e a r , and have s a l i n i t i e s which are 'normal marine' for the S t r a i t of Georgia. The grain s i z e of the surface sediments varies l i t t l e , c o n s i s t i n g almost e n t i r e l y of very fin e to f i n e , w e l l to very well sorted sands, which show a gradual f i n i n g shorewards trend. There are f i v e f l o r a l / s e d i m e n t o l o g i c a l zones on the t i d a l f l a t s , which are, from the shoreline seawards, the saltmarsh zone, the a l g a l mat zone, the upper sand wave zone, the eelgrass zone and the lower sand wave zone. The zones seaward of the saltmarsh have d i s t i n c t i v e macrofaunal commu-n i t i e s , producing assemblages of biogenic sedimentary structures diagnostic of each zone. The lower l i m i t of the saltmarsh l i e s at the lower l i m i t of the upper atmozone, a l e v e l above which the maximum duration of exposure r i s e s abruptly from about 12 to 40 days. The lower l i m i t of the a l g a l mat zone l i e s at the lower l i m i t of the lower atmozone, a l e v e l above which the maximum duration of exposure jumps from less than one to nearly two lunar days. The upper l i m i t of the eelgrass zone l i e s at the upper l i m i t of the lower amphi-zone, a l e v e l above which the maximum duration of exposure jumps from about 0.5 to ;QT7 lunar days. These "correlations"' are "beTi'eved' to be -causal. Topography of small and large scale creates l a t e r a l heterogeneity within the b i o f a c i e s of each zone. The a l g a l mat zone and the eelgrass zone a r e . f l a t , on the large scale.-However, i n both zones microtoppgraphy of biogenic o r i g i n , only a few centimeters high, profoundly influences faunal d i s t r i b u t i o n patterns on the l o c a l s c a l e . The upper sand wave zone i s characterized by symmetrical sand waves with wavelengths of the order of 30 m and very low amplitudes (<0.1 m), which probably form i n response 29 t o s e a waves d u r i n g s e v e r e w i n t e r s t o r m a c t i v i t y and r e m a i n dormant f o r most o f the t i m e . F a u n a l d e n s i t i e s m a x i m i z e i n t h e s h a l l o w w a t e r - f i l l e d t r o u g h s o f t h e s e s a n d w a v e s . The s a n d waves o f t h e l o w e r s a n d wave zone a r e o f h i g h e r a m p l i t u d e (up t o 0 . 5 m ) , have l o n g e r w a v e l e n g t h s (60 m) , a r e o f t e n l u n a t e i n o u t l i n e and p r o b a b l y f o r m i n r e s p o n s e to t i d a l c u r r e n t s . F a u n a l d e n s i t i e s i n t h i s zone a r e l o w , and p h y s i c a l s e d i m e n t a r y s t r u c t u r e s d o m i n a t e o v e r b i o g e n i c . I n w i n t e r t he f l o r a l zones r e t r e a t . The e n v i r o n m e n t a l v e n e r g y o f t h e Bay and s u r f a c e s e d i m e n t t r a n s p o r t i n c r e a s e due t o w i n t e r s t o r m a c t i v i t y . The d e n s i t i e s o f e i g h t m a c r o f a u n a l o r g a n i s m s w h i c h p r o d u c e d i s t i n c t i v e b i o g e n i c s e d i m e n t a r y s t r u c t u r e s were d e t e r m i n e d on two s u r v e y e d t r a n s e c t s . The o r g a n i s m s i n v e s t i g a t e d were C a l l i a n a s s a c a l i f o r n i e n s i s and U p o g e b i a p u g e t t e n s i s , b o t h t h a l a s s i n i d e a n b u r r o w i n g s h r i m p s , t h r e e p o l y c h a e t e wo rms , A b a r e n i c o l a s p . , S p i o s p . and P r a x i l l e l a s p . , t he b i v a l v e Mya a r e n a r i a and t he g a s t r o p o d s B a t i l l a r i a a t t r a m e n t a r i a and N a s s a r i u s m e n d i c u s . The d i s t r i b u t i o n p a t t e r n s o f e a c h o r g a n i s m and t he n a t u r e o f the b i o g e n i c s e d i m e n t a r y s t r u c t u r e s t h e y p r o d u c e a r e d e s c r i b e d . C a l l i a n a s s a b u r r o w s a r e i n t e r p r e t e d as b e i n g t e m p o r a r y f e e d i n g s t r u c t u r e s whe reas U p o g e b i a b u r r o w s , w h i c h a r e m u d - l i n e d , a r e s u g g e s t e d t o be permanen t d w e l l i n g b u r r o w s . 3 0 INTRODUCTION The t o p i c o f a n i m a l - s e d i m e n t r e l a t i o n s h i p s h a s b e e n r e c e i v i n g i n c r e a s i n g a t t e n t i o n i n s e d i m e n t o l o g i c a l l i t e r a t u r e , b o t h i n r e c e n t and f o s s i l s e d i m e n t s . I n r e c e n t s e d i m e n t s i n t e r e s t has f o c u s e d on i n t e r t i d a l e n v i r o n m e n t s , b e c a u s e o f t h e i r e a s e o f a c c e s s , a l t h o u g h s e v e r a l s t u d i e s have b e e n c a r r i e d o u t i n s u b t i d a l a r e a s (Rhoads and Y o u n g , 1 9 7 0 ; A l l e r and D o d g e , 1 9 7 4 ) . Work on c l a s t i c t i d a l f l a t s has c e n t r e d on t he e a s t e r n s e a b o a r d o f t he U . S . A . , i n p a r t i c u l a r a t S a p e l o I s l a n d , G e o r g i a ( F r e y and H o w a r d , 1 9 6 9 ; Howard and D b r j e s , 1 9 7 2 ; Howard and F r e y , 1 9 7 3 ) , and s t u d i e s a r e a l s o b e i n g c a r r i e d o u t on t he e a s t e r n s e a b o a r d o f Canada a t M i n a s B a s i n , Bay o f Fundy ( R i s k and M o f f a t , 1 9 7 7 ; F e a t h e r s t o n e and R i s k , 1 9 7 7 ; R i s k e t a l . , 1 9 7 6 ) . B i o g e n i c s e d i m e n t a r y s t r u c t u r e s h a v e b e e n s t u d i e d by E u r o p e a n w o r k e r s , i n p a r t i c u l a r t he German and D u t c h , f o r many y e a r s (Van S t r a a t e n , 1 9 5 2 ; R e i n e c k , 1 9 5 8 ; S e i l a c h e r , 1 9 6 4 ; D o r j e s , 1 9 7 0 ; S c h a f e r , 1972) . '-T h i s s t u d y , c e n t r e d on Bounda ry Bay t i d a l f l a t s on t he i n a c t i v e s o u t h -e r n f l a n k o f t h e F r a s e r D e l t a , i s one i n a s e r i e s o f s t u d i e s b e i n g c a r r i e d ou t on t he t i d a l f l a t s o f t h e F r a s e r D e l t a on t he w e s t c o a s t o f Canada ( S w i n b a n k s , 1979). . . ' Bounda ry Bay t i d a l f l a t s a r e u n i q u e , i n t h a t v a r i a t i o n s i n g r a i n s i z e o f t he s u r f a c e s e d i m e n t s o v e r most o f t h e f l a t s a r e s l i g h t , and a d i s t i n c t f l o r a l / f a u n a l z o n a t i o n e x i s t s , w h i c h i s p r i m a r i l y c o n t r o l l e d by e l e v a t i o n and e x p o s u r e (Sw inbanks and M u r r a y , 1 9 7 7 ) . T h i s i s u n l i k e most o t h e r t i d a l f l a t s d e s c r i b e d i n t he l i t e r a t u r e , where v a r i a t i o n s i n g r a i n s i z e o f t he s u b s t r a t e can p l a y an i m p o r t a n t r o l e i n d e t e r m i n i n g f l o r a l / f a u n a l z o n a t i o n , s u c h as M i n a s B a s i n , Bay o f Fundy ( R i s k and M o f f a t , 1977) and t he a c t i v e t i d a l f l a t s o f t he F r a s e r D e l t a ( L e v i n g s and C o u s t a l i n , 1 9 7 5 ) . 31 Boundary Bay l i e s on the southern flank of the Fraser Delta ( F i g . 1). The Bay i s protected from the sediment plume of the Fraser River by the Pleistocene peninsula of Point Roberts. As a r e s u l t rates of sedimentation are low (0.42 mm/year, K e l l e r h a l s and Murray, 1969), and the Bay !waters are clear and have s a l i n i t i e s which are 'normal marine' (24-29 %•) f o r the southern S t r a i t of-Georgia. Sediment i s transported i n t o the Bay by long-shore d r i f t along the western and eastern margins of the Bay, as evidenced by the accretion s p i t s at Beach Grove and Crescent Beach, and by the r e l i c t beaches j u s t south of Beach Grove, a l l of which indicate longshore d r i f t towards the north (Figs. 2 & 3). The unconsolidated Pleistocene c l i f f s at Point Roberts are an important present day source of sediment to the Bay (Kellerhals and Murray, 1969). Two small r i v e r s discharge minor amounts of sediment into the eastern end of the Bay (Fig. 1). In the western part of the Bay the saltmarsh i s prograding, whereas to the east i t has receded at least.1.2 km since 4,350 yrs. B.P. (Kellerhals and Murray, 1969). K e l l e r h a l s and Murray (1969) divided the f l a t s into four zones each having d i s t i n c t i v e sedimentological and f a u n a l / f l o r a l c h a r a c t e r i s t i c s . The four zones described by K e l l e r h a l s and Murray (1969) are the saltmarsh, the high t i d a l f l a t s , the intermediate t i d a l f l a t s and the low t i d a l f l a t s . The fauna and f l o r a of Boundary Bay have been described by O'Connell (1975). In this paper the zonation described by K e l l e r h a l s and Murray (1969) i s revised and mapped using a e r i a l photographs and data c o l l e c t e d from surveyed transects. D i s t r i b u t i o n of fauna on the t i d a l f l a t s i s given i n quantita-t i v e terms and the biogenic sedimentary structures they produce are described..^ \_ _ 32 Figure 1. Map of the Fraser Delta area, showing the l o c a t i o n of Boundary Bay t i d a l f l a t s (reproduced from K e l l e r h a l s and Murray, 1969). 33 METHODS Two transects were established traversing the Boundary Bay t i d a l f l a t s (Fig. 2). Transect A, which contained 22 s t a t i o n s , was set up and analysed during June and early July, 1976. Transect B, which contained 38 s t a t i o n s , was established and analysed, i n Julyaarid August,J.1976. Further observations were made on the transects i n November, 1976, and at various times of the-year i n 1977 and 1978 (Table I ) . The two transects, A and B, were surveyed from bench marks with the use of a t r a n s i t and an alidade. The transects run north/south from the edge of the saltmarsh to low water mark with stations taped at 91.4 m (300 f t ) i n t e r v a l s and marked by wooden stakes. The change i n elevation between successive stations was determined to an accuracy of about + 5 mm. High accuracy was required because errors are cumulative along the transect. Some burrowing organisms produce v i s i b l e evidence of t h e i r density at the sediment surface, e i t h e r d i r e c t l y by t h e i r presence i n the case of epifauna, or i n d i r e c t l y i n the form of burrow e x i t s or f e c a l mounds i n the case of infauna.. Using this surface evidence i t was possible to determine the density of the following organisms by quadrat sampling: Callianassa c a l i f o r n i e n s i s and Upogebia pugettensis, both thalassinidean burrowing shrimps, three polychaete worms, Abarenicola rsp., Spio sp. and P r a x i l l e l a sp., the bivalve My a arenaria and the gastropods B a t i l l a r i a  attramentaria and Nassarius mendicus. Direct counts of organisms were made i n the cases of B a t i l l a r i a , Nassarius, Spio, P r a x i l l e l a and Mya, whereas i n d i r e c t counts of f e c a l mounds or burrow openings were made i n the cases of Abarenicola,Upogebia and Callianassa . Quadrats of four d i f f e r e n t sizes were used — 1 m2, 0.25 m2, 100 cm2, and 4 cm2. The quadrat si z e selected depended on the s i z e and density of Figure 2. Floral/sedimentological zonation of Boundary Bay with the locations of transects A and B indicated. Between the t i d a l channels the waterline approximates to the -2.4 m (-8.0 f t ) contour (Geodetic Datum). The waterline i n the t i d a l channels does not follow a s p e c i f i c contour since these depressions remain w a t e r - f i l l e d during low t i d e , despite the fact that they are w e l l above sea l e v e l . 1000^ 2000 3000 m o t o r s C A N A D A U.S.A. S o u r c e > Intogratod R o s o u r c s s Photography , F l i g h t No. 142 , J u l y , 1974. F L O R A L / S E D I M E N T O L O G I C A L Z O N E S of BOUNDARY BAY A N D L O C A T I O N of T R A N S E C T S ( C o m p i l e d from Colour Aorta l P h o t o g r a p h s of July, 1974) LEGEND Sa l t M a r s h Zone | > :! | A lgal Mat Zone | • | Upper Sand Wave Zone Al—'—• Transect Line Eelgrass Zone u u Dyke Lower Sand Wave Zone •=3.^' 8end Weve Troughs ? ? Ares Not Covered by Aerial Photo Mosaic • • « o = : Relict Features Send Weve Fixed by Eelgrass .'<J>.-TABLE f; Dates and Locations of F i e l d Observations June 5-16, 1976 June 17-July 5, 1976 July 6-16, 1976 Aug. 2-22, 1976 Nov. 8, 1976 Feb. 10, 1977 A p r i l 5-7, 1977 Sept. 21,- 1977 Sept. 27, 1977 March 25-26, 1978 A p r i l 8-9, 1978 * May 14, 18-21, 26, July 17, 1978 Transect A established and surveyed. Sampling on transect A. Transect B established and surveyed. Sampling on transect B. S a l i n i t y measurements at stations A5-A12. Observations of f l o r a l zone l i m i t s on transect A. Observations of eelgrass and juvenile Abarenicola on transect A. S a l i n i t y measurements at stations A1-A22 and B1-B17. S a l i n i t y measurements at stations A6-A13. Observations of juvenile Abarenicola at stations A1-A6, and eelgrass at stations A13-A17 and B12-B30. Observations of ju v e n i l e Abarenicola at stations A1-A6. Quantitative observations of juvenile Abarenicola at stations A1-A6. * Quantitative r e s u l t s to be published elsewhere (Swinbanks, i n preparation). 37 the organism being investigated. The quadrat was thrown down randomly within 5 m of the s t a t i o n , and organisms or burrow numbers within the quadrat counted. When the o r i g i n of a burrow was i n doubt, the burrow was excavated with a spade or trowel to determine the organism respon-s i b l e . Sample specimens of each organism studied were c o l l e c t e d and preserved i n 10% formaldehyde for l a t e r i d e n t i f i c a t i o n . Light's Manual (Smith and Carlton, 1975) and the descriptions of Boundary Bay fauna by O'Connell (1975) were used i n organism i d e n t i f i c a t i o n s . Several of the organisms studied are s e n s i t i v e to desiccation i n the upper layers of sediment. To di s t i n g u i s h 'wet' sampling s i t e s from 'dry,' a depression about 1 cm deep was made i n the sand with a finger a f t e r ten hours of exposure. I f the depression formed immediately f i l l e d with water, then the sediment was considered 'wet.' This procedure simply establishes whether or not the water table l i e s within 1 cm of the surface. For a l l transects the densities of Abarenicola, B a t i l l a r i a , Nassarius, Upogebia, Callianassa and Mya were determined by sampling a 2 m2 area at each s t a t i o n with e i t h e r a 1 m2 or 0.25 m2 quadrat (8 x 0.25 m2 for stations A1-A22 and B1-B12, 2 x 1 m2 for stations B12-B38). P r a x i l l e l a densities , — 2 were s i m i l a r l y determined, except where densities became excessive (>10O m ), i n which case densities were determined by taking eight readings with a 100 cm2 quadrat. The density of Spio, a small tube-dwelling polychaete worm, was determined by taking four readings at each s t a t i o n with a 4 cm2 quadrat. To determine the re l a t i o n s h i p between burrow density and organism density, i n the case of Callianassa, a l l the sediment within a 0.25 m2 open ended metal box was excavated to a depth of 50 cm, and the sediment examined for shrimps with the aid of a coarse sieve (2 mm mesh). Six resu l t s were obtained by sampling twice at stations A12, A13 and A14. For Abarenicola the r e l a t i o n s h i p between f e c a l cast density and worm density was determined using a box core constructed from a two gallon can, which gives a core of rectangular cross section (15 x 20 cm), 30 .cm deep. Th i r t y cores were taken. Only two casts were enclosed within each core to ensure that the worms responsible for the casts were enclosed within the core. Burrow morphologies were examined using two box cores, one giving a core 30 cm deep and the other 1 m. The burrow morphologies of the shrimps Callianassa and Upogebia were investigated by taking r e s i n casts using the method developed by Shinn (1968). In several casts shrimps were v i s i b l e , entombed i n the r e s i n cast. This gave a means of checking the shrimp to burrow r a t i o determined as described above. Grain s i z e samples of the surface sediment were c o l l e c t e d at a l l stations using a 2 cm deep rectangular can. The s a l i n i t y of surface waters at low tide were recorded at stations with a refractometer. In the lab, grain s i z e samples were washed free of s a l t , treated with 30% H 2 O 2 , wet sieved through a 63 pm sieve for extraction of the s i l t / c l a y f r a c t i o n , and then dry sieved at 0.5 0 i n t e r v a l s . Between 10 and 30 g of sample was used. Colour a e r i a l photographs of Boundary Bay taken i n July, 1974 (Integrated Resources Photography, F l i g h t No. 142) were used to map the d i s t i n c t i v e f l o r a l / s e d i m e n t o l o g i c a l zones of the exposed t i d a l f l a t s . A topographic base map with a 0.3 m contour i n t e r v a l (Kellerhals and Murray, 1969), was used to determine the elevation of zone boundaries between the surveyed transects. FLORAL/SEDIMENTOLOGICAL ZONATION OF THE TIDAL FLATS D e s c r i p t i o n A e r i a l photographs taken i n J u l y , 1974 of Boundary Bay r e v e a l f i v e f l o r a l / s e d i m e n t o l o g i c a l zones on the exposed t i d a l f l a t s ( F i g . 2 ) , which can be recognized on the b a s i s of t h e i r d i s t i n c t i v e f l o r a l cover or by the presence of l a r g e s c a l e bedforms. The zones v i s i b l e i n the photo-graphs are, from the s h o r e l i n e seawards, the saltmarsh zone, the a l g a l mat zone, the upper sand wave zone, the eelgrass zone and the lower sand wave zone. Dense eelgrass growth a l s o occurs i n the t i d a l channels and extends down the channels i n t o the s u b t i d a l zone below the lower sand wave zone. The f i v e zones are present throughout the Bay, except at the eastern and western e x t r e m i t i e s immediately south of Beach Grove and Crescent Beach ( F i g . 2 ) , where sand waves cover the e n t i r e i n t e r t i d a l zone, and no eelgrass beds, a l g a l mats or saltmarsh have developed, because of the coarseness and probable i n s t a b i l i t y of the sands i n both these areas. In these two areas the e n t i r e . i n t e r t i d a l zone has the c h a r a c t e r i s t i c s of the lower sand wave zone. The f i v e - f o l d zonation i s al s o absent i n the f i n e r grained sediments of Mud Bay. Table I I tabulates the e l e v a t i o n s of the various zone boundaries w i t h respect to Canadian Geodetic Datum, on both t r a n s e c t s , as determined by surveying i n summer. The boundary between the a l g a l mat zone and the upper sand wave zone i s defined to be at the lower l i m i t of continuous a l g a l mat growth. I s o l a t e d a l g a l mat hummocks are present below t h i s l e v e l on both t r a n s e c t s . A l l boundaries are e l e v a t i o n d e l i m i t e d except that between the eelgrass and lower sand wave zones, which i n places extends down to low water l i n e (- 2.4 m. Geodetic Datum, F i g . 2). In w i n t e r the f l o r a l zones r e t r e a t . The a l g a l mats die back and TABLE i l l Elevations of Zone Boundaries Transect A Transect B Boundary Station Elevation (m) Station Elevation (m) Lower Limit of Saltmarsh Zone T6 + L I S : 0.05 2 + 1.10 + 0.01 Upper Limit, of A l g a l Mat Zone A l + 0.98 + 0.06 BI + 1.04 + 0.01 : A l g a l Mat Zone/Upper Sand Wave Zone A5/A6 + 0.75 + 0.08 B4/B5 + 0.76 + 0.07 Upper Sand Wave Zone/Eelgrass Zone A13 -• 0.10 + 0.12 B12 0.00 + 0.05, Eelgrass Zone/Lower Sand Wave Zone — Approximately - 1.2 m N.B. A l l elevations with respect to Canadian Geodetic Datum (to convert to 1978 Chart Datum at Tsawwassen, add 2.95 m). This boundary i s defined to he at the lower l i m i t of continuous a l g a l mat growth. 41 only a few i s o l a t e d hummocks remain. The eelgrass retreats seawards. Observations i n mid-February on transect A revealed that the eelgrass had retreated 350 m seawards from i t s summer p o s i t i o n , to an el e v a t i o n of - 0.5 m (Geodetic Datum). On transect B only i s o l a t e d patches of eelgrass remain during winter. The eelgrass which dies back i n winter consists l a r g e l y of the smaller species Zostera americana, while a permanent growth i s maintained by the la r g e r species Zostera marina. Zostera americana reappears from seedlings i n the spring. Considerable quantities of both species of eelgrass are uprooted by winter storms and r a f t e d i n t o the edge of the saltmarsh zone, forming an organic r i c h mat on which saltmarsh plant seedlings sprout i n the spring. Figure 2 depicts sand wave troughs i n p o s i t i o n as traced from a e r i a l photographs. The sand waves of the upper sand wave zone are d i s t i n c t l y d i f f e r e n t from those of the lower sand wave zone. Those of the upper sand wave zone have wavelengths averaging 30 m (range 20-70 m) and ampli-tudes of <0.1 m, giving a wavelength:amplitude r a t i o greater than 300:1. From q u a l i t a t i v e observations the waves appear to be symmetrical i n p r o f i l e . The sand waves of the lower sand wave zone have wavelengths averaging 60 m (range 40-80 m), and, according to K e l l e r h a l s and Murray (1969), amplitudes from 0.3 m to 0.5 m. In plan view these sand waves have both s t r a i g h t -crested and lunate out l i n e s . The lunate forms are predominantly concave seawards. At f a i r l y regular i n t e r v a l s along the lower l i m i t of the upper sand wave zone, sand waves s i m i l a r to those of the lower sand wave zone occur. They are lunate i n outline and concave seawards (Fig. 2 & 3). The a l g a l mat zone i s devoid of sand waves. This may i n part be due to the sediment binding e f f e c t s of the a l g a l mats. A few sand waves occur i n the eelgrass zone. Some are completely overgrown by eelgrass during summer (Fig. 2). 42 Discussion of Sand Waves The j u s t i f i c a t i o n f o r c a l l i n g the large scale'bedforms of the upper sand wave zone 'sand waves' l i e s i n the fact that they meet the loose s p e c i f i c a t i o n s of sand waves given by Harms et a l . (1975) — v i z . spacing between 5-100 m, s t r a i g h t .to sinuous crested and having a . r e l a t i v e l y small height/spacing r a t i o — and .they .exhibit a charac t e r i s t i c . p r o p e r t y of waves, namely interference patterns. The bedforms are thought to be produced i n response to surface sea waves. The wave properties of these bedforms and t h e i r response to surface sea wave a c t i v i t y i s best i l l u s t -rated i n the western part of the Bay next to Beach Grove, where r e f l e c t i o n interference patterns are v i s i b l e ( F ig. 3). A concrete breakwater i s present along the waterfront , of Beach Grove, and the beach p r o f i l e i n th i s area of the Bay i s steeper than elsewhere. Surface sea waves are r e f l e c t e d by the concrete breakwater, whereas i n the rest of the Bay wave energy i s diss i p a t e d i n the saltmarsh zone without r e f l e c t i o n . The sand waves mirror the r e f l e c t i o n interference patterns of the surface sea waves. The lunate sand waves near the lower edge of the upper sand wave zone could perhaps be produced by r i p currents set up i n response to wave action (Fig. 3). Between the period of June, 1976, to July, 1978, the sand waves of the upper sand wave zone on transect A have not moved or changed shape noticeably with respect to the positions of stakes placed along the tran- ' sect. The sand waves of the upper sand wave zone are probably only active during winter storms, and when active may be i n a state of dynamic e q u i l i -brium. The sand waves indic a t e storm wave propagation from the south. Twenty-two percent of winds i n excess of 48 km/hour (30 m.p.h.) during winter (October - March) are from the south, and on average occur for 5.5 hours per month (Swan Wooster, 1968). Southerly winds have a greater fetch than any others, and should produce the most severe storm waves. 43 Hwy. 499 0" o • LEGEND Longshore D r i f t Rip Currents Dyke Saltmarsh Zone A l g a l Mat Zone Upper Sand Wave Zone Eelgrass Zone Lower Sand Wave Zone Sand wave troughs Area not covered by a e r i a l photo mosaic X Transect l i n e A 1000 N i B e a c h G r o v e r e f l e c t i o n i n t e r f e r e n c e p a t t e r n s a c c r e t i o n sp i t r e l i c t b e a c h e s 1000 2000 meters Source - . I n t e g r a t e d R e s o u r c e s P h o t o g r a p h y , F l i g h t N o . 142, J u l y , 1974 Figure 3. Ref l e c t i o n interference patterns i n the upper sand wave zone i n the area of Beach Grove. Also indicated are the probable direc-tions of wave induced currents. The o r i e n t a t i o n of t i d a l channels i n the lower sand wave and eelgrass zones give a good i n d i c a t i o n of t i d a l current d i r e c t i o n s , i n p a r t i c u l a r of ebb currents (Kellerhals and Murray, 1969; Weir, 1963). In the lower sand wave zone i t can be seen that sand waves are aligned approximately perpendicular to the axis of the t i d a l channels, i n d i c a t i n g that the sand waves may be produced by t i d a l current action. Weir (1963), during the summer of 1959, recorded maximum flood and ebb t i d a l currents, during spring and mean ti d e s , ranging from 24 cm sec" 1 to 49 cm s e c - 1 (average of several readings through water column) i n the four major t i d a l channels west of the "Great Channel" (beside Crescent Beach). Several of the readings for both flood and ebb currents exceeded 35 cm s e c - 1 . The water depths i n the lower sand wave zone at the time of these measurements ranged between 0.0 m and 1.7 m. Current v e l o c i t i e s i n excess of about 35 cm s e c - 1 i n these water depths are capable of forming sand waves (Harms et a l . , 1975). However, to determine conclusively whether the sand waves are of t i d a l o r i g i n , current v e l o c i t y data would have to be c o l l e c t e d from the lower sand wave zone i t s e l f , rather than extrapolating from data c o l l e c t e d i n the adjacent t i d a l channels. The sand waves could be the r e s u l t of currents induced by the combined action of tides and storm waves. ENVIRONMENTAL FACTORS AND Z0NATI0N There are three factors i n the p h y s i c a l environment which have the p o t e n t i a l of being primary agents i n causing f l o r a l / f a u n a l zonation on t i d a l f l a t s . These are the grain s i z e c h a r a c t e r i s t i c s of the substrate, the properties of the covering waters at. high arid.vlow'.tides,;Lin-ip.articular t h e i r s a l i n i t y and t u r b i d i t y l e v e l s , and the duration of exposure, which i s a function of t i d e s , elevation and topography. 45 Grain Size of Surface Sediments Figures 4 and 5 present the grain s i z e c h a r a c t e r i s t i c s of surface sediments on transects A and B. In addition to these r e s u l t s , one surface sample (upper 2 cm) from the saltmarsh zone, c o l l e c t e d on transect A 100 m landward from the seaward perimeter of the marsh, was analysed. It proved to be a moderately w e l l sorted very f i n e sand (Graphic Mean 3.6 0) , containing 25% s i l t and clay, and 8% organic matter h a l f of which consisted of fibrous peat. On both transects A and B the surface sediments gradually coarsen seawards from very f i n e sands to fine sands i n going from the saltmarsh zone to the lower eelgrass zone, giving a very l i m i t e d mean grain s i z e range of less than 1 0 (Figs. 4 & 5). This coarsening i n mean grain s i z e i s best i l l u s t r a t e d on transect B (Fig. 5). Towards the lower end of transect A there i s a s l i g h t r e v e r s a l i n the trend, but this i s due to an increase i n the mud content of the sediment rather than being due to a f i n i n g i n the grain s i z e of the sand f r a c t i o n . The sands on transects A and B are predominantly w e l l to very w e l l sorted ( I n c l . Graphic Std. Dev. <0.5 0 ) . Fluctuations i n the s o r t i n g values are la r g e l y a function of mud content, as indicated by the fact that s o r t i n g and mud content values fluctuate i n harmony (Figs. 4 & 5). With the exception of the saltmarsh zone, mud contents are low, amounting to only a few percent. Box cores from both transects reveal monotonous sequences of sand with l i t t l e v a r i a b i l i t y i n grain s i z e with depth. A f i n i n g i n grain s i z e shorewards i s t y p i c a l of t i d a l f l a t s ( K l e i n , 1971), but the v a r i a t i o n i s usually much more extreme, ranging from mud i n the upper i n t e r t i d a l zone to sand i n the lower i n t e r t i d a l zone (Linke, 19 39; 'Ey'anf^l90 Risk^and'Moffat /l9,77).V ' The processes responsible for th i s t e x t u r a l trend have been extensively 46 N O < ac a 3.30 3.20 3.10-1 2 ~ 3.00 2.80 2.70 2.60 — i — 10 — i — 15 — i — 20 o 0.60 I 1 1 °- 5 ° £ ° * 0.40 ° o » » c 0.30 I— 10 I— 15 —I— 20 10 a i _ 8 !- =s 6 z n in (0 o v 4 E ~ UJ -a. 2 | l I I 1 J I I I I 1 I I l 10 15 20 1.50 ; I.OOH * 0 .50 5" **\ >- - • > 0 .50 IM -J IU 1.00 1.50 r 6 A , A L G A L M A T Z O N E U P P E R S A N O W A V E Z O N E E E L G R A S S Z O N E M.L.L.W. 1 0 0 0 D I S T A N C E , m a t e r s 1 5 0 0 2 0 0 0 Figure 4. Variations i n mean grain s i z e , s o r t i n g and mud content on transect A. 47 o a > 0.50 i z • • 5 ° 0.40-CC SO U CO e 0.30-— 1 a 8-S — 6 h- a» ' Z u <•> . co 4-u V K ~ 2 IU a. 0- I i l i i . ! . f . • . . T | 10 15 20 25 —f-30 35 • 1.00 • o.so 6 • « _ ° 2 0.50 -< 1.00 -" 1.50-M.H.H.W. A L G A L M A T ZONE UPPER SAND WAVE Z O N E B25 E E L G R A S S 1000 1500 2000 D I S T A N C E , m a t e r s 3000 Figure 5. Variations i n mean grain s i z e , s o r t i n g and mud content on transect B. discussed i n the l i t e r a t u r e . They include "scour l a g " (Van Straaten and Kuenen, 1958) and . " s e t t l i n g l a g " (Postma, 1961) , combined with a net transport of sediment shorewards due to t i d a l v e l o c i t y asymmetry (Postma, 1961; Groen, 1967), under the influence of a decreasing wave and current energy regime (Reineck, 1967). To t h i s l i s t the authors would add that the i n i t i a l sheet flow of water onto the dry t i d a l f l a t on flood tide i s a potent sediment transporting agent, which may play an important r o l e i n producing the f i n i n g shorewards trend. On the i n i t i a l i n f l u x of fl o o d waters any protruding sediment mounds (e.g. f e c a l casts) are swept f l a t by the advancing waterline. This sediment gradually s e t t l e s back onto the substrate. The f i n e r grains t r a v e l further inshore because t h e i r s e t t l i n g v e l o c i t i e s are lower. Dry sediment also has an a b i l i t y to f l o a t . buoyed up by surface tension and foam, p a r t i c u l a r l y f i n e r sediment because of i t s l a r g e r surface area to volume r a t i o . The f i l m of foam and sediment car r i e d by the advancing waterline i s deposited along with organic debris at high water mark. This flood tide process i s not reciprocated by the l a t e stages of ebb t i d e , because, on ebb, subsurface water continually seeps o f f the waterlogged t i d a l f l a t , no foam or pronounced waterline forms, and strong current action due to l a t e stage runoff i s l o c a l i s e d i n channels. The lack of mud and high degree of s o r t i n g i n the sands of Boundary Bay t i d a l f l a t s i s probably due i n part to reworking by winter storm waves, although i t i s also due to the lack of a s i g n i f i c a n t source of suspended mud si z e sediment i n the Bay. On the active t i d a l f l a t s of the Fraser Delta, which experience tides and wave action s i m i l a r to Boundary Bay, mud contents of the surface sediments are an order of magnitude higher, due to the i n f l u x of mud from the Fraser River (Swinbanks, ,1979").. - - . 49 Superimposed on the gradation i n mean grain s i z e , which i s co n t r o l l e d by p h y s i c a l processes, i s a b i o l o g i c a l l y c o n t r o l l e d f l u c t u a t i o n i n the percentage of s i l t and clay (material <63 ym), due to the a b i l i t y of vegetation to entrap fine grained material, much as described by Ginsburg and Lowenstam,(1958). Mud contents decrease i n a s t e p l i k e fashion from about 25% to 5% to 1%, i n passing from the saltmarsh zone through the a l g a l mat zone to the upper sand wave zone, as the extent of f l o r a l cover decreases abruptly. In the lower eelgrass zone on transect A (Stations A17-A22, F i g . 4), where a continuous and extensive mat of eelgrass i s present throughout the year, the percentage of mud r i s e s abruptly to ' between 2.5 and 7%. In conclusion, because var i a t i o n s i n the grain s i z e of the substrate are s l i g h t and gradual, grain s i z e of the sediments i s not considered to play a primary r o l e i n determining f l o r a l / f a u n a l zonation i n Boundary Bay. The s t e p l i k e v a r i a t i o n i n mud contents of the sediments, rather than being a cause of zonation, i s considered to be an e f f e c t of f l o r a l zonation. S a l i n i t y and Turbidity The plume of turbid, brackish water from the Fraser River seldom i f ever enters Boundary Bay. The plume i s only directed south-eastwards towards the Boundary Bay area when north-westerly winds blow i n conjunction with an ebbing tide (Giovando and Tabata, 1970). North-westerly winds only occur about 13% of :the .time (Luternauer and Murray, 1973). Even on these occasions the Point Roberts peninsula prevents the plume from entering the Bay, and on the subsequent floo d the plume i s not flushed into the Bay, because i n i t i a l flood currents at the entrance to the Bay are from the south-east (Weir, 1963). Analysis of s a t e l l i t e imagery and a e r i a l photographs reveals that the plume of the Fraser does not enter the Bay and the Bay waters are clear (Table III);. A number of s a l i n i t y TABLE LLI A e r i a l photographs and s a t e l l i t e imagery i l l u s t r a t i n g the low t u r b i d i t y l e v e l s of Boundary Bay waters )Film R o l l No. >' <*A 37597 :A 37170 **IRP 142 . .A 30339 ERTS EMG-1283-A ERTS-1 Frame No, 132 38 & 39 19 - 27 113, 114 & 116 mosaic Emulsion nat. colour nat. colour nat. colour nat. colour i n f r a r e d i n f r a r e d F l i g h t A l t i t u d e (m) 10976 9451 3810 12195 s a t e l l i t e s a t e l l i t e Date June 20, 1978 June 11, 1975 July, 1974 July 16, 1971 1973 - 1974 11:36 a.m. July 30, 1972 Remarks Contrast frame 132 with the muddy plumes i n frame 146. Taken at low t i d e . Frame 38 shows clear demarca-t i o n between muddy Fraser plume and blue Bay waters i n v i c i n i t y of Point Roberts. Taken at low t i d e . Taken at low t i d e . Contrast frames 113 & 114 with frame 116. Taken at mid-tide. Mosaic with Fraser plume c l e a r l y v i s i b l e . Taken at mid-tide on ebb, three hours a f t e r lower high water.. * Film'»rolls prefixed (A) are federal government photographs, available through the National A i r Photo Li b r a r y , Ottawa, Ontario, Canada. ** The r o l l prefixed IRP i s available through Integrated Resources Photography Ltd., Vancouver, B.C., sCanada. Note: A l l of these photographs except ERTS EMG-1283-A were taken i n June or July when the Fraser River reaches i t s peak discharge. 51 measurements taken i n t i d a l pools at low tide confirm that brackish water from the Fraser does not enter Boundary Bay, even i n June when the Fraser i s i n freshet (Table IV)V, The s a l i n i t i e s measured nearest to low tide l i n e are probably most i n d i c a t i v e of the s a l i n i t i e s of water entering the Bay, since these are l e a s t affected by exposure or fresh water drainage from the marsh (Table IVy,Stations A12, *A9, A22, B17 3&'A13). They indi c a t e that the Bay water s a l i n i t i e s probably l i e i n the range of 24 to 29% 0, which can be considered 'normal marine' for the southern S t r a i t of Georgia as they are t y p i c a l of the s a l i n i t i e s of surface waters of the ' v e r t i c a l l y mixed' water mass of the southern S t r a i t of Georgia (Waldichuk, 1957), as opposed to the 'brackish s t r a t i f i e d ' water mass of the c e n t r a l S t r a i t of Georgia (Waldichuk, 1957). The exceptionally high value of 39%» recorded next to the saltmarsh (Station *A1) by 0'Connell (1975), was probably due to evaporation during prolonged exposure on a warm day (Air Temp. 20-24° C). S a l i n i t y does not vary appreciably'oyer the exposed t i d a l f l a t s (Table IV.)) and thus does not influence zonation. This i s i n complete contrast to the t i d a l f l a t s on the active Fraser Delta front, where surface water s a l i n i t i e s at low tide range between 1 and 33%« , and markedly i n f l u -ence faunal-and f l o r a l di'stributipn;.patterns (Swinbanks ,1979) . Exposure Time Exposure time on a t i d a l f l a t i s an important parameter i n f l u e n c i n g f l o r a l / f a u n a l zonation, although i t s e f f e c t s can be masked or even overridden by other f a c t o r s , such as v a r i a t i o n s i n the grain s i z e of the substrate and/or v a r i a t i o n s i n s a l i n i t y or t u r b i d i t y l e v e l s of the covering waters. However, as already stated, these parameters do not vary appreciably over most of Boundary Bay t i d a l f l a t s . As a r e s u l t on these t i d a l f l a t s exposure 52 TABLE IV Salinity..-of Water i n T i d a l Pools" at,Low "Tide -Date Station Distance from saltmarsh S a l i n i t y (meters) (%o) Remarks 8/11/76 A5 366 26.0 A6 457 26.5 A7 A8 549 640 26.0 26.5 Cool and Cloudy A9 732 27.0 AlO 823 27.5 (Air Temp. 4-11° C) A l l 915 28.5 A12 1006 29.0 10/6/75 *A1 50 39.0 *A2 200 33.0 *A3 350 32.0 Warm *A4 ' 500 33.0 *A5 650 32.0 ' *A7 975 33.0 (Air Temp. 20-24° C) 11/6/75 *A8 1275 30.0 *A9 1575 28.0 21/9/77 21/9/77 Al 0 23.0 A2 91 23.0 A3 183 25.5 A4 274 26.0 A5 366 26.5 A6 457 25.5 A7-A14 549-1189 24.5 (8 readings) A15-A22 1280-1921 24.0 (8 readings) BI 0 20.0 B2 .91 21.0 B3 183 22.0 B4 274 22.0 B5 366 23.0 B6-B17 457-1463 24.0 (12 readings) Cool and Cloudy 27/9/77 A6-A13 457-1098 28.0 C o o l a n d c (8 readings) * Station locations and data from O'Connell (1975) 53 can be seen to play a. major r o l e i n determining f l o r a l / f a u n a l zonation, and a deta i l e d analysis of the r e l a t i o n s h i p between exposure time and elevation i s warranted. At t h i s point i t should be stressed that i t i s the e f f e c t s of exposure rather than exposure i t s e l f which probably most influence the d i s t r i b u t i o n of f l o r a and fauna. The e f f e c t s include such things as dehydration, and i n s t a b i l i t y of oxygen l e v e l s , pH, temperature and s a l i n i t y , which may d i r e c t l y k i l l f l o r a or faunal o f f s p r i n g or, a l t e r n a t i v e l y , c r i t i c a l l y impair t h e i r a b i l i t y to cope with competing, predatory or non-compatible organisms. I t should be borne i n mind that other parameters, i n p a r t i c u l a r small and large scale topography and drainage, can d r a s t i c a l l y a l t e r the e f f e c t s of exposure, and that the i n t e r a c t i o n of tides and elevation do not alone determine exposure e f f e c t s . Ginsburg et a l . (1970) suggested that for any t i d a l f l a t an exposure vs. elevation curve should be computed, and then at any given elevation on the t i d a l f l a t an 'exposure index' can be assigned, which i s equal to the mean exposure expressed as a percentage. Such a curve, with some additions, i s presented i n Figure 6 for Boundary Bay. Ginsburg et a l . (1970) attempted to r e l a t e the occurrence of mudcracks, a l g a l stromatolites and other biogenic sedimentary structures to t h e i r 'exposure index.,' However, the 'exposure index' curve has a number of d e f i c i e n c i e s which severely l i m i t i t s usefulness i n i n t e r p r e t i n g the d i s t r i b u t i o n of f l o r a and fauna. The curve's major deficiency i s that i t i s an average, and as such can provide no information regarding the range of possible exposures at any given elevation. From the point of view of florauand fauna the most extreme exposures are probably of more concern than the average. Even for some physi c a l sedimentary structures, such as mudcracks, i t i s the longest periods of exposure which determine whether or not they tform,not 54 K E Y Mean Daily Exposure -Bars indicate one Standard Deviation from Mean \ Dotted envelope encloses the \^ complete range of values SN obtained at any given elevation. i — I — I 1—I r .50 1.00 +0.50 E L E V ATI O N , m e t e r s (Geodetic Datum) Figure 6. Mean d a i l y exposure with respect to elevation f o r Boundary Bay • tid e s . This was compiled from 16 representative d a i l y t i d a l curves (eight mean t i d e s , four spring tides and four neap tides) selected from 25 ava i l a b l e i n the data of Weir (1963) covering the period of June to September, 1959. Using more than 16 days of t i d a l . records would probably reduce some of the standard deviations, but i t would i f anything increase the ranges of possible values. 55 the average. In Figure 6 an attempt has been made to overcome these d e f i c i e n c i e s by deli m i t i n g the range of d a i l y exposure values which are possible at any eleva-t i o n . A measure of the natural day-liojday v a r i a b i l i t y i n exposure i s also given by including the standard deviation from each mean. I t can be seen t that at most l e v e l s , and i n p a r t i c u l a r i n the uppermost and lowermost i n t e r -t i d a l regions, exposure varies tremendously from day to day and 'exposure index' i s a misleading i n d i c a t i o n of exposure. Even with the addition of range and standard deviation Figure 6 can give no i n d i c a t i o n of the maximum duration of continuous exposure possible at any elevation, because i n i t s computation no d i s t i n c t i o n i s made between continuous and discontinuous ex®©sur exposure. Swinbanks (1979) has recently elaborated on the concepts of c r i t i c a l t i d a l l e v e l s (Doty, 1946),.and has advocated t h e i r use i n the desc r i p t i o n of i n t e r t i d a l exposure and i n the subdivision of,the i n t e r t i d a l zone. A c r i t i c a l t i d a l l e v e l i s a p a r t i c u l a r t i d a l e levation at which the duration of c o n t i -nuous exposure or continuous submergence changes abruptly i n a s t e p - l i k e fashion. There are d a i l y , monthly, annual and longer term c r i t i c a l t i d a l l e v e l s , which can be defined depending on the time scale considered (Swinbanks, 1979). As the concepts of c r i t i c a l t i d a l l e v e l s are probably unfamiliar to most geologists, they w i l l be outlined below as they apply i n the s p e c i f i c case of Boundary Bay t i d e s , although much of what i s sa i d applies to a l l astronomically c o n t r o l l e d t i d e s . The tides i n Boundary Bay are of mixed-semi-diurnal type. This means that there are two high tides and two low tides a day, but that successive high tides and successive low tides are of d i f f e r e n t height (Fig. 7a). As a r e s u l t on any given day there are f i v e d i f f e r e n t l e v e l s of exposure which can be experienced, depending on the elevation considered. The duration of 56 continuous exposure or submergence jumps on passing from one l e v e l to the next. The f i v e 'exposure l e v e l s ' (Swinbanks, 1979) as indicated i n Figure 7a are: Level 1. At an elevation above higher high water continuous exposure i s at l e a s t nearly two lunar days. Level 2. At a l e v e l intermediate between higher high water and lower high water, exposure occurs once and i s greater than h a l f a lunar day, but less than one. Level 3. At a l e v e l between lower high water and higher low water, exposure i s s p l i t i n t o two periods by lower high water, each period of exposure or submergence being less than h a l f a lunar day. The t o t a l d a i l y exposure may or may not exceed h a l f a lunar day, but the length of each period of continuous exposure does not. Level 4. Below higher low water exposure occurs once i n the lunar day, and exposure i s l e s s than h a l f a lunar day. Submergence i s greater than h a l f a lunar day, but less than one. Level 5. Below lower low water the t i d a l f l a t i s continuously sub-merged f o r at l e a s t nearly two lunar days. The above f i v e statements are based on the assumption that the time between successive high or low tides i s h a l f a lunar day. This i s not s t r i c t l y true, because there i s a«time asymmetry to mixed tides caused by a lag between the p o s i t i o n of the moon and the response of the t i d e , which i s dependent on t i d a l range. However, examination of twenty-five t i d a l curves of a l l ranges reveals that despite this f a c t the f i v e statements s t i l l always hold true, f o r Boundary Bay t i d e s , except for Level 3 expo-sures f o r which continuous exposure can on occasion marginally exceed h a l f a lunar day (Figs. 7b & 7c). 57 Lunar D a y - " Key H.H.W. - H i g h e r H i g h W a t e r L.H.W. - Lower H i g h Wate r H.L.W. - H i g h e r Low W a t e r L.L.W. - Lower Low W a t e r 4 * E x p o s u r e L e v e l 4 (a) 2.0 • 1.5M 3 o u Is -> 1.0 5 S «• o •- K 0.5 -2.0 —I r— 1 2 3 4 5 Exposure Level Kay m Range I - Absolute L imit a - Average L lmlt • m Extreme L imi t A - Un l imi ted (b) • • 1 2 3 4 Exposure Level Key • • Rang* i • Absolute Limit • B Average Limit • • Extreme Limit * • Unlimited (c) r-u e o re 11 -1.0 c o -0.5 £ l ,22 ,23 l2'. l25'26 l27 l28 l29 l3o! 1 ' 2 ' 3 '.4 ' 5 ' 6 ' 7 ' 8 ' 9 ' l o ' l l ' 12 'u ' t -MS ' l f i /w ' I S ' w ' l o ' u ' l l Z l ' z k ' v ' l b l T i a ' 2 9 V 3 1 " : 2.0 • -E o *i.oH a o z o LO H " 2.0 June, 1959 J u l y . 1959 E X P O S E D Leve l 2 Level 3 * \ N  Leve l 1 • L e v e l 4 S U B M E R G E D (d) Figure 7. (a) The f i v e 'exposure l e v e l s ' possible f o r mixed semi-diurnal tide s . (b) The ranges of duration of continuous exposure f o r each of the f i v e exposure l e v e l s . (c) The ranges of duration of continuous submergence for each of the f i v e exposure l e v e l s . (d) The monthly modulation of the f i v e exposure l e v e l s f o r the period June 21-July 31, 1959. T i d a l data from Weir (1963). 58 The four boundaries between exposure l e v e l s are 1st order (daily) c r i t i c a l t i d a l l e v e l s , as they are defined by the d a i l y t i d a l cycle. Higher order c r i t i c a l t i d a l l e v e l s , defined by monthly, annual and longer term t i d a l cycles, are common to a l l astronomically c o n t r o l l e d tides (Swinbanks, 1979). Over a period of months exposure at any elevation i s a combination of the f i v e exposure l e v e l s ( F ig. 7d). Figure 8 graphs the frequency of each expo-sure l e v e l at 6.1 cm (0.2 f t ) elevation i n t e r v a l s . The lowest elevation attained by Level 1 exposures defines the 'atmozone'; the i n t e r t i d a l exposure zone i n which the maximum duration of.continuous exposure exceeds at le a s t nearly two lunar days (Swinbanks, 1979). S i m i l a r l y the highest l e v e l attained by Level 5 exposures defines the upper l i m i t of the 'aquazone' (Swinbanks, 1979) i n which the maximum duration of continuous submergence exceeds at least nearly two lunar days. Between l i e s the core to the i n t e r t i d a l region, the 'amphizone' (Swinbanks, 1979), i n which the maximum duration of continuous exposure or submergence i s always les s than one lunar day. The lowest l e v e l reached by Level 2 exposures defines the boundary between upper and lower amphizones. Monthly (2nd order) c r i t i c a l t i d a l l e v e l s , at which the maximum duration of continuous exposure or submergence begins to r i s e abruptly from about 10 to 20 days subdivide the atmozone and aquazone i n t o upper and lower parts (Fig. 8). Superimposed on Figure 8 are the flo r a l / s e d i m e n t o l o g i c a l zones previously described. Some corr e l a t i o n s between these zones and the exposure zones are immediately apparent, but discussion of any causal r e l a t i o n s h i p s between expo-sure and zonation must await the presentation of faunal d i s t r i b u t i o n data i n the following section. Although Figure 8 provides an extremely useful frame-work f o r describing f l o r a l / f a u n a l zonation with respect to t i d a l exposure i t should be r e a l i z e d that computation of Figure 8 assumes a p e r f e c t l y f l a t t i d a l f l a t which drains free of water at each low t i d e . This i s not the case. Topographic depressions of small and large scale cause water cover Figure 8 . U p p e r S a n d W a v e Z o n a L o w e r S a n d W a v e Z o r 0.50 +0.25 0 -0 .25 0.50 0.75 1.00 E l e v a t i o n in M e t e r s (Geode t i c Datum) T" 1.25 1.50 1.75 2.00 2.25 2.50 Data f o r Doundary Bay, B.C., 21 June - k S e p t . , 1959 The* f n . J t m e " 1 S e p t . , 19";9 hand scale i n percent Also „ i ^ ^ J " s o u r c e . Weir, 1963). Left hand scale i n days r i g h t Ln to be "maintained ~during.;16w,:tide i n i t i d a l ' pools within:', the:: depressions. /. This has a profound influence on the d i s t r i b u t i o n of f l o r a and fauna on the l o c a l s c a l e , p a r t i c u l a r l y of the smaller macrofauna. Eelgrass can be found growing i n the w a t e r - f i l l e d troughs of sand waves i n the upper sand wave zone, and i s o l a t e d a l g a l mats may be found growing on the sand wave crest s , defying t h e i r respective zone limits.'.. FLORA FAUNA AND THEIR BIOGENIC SEDIMENTARY STRUCTURES Saltmarsh Zone The saltmarsh l i e s at an elevation which i s upper atmozonal i n exposure (Fig. 8). A t r i p l e x patula, G r i n d e l i a i n t e g r i f o l i a , Rumex crispus, A c h i l l e a m i l l e f o l i u m , and Aster sp. dominate on the landward portion of the marsh, whereas on the seaward side halophytesfthat are more s a l t tolerant predominate, such as S a l i c o r n i a sp., T r i g l o c h i n maritima and Spergularia maritima,(Kellerhals and Murray, 1969; O'Connell, 1975; Parsons, 19:75) .. :•; Dis t i c h l i s spicata':is, ab undarit a t ~ a l l levelslXParsohs, A19.75) . , . The sediments accumulating i n t h i s region consist of i r r e g u l a r l y . s t r a t i f i e d sand, peat, s i l t and clay. Sand and organic debris (eelgrass and driftwood) are transported onto the saltmarsh by winter storms, whereas peat and f i n e r grained sediment accumulate during the summer. However, due to disruption of bedding by vigorous r o o t l e t growth, seasonal s t r a t i f i c a t i o n i s f a i r l y poorly developed. I t i s noteworthy that, even within t h i s densely vegetated zone of the uppermost i n t e r t i d a l region, sand i s s t i l l the dominant sediment component. None of the organisms investigated i n t h i s study occur within the saltmarsh zone. The small shore crab Hemigrapsus.r.Qrgg°.nensis<is' abundant ;.-along the saltmarsh perimeter;' 61 A l g a l Mat Zone The a l g a l mat zone i s lower atmozonal i n exposure ( F i g 8). I t i s characterised by an almost continuous growth of cyanophyte a l g a l mats i n summer. The a l g a l mats consist predominantly of Microcoleus sp. and Phormidium sp. with minor amounts of the chlorophytes Enteromorpha sp. . .>! and Rhizoclonium sp. (Kellerhals and Murray, 1969). In winter, storms smother the a l g a l mats with sand. Thus, an annual s t r a t i f i c a t i o n of organic r i c h and sandy laminae i s produced (Kellerhals and Murray, 1969). The following organisms and biogenic sedimentary structures are character-i s t i c of the zone. B a t i l l a r i a B a t i l l a r i a attramentaria (Sowerby),>is _a herbivorous, d e p o s i t - T ' feeding gastropod (Fig. 10a). According to Whitlach (1974), i t feeds mainly on diatoms. B a t i l l a r i a i s found throughout most of the i n t e r t i d a l area i n summer, but i t s numbers decrease to zero i n the lower eelgrass zone (Figs. 9a & 9b). B a t i l l a r i a d ensities maximize i n shallow t i d a l pools (Fig. 9a). B a t i l l a r i a ' s presence i s most noticeable i n the a l g a l mat zone, where i t s traces are w e l l preserved i n the cohesive a l g a l mats. B a t i l l a r i a produces both r e s t i n g traces and grazing traces. Resting T r a c e s — B a t i l l a r i a produces a r e s t i n g trace by burying i t s e l f head f i r s t i n the sand using a corkscrew s p i r a l l i n g a c t i o n . Often only the pointed t i p of i t s s h e l l remains protruding. This behaviour i s probably a protective response against desiccation. Resting trace .pitsv are preserved between t i d a l cycles, while the grazing traces leading to them (Fig. 10b), which are much shallower depressions, are usually removed by the incoming t i d e . Hence, extensive areas of dense p i t t i n g r e s u l t with l i t t l e evidence of the o r i g i n a t o r remaining (Fig. 10c). P i t t i n g by B a t i l l a r i a i s most prevalent on the upraised a l g a l mats. 62 DENSITY (No. m-2) 120 C UNDER WATER SITES D DRY SITES D AVERAGE P = PRESENT 1.50-1 ; i.oo-I 0 .50 . • ^ I 0 h- -> 0 .50H Ul 1 - l w 1.00 1.50H '6 A , A L G A L M A T Z O N E U P P E R S A N D W A V E Z O N E M.H.H.W. E E L G R A S S Z O N E 1 0 0 0 D I S T A N C E , m a t e r s 2 0 0 0 B 60r DENSITY ( N a m - 4 ) 4 0 20 11 B a t i l l a r i a 1 i 1 t i l l _i_L STATION 20 25 35 « 1.50-1 _ 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0 D I S T A N C E , m a t o n Figure 9. a) Densities of B a t i l l a r i a sp. on transect A, d i f f e r e n t i a t i n g between dry s i t e s and under water s i t e s (shallow t i d a l pools) b) Densities of 'Batillaria''sp. on transect B. 63 Figure 10 (a) B a t i l l a r i a attramentaria producing a grazing t r a i l . Its s h e l l i s about 3 cm long. (b) B a t i l l a r i a grazing t r a i l s and r e s t i n g traces ( p i t s ) . Note the t r a i l s leading to p i t s . Trowel head i s about 5 cm wide. (c) Resting trace p i t s produced by B a t i l l a r i a sp. Four B a t i l l a r i a sp. can be seen s t i l l occupying p i t s . Trowel head i s about 5 cm wide. 64 65 Grazing T r a c e s — B a t i l l a r i a , as i t grazes on the sediment, produces a simple furrow with upraised edges (Figs.I0a,,& 10b) . B a t i l l a r i a does not e x h i b i t phobotactic behaviour—avoidance of previously grazed sediment. In f a c t , on occasion B a t i l l a r i a were observed to follow p r e c i s e l y grazing t r a i l s only minutes o l d , with as many as three B a t i l l a r i a on one t r a i l . In the presence of weak currents, f o r example where water drains out of sand wave troughs, B a t i l l a r i a heads upstream grazing on the sediment, leaving grazing t r a i l s p a r a l l e l i n g the current d i r e c t i o n ( F i g . 11a). The abrupt termination of t r a i l s due to r o l l i n g of the gastropod by the current indicates current sense as w e l l as d i r e c t i o n (Fig. l i b ) . The currents i n which this behaviour was observed were too weak to remove the t r a i l s . In stronger currents B a t i l l a r i a dives head f i r s t into the sediment with i t s pointed t i p pointing downcurrent, and ceases moving. Alignment of biogenic structures with a current has been described by Rhoads (1975) for b i v a l v e siphon openings and polychaete dwelling tubes. I t has also been described i n the geological record by Seilacher (1964) for t r i l o b i t e t r a i l s . On the rare occasions i n summer that strong winds blow from the south to southeast, waves are blown into Boundary Bay from the open S t r a i t of Georgia, and f a i r l y considerable movement of sediment by wave-formed ri p p l e s occurs. On these occasions B a t i l l a r i a burrows i t s e l f out of sight about 1 cm below the surface to avoid being r o l l e d over by wave induced currents. Such occurred on August 4, 1976, when on f i r s t sight the t i d a l f l a t s appeared to be almost completely devoid of B a t i l l a r i a , but on scraping the r;surface with a trowel B a t i l l a r i a was revealed i n i t s usual d e n s i t i e s . Spio Spio sp. i s a small tube-dwelling polychaete worm (Figs. 12a, 12b & 12c). It constructs an agglutinated sand tube. I t . l i v e s upright i n the tube and draws food and sediment into i t s tube with i t s two t e n t a c l e - l i k e palps 66 Figure 11. a) B a t i l l a r i a sp. heading upstream producing grazing t r a i l s p a r a l -l e l i n g the current d i r e c t i o n (eelgrass at top of photo in d i c a t e s current d i r e c t i o n ) . B a l l p o i n t pen i s about 15 cm long, b) Behaviour of B a t i l l a r i a sp. i n weak currents: the gastropod heads upstream grazing, and produces a t r a i l p a r a l l e l i n g the current. Occasionally the current causes the gastropod to r o l l . Once s t a b i l i z e d again the gastropod turns i n towards the current and reverts to grazing i n an upstream d i r e c t i o n . 67 2. cm y * (b) Figure 12. a) Mounds produced by the feeding a c t i v i t i e s of Spio sp. Pen i s about 15 cm long. b) Plan view of Spio sp. Two t e n t a c l e - l i k e palps draw food i n t o i t s tube and are a l s o used to v o i d sandy pseudo-fecal s t r i n g s i n a r a d i a l p a t t e r n . c) Cross s e c t i o n of Spio sp. i n i t s d w e l l i n g tube. 68 (Figs. 12b & 12c). Sediment, p a r t i c u l a r l y l a r g e r sand grains, tends to catch on the t i p of the tube as the worm draws i n i t s food laden palps. As a r e s u l t a mound up to 0.5 cm i n height forms around the tube (Fig. 12a). The worm also voids f r a g i l e , elongate, sandy pseudo-fecal s t r i n g s . These are extruded r a d i a l l y around the tube (Fig. 12b). The sediment mounds are fl a t t e n e d by the incoming t i d e . Spio i s abundant i n the a l g a l mat zone (Figs. 13a & 13b) a t t a i n i n g i t s maximum density of about 10^ m - 2 next to the saltmarsh (Al & B l ) . However, Spio only occurs i n t i d a l pools between the upraised a l g a l mats. In the upper sand wave zone Spio decreases i n density by an order of magnitude (Figs 13a & 13b). There are two possible reasons for the lower densities of Spio i n the upper sand wave zone. F i r s t l y , the sand wave crests, although of very low amplitude, make an inhospitable, dry environ-ment for Spio during low tide and Spio Is r e s t r i c t e d to the w a t e r - f i l l e d troughs of the sand waves. Secondly, i n winter, storm waves cause wave-formed r i p p l e s to disturb the upper 2-3 cm of sediment uprooting many Spio i n the process. In the eelgrass zone Spio increases markedly i n density a t t a i n i n g 5 x 10^ m - 2, but Spio does not colonize the co n i c a l mounds formed by Callianassa c a l i f o r n i e n s i s . Comparable densities of Spiophanes w i g l e y i , another spionid worm, have been reported by Featherstone and Risk (1977) i n Minas Basin, Bay of Fundy. Measurements of sediment turnover by Spio could not be made d i r e c t l y , however, an estimate can be made. During exposure at low tide (about 20 hours i n the a l g a l mat zone) Spio produces a mound of about 0.02 cm3 of sediment. At a density of 10^ m - 2 t h i s turnover amounts to 200 cm3 per m2. This i s equivalent to reworking a monogranular surface layer of sand 100 um thick i n that one square meter twice between t i d e s . 69 40 ,000 DENSITY (Na m-*) 20,000 Spio I • • • STATION 20 ; 1.00 I 0 . 0 , 1 o-l K - • > 0.50 -j ty 1.00 1.50 A L G A L M A T Z O N E U P P E R S A N D W A V E Z O N E M.H.H.W. E E L G R A S S Z O N E 5 0 0 1 0 0 0 D I S T A N C E , m e t e r s T B 20,000 DENSITY (No. m'z) lopoo ' p ' • P P • -P.N.D.-15 20 S T A T I O N P = P R E S E N T RN.D. = P R E S E N T , BUT NOT DETERMINED i l l ! JL I l . i I p p p 30 • 1.00 -• 0 . 5 0 -E 0 - A L G A L z o 0.50 - M A T VAT 1.00 -Z O N E Ul - J Ul 1.50 -U P P E R S A N D W A V E Z O N E 6 2 0 B 3 0 E E L G R A S S 5 0 0 1— 1 5 0 0 2 0 0 0 D I S T A N C E , m o t o r s Figure 13. a) Densities of Spio sp. on transect A. b) Densities of Spio sp. on transect B. 70 Fly Larvae In the upper h a l f of the a l g a l mat zone the raised a l g a l mat platforms are r i d d l e d with small ' U' shaped burrows. The burrows are 1.5 to- . • TO cm deep and 0.25- to 1.0 mm in. diameter.^ They only, occur, on -..•'> the upraised a l g a l mats and t h e i r densities are highly v a r i a b l e , ranging from .0 to 10 5 m - 2 (extrapolating from measurements taken with a 4 cm2 quadrat), but i n general of the order of lO1* m - 2. Despite thorough searching through numerous box core samples the organism responsible for the burrows could not be found. The burrows are thought to be produced by f l y larvae. The f l i e s lay eggs i n or on the a l g a l mats. The larvae hatch, burrow downward feeding on the n u t r i t i o u s a l g a l mat seams, then burrow out and f l y o f f without a trace. Upper Sand Wave Zone The upper sand wave zone i s upper amphizonal i n exposure (Fig. 8). I t i s characterized by very low amplitude sand waves, lacks any extensive f l o r a l mat, and i s dominated by the following organisms and biogenic sedimentary structures. Abarenicola . Abarenicola p a c i f i c a 'Healy and Wells i s a deposit-feeding ' -polychaete worm (Hobson, 1967), which constructs a v e r t i c a l l y orientated ' J ' shaped burrow ( F i g . 14a). j The t a i l shaft reaches the surface and f e c a l casts are excreted here i n the form of sand c o i l s . The head shaft consists of a collapse cone produced by the feeding a c t i v i t i e s of the worm, and this i s occasionally v i s i b l e as a depression at the surface. Abarenicola  p a c i f i c a c i r c u l a t e s water through i t s burrow system (Hylleberg, 1975). The r a t i o of f e c a l cast density to worm density was determined to be 1.07:1 by taking 30 box cores betweenC5.tatio,nsAA6TAll. Sixty.'fecal casts (30 x 2) were found to be associated with 56 worms (26 x 2, 4 x 1 ) . 71 (b) Figure 14. (a) Morphology of an Abarenicola burrow ( a f t e r Hylleberg, 1975). Arrows i n d i c a t e d i r e c t i o n of r e s p i r a t i o n current, (b) Patchy d i s t r i b u t i o n of Abarenicola f e c a l casts. The highest d e n s i t i e s of casts occur i n and around t i d a l pools. Trowel i s about 25 cm high. 72 For a l l intents and purposes the r a t i o can be taken as 1:1. Abarenicola appears abruptly near the lower edge of the a l g a l mat zone. Densities r i s e from zero to about 20 m - 2 (A5 x = 19 m - 2, a = 12 m - 2; B5 x = 2015 m~2, a = 37 m - 2) i n the distance of less than 100 m (Figs. 15a & 15b). S i x t y -four quadrat readings between Stations A-1-A4 and 31-B4, sampling i n t o t a l 16 m2, registered zero Abarenicola. The upper l i m i t of the worms' occur-rence on transect A l i e s between A4 at + 0.87 m elevation (Geodetic Datum) and A5 at + 0.75 m elevation, and on transect B between B4 (+ 0.83 m) and B5 (+ 0.70 m) . There i s no change i n grain s i z e over the 100 m i n t e r v a l i n which the worms appear. Abundant juvenile Abarenicola have been observed w e l l above t h i s l i m i t , up to the saltmarsh perimeter, i n early spring. However, they disappear from t h i s area by July, probably because desicca-t i o n during prolonged exposure on warm, sunny days at neap t i d a l periods r e s u l t s i n high mortality amongst ju v e n i l e Abarenicola within the atmozone (Fig. 8), whereas mortality w i t h i n the amphizone (Fig. 8) i s reduced because the t i d a l f l a t i s inundated with fresh sea water at le a s t once everyday (Swinbanks, i n preparation). Abarenicola attains i t s maximum densities i n the upper sand wave zone and extends i n t o the eelgrass zone (Figs. 15a & 15b). Its densities are higher i n wet sediments than dry (Fig. 15a). Abarenicola congregates i n the troughs of sand waves and i n t i d a l pools which are wet or under water (Fig. 14b). This i s probably the r e s u l t of d i f f e r e n t i a l m o r tality amongst juvenile Abarenicola. In the upper sand wave zone Abarenicola excretes on average about 1-5 wet ml worm ^ day ^  (1 wet ml = 1.5 g dry weight), but rates vary depending on the wetness of the sediment. In dry sediment average rates can be as low as 0.02 ml worm ^ day while i n t i d a l pools average rates can be as high as 8.4 ml worm day Tn the eel grass'zone large Abarenicola 73 DENSITY (No. m - 2 ) Abarenicolo DENSITY (No. •»-») 1 111 | WET SITES Q DRY SITES P a PRESENT J _ J L Abarenico la I I 1 11 1 B i 9 • i P P P . P P m 20 DENSITY (No. Abarenico la I 1 1 I i I . . . P . , I . I I P - I P - 1 1 5 0 0 3 0 0 0 DISTANCE, m o t o r I Figure 15. a) Densities of Abarenicola f e c a l casts on transect A. Upper histogram distinguishes between wet and dry s i t e s , and i s based on four wet s i t e readings and four dry s i t e readings at each s t a t i o n with a 0.25 m quadrat. Stations A8 and A9 had no wet s i t e s which could be sampled. Stations A11-A17 had no dry s i t e s . Station A5 had dry s i t e s , but Abarenicola was absent from them. Lower histogram presents the average d e n s i t i e s , based on random quadrats. . ' j . b) Densities of Abarenicola f e c a l casts on transect B. 74 excrete on average 29 wet ml worm day (Swinbanks, 1979). Abarenicola p a c i f i c a i n the course of eating r e j e c t s coarse grains (Hylleberg, 1975) and through i r r i g a t i o n of i t s burrow can f l o a t clay towards the surface i n the head shaft i r r i g a t i o n current (Swinbanks, 1979). This could r e s u l t i n biograded bedding as described by Rhoads and Stanley (1965). However, the f i n e grain s i z e and low mud content of Boundary Bay sands renders these processes v i r t u a l l y indetectable. My a Mya arenaria Linna^uV,fa";^us"pensioh feeding bivalve,'Jqccurs ' i n the upper sand wave zone (Fig. 16a). A few i n d i v i d u a l s also occur i n the t i d a l pools of the a l g a l mat zone, but i n d e n s i t i e s l e s s than -2 0.5 m . Mya was not observed i n the eelgrass zone. The maximum density -2 Mya attains i s about 4 m . I t occurs i n the upper sand wave zone on -2 transect B but i n densities l e s s than 0.5 m Mya constructs a simple v e r t i c a l tube up to 15 cm i n depth (Fig. 16b). Movement of Mya within i t s burrow causes downward warping of a l g a l mat laminations (Fig. 16b). The v e r t i c a l tube constructed by Mya accommodates the inhalent and exhalent siphons which are fused together. Fecal p e l l e t s are voided from the exhalent siphon. Because of i t s low densities Mya i s not considered to contribute s i g n i f i c a n t l y to the sediments of Boundary Bay through defecation. Callianassa Callianassa c a l i f o r n iensis ' Dana.is a burrowing thalassinidean,, . ^ L ' ^ j shrimp. Figures 17a and 17b present the density d i s t r i b u t i o n of Callianassa burrow openings on transects A and B. Isolated i n d i v i d u a l Callianassa burrows occur i n the a l g a l mat zone, but i n very low d e n s i t i e s . Eighty quadrat readings on transects A and B taken wi t h i n the a l g a l mat zone, 2 sampling i n t o t a l 20 m , registered zero Callianassa burrow openings. 75 (b) Figure 16. (a) Densities of Mya sp. on transect A. (b) Spreite traces l e f t by Mya sp., and downwarping of lamina-, tions caused by movement- of the clam within i t s burrow due to i t s growth or changes i n the l e v e l of the sediment water in t e r f a c e ( a f t e r Reineck, 1958). 76 DENSITY (No. m"*) 40 20 Collianossg / Upoqebio p p p p p Q CALLIANASSA ONLY H CALLIANASSA a UPOGEBIA | UPOGEBIA ONLY P » PRESENT " - p - n IL STATION 1.50 ; LOO J 0 .50 H . • z O 0-) < > 0.50-ui -j Ul 1.00 1.50' A L G A L M A T Z O N E M.H.H.W. U P P E R S A N O W A V E Z O N E E E L G R A S S 5 0 0 1 0 0 0 D I S T A N C E , m o t o r s 1 5 0 0 2 0 0 0 B 10 D E N S I T Y (No. m" 2) Collionasso P = PRESENT 1 5 20 25 30 35 STATION 1 1.00 • 0 . 5 0 H ::» 2 0.50 < LOO H 3 1-50 M.H.H.W. A L G A L M A T Z O N E *10 U P P E R S A N D W A V E Z O N E E E L G R A S S 5 0 0 3 0 0 0 D I S T A N C E , m o t o r s Figure 17. a) Densities of Ca l l i a n a s s a and Upogebia burrow openings on transect A. • b) Densities of Ca l l i a n a s s a burrow openings on transect B. 77 Walking seawards along the' t^ans.ect\and scanning. approximately ^ 2 m'on either side of the transect l i n e the f i r s t C a l l i a n a s s a burrow encountered was at A2 (+ 0.94 + 0.07 m, Geodetic Datum) on transect A, and at B4 (+ 0.83 + 0.02 m, Geodetic Datum) on transect B (Figs. 17a & 17b). On exposure the dissolved oxygen content of Upogebia burrows, which are mud-lined, decreases r a p i d l y and anoxic conditions can p r e v a i l within one hour (Thompson and P r i t c h a r d , 1969). Anoxic conditions are probably reached more rap i d l y i n the burrows of Callianassa c a l i f o r n i e n s i s because the lack of a firm burrow l i n i n g exposes the burrows to hypoxic i n t e r -s t i t i a l waters (Thompson and P r i t c h a r d , 1969). Callianassa c a l i f o r n i e n s i s can survive approximately 5.7 days of anoxia , (range 3.2-7:-8 days, N=35, Thompson and P r i t c h a r d , 1969). At + 0.79 m (Geodetic Datum) the maximum duration of continuous exposure i s 4 lunar days, at + 0.85 m i t i s 5 lunar days while at +>0-.-91 mVarid jff 0v97 m'it is-(9 lunar days (Fig. 8). In Boundary Bay the upper l i m i t of Cal l i a n a s s a , l y i n g at about + 0.9 m (Geodetic Datum), i s , therefore, almost c e r t a i n l y determined by exposure because above this l e v e l the maximum duration of ;anOxia }due to exposure exceeds the l e t h a l l i m i t for C a llianassa . Conceivably the occasional Callianassa i n d i v i d u a l could survive above t h i s l e v e l , i f a t i d a l pool with oxygenated water happened to o v e r l i e i t s burrow entrance, because Callianassa could then draw oxygen-ated surface water i n t o i t s burrow by r a p i d l y fanning i t s pleopods, as reported by Farley and Case (1968). Rare i n d i v i d u a l C a l l i a n a s s a burrows have been observed o f f transect i n t i d a l pools next to the saltmarsh perimeter, and these may f a l l i n t o the above category. However, the precise elevations of these pools are^unknown"; In the upper sand wave zone Callianassa burrow openings appear i n clusters covering areas of about one square meter;. Burrow opening density within a c l u s t e r may be 10 m-2, but c l u s t e r s are separated by tens of 78 meters, and as a r e s u l t average densities are l e s s than 0.5 m - 2. Below about +0.6 m elev a t i o n (Geodetic Datum) Callianassa's d i s t r i b u t i o n i s more uniform and greater than 0.5 m ? Callianassa probably requires the day-to-day r e l i a b i l i t y of t i d a l inundation found within the amphizone in order to t h r i v e , unhindered by the stress of periods of excessively prolonged •;anoxia - (Torres et al.j,'XL977}> as3mus_t"'"6ccu'rr'"in"the" atmozone. Average Callianassa burrow densities maximize at about 20 m - 2^around the edge of the eelgrass zone. Callianassa densities are very low below - 0.6 m elevation. Seventy-eight quadrat readings taken on both transects below - 0.6 m ele v a t i o n , sampling i n t o t a l 54 m2, registered zero Ca l l i a n a s s a burrow openings. These low densities may be the r e s u l t of the dense r o o t l e t s of ^ Zostera 'marina^irihib^itirig- the mining a c t i v i t i e s of the shrimps, thereby l i m i t i n g t h e i r population. Using an open ended metal box the r a t i o of burrow openings to shrimp density was determined to be 2.5 (arithmetic mean deviation 0.8, N=6) to 1. In several r e s i n casts shrimps were v i s i b l e entombed within-thev.) casts. Invariably each burrow system was occupied by one shrimp. There are usually two openings to each burrow system, although occ a s i o n a l l y there may be three and r a r e l y four openings. Hence,'the average burrow opening to shrimp r a t i o i s 2.5 to 1. This agrees with the findings df 0 t 1 : e t a l - (1976) and Hertweck (1972) for other thalassinidean shrimps. Burrow Morphology—Since the development of a r e s i n casting technique by Shinn (1968) , burrow morphology of thalassinidean shrimps has been studied quite extensively. Frey and Howard (1975) c i t e numerous references. Figures 18a, 18b and 18c i l l u s t r a t e the morphology of these c a l l i a n a s s i d burrows. They extend 20-30 cm down in t o the sediment and then branch h o r i z o n t a l l y f o r distances of up to a meter. Each system usually has two exits which j o i n as a bulbous chamber at from 5 cm to 10 cm depth. The 7.9 Figure 18. a) Plan view of a Ca l l i a n a s s a burrow cast, showing bulbous 'turnarounds.' Cast i s about 60 cm i n plan view length. Metric r u l e r (lm) with centimeter subdivisions provides scale. b) Side view of a Callianassa burrow cast showing h o r i z o n t a l mine-like nature of burrow system. Burrow extends to about 30 cm depth. Metric r u l e r (1 m) with centimeter subdivisions provides scale. ^Overflow of r e s i n produced 'heads' on cast. c) Plan view of a large Callianassa burrow cast which i s j u s t over 1 m long. Metric r u l e r (1 m) provides scale. 81 ex i t s have constricted apertural necks. Branching i s dichotomous. There are bulbous turnarounds within the systems and b l i n d a l l e y s . There i s no d i s t i n c t l i n i n g to the burrow walls except that the sediment i s oxidized and l i g h t e r i n colour. The lack of a firm burrow l i n i n g and the h o r i z o n t a l , branching nature of the burrows suggest they are temporary feeding burrows rather than permanent dwelling burrows. Ott et a l . (1976) came to a s i m i l a r conclusion regarding the burrows of Callianassa stebbingi. The geometry of the burrows of Callianassa c a l i f o r n i e n s i s have a l l the c h a r a c t e r i s t i c s of a mine used for deposit feeding. However, the presence of a bulbous chamber close to the surface does suggest that the shrimps may suspension feed while the tide i s i n . A l l organisms to some extent a l t e r t h e i r environment to the ben e f i t of some and detriment of others. This i s nowhere more apparent i n Boundary Bay than i n the case of Callianassa. For example Cryptomya c a l i f o r n i c a , a small b i v a l v e , uses the sediment-water in t e r f a c e of the Callianassa burrow as a surface for suspension feeding, c l u s t e r i n g around the bulbous chamber 5-10 cm below the surface. On the other hand, on the surface, Spio cannot colonize the mounds heaped up by Callianassa because they dry out during low t i d e , and Callianassa's excavation a c t i v i t i e s probably choke surface suspension feeders l i k e Mya arenaria. Eelgrass Zone The eelgrass zone i s l a r g e l y lower amphizonal to upper aquazonal i n exposure (Fig. 8), and i s , on the large scale, f l a t , except where the upper reaches of t i d a l channels cross the zone producing broad, shallow, w a t e r - f i l l e d depressions (e.g., between Stations B18 to B25 on transect B). The uppermost part of this zone i s dominated by a summer growth of Zostera  americana, while i n the re s t of the zone a perennial-growth, of Z. marina 82 i s present. The following organisms and biogenic sedimentary structures are t y p i c a l of the Z_. marina subzone. Upogebia Upogebia pugettensis (Dana), l i k e Callianassa, i s a thalassinidean burrowing shrimp. I t has been studied i n d e t a i l by Thompson (1972). Upogebia only occurs on transect A, appearing abruptly at the edge of the Z. marina subzone at A16, and a t t a i n i n g a maximum burrow opening density of 44 m - 2 (Fig. 17a). There i s a region of overlap where Callianassa and Upogebia burrows occur side by side . Data c o l l e c t e d from the t i d a l f l a t s of the active Fraser Delta front indicates that Upogebia prefers muddy substrates (Swinbanks, T?79^ v'Part,;4'A)'X.' . . *T -,, ': Upogebia i s probably r e s t r i c t e d to the Z. marina beds on transect A i n Boundary Bay because of t h e i r higher mud content. Upogebia uses mud to l i n e i t s burrow, and so i t i s not s u r p r i s i n g that Upogebia's occurrence i s r e s t r i c t e d by the mud content of the sediment, p a r t i c u l a r l y i n an environment such as Boundary Bay where mud contents are only a few percent. Amongst the organisms studied Upogebia i s an exception to the e a r l i e r contention that exposure time, rather than substrate, i s the prime co n t r o l -l i n g agent of f l o r a l / f a u n a l zonation.in Boundary Bay. Burrow Morphology—The Upogebia burrow i s a 'Y' shaped ( F i g . 19a). The two branches of the 'Y' system meet 20-30 cm below the surface, and the burrow stem continues down to depths of 50 to 60 cm. In contrast to Callianassa burrows, Upogebia burrows are predominantly v e r t i c a l l y oriente'd,;"^ do not have constricted entrances, and lack bulbous turnarounds. The i n t e r n a l walls of the burrow are smooth and l i n e d with mud. Upogebia burrows very seldom have sediment mounds outside t h e i r entrances, i n d i c a t i n g that the burrows are probably not used for mining purposes. Upogebia burrows appear to be permanent dwelling burrows. Qtt et al.•(1976) reached Figure 19. a) Cast of two Upogebia 'Y' shaped burrows joined by a constricted neck. Cast i s j u s t over 50 cm i n depth. Metric r u l e r (1 m) with centimeter subdivisions provides scale. b) Side view of cast i n (a.) showing shrimp entombed within the cast. ," '.••<•/ r i oo 84 85 a s i m i l a r conclusion regarding the burrows of Upogebia l i t o r a l i s . Adjacent 'Y' burrow systems are often interconnected by cons t r i c t e d apertiial necks much l i k e those described by Frey and Howard (1975) for Upogebia a f f i n i s (Fig. 19a). The excavated casts are free of any sand coating, because of the mud l i n i n g to the burrows, and shrimps are c l e a r l y v i s i b l e entombed within the casts (Fig. 19b). Five 'Y' tube casts were obtained and i n v a r i a b l y each 'Y' tube contained one shrimp, giving a burrow opening to shrimp r a t i o of 2 to 1. The body width of the shrimp determines the i n t e r n a l diameter of the burrow. P r a x i l l e l a P r a x i l l e l a af f i n i s p a c i f i c a Berkeley i ^ a tub~e-dwelling mald_ani'V£ polychaete worm, and i s clo s e l y r e l a t e d to Clymenella torquata described by Rhoads and Stanley (1965) and Featherstone and Risk (1977). P r a x i l l e l a constructs an agglutinated sand tube up to 15 cm i n length and the worm lives.upside down i n the tube and excretes unconsolidated sandy feces onto the surface ( F i g . 21). I t occurs i n the same area of the eelgrass zone as Upogebia, but i s present on both transects. I t attains densities of 650 m - 2 (Figs. 20a & 20b), and extends into the lower sand wave zone. No evidence of biograded bedding as described by Rhoads and Stanley (1965) f o r Clymenella was found i n the case of P r a x i l l e l a . However, th i s i s not s u r p r i s i n g , since the sands i n which P r a x i l l e l a l i v e s on these t i d a l f l a t s are fine grained and the worm need not be s e l e c t i v e about the grain si z e of sediment i t eats. The grain s i z e d i s t r i b u t i o n of the dwelling tubes of P r a x i l l e l a were found to have an i d e n t i c a l grain s i z e d i s t r i b u t i o n to that of the surrounding sediment, within the experimental errors of grain si z e a n a l y s i s . This i s contrary to the findings of Featherstone and Risk (1977) who found that the grain s i z e d i s t r i b u t i o n of Clymenella tubes was 86 DENSITY (No. ru"*) Nassarius 1 1 I I I 1 R DENSITY (No. ' s o < * 1 0 0 0 1300 D I S T A N C E , m a t e r s Z . _ L = _ 2 0 0 0 Nassar ius P =• PRESENT | P P | P J j I P P | | P i' P P P I I I DENSITY (No. in-*) Pranilleifl t i l l 1500 2000 D I S T A N C E , fliitin Figure 20. a) Densities of P r a x i l l e l a sp. and Nassarius sp. on transect A. b) Densities of P r a x i l l e l a sp. and Nassarius sp. on transect B. 87 PRAXILLELA. -7T E in U * <r—» 3-4 mm Figure 21. ( V e r t i c a l agglutinated.• ~sand;tube' of.,Eraxi 11 e l a sp. 88 s i g n i f i c a n t l y coarser than'that of the surrounding sediment. Perhaps :the s l i g h t l y coarser grain s i z e of the sands i n Minas Basin (median grain si z e about 2.2 0 as opposed to 2.7 0 i n Boundary Bay) induces s e l e c t i o n of coarser grains during tube b u i l d i n g . The coarser grains possibly being derived from those rejected during feeding. Nassarius Nassarius mendicus (Gould)-is a gastropod which occurs alongside " Upogebia and P r a x i l l e l a (Figs. 20a & 20b). I t i s more active and f a s t e r moving than B a t i l l a r i a but i t does not produce any d i s t i n c t i v e traces, apart from a very s u p e r f i c i a l grazing t r a i l . On transect B the d i s t r i b u t i o n s of Nassarius and P r a x i l l e l a are bimodal (Fig. 20b). This i s due to the presence of a topographic high at B25. Water drains o f f this elevated region from both sides and i t r a p i d l y dries out during low t i d e , whereas the depression centred on B20 remains w a t e r - f i l l e d , despite the fact that i t l i e s above sea l e v e l , because water constantly drains into i t . As a r e s u l t the region near B25 has the c h a r a c t e r i s t i c s of higher elevations on the t i d a l f l a t , whereas the depression around B20 has the c h a r a c t e r i s t i c s of lower elevations. A bimodal d i s t r i b u t i o n of P r a x i l l e l a and Nassarius r e s u l t s , the modes being s p l i t by the topographic high. This emphasizes the fact that the method of subdividing the i n t e r t i d a l zone into exposure zones, presented e a r l i e r , only holds true i f the slope of the t i d a l f l a t i s r e l a t i v e l y constant, with no topographic highs or lows producing abnormal exposure or prolonged submergence due to drainage e f f e c t s . Lower Sand Wave Zone The lower sand wave zone i s aquazonal i n exposure (Fig. 8):and i s characterized by large sand waves - and by the lack of a f l o r a l cover. : Some q u a l i t a t i v e observations of this zone have been made, p a r t i c u l a r l y at the end of transect B which encroaches upon i t (Fig. 2). P r a x i l l e l a and 89 large.Abarenicola "\ are present i n t h i s zone, as are sand d o l l a r s (Kellerhals and Murray, 1969), but a l l other organisms considered i n this study are absent or present i n very low d e n s i t i e s . The zone i s dominated by p h y s i c a l sedimentary structures. Dunes l i n e the sides of the t i d a l channels . (Kellerhals and Murray, 1969). Ripples and sand waves are the c h a r a c t e r i s t i c bedforms of the rest of the zone. K e l l e r h a l s and Murray (1969) report coarse sands along the lower perimeter of this zone. The t i d a l channels which dissect t h i s zone are l i n e d with a dense growth of eelgrass (Z. marina), and s h e l l lag deposits are also present (Kellerhals and Murray, 1969). DISCUSSION OF ZONATION The cause of i n t e r t i d a l zonation has been a topic of great debate amongst b i o l o g i s t s f or many years, and the extent of the r o l e which tides play i n zonation has been a matter of much controversy ('Doty, 1957; Ricketts and'Calvin;, 19"&8^"Chapman and Chapman, .1973I; Chapman,:1974;"''Carefoot, 1977)'.^ _In B o u n d a r y "\ Bay the evidence suggests that tides,and i n p a r t i c u l a r c r i t i c a l t i d a l l e v e l s , are a major cause of zonation. There are three f l o r a l zone l i m i t s which surveying and topographic maps have revealed to be delimited by elevation. These are the lower l i m i t of the saltmarsh zone, the lower l i m i t of the a l g a l mat zone and the upper l i m i t of the eelgrass zone. Figure 8 can o f f e r explanations for a l l three. The lower l i m i t of the saltmarsh l i e s at + 1.15; m (Geodetic Datum) on transect A and + 1.10 m on transect B (Table I I ) . This i s coincident, within the errors of surveying, with the lower l i m i t of the upper atmozone (+ 1.16 m, Fig. 8), which i s a l e v e l at which the maximum duration of continuous exposure begins to r i s e abruptly from 12 to 40 days. I t i s also the upper l i m i t of Level 3 exposures and the elevation of the highest lower high water. As a r e s u l t sea water only covers this area during the l a t e afternoon, evening or 90 at night i n summer. The saltmarsh apparently thrives under conditions of prolonged daylight exposure. The period that a plant, i s continuously "flooded and the duration of continuous exposure are both l i m i t i n g factors i n t h e i r own r i g h t (Chapman, 1974). Continuous flooding l i m i t s plant growth by water-logging roots reducing r e s p i r a t i o n due to lack of oxygen, imposing s a l i n i t y stress and by reducing l i g h t a v a i l a b i l i t y . A s a l i n e environment imposes p h y s i o l o g i c a l s t r e s s . Nutrient deficiency ,, stress i s caused by uptake of sodium ions i n preference to potassium ions. Hormonal stress i s induced by s a l t stressed roots being i n h i b i t e d i n transport of hormones to leaves, and osmotic pressure also increases root resistance thereby decreasing water delivery to leaves, a l l of which re s u l t s i n growth reduction ( L e v i t t , 1972; Waisel, 1972)~V This may not apply_to S a l i c o r n i a sp. , the pre- ' 5' dominant halophyte in-the lower saltmarsh zone (Parsons, 1975), as some consider S a l i c o r n i a sp. to be an obligate halophyte (Chapman, 1 9 7 4 ) — i . e . a halophyte which requires s a l t for optimum growth. However, the existance of obligate halophytes i s questionable (Barbour, 1970; Ungar, 1966). Ungar, (1966) suggested that halophytes grow i n s a l i n e s o i l s simply because they cannot compete e f f e c t i v e l y with t e r r e s t i a l plants i n non-saline s o i l s . Parsons (1975) found that D i s t i c h l i s s picata, which i s abundant throughout the saltmarsh zone i n Boundary Bay, grows better i n s o i l saturated with tap water than with d i l u t e seawater or seawater. I t i s therefore reasonable to suggest that flooding frequency: i s a l i m i t i n g factor i n the Boundary Bay saltmarsh, at l e a s t i n the case of D i s t i c h l i s s p i c a t a , because flooding imposes p h y s i o l o g i c a l s t r e s s . Chapman (1974)'stressed' the importance of also con-. . / -s i d e r i n g the maximum duration of exposure, since saltmarsh plant seedlings require several days of continuous exposure without flooding i n order to germinate and root s u c c e s s f u l l y . S a l i c o r n i a s t r i c t a requires two to three days while Aster t r i p o l i u m requires f i v e (Chapman, 1974). Continuous exposure 91 must coincide with germination. The c r i t i c a l tides which define the boundary between the upper and lower atmozones occur at the spring and autumn equinoxes (Swinbanks, ;1979)". Saltmarsh /plant seedlings 'have'beeh^obs'erve'd " } sprouting in Boundary Bay in March close to the time of the spring equinox, and so the long periods of continuous exposure which occur i n the upper atmozone at this time have a high probability of coinciding with seedling germination. Thus there are reasonable physiological grounds for suggesting that the break in exposure duration and submergence frequency between the upper and lower atmozones is a causative factor i n limiting the saltmarsh zone rather than a mere coincidence. The lower limit of the algal mat zone coincides within the errors of surveying with the lower limit of the lower atmozone (Fig. 8). Apparently the cyanophyte algal mats thrive in an area subject to the prolonged periods of exposure associated with Level 1 exposures. Many blue-green algae are obligate photoautotrophs (Fogg,et a l . , 1973)—i.e. they cannot grow without light. Phormidium sp. can grow very slowly i n the dark on a medium of glucose and yeast autolysate (Allen, 1952), but for any blue-green algae to thrive, sufficient light i s essential. However, as algal mats can be found growing on the crests of sand waves, well below the lower limit of the algal mat zone i t would seem unlikely that the algal mat zone is light limited, but rather that desiccation for some reason is necessary for the algal mats to thrive. An a b i l i t y to withstand desiccation i s a characteristic feature of blue-green algae, and the vegetating cells of Oscillatoriaceae (the family; tol which. Phormidium and Microcoleus belong) which have no perennating cells, survive desiccation better than other families of blue-green algae (Fogg.-et a l . , 1973). Although some species show great resistance to desiccation, growth of these does not occur at relative humidities of less than 80% (Hess, 1962), and therefore desiccation must only be indirectly beneficial.' 92 I t has been demonstrated that c e r i t h i d gastropods, such as B a t i l l a r i a , destroy blue-green a l g a l mats by t h e i r grazing a c t i v i t i e s (Garrett, 1970), and i t has been suggested that a l g a l mats are r e s t r i c t e d to the uppermost i n t e r t i d a l to su p r a t i d a l regions because grazing gastropods and burrowing organisms are absent or present i n low densities i n these areas (Garrett, 1970). In Boundary Bay B a t i l l a r i a alone cannot l i m i t the extent of the a l g a l mat zone, because B a t i l l a r i a densities are the same or of the same order of magnitude within the a l g a l mat zone as without i t (see average densities Figs. 9a & 9b). However, the grazing a c t i v i t i e s of B a t i l l a r i a combined with the intense \._ reworking a c t i v i t i e s of Abarenicola and Callianassa might be s u f f i c i e n t to l i m i t the extent of the a l g a l mats. The densities of Abarenicola and .: Callianassa increase abruptly near the lower l i m i t of the a l g a l mat zone (Figs. 15a & 15b, Figs. 17a & 17b). The a l g a l mats and these organisms tend to be mutually exclusive, because the raised a l g a l mat platforms are dry and inhospitable to the organisms, i n p a r t i c u l a r Abarenicola, while the reworking and grazing a c t i v i t i e s of the organisms i n h i b i t a l g a l mat formation. I f burrowing and grazing organisms were absent from Boundary Bay, a l g a l mats would probably develop at lower i n t e r t i d a l l e v e l s as demonstrated by Garrett (1970). I t i s therefore.suggested that the step i n exposure duration between the amphizone and the atmozone sets the l i m i t to the a l g a l mat zone because desiccation associated with Level 1 exposures during neap tides prevents extensive population by burrowing organisms, i n p a r t i c u l a r Abarenicola.•': The i n t e r a c t i o n between f l o r a and fauna i s an e s s e n t i a l element i n the r e s t r i c t i o n of the a l g a l mat zone, but the abruptness of the zone's lower l i m i t i s caused by the s t e p - l i k e nature of i n t e r t i d a l exposure. The'upper l i m i t of the eelgrass zone terminates at the upper l i m i t of the lower amphizone and thus never experiences Level 2 exposures (Fig. 8). K e l l e r and Harris (1966) found a d i r e c t correlation, between ..the extent df Z. marina coverage and elevation i n Humboldt Bay, C a l i f o r n i a . The eelgrass showed a pronounced upper l i m i t at a l e v e l of 15% mean exposure 0.3 m above MLLW. They suggested that the upper l i m i t of eelgrass i s co n t r o l l e d by t i d a l exposure, because desiccation during exposure decreases the vigor and vegeta-tive reproduction of the eelgrass. The upper l i m i t of eelgrass i n Boundary Bay l i e s at a much higher t i d a l e levation (mean sea lev e l ) and i s exposed almost 50% of the time ( F i g . 6). However, the uppermost part of the eelgrass zone i n Boundary Bay consists e n t i r e l y of the smaller species Z. americana. We therefore suggest that the upper l i m i t of the eelgrass zone terminates at the upper l i m i t of the lower amphizone because Z_. americana cannot- t o l e r a t e Level 2 exposures, which are always w e l l i n excess of h a l f a lunar day and occur during daylight hours i n summer, and Z. americana requires the i n f l u x of seawater brought by lower high water i n order to survive. In Boundary Bay the upper l i m i t of Z. marina does not appear to be c o n t r o l l e d by el e v a t i o n but rather seems to be strongly influenced by the d i s t r i b u t i o n of t i d a l channels. This i s w e l l i l l u s t r a t e d i n the map of Z_. marina d i s t r i b u t i o n presented by O'Connell (1975). The t i d a l channels remain w a t e r - f i l l e d during low tide despite the fact that they l i e w e l l above sea l e v e l because water constantly drains into them. Z. marina growth extends up the fl o o r s and flanks of the channels and has thus attained elevations which i n theory have as much as 30% exposure (Station A17 at - 0.5 m Geodetic Datum), but i n fact usually remain under several centimeters of water throughout low ti d e , because of the presence of a topographic low and because the dense mat of Z. marina i t s e l f i n h i b i t s drainage. The lower l i m i t of the eelgrass zone on the t i d a l f l a t s i s not delimited by elevation. I t may be l i m i t e d by the presence or absence of sand waves or by the 'current or'~wave regime^ associated, with"-sand-w.aves (Fig. 2). 94 SUMMARY E a c h o f t h e t h r e e f l o r a l / s e d i m e n t o l o g i c a l zones l y i n g b e t w e e n t h e s a l t m a r s h zone and the l o w e r s a n d wave zone h a s a d i s t i n c t i v e m a c r o f a u n a l a s s e m b l a g e , and as a r e s u l t e a c h zone h a s a c h a r a c t e r i s t i c a s s e m b l a g e o f b i o g e n i c s e d i m e n t a r y s t r u c t u r e s ( F i g . 2 2 ) . A l t h o u g h l i v i n g p o p u l a t i o n d e n s i t i e s c a n n o t be c o r r e l a t e d d i r e c t l y w i t h t r a c e f o s s i l d e n s i t i e s , i t i s assumed t h a t .the. distribution.'patterns:-and '.assemblages o f l i v i n g : . o r g a n i s m s • w i l l be p r e s e r v e d i n the t r a c e f o s s i l r e c o r d . Topography o f s m a l l and l a r g e s c a l e c r e a t e s l a t e r a l h e t e r o g e n e i t y w i t h i n t he b i o f a c i e s o f e a c h zone ( F i g . 2 3 ) . The t o p o g r a p h y may be p h y s i c a l o r b i o g e n i c i n o r i g i n . The c h a r a c t e r i s t i c b i o g e n i c s e d i m e n t a r y s t r u c t u r e s o f e a c h zone a r e as f o l l o w s . A l g a l Mat Zone The r a i s e d a l g a l mat p l a t f o r m s c r e a t e l a t e r a l h e t e r o g e n e i t y w i t h i n t h i s z o n e ' s b i o f a c i e s , b e c a u s e t h e y q u i c k l y become d r y and i n h o s p i t a b l e to o r g a n i s m s d u r i n g l o w t i d e ( F i g . 2 3 a ) . The p l a t f o r m s have an i n t e r n a l s t r a t i -f i c a t i o n c o n s i s t i n g o f a l t e r n a t i n g sandy and o r g a n i c r i c h l a y e r s , w h i c h a r e r i d d l e d w i t h ' U ' shaped b u r r o w s (up t o 1 0 0 , 0 0 0 m - 2 ) , s u s p e c t e d t o be o f f l y l a r v a l o r i g i n . B a t i l l a r i a p i t s a l s o abound on t he r a i s e d p l a t f o r m s . I n w a t e r - f i l l e d d e p r e s s i o n s S p i o a r e p r e s e n t i n h i g h d e n s i t i e s (10^ m - 2 ) , and B a t i l l a r i a g r a z i n g t r a i l s a r e a b u n d a n t . C a l l i a n a s s a and Mya b u r r o w s o c c u r i n t h i s z o n e , b u t i n v e r y l ow d e n s i t i e s . A b a r e n i c o l a a p p e a r s a b r u p t l y n e a r t he l o w e r l i m i t o f t he a l g a l mat z o n e . The mean g r a i n s i z e o f t he sands o f t h i s zone u s u a l l y l i e i n the range o f 3 . 1 - 3 . 3 0. The s a n d s a r e w e l l s o r t e d ( I n c l . G r a p h i c S t d . D e v . - 0 . 3 5 - 0 . 5 0 0) and c o n t a i n a b o u t 5% mud ( r a n g e 3 . 6 - 8 . 0 % ) . ALGAL Z u O Z DISTANCE FROM SALTMARSH, m a l a r . 2000 S T R A T I G R A P H I C S U C C E S S I O N UPPER 1 0 AQUAZONE u i ALGAL MAT ZONE UPPER SAND WAVE ZONE EELGRASS ZONE MEAN GRAIN SIZE ( & ) *? , * ; 5 , 2 7 3 - 1 « ' I I I i i i _ | ^ SORTING (Inclutiv. Graphic Std. Dev.) M U O C O N T E N T { '/•< S3 JI | 0 1 2 3 4 5 6 7 8 ' " t v i ' ' ' ALGAL MAT ZONE UPPER SAND WAVE ZONE EELGRASS ZONE L E G E N D A l g « I Hat l a m i n a t i o n * 3 a t 1 1 l a r l a p i t s *^ 3fc> B a t H l a r l i g r a z i n g t r a i l s \f ' U ' shaped burrows {t F l y l a r v a e ) (1 Sp to d w e l l i n g tubes A b a r e n i c o l a burrow;. VERY WELL WILL MOO WEIL SOHTED SORTED SOHTCO C a l l l a n a s s a burrows Hye burrows ^ Zos te ra americana r o o t l e t s Z o s t e r a marina r o o t l e t s P r a x l l I d a d w e l l i n g tubes Upogebia burrows Nassar ius g r a c i n g t r a i l s Gra in s i z e data based on Transec t A Gra in s i z e data based on Transec t 9 Break In expected sequence: due to the presence of a topograph ic h igh on Transect 8 which w i l t not be preserved In the s u c c e s s i o n . Figure 22. Zonation of biogenic sedimentary structures i n three of the floral/sedimentological zones of Boundary Bay t i d a l f l a t s , and the expected s t r a t i g r a p h i c succession of biogenic sedimentary structures and grain s i z e parameters, i f the t i d a l f l a t s are prograding seawards without subsidence. b) a) 'U' shaped burrows (7 Fly Larva*) Bat i l l a r i a pits Batl1larla grazing t r a i l s Algal mat laminations Sp lo tubas Ea I grass AbarenI col a burrow x x Algal mat pla tforn C a l l i a n a s s a burrow Cryptomya sp. c) Figure 23. L a t e r a l heterogeneity within zonal b i o f a c i e s caused by topography of small and large scale, a A l g a l mat zone: L a t e r a l heterogeneity created by upraised a l g a l mat platforms. b) Upper sand wave zone: L a t e r a l heterogeneity created by sand waves. c) Eelgrass zone: L a t e r a l heterogeneity created by Cal l i a n a s s a mounds and burrows ON 97 Upper Sand Wave Zone This zone i s characterized by the presence of Abarenicola and by the lack of a f l o r a l cover. Mya a t t a i n t h e i r maximum density i n the lower part of this zone. L a t e r a l heterogeneity within the b i o f a c i e s of this zone i s created by the low amplitude sand waves ( F i g . 23b). Abarenicola, B a t i l l a r i a and Spio congregate i n the w a t e r - f i l l e d troughs of the sand waves, and patches of Zostera americana are also present here. Weak currents produced by water draining along the axis of the sand wave troughs cause B a t i l l a r i a to head upstream, producing grazing t r a i l s p a r a l l e l i n g the current d i r e c t i o n . On the sand wave crests, which dry out during low t i d e , B a t i l l a r i a grazing t r a i l s are sinuous, p i t t i n g by B a t i l l a r i a i s evident, Abarenicola densities are low and Spio are absent. A l g a l mat platforms may be present on the crests. The d i s t r i b u t i o n of Callianassa burrows i s not influenced by the sand waves, because Callianassa i s a deeper burrowing organism, and i t s burrow system i s always w a t e r - f i l l e d , whether or not i t occurs i n the sand wave troughs.. The sands of this zone have a mean grain s i z e i n the range of 311-2.'..7 0. They are w e l l to very w e l l sorted ( I n c l . Graphic Std. Dev. 0.33-0.39 0 ) , and contain about 1% mud, except at the zones upper l i m i t where values r i s e to 4%. Eelgrass Zone The eelgrass zone can be subdivided into an upper and lower part. The upper eelgrass zone i s characterized by a ^Zostera americana-Callianassa- A b a r e n i c o l a - B a t i l l a r i a 'community, )while the lower eelgrass zone i s charac-t e r i z e d by a '-Zostera marina-Upogebia-Praxillela-Nassarius' community. Callianassa a t t a i n t h e i r maximum densities i n the upper eelgrass zone. Their sediment mounds and burrows create l a t e r a l heterogeneity w i t h i n the b i o f a c i e s of this zone (Fig. 23c). In the lower eelgrass zone Zostera marina growth i s present throughout the year, and, i f i t forms an extensive and permanent 98 f l o r a l mat, mud accumulates i n this zone. The mean grain s i z e i n the eelgrass zone i s i n the range of 2.4-2.8 0. The sands are very w e l l to moderately w e l l sorted ( I n c l . Graphic Std. Dev. 0.29-0.60 0 ) , and i n the upper eelgrass zone contain about 1% mud, while i n the lower eelgrass zone mud contents can a t t a i n 7% i f Z. marina forms an extensive f l o r a l mat, i f not, values- drop w e l l below 1%. The lower h a l f of Figure 22 i l l u s t r a t e s the expected s t r a t i g r a p h i c succession of b i o f a c i e s and grain s i z e parameters i f the t i d a l f l a t s of Boundary Bay are prograding without subsidence, and a core were sunk i n the a l g a l mat zone. This succession might be expected i n the western h a l f of the Bay, where there i s evidence that the saltmarsh i s advancing (Kellerhals and Murray, 1969). CONCLUSIONS In Boundary Bay a d i s t i n c t f l o r a l / f a u n a l zonation e x i s t s , which i s c o n t r o l l e d p r i m a r i l y by exposure, although f l o r a l / f a u n a l i n t e r a c t i o n s also play an important r o l e . The extent of exposure which a given l o c a t i o n on the t i d a l f l a t experiences-) i s a function of the r e l a t i o n s h i p between elevation and t i d e s , and i s also a function of l o c a l topography, which may be of p h y s i c a l or biogenic o r i g i n . For t i d a l f l a t s experiencing astronomically c o n t r o l l e d t i d e s , the i n t e r t i d a l zone can be divided i n t o three d i s t i n c t exposure zones. The duration of maximum continuous exposure or maximum continuous submergence 'jumps' on passing from one.zone to the next. In Boundary Bay these exposure zones to a large extent delimit the f l o r a l zonation of the t i d a l f l a t s on the macroscopic s c a l e . However, on-the more l o c a l scale, topography can be seen to profoundly influence faunal and f l o r a l d i s t r i b u t i o n patterns, since topo-:.:--. :.c graphic-,highs dry :out rapidly, during low t i d e , while depressions remain: . w a t e r - f i l l e d . 99 Boundary Bay t i d a l f l a t s are exceptional, i n comparison with those previously described i n the l i t e r a t u r e , because grain s i z e varies l i t t l e over the bulk of the t i d a l f l a t s . The f l a t s are mantled with clean, w e l l to very w e l l sorted fine to very f i n e sand. The homogeneity i n grain s i z e continues to a depth of at le a s t 30 cm, as revealed from box cores. Because of this homogeneity i n grain s i z e Boundary Bay t i d a l f l a t s are to some extent comparable with rocky i n t e r t i d a l shorelines where precise e l e v a t i o n a l delimi-tation of f l o r a l / f a u n a l zonation i s w e l l documented (Carefoot, 1977). In attempting to elucidate the e f f e c t s of other parameters on faunal d i s t r i b u t i o n patterns (e.g. grain s i z e , mud content or s a l i n i t y ) , one must f i r s t eliminate the elevation parameter. This can be done by s e t t i n g up stations p a r a l l e l to the waterline at some chosen t i d a l height, rather than using data c o l l e c t e d from transects set up perpendicular to the shoreline. In addition to e l i m i -nating the elevation parameter, one must take care to compare stations with comparable topographic s i t u a t i o n s . The most s t r i k i n g feature of Boundary Bay t i d a l f l a t s i s the fact that from shoreline to low water mark the t i d a l f l a t s are mantled with sand. As a r e s u l t the s t r a t i g r a p h i c succession preserved by this t i d a l f l a t would consist of a monotonous sequence of sand, and the only means by which a detailed i n t e r p r e t a t i o n of the succession could be made, would be through the study of biogenic sedimentary structures (both f l o r a l and faunal), and by grain s i z e analysis techniques. ACKNOWLEDGEMENTS Mrs. M. Muhlert and Ms. N. Hayakawa ably assisted i n c o l l e c t i o n of f i e l d data. This project was financed by Geological Survey of Canada contract D.S.S. No. 0SS76-02075 from the Department of Supply and Services, Ottawa, Ontario, 100 Canada. We are indebted to W. Weir of C.B.A. Engineering Ltd., Vancouver, for providing t i d a l data from the Boundary Bay area. Many p r o f i t a b l e discus-sions resulted from contact with Dr. J. L. Luternauer, Geological Survey of Canada. We thank Dr. W. C. Barnes, Dr. C. D. Levings and Dr. J . P. S y v i t s k i ' for c r i t i c a l l y reading the manuscript. Especial thanks are due to Dr. P. G. Harrison, Botany Department, University of B r i t i s h Columbia for assistance i n the species i d e n t i f i c a t i o n of eelgrass, and to Dr. T. H. Carefoot, Zoology Department, University of B r i t i s h Columbia, for assistance i n the di s s e c t i o n and i d e n t i f i c a t i o n of Abarenicola sp. We thank Mrs. C. M. Armstrong, Mr. B. von Spindler and Mr. G. D. Hodge for d r a f t i n g the diagrams, and Ms. N. Hayakawa for typing the s c r i p t . L a s t l y , thanks are due to Dr. J. D. Milliman, currently of Woods Hole Oceanographic I n s t i t u t i o n , Woods Hole, Massachusetts, for i n i t i a t i n g f i n a n c i a l support for th i s project. 101 REFERENCES A l l e n , M. B., 1952, The c u l t i v a t i o n of Myxophyceae: Arch. Mikrobiol., • v. 17, p. 34-53. A l l e r , R. C. and Dodge, R. 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Petrology, v. 47, p. 1425-1436. SchSfer, W., 1972, Ecology and palaeoecology of marine environments: O l i v e r and Boyd and University Chicago Press, Edinburgh and Chicago, 568 p. Seilacher, A., 1964, Biogenic sedimentary structures: In Imbrie, J . , and Newell, N. D. (eds), Approaches to paleoecology, John Wiley, New York, p. 296-316. Shinn, E. A. , 1968, Burrowing i n recent lime sediments of F l o r i d a and the Bahamas: Jour. Paleontology, v. 42, p. 879-894. Smith, R. I. and Carlton, J . T., (eds.), 1975, Light's Manual: i n t e r t i d a l invertebrates of the ce n t r a l C a l i f o r n i a coast: Third E d i t i o n , University of C a l i f o r n i a Press, Berkeley, Los Angeles, 716 p. Swan Wooster, 1968, Roberts Bank—Stage 1 dredging and reclamation: wind information: report of Swan Wooster Enginnering Ltd. to National Harbours Board, Vancouver, B. C. Swinbanks, D. D., 1979, Environmental factors c o n t r o l l i n g f l o r a l zonation and the d i s t r i b u t i o n of burrowing and tube-dwelling organisms on Fraser Delta t i d a l f l a t s , B r i t i s h Columbia: unpub. Ph.D. t h e s i s , University of B r i t i s h Columbia, Vancouver, B. C , 274 p. and Murray, J. W., 1977, Animal-sediment r e l a t i o n s h i p s of Boundary Bay and Roberts Bank t i d a l f l a t s , Fraser River Delta: Geol. Assoc. Canada Program with Abstracts, v. 2, p. 51. Thompson, R. K., 1972, Functional morphology of the hind-gut of Upogebia pugettensis (Crustacea, Thalassinidea) and i t s role i n burrow construction: unpub. Ph.D. t h e s i s , University of C a l i f o r n i a , Berkeley, 202 p. and P r i t c h a r d , A. W., 1969, Respiratory adaptions of two burrowing crustaceans, C a l l i a n a s s a c a l i f o r n i e n s i s and Upogebia pugettensis (Decapoda, Thalassinidea): B i o l . B u l l . , v. 136, p. 274-287. Torres, J . J . , Gluck, D. L. and Childress, J . J . , 1977, A c t i v i t y and physio-l o g i c a l s i g n i f i c a n c e of the pleopods i n the r e s p i r a t i o n of Callianassa c a l i f o r n i e n s i s (Dana) (Crustacea: Thalassinidea): B i o l . B u l l . , v. 152, p. 134-146. Ungar, I. A., 1966, Salt tolerance of plants growing i n s a l i n e areas of Kansas and Oklahoma: Ecology, v. 47, p. 154-155. Van Straaten, L.M.J.U., 1952, Biogene textures and the formation of s h e l l beds i n the Dutch Wadden Sea, I & I I : Proc. Koninkl. Ned. Akad. " Wetenschap., B55, p. 500-516. 105 Van Straaten, L.M.J.U., and Kuenen, PH.H., 1958, T i d a l a ction as a cause of clay accumulation: Jour. Sed. Petrology, v. 28, p. 406-413. Waisel, Y. X-ed.), 1972, Biology of halophytes: Academic Press, New York, 395 p. Waldichuk, M., 1957, Physical oceanography of the S t r a i t of Georgia, B r i t i s h Columbia: Jour. F i s h . Res. Brd. Canada, v. 14, p. 321-486. Weir, W., 1963, Boundary Bay reclamation: Part I I I — R e p o r t on current measurements and t i d a l analysis. Private report of C.B.A. Engineering Ltd., Vancouver, B. C. Whitlach, R. B., 1974, Studies of the salt-marsh gastropod B a t i l l a r i a z o n a l i s : V e l i g e r , v. 17, p. 47-55. Part 3 SEDIMENT REWORKING AND THE BIOGENIC FORMATION f OF CLAY LAMINAE BY ABARENICOLA PACIFICA 107 ABSTRACT g An estimated 4.25 x 10 Abarenicola populate the t i d a l f l a t s of Boundary Bay on the southern flank of the Fraser Delta and annually rework about one m i l l i o n cubic meters of sand. In t i d a l pools, where Abarenicola a t t a i n -2 densities of 200 m , the worms completely rework the substrate they l i v e i n to a depth of 10 cm i n 100 days. In the laboratory Abarenicola can separate a sand/clay mixture, by f l o a t i n g the clay out i n suspension i n the head shaft i r r i g a t i o n current. The clay then s e t t l e s as a b i o g e n i c a l l y formed lamina, which i s subsequently buried and reworked by the worm. In the natural i n t e r t i d a l environment the clay would be ca r r i e d away by t i d a l currents, and by using t h i s process Abarenicola could 'clean' mud out of a mud/sand mixture creating a better sorted sand. 108 INTRODUCTION The top i c of bioturbation and biodeposition has recently been r e c e i v i n g increasing attention i n the l i t e r a t u r e (Rhoads and Stanley, 1965; Rhoads and Young, 1970; A l l e r and Dodge, 1974; Risk and Moffat, 1977). Much of the c l a s s i c work i n this f i e l d has been by German and Dutch workers (Schwarz,.1932; Van Straaten, 1952; Reineck, 1958; Schafer, 1972). Organisms can s i z e - s o r t sediment and create biogenic graded bedding (Van Straaten, 1952; Rhoads and Stanley, 1965; Featherstone and Risk, 1977). By constantly reworking s e d i -ments, organisms can d r a s t i c a l l y a l t e r i t s p h y s i c a l properties (e.g., r e l i e f , water content, compressibility, e t c . ) . This has both sedimentological and b i o l o g i c a l consequences. For example, deposit feeders through t h e i r burrow-ing and feeding a c t i v i t i e s can produce an unstable substrate that i s e a s i l y reworked by currents and which thereby tends to exclude f i l t e r feeding orga-nisms from the area (Rhoads and Young, 1970; A l l e r and Dodge, 1974). During a study of biosedimentological zonation on Boundary Bay t i d a l f l a t s (Swinbanks, 1979) on the southern flank of the Fraser Delta ( F i g . 1), one of the organisms studied which proved to be e s p e c i a l l y i n t e r e s t i n g was Abarenicola p a c i f i c a Healy and Wells. This pply^chaete produces the ; } most v i s i b l e evidence of bioturbation i n Boundary Bay, continually excreting mounds of loosely c o i l e d sediment onto the surface. By constantly reworking the surface sediments Abarenicola may l i m i t the extent of the a l g a l mat zone -one of the f i v e major f l o r a l / s e d i m e n t o l o g i c a l zones of the t i d a l f l a t s (Swinbanks, 1979) (Fig. 1). I t i s thus of i n t e r e s t from both the e c o l o g i c a l and sedimentological point of view to determine the rate at which t h i s orga-nism reworks sediment. Abarenicola p a c i f i c a i s the P a c i f i c coast equivalent of the well known lugworm, Arenicola marina, which i s found on the A t l a n t i c coasts of North 109 I LOWER SAND WAVE ZONE <;;;"•;;, SAND W A V E FIXED BY E E L G R A S S Figure 1. Location of study area. * Upper mapsnw of Boundary Bay on the Fraser Delta and the lower maps the f l o r a l / sedimentological zones of the t i d a l f l a t s . The two transects A and B were set up i n 1976 (Swinbanks, 1979). 110 America and Europe, and which i s mentioned i n several i n t e r t i d a l studies by sedimentologists (Van Straaten, 1952; Reineck, 1958; Evans, 1965). Abarenicola p a c i f i c a i s a deposit feeder but there i s some evidence that, as i n the case of Arenicbla marina, i t may also suspension feed by f i l t e r i n g the sea water that i t c i r c u l a t e s through i t s burrow f o r r e s p i r a t i o n purposes (Hobson, 1967). I t constructs a ' J ' shaped burrow with a v e r t i c a l t a i l s h a f t , and h o r i z o n t a l g a l l e r y that ends i n a feeding chamber above which l i e s a cone of collapsed sediment on which i t feeds (Hylleberg, 1975; Swinbanks, 1979). Hylleberg (1975) suggested that Abarenicola 'gardens' the sediment by i r r i g a t i n g i t s burrow, creating an o x i d i z i n g micro-environment i n which the micro-organisms (e.g., c i l i a t e s , f l a g e l l a t e s and nematodes), on which i t feeds, f l o u r i s h . Hylleberg (1975) found that Abarenicola l o c a l l y increases the percentage of coarse grains around i t s feeding chamber much as reported by Van Straaten (1952) f or Arenicola marina and by Rhoads and Stanley (1965) fo r Clymenella tOrquata. Hylleberg (1975) a t t r i b u t e d - t h i s to Abarenicola s e l e c t i v e l y feeding on sediment less than 80 um i n s i z e . Another mechanism of biogenic s i z e - s o r t i n g , caused by Abarenicola i r r i g a t i n g i t s burrow;-is reported here. There are f i v e f l o r a l / s e d i m e n t o l o g i c a l zones on the Boundary Bay t i d a l f l a t s (Fig. 1). These are, from the shoreline seawards, the saltmarsh zone, the a l g a l mat zone, the upper sand wave zone, the eelgrass zone and the lower sand wave zone. Abarenicola i s most abundant! i n the upper sand .wave zone and appears abruptly near the lower l i m i t of the a l g a l mat' zone. I t s density -2 -2 r i s e s from much less than 0.5 m to about 20 m i n the distance of l e s s than 100 m. They are no changes i n grain s i z e parameters (mean s i z e , s o r t i n g or mud content) over t h i s i n t e r v a l (Swinbanks, 1979). Hobson (1967) and Healy and Wells (1959) consider that sediment type rather than i n t e r t i d a l exposure governs the d i s t r i b u t i o n .of Abarenicola p a c i f i c a and Abarenicola claparedi I l l vagabunda Healy andJJelXs_ in .False.'.Bay., on San Juan .Island, p a c i f i c a p r e f e r r i n g muddy substrates and A., vagabunda clean sand. In Boundary Bay A. p a c i f i c a i s abundant i n pure sands c o n t a i n i n g only about 1% mud, and the abrupt upper l i m i t to t h e i r d i s t r i b u t i o n occurs at a constant e l e v a t i o n that i s probably determined by exposure (Swinbanks, 1979). METHODS Two tr a n s e c t s running north/south from the saltmarsh edge to low water mark were e s t a b l i s h e d i n the summer of 1976 (Swinbanks, 1979) ( F i g . 1 ). S t a t i o n s marked w i t h wooden stakes were placed at 91.4 m (300 f t ) i n t e r v a l s and t h e i r e l e v a t i o n s determined by surveying. The r a t e at which A b a r e n i c o l a reworks sediment was determined i n June, 1976, November, 1976 and August, 2 1978. A 1 m area.was staked out at s t a t i o n s A5 to A14 on transect,A ( F i g . 1) j u s t as the: w a t e r l i n e r e t r e a t e d from theoarea. _ A l l A b a r e n i c o l a casts were c a r e f u l l y removed and counted. The s t a t i o n s were reoccupied about 10 hours l a t e r j u s t before the t i d e returned and the volume of casts accumulated was measured w i t h a graduated c y l i n d e r . To d i s t i n g u i s h 'wet' s i t e s from 'dry' a depression about 1 cm deep was made i n the sand w i t h a f i n g e r a f t e r about 10 hours of exposure. I f the depression immediately f i l l e d w i t h water the s i t e was considered 'wet;' In the la b o r a t o r y A b a r e n i c o l a was kept and observed i n sandwich tanks usi n g the running sea water f a c i l i t i e s o f the P a c i f i c Environment I n s t i t u t e , West Vancouver, during the wi n t e r of 1977. Worms were placed i n wet sieved Boundary Bay sand (>63-%ijm) , which had been homogenized w i t h about 10% (by weight) f i n e l y powdered (<63 um) m o n t m o r i l l o n i t e . Abarenicola's a b i l i t y to s i z e s o r t sediment was c l e a r l y v i s i b l e using t h i s technique as concentra-t i o n s of the f i n e grained white mont m o r i l l o n i t e stood out i n c o n s t r a s t to the dark grey sand. 112 RESULTS F i e l d Results Sediment Reworking Rates The rate at which Abarenicola reworks sediment during various times of the year i s presented i n Tables I and I I . .In the upper sand wave zone (stations A5 to A12), the factor which seems to influence the rate of s e d i -ment turnover most i s the wetness of the sediment. The highest rates were recorded i n t i d a l pools where the worms remain under water during low t i d e . They excrete 5.1 ± 2.4 wet ml worm ^ day (1 wet ml=1.5 g dry weight) on average. Rates were lower i n sediment which remained wet but not under water, the average rate being 1.8 ± 0.9 wet ml worm ^ day and lowest i n dry sediment, averaging 0.5 ± 0.6 wet ml worm ^ day ^ . In the lower i n t e r t i d a l regions (Stations A13 and A14) rates were an order of magnitude higher, averaging 29 ± 18 wet ,mr;worm ^ %ay, The "rates ^ of :sediment turnover at stations A5 to A12 do not decrease greatly between summer and winter. The wet rate i n June averaged 2.5 ± 0 . 5 wet ml worm ^ day ^  while i n November i t dropped to 0.9 ± 0.4 wet ml worm ^ day ^ , but the under water rate did not decrease (June, 4.4 ± 2.7 wet ml worm day November, 6.6 + 2.3 wet ml worm ^ day "S . On the other hand f o r the worms producing high rates of se d i -ment turnover i n summer at A13 and A14, there i s such a marked decrease i n rate of turnover i n winter that f e c a l casts are very hard to f i n d . When present" they only contain 1 or 2 ml of sediment as opposed to 20 to 50 ml i n summer. Budget of Sediment Turnover Taking into account the above va r i a b l e rates, an annual budget-^of s e d i -ment turnover by Abarenicola can be calculated u t i l i z i n g density data 113 TABLE I Rates of Sediment Turnover by Abarenicola June 28, 1976 Mean A i r Temperature 18.5 °C (range 16-21 °C)_ Station State of Substrate (wet ml Rate_ 1 worm day ) H A5 dry 0.02 43 A6/A7 dry 0.72. 102 A8 dry 0.08 18 *A9 1/3 wet 3.05 ' 37 A l l dry 0.30 10 A13 u.w. 18.00 5 * Between A9 and A10 30.5 m (100 f t ) from A9 • June 30, Mean A i r Temperature 17 1976 °C (range 13-20 °C) Station State of Rate 1 1 N Subs trate (wet ml worm day ) A5 u.w. 8.4 63 A6 u.w. 2.8 54 A7 u.w. 4.0 43 A8 u.w. 2.5 19 A9 dry 1.4 130 A10 wet 2.6 66 A l l wet 1.9 96 A12 wet - 2.4 52 A13 u.w. 21.8 18 A14 u.w. 19.6 23 N = number of f e c a l casts i n sampled area (1 m") u.w. = under water Note: A i r temperature data based on records during the hours of sampling at . Vancouver International A i r p o r t approximately 20 km from the study s i t e (source: Monthly meteorological summary, Atmospheric Environment Service, F i s h e r i e s and Environment Canada). 114 • TABLE II Rates -of Sediment Turnover by Abarenicola November 8, 1976 Mean A i r Temperature 9.5 C (range 9-10 °C)_ Station State of Substrate Rate 1 (wet ml worm day ) N A5 wet 1.4 30 A5/A6 wet 0.4 52 A6 wet 1.2 47 A5 u.w. 8.2 28 A6 u.w. 5.0 25 August 22, 1978 Mean A i r Temperature 17.5 °C (range 16.5-18.5 °C) Station Substrate Temperature (°C) cm depth) State of Substrate R a t e -1 -1 (wet ml worm day ) N AlO A13 19- 21 20- 22 u.w. u.w. 5.1 56.0 211 9 N = number of f e c a l casts i n sampled area (1 m*") u.w. = under water Note: A i r temperature data based on records during the hours of sampling at Vancouver International A i r p o r t approximately 20 km from the study s i t e (source: Monthly meteorological summary, Atmospheric Environment Service, F i s h e r i e s and Environment Canada). 115 c o l l e c t e d by Swinbanks (1979). The f o l l o w i n g assumptions and estimates are made (Table I I I ) : (1) Most Ab a r e n i c o l a occupy the upper sand wave zone which l i e s bet-7 2 ween mean sea l e v e l and +0.75 m Geodetic Datum and has an area of 0.8 x 10 m . The area i s exposed on average 70% of the time (estimated from mean exposure curve presented by Swinbanks, 1979). In summer during exposure about one t h i r d of t h i s area remains under water due to the presence of t i d a l pools. The remaining two t h i r d s i s 'wet' during exposure, except during d a y l i g h t summer exposure when one t h i r d i s 'wet' and the other 'dry.' This was e s t i -mated from 77 random quadrats taken on tr a n s e c t A i n June, 1976, of which 24 were under water, 27 were 'wet' and 26 'dry.' (2) The average density of Ab a r e n i c o l a at wet or under water s i t e s , -2 which c o n s t i t u t e two t h i r d s of the area above mean sea l e v e l , i s 57 m , w h i l e the average d e n s i t y i n dry areas, which c o n s t i t u t e one t h i r d of the —2 8 area i s 10 m . Hence the under water + wet po p u l a t i o n amounts to 3 x 10 g i n d i v i d u a l s , w h i l e the dry p o p u l a t i o n i s 0.25 x 10 i n d i v i d u a l s . (3) I n the upper sand wave zone, Abar e n i c o l a turns over about 5.1 ml worm day when under water, throughout the year. At wet s i t e s i n summer they average 2.5 ml worm day and i n w i n t e r 0.9 m l worm ^ day" 1. The summer dry r a t e i s 0.5 ml worm day g (4) There i s an estimated p o p u l a t i o n of 10 Aba r e n i c o l a i n the e e l -grass and lower sand wave zones below mean sea l e v e l , which t u r n over s e d i -ment at an average under water r a t e of 29 ml worm ^ day during summer. The rate of sediment t u r n o v e r . i n w i n t e r i s n e g l i g i b l e . (5) 'Summer' co n d i t i o n s l a s t from A p r i l to September and 'winter' c o n d i t i o n s from October to March. 6 3 The t o t a l annual budget i s about 1 x 10 m (Table I I I ) . Of t h i s budget more than h a l f i s reworked i n the area below mean sea l e v e l during summer. TABLE I I I Annual Budget of Sediment Turnover for Abarenicola i n Boundary Bay State , Rate „ , . Turnover Zone _ _. , No. of days , n _ i , _ i s Population , q . of Tide J (ml worm } day L) r (mJ) UPPER SAND WAVE ZONE IN OUT OUT OUT H Q 255 255 { 255 { 127.5 127.5 127.5 127.5 u.w. 5.1 u.w. 5.1 winter wet 0.9 summer wet 2.5 winter wet 1.00 summer dry 0.25 Mean Sea Level 3.25 x 10 8 ( t o t a l pop.) 1.50 x 10 8 ( t i d a l pools) 1.50 x 10 8 1.50 x 10 8 ('wet1 pop.) 0.25 x 10 8 0.25 x 10 8 ('dry' pop.) 1.82 x 10 5 1.95 x 10 5 0.17 x 10 5 0.48 x 10 5 0.03 x 10 5 0.01 x 10 5 4.46 x 10 5 EELGRASS ZONE IN/OUT 182.5 u.w. 29 10 8 5.30 x 10 5 Total Budget 9.76 x 10 5 117 To place t h i s budget i n perspective, i t can be compared with the annual 6 3 sediment discharge of the Fraser River which amounts to about 20 x 10 m (Mathews and Shephard, 1962). On the smaller scale, Abarenicola can a t t a i n densities as high as _2 200 m i n t i d a l pools, and the worms turn over sediment at the under water rate of about .5 ml worm day ^ . Each day a layer of sediment about 1 mm thick i s extruded onto the surface by Abarenicola. This i s equivalent to a layer about eight grains thick. In 100 days Abarenicola reworks a l l the sediment i t l i v e s i n to a depth of 10 cm. However, i t should be remembered that t h i s rate i s under optimum conditions. Laboratory Results In the laboratory Abarenicola demonstrates an amazing capacity to s i z e - s o r t sediment. When placed i n a mixture of homogenized sand and clay Abarenicola quickly segregates the two through i t s i r r i g a t i o n a c t i v i t i e s (F i g . 2). Water i s drawn i n through the t a i l shaft and returns to the surface through the head shaft (Fig. 3). Fine grained clay p a r t i c l e s f l o a t to the surface c a r r i e d i n suspension by the head shaft current and i n the low energy environment of the laboratory the clay s e t t l e s out on the substrate forming a thick lamina within 24 hours ( F i g . 2b). Within a few days the '• lamina i s buried by f e c a l casts excreted through the t a i l shaft, and i s then deformed by the feeding and r e s p i r a t i o n a c t i v i t i e s of the worm ( F i g . 2c). DISCUSSION The rates of sediment turnover reported here are comparable with those determined by Hobson (1967) and Healy and Wells (1975) f o r A. p a c i f i c a , with the exception of the very high rates recorded i n the eelgrass zone (Tables I 118 (c) Figure 2. Grain size sorting by Abarenicola: a) Start of the experiment. An individual Abarenicola (-2 g) was placed in a homogenized mixture of sand (>63/«"\) and montmorillonite (<63^). b) After 24 hours a thick, biogenically formed lamina of montmoril-lonite (white) has developed as a result of the irrigation activities of the worm. c) After three days the lamina has been buried by fecal casts and the lamina has been deformed and bioturbated by the feeding and irrigation activities of the worm. 119 respi ra t ion current respiration current head shaft pocket sand tail shaft 5cm Figure 3. Sketch of a comparable s i t u a t i o n to that i l l u s t r a t e d i n Figure 2b. The important features of the Abarenicola burrow are l a b e l l e d and the d i r e c t i o n of flow of the r e s p i r a t i o n current which i r r i g a t e s the burrow i s indicated. A clay lamina has developed as a r e s u l t of fi n e grained clay p a r t i c l e s f l o a t i n g out i n suspension i n the head shaft i r r i g a t i o n current and then s e t t l i n g on the substrate. 120 and I I ; stations A13 and A14). These high rates, ranging between 18-56 ml worm day \ are a puzzle which remains unsolved. The worms i n the eelgrass zone appear to be larger and this may account for the higher rates. I t may be that Abarenicola migrates to lower i n t e r t i d a l l e v e l s as i t gets older and larger as reported for Arenicola marina by Werner (1954a, b ). But why do reworking rates decrease so dramatically from an average of 29 ml worm ^ day ^  -1 -1 during summer to at most 1 or 2 ml worm day during winter, while i n the upper sand wave zone rates only decrease s l i g h t l y or not at a l l (Tables I and II)? The p o s s i b i l i t y that another species of Arenicolidae i s present should not be ruled out. Of nineteen specimens c o l l e c t e d i n the v i c i n i t y of A13 seven-teen proved to be Abarenicola p a c i f i c a upon d i s s e c t i o n ; having between 4 to 6 pairs of esophageal caeca each. However, two specimens had only one large pair of esophageal caeca each, which i s diagnostic of the genus Ar e n i c o l a (Smith and Carlton, 19 75). Arenicola marina i n the Dutch Wadden Sea demonstrate seasonal v a r i a t i o n i n reworking rates very comparable to those reported above f o r the eelgrass zone (26-29 ml worm ^ day ^  i n summer, av. a i r temp. 17°C, 2.4 ml ••;?.:•. worm ^ day ^  i n winter, av. a i r temp. 3°C, Cadee, 1976). The phenomenal rates at which Abarenicola rework sediment i n Boundary ; Bay probably has a considerable influence on the sedimentology and ecology of the t i d a l f l a t s . The rates of biogenic reworking of up to 1mm per day are f ar i n excess of estimated rates of sedimentation by physi c a l processes. K e l l e r h a l s and Murray (1969) estimated a long term sedimentation rate of 0.41 mm year , based on radio-carbon dating, and a short-term rate of 5 mm year \ based on the thickness of seasonal a l g a l mat laminae. As Swinbanks (1979) has suggested the intense reworking a c t i v i t i e s of Abarenicola may act to l i m i t the extent of the a l g a l mat zone - one of the f i v e major f l o r a l / sedimentological zones of the Boundary Bay t i d a l f l a t s ( F i g . 1). The worms may i n h i b i t the formation of blue-green a l g a l mats by t h e i r constant 121 turnover of surface sediments or through b u r i a l of algae by feces. Abarenicola has the p o t e n t i a l of generating biogenic graded bedding and bi o g e n i c a l l y formed clay laminae through i t s i r r i g a t i o n a c t i v i t i e s . In the i n t e r t i d a l environment where the surface i s constantly reworked by the inflow and outflow of water, the lamina of clay seen i n Figure 2 would probably be washed away as fa s t as i t forms, and using t h i s process Abarenicola could conceivably 'clean' the mud out of a loose mud/sand mixture, creating a better sorted sand. The high degree of s o r t i n g of sands i n the upper sand wave zone i n Boundary Bay ( I n c l . Graphic Std. Dev. 0.30-0.39 0; Swinbanks, 1979) may at l e a s t i n part be due to the reworking a c t i v i t i e s of th i s worm. This i s a quite a separate process from that of r e j e c t i o n of coarse grains during feeding, reported by Hylleberg (1975) for Abarenicola, by Rhoads and Stanley (1965) for Clymenella and by Van Straaten (1952) for Arenicola. But the two processes should produce the same r e s u l t as they both transport f i n e grains towards the surface e i t h e r i n the head shaft i r r i g a t i o n current or as feces. ACKNOWLEDGEMENTS This work was financed i n part by Geological Survey of Canada Contract D.S.S. No. 0SS76-02075 from the Department of Supply and Services, Ottawa, Ontario, Canada. Dr. CD. Levings k i n d l y made available the laboratory f a c i l i t i e s at the P a c i f i c Environment I n s t i t u t e , West Vancouver, B.C., Canada. I would l i k e to thank Dr. T.H. Carefoot, of the Zoology Department, University of B r i t i s h Columbia for helping i n the d i s s e c t i o n and i d e n t i f i -cation of Abarenicola, and Dr. J.W. Murray and Dr. W.C. Barnes, Un i v e r s i t y of B r i t i s h Columbia, Dr. CD. Levings, P a c i f i c Environment I n s t i t u t e and Dr. J.L. Luternauer, Geological Survey of Canada, for c r i t i c a l l y reading the manuscript. 122 REFERENCES A l l e r , R.C. and Dodge, R.E., 1974, Animal-sediment r e l a t i o n s i n a t r o p i c a l lagoon, Discovery Bay, Jamaica: Jour. Mar. Res., v. 32, p. 209-232. Cadee, G.C, 1976, Sediment reworking by Arenicola marina on t i d a l f l a t s i n the Dutch Wadden Sea: Neth. Jour. Sea Res'.,. v. 10, p. 440-460. Evans, C , 1965, I n t e r t i d a l f l a t sediments and t h e i r environments of deposi-t i o n i n the Wash: Quart. Jour. Geol. Soc. London, v. 121, p. 209-245. Featherstone, R.P. and Risk, M.J.,1977, E f f e c t of tube-building polychaetes on i n t e r t i d a l sediments of the Minas Basin, Bay of Fundy: Jour. Sed. Petrology, v. 27, p. 446-450. Healy, E.A. and Wells, CP., 1959, Three new lugworms (Arenicolidae, poly-chaeta) from the North P a c i f i c area: Proc. Zool. Soc. London, v. 133, p. 315-335. Hobson, K.D., 1967, The feeding and ecology of two North P a c i f i c Abarenicola species (Arenicolidae, Polychaeta): B i o l . B u l l . , v. 133, p. 343-354. Hylleberg, J . , 1975, S e l e c t i v e feeding by Abarenicola p a c i f i c a with notes on Abarenicola vagabunda and a concept of gardening i n lugworms: Ophelia, v. 14, p. 113-137. K e l l e r h a l s , P. and Murray,' J.W., 1969 , Tidal, f l a t s .at Boundary .Bay, Fraser , River Delta, B r i t i s h Columbia: B u l l . Can. Pet. Geol, v. 17, p. 67-91. Mathews, W.H. and Shephard, F.P., 1962, Sedimentation of Fraser River Delta, B r i t i s h Columbia: B u l l . Amer. Assoc. Pet. Geol., v. 46, p. 1416-1443. Reineck, H.E., 1958, Wlihlbau-Geflige i n Abhangigkeit von Sediment-Umlagerungen: Senckenbergiana Lethaea, B. 39, p. 1-23, 54-56. Rhoads, D.C. and Stanley, D.J., 1965, Biogenic graded bedding: Jour. Sed. Pet., v. 35, p. 956-963. and Young, 1970, The influence of deposit-feeding benthos on bottom s t a b i l i t y and community trophic structure: Jour. Marine Res. v. 28, p. 150-178. Risk, M.J. and Moffat, J.S., 1977, Sedimentological s i g n i f i c a n c e of f e c a l p e l l e t s of Macoma b a l t i c a inc.Mi'nas Basin, Bay of Fundy: Jour. Sed. Petrology, "v. 47, p. 1425-1436. Schafer, W. 1972, Ecology and palaeoecology of marine environments: O l i v e r and Boyd and University Chicago Press, Edinburgh and Chicago, 568 p. Schwarz, A., 1932, Der t i e r i s c h e E i n f l u s s auf die Meeressedimente (besonders auf die Beziehungen zwischen Frachtung, Ablagerung und Zusammen-Setzung von Wattensedimenten): Senckenbergiana, v. 14, p. 118-172. 123 Smith, R.I. and Carlton, J.T., (eds.), 1975, Light's Manual: i n t e r t i d a l invertebrates of the ce n t r a l C a l i f o r n i a coast: Third E d i t i o n , University of C a l i f o r n i a Press, Berkeley, Los Angeles, 716 p. Swinbanks, D.D., 1979, Environmental factors c o n t r o l l i n g f l o r a l zonation and the d i s t r i b u t i o n of burrowing and tube-dwelling organisms on Fraser Delta t i d a l f l a t s , B r i t i s h Columbia: unpub. Ph.D. th e s i s , University of B r i t i s h Columbia,_Vancouver, B.C., 274 p. Van Straaten, L.M.J.U., 1952, Biogene textures and the formation of s h e l l beds i n the Dutch Wadden Sea, I and I I : Proc. Koninkl. Ned. Akad. Weternschap., B55, p. 500-516. Werner, B., 1954a, Eine Beobachtung uber die Wanderung von Arenicola marina (Polychaeta sedentaria): H e l g l . Wiss. Meeresuntersuch, v. 5, p. 93-102. , 1954b, Uber die Winterwanderung von Arenicola marina L. (Poly-chaeta sedentaria): H e l g l . Wiss. Meeresuntersuch, v. 5, p. 353-378. Part 4 A ENVIRONMENTAL CONTROLS ON THE DISTRIBUTION OF THALASSINIDEAN BURROWING SHRIMPS ON FRASER DELTA TIDAL FLATS, BRITISH COLUMBIA A Marine T i d a l F l a t Between Two Man-Made Causeways on Southeastern Roberts Bank 125 ABSTRACT The thalassinidean burrowing shrimps Callianassa c a l i f o f n i e h s i s and Upogebia pugettensis are abundant on a sandy t i d a l f l a t , which l i e s between two man-made causeways on the south-eastern t i d a l f l a t s of the Fraser Delta-_2 front, a t t a i n i n g densities as high as 446 burrow openings m . This t i d a l f l a t i s 'marine' i n character and can be divided into four major f l o r a l / sedimentological zones. These are, from the shoreline seawards, the saltmarsh zone, the a l g a l mat zone, the sandflat zone and the eelgrass zone. T h a l a s s i -nidean burrowing shrimps are most abundant i n the sandflat zone. At high i n t e r t i d a l l e v e l s C a llianassa d i s t r i b u t i o n i s l i m i t e d by the presence of the saltmarsh, the lower l i m i t of which l i e s at the lower l i m i t of the upper atmozone, while dense eelgrass cover l i m i t s C a llianassa d i s t r i -bution at low i n t e r t i d a l l e v e l s . The upper l i m i t of the eelgrass zone l i e s at the upper l i m i t of the lower aquazone. Callianassa are abundant (>50 -2 burrow openings m ) i n sediments which range from 5-50% i n mud content and from 2.6-4.0 0 i n median grain s i z e . Upogebia extend up to the base of the upper amphizone (about mean sea l e v e l ) , a l e v e l above which the maximum duration of anoxia due to exposure probably exceeds the l e t h a l l i m i t f o r postmolt Upogebia. Upogebia show a d i s t i n c t preference f o r muddy substrates and only a t t a i n high densities -2 (>20 burrow openings m ) i n sediments containing more than 40% mud. The re l a t i o n s h i p between percent mud and Upogebia density can be approximated by a s t r a i g h t l i n e , the slope of which i s dependent on t i d a l e l e v a t i o n . There i s some evidence to suggest that Callianassa and Upogebia densities are negatively correlated at t i d a l elevations where both Upogebia and Callianassa densities have the p o t e n t i a l of being high. I t i s speculated that t h i s may be a consequence of increased mortality amongst p o s t l a r v a l 126 suspension feeding Upogebia r e s u l t i n g from the surface reworking a c t i v i t i e s of adult deposit-feeding Callianassa, and/or as a r e s u l t of predation by the carnivorous p l a n k t i c larvae of Upogebia on the p l a n k t i c larvae of Calli a n a s s a . I t may be that t h i s negative i n t e r a c t i o n between Upogebia and Callianassa overrides and masks a possible preference on the part of Callianassa for muddy substrates. Callianassa reworks sediment at the rate of 18 ± 9 ml/shrimp/day. An estimated 100 m i l l i o n C a llianassa on the intercauseway t i d a l f l a t rework about 0.2 m i l l i o n cubic metres of sand during the three months of summer. _2 In the area of t h e i r peak density (446 burrow openings m ) Ca l l i a n a s s a . rework the sediment they l i v e i n to a depth of 50 cm i n about f i v e months. I t i s suggested that bioturbation of a shallow subsurface horizon of clayey:' , mud by the deep burrowing of Upogebia has l o c a l l y increased the mud content of surface sediments on the southeastern side of the sandflat zone, r e s u l t i n g i n an anomalous patch of muds with i n this otherwise sandy zone. 127 INTRODUCTION Thalassinidean burrowing shrimps produce the very d i s t i n c t i v e trace f o s s i l s Ophiomorpha and Thalassinoides (Weimer and Hoyt, 1964; Frey and Howard, 1975; Pemberton, 1976). The burrows of these shrimps extend deep int o the substrate and, as a r e s u l t , stand very good chances of being preserved as trace f o s s i l s i n the geological record. Their p o t e n t i a l as paleoenvironmental i n d i c a t o r s has already been pointed out (Weimer and Hoyt, 1964; Dewindt, 1974), but such p o t e n t i a l w i l l remain l i m i t e d as long as data on the influence of pertinent environmental factors on thalassinidean shrimp d i s t r i b u t i o n i s la c k i n g . The primary aim of t h i s paper i s to assess the e f f e c t s of various environmental factors on the d i s t r i b u t i o n of Callianassa  californiensis'Dana and Upogebia pugettensis (Dana) on a marine t i d a l f l a t that : lies-between two man-made causeways on the southeastern Fraser-Delta -. the Tsawwassen f e r r y terminal causeway and the Coalport causeway (Fig. 1). The Coalport causeway has cut o f f the supply of brackish s i l t - l a d e n waters from the Fraser River to th i s t i d a l f l a t , and has thus probably enhanced i t s 'marine' c h a r a c t e r i s t i c s (Levings and Coustalin, 1975). Because of the r e l a t i v e l y stable s a l i n i t y regime of the t i d a l f l a t , the combined e f f e c t s of t i d a l elevation, grain s i z e and b i o - i n t e r a c t i o n s on thalassinidean shrimp d i s t r i b u t i o n can be systematically broken down in t o t h e i r component e f f e c t s . B io-interactions include the e f f e c t s of f l o r a l cover (saltmarsh, a l g a l mats and eelgrass) and any ammensalistic i n t e r a c t i o n s between Upogebia, a suspension feeder, and Callianassa, a deposit feeder. Rhoads and Young (1970) introduced the concept of trophic group ammensalism to describe the negative i n t e r a c t i o n between deposit feeders and suspension feeders i n a subtidal environment, where the unstable bottom created by the reworking a c t i v i t i e s of deposit feeders were tending to exclude suspension feeding 128 Figure 1. Location of the study area.\is indicated .by hpIrUzmtal cross-hatching T i d a l f l a t s are s t i p p l e d , land area of Recent alluvium i s blank, and older deposits diagonally. cross-hatchecL (adapted from Luternauer and Murray, 1973). 129 organisms. As an essential part of the study of the interactions between Callianassa and Upogebia the rates at which these shrimps rework sediment were measured." Armed with the information from this ti d a l f l a t , in Part £B the com-plexities of the environment of central and northern Roberts Bank is tackled, where salinity becomes the overriding factor among the variables influencing thalassinidean shrimp distribution, due to the influx of freshwater from the Fraser River. The environment of Roberts Bank divides naturally into a brackish environment to the northeast, and a marine environ-ment to the southeast and i t was f e l t appropriate to s p l i t Parts 4A..iarid,B in this way. The presence of Callianassa californiensis on the active t i d a l flats of the Fraser Delta was f i r s t reported by Bawden et a l . (1973) and Luternauer and Murray (1973) and again by Levings and Coustalin (1975). The distribution of Callianassa and Upogebia in Boundary Bay on the inactive southern flank of the Fraser Delta (Fig. 1) has been described qualitatively (Kellerhals and Murray, 1969; O'Connell, 1975) and quantitatively (Swinbanks, '1979). Thalassinidean burrowing shrimps are frustrating organisms to study because they prove so d i f f i c u l t to catch, as others have noted (Risk et a l . , 1978). With experience one is quickly able to recognize the very characteristic burrows, burrow entrances, mounds, fecal pellets and discarded exoskeletons (exuviae) of these shrimps. Callianassa burrows usually have two entrances to each system although occasionally Tthey: may have three and rarely four openings, while Upogebia burrows usually have two (Thompson,.1972; Swinbanks, 1979). Thus, one can quantitatively assess the distribution of the shrimps by counting burrow entrances at the surface rather than catching the shrimps themselves. It would be more desirable-to-obtain direct shrimp counts and shrimp biomass data, but to do so would have severely limited the geographical 130 extent of th i s survey. A number of studies of Callianassa c a l i f O r n i e n s i s and Upogebia pugettensis have been made by b i o l o g i s t s . MacGinitie (1930, 1934) recounts i n v i v i d d e t a i l the burrowing, feeding, reproduction and day-to-day l i f e s t y l e s of both shrimps. MacGinitie and MacGinitie (.1968) l i s t nine commensal organisms, incl u d i n g pea-crabs, amphipods, bi v a l v e s , goby f i s h and polychaetes, which use the burrows of Callianassa as a protective haven or as a sediment-water i n t e r f a c e on which to feed. L. Thompson and Pritc h a r d (1969) have c a r r i e d out p h y s i o l o g i c a l studies on the osmoregulatory capacities of both _C. c a l i f o r n i e n s i s and JJ. pugettensis as have Torres et a l . (1977). R. Thompson and Pr i t c h a r d (1969) have studied anoxia tolerance i n both shrimps. Thompson (1972) has studied JJ. pugettensis i n d e t a i l for a doctoral t h e s i s . METHODS Three transects, A, B and C, were established perpendicular to the shoreline on the inter-causeway t i d a l f l a t ( F i g . 2) i n June and July, 1977. In addition to these, two transects, D and E, were established between transects A and B roughly p a r a l l e l to the shoreline, because q u a l i t a t i v e observations indicated that l a t e r a l changes i n grain s i z e were pronounced i n this region. Stations were taped at 100 m i n t e r v a l s on the transects and marked by wooden stakes. The elevations of a l l stations on transects A, B and C were determined by surveying from three bench marks which lay close to the ends of each l i n e ( F i g . 2). Elevations at stations were determined by standard l e v e l i n g techniques using a T2 t r a n s i t ( S c i e n t i f i c Instruments Ltd., Ottawa, Ontario), which i s capable of determining elevation differences to an accuracy of 5 mm over a distance of 200 m (the maximum sigh t i n g distance employed). A cross survey between transects A and B 131 « « * * 1M*M' til Ot I1*«T' T ) ) " M • 1000 0 1000 2000 I Metere Saltmarsh Zone j \ Algal Mat Zona ~] Sandi la i Zona E:;;:3 z o n e ~] Patchy Ealgraaa Cover a Stations (or elevation flaee on tower limit of • altmatah 2^ Ceueeway Zona fc£>£l Cauaaway Rip Rap Ic'jyj Driftwood .•**t:"* Gravel road Dyko. with dyka road >•'' (dashed) and landward embankment (hachured) fflgH Drainaga Channe l ! Sand Sara Re l ic t eand bars fixed by colonfi lng vegetetlon Imarah and algal mate) • B M * Bench Hark for Transect A' * Railway A • Traneect and station location Figure 2. The f l o r a l / s e d i m e n t o l o g i c a l zones of the inter-causeway t i d a l f l a t with the locations of transects, stations and bench marks indicated. 132 was made along transect D, tying bench mark B (Fig. 2) to bench mark A by surveying over a distance of approximately 2 km. The discrepancy between the expected e l e v a t i o n for the bench mark A and the observed el e v a t i o n was 6 cm (observed 6 cm higher than expected). This gives an estimate of the accuracy of the surveying technique over a t y p i c a l transect length, and provided e l e v a t i o n data for transect D. A further check of the survey data using observed t i d a l heights at Tsawwassen i n d i c a t e s t t h a t .the .datan.is accurate to about ±4 cm (range 0.2-9.4 cm, N=19, Appendix 3 ). Transect E was not surveyed because observations indicated that i t lay almost p a r a l l e l to the waterline and the sta t i o n s on the transect were therefore of very s i m i l a r e l e vation. The survey r e s u l t s confirm t h i s conclusion as the eleva-tions of stations B16 and A l l at e i t h e r end of transect B/A only d i f f e r i n elevation by 13 ± 2.5 cm. In addition to the transects mentioned above s i x stations were established adjacent to the causeways 20 m from the edge of the ' r i p rap,' ( F i g . 2). They were not surveyed. At a l l stations shrimp burrow opening de n s i t i e s were determined by 2 sampling eight times at each s t a t i o n with a 0.25 m quadrat. Upogebia burrows were e a s i l y distinguished from those of Callianassa, because of t h e i r f i r m mud l i n i n g , lack of co n s t r i c t e d apertural necks, and d i s t i n c t i v e geometry (Swinbanks, 1979). Surface grain s i z e samples were obtained at a l l stations using a 2 cm deep rectangular box, and surface substrate s a l i n i t i e s were recorded at low tide using a refractometer (Endeco type 102). Twenty-one substrate s a l i n i t y • p r o f i l e s were taken on transects A, B and C sampling at 2.5 cm, 5 cm, 7.5 cm, 10 cm, 15 cm and 45 cm over the period July 1-7, 1977, using a sampler which draws i n t e r s t i t i a l waters from these depths (Appendix 5;). Fortunately, because of the sandy nature of the substrate, i n t e r s t i t i a l waters were clear and s a l i n i t y could be measured d i r e c t l y with a refractometer. In order to determine the rate at which the shrimps turnover sediment, 133 protected enclosures were constructed and a placed around a number of burrow openings (usually about 10) i n order to prevent currents from washing away the mounds which accumulate outside the burrow entrances. The enclosures consisted of open ended metal boxes 50 x 50 x 50 cm. They were pushed about 10 cm into the substrate. Twelve holes (5 mm diameter) placed around the box at the l e v e l of the sediment-water i n t e r f a c e allowed easy inflow and outflow of water and prevented 'geysers' from forming on flood t i d e . A layer of gravel was used to mark the i n i t i a l l e v e l of the sediment-water i n t e r f a c e . Preliminary tests indicated that the boxes could only be l e f t i n for one t i d a l cycle (25 hours), because, i f wave action at the beginning of flood tide was s i g n i f i c a n t (>10 cm amplitude) waves breaking over the box would r e d i s t r i b u t e the accumulated mounds to the sides of the enclosure and on the subsequent ebb tide sand would drain out of the boxes. For t h i s reason the boxes were emplaced j u s t before the flooding waterline reached the s t a t i o n and data was c o l l e c t e d the following day, i f undisturbed mounds were found within the box ( i . e . , there had been no s i g n i f i c a n t wave action at the l a t e stages of ebb). In the laboratory, grain s i z e samples were washed free of s a l t , wet sieved through a 63 um sieve to extract the s i l t / c l a y f r a c t i o n and dry sieved at 0.5 0 i n t e r v a l s . Approximately 10 g of sample was used. The d i s t i n c t i v e f l o r a l / s e d i m e n t o l o g i c a l zones of the inter-causeway t i d a l f l a t were mapped using low l e v e l colour a e r i a l photographs (scale 1:12,000) of July 29, 1977 (A31164, National A i r Photo L i b r a r y , Ottawa, Canada). Detailed descriptions of these zones are included i n t h i s paper because i t i s e s s e n t i a l to place the thalassinidean shrimp d i s t r i b u t i o n data within the o v e r a l l zonal framework of the t i d a l f l a t , as the two are intimately i n t e r -r e l a t e d . 134 THE INTER-CAUSEWAY TIDAL FLAT The f i r s t study of animal-sediment re l a t i o n s h i p s c a r r i e d out on the inter-causeway t i d a l f l a t was that of Levings and Coustalin (1975). They made extensive baseline studies on a l l the t i d a l f l a t s of the Fraser Delta-front, concentrating on the near surface benthic organisms (sampling depth 2 cm). Their r e s u l t s demonstrated that the number of species of organisms on t h i s t i d a l f l a t i s the highest f o r the whole d e l t a - f r o n t ; cumaceans were recorded at almost a l l s t a t i o n s . Both indicate that marine influences are prominent over the e n t i r e t i d a l f l a t (Levings and Coustalin, 1975). The Canadian National Harbours Board has recently proposed to expand the Coalport, and an environmental impact study has been c a r r i e d out (Beak-Hinton, 1977). The report includes d e t a i l e d maps and descriptions of the eelgrass bed and saltmarsh on this t i d a l f l a t . H i l l a b y and Barrett (1976) have also studied and mapped the saltmarsh. R. Moody (1978) studied an eelgrass bed on the southern side of the fe r r y causeway. His study included monthly monitoring of s a l i n i t y and t u r b i d i t y l e v e l s adjacent to the eelgrass bed. Floral/Sedimentological Zones F i e l d observations and a e r i a l photographs of the inter-causeway t i d a l f l a t reveal four major f l o r a l / s e d i m e n t o l o g i c a l zones which can be recognized on the basis of t h e i r d i s t i n c t i v e f l o r a l cover and/or drainage and sediment c h a r a c t e r i s t i c s ( F i g . 2). These are, from the shoreline seawards, the s a l t -marsh zone, the a l g a l mat zone, the sandflat zone and the eelgrass zone. There are also two d i s t i n c t i v e zones of minor areal extent, which p a r a l l e l the two causeways, here c a l l e d the 'causeway zones' (Fig. 2). The sediments 135 i n the causeway zones are coarser and contain less mud than the adjacent t i d a l f l a t , sand bars are present, the beach p r o f i l e i s much steeper than elsewhere, and, at the seaward end of the causeways, these zones lack any eelgrass cover, despite l y i n g at elevations which could be colonized by eelgrass. The sediments of the causeway zones have i n part been derived from sand f i l l during causeway construction. Shoreward of the causeway zones a narrow s t r i p of ' r i p rap' p a r a l l e l s each causeway. Figure 3 presents the e l e v a t i o n a l l i m i t s of the four major f l o r a l / sedimentological zones with respect to Geodetic Datum and with respect to the average exposure zone l i m i t s for observed tides between 1968-1978 at the Tsawwassen tide gauge. Swinbanks (1979) advocates the subd i v i s i o n of the i n t e r t i d a l region into exposure zones on the basis of c r i t i c a l t i d a l l e v e l s at which the maximum duration of exposure or submergence changes abruptly i n a s t e p - l i k e fashion. Exposure zone terminology (atmozone, amphizone, aquazone) has been introduced and defined elsewhere (Swinbanks, 1979). The elevation of the saltmarsh/algal mat zone boundary was determined at 7 locations ( F i g . 2), where the marsh showed signs of prograding ( i . e . , clumps forming seaward of the marsh and no evidence of erosion). The upper l i m i t of the a l g a l mat zone l i e s at +0.88 ± 0.04 m (range 0.83-0.92 m, N=7) Geodetic Datum, which i s 13 cm lower than i n Boundary Bay (Swinbanks', 1979) (Fig. 1). The saltmarsh plants grow on mounds and plateaus which are elevated about 20 cm above t h i s l e v e l , and thus the lower l i m i t of the saltmarsh l i e s at +1.07 m (range 1.066-1.071 m, N=2) Geodetic Datum, which i s almost iden-t i c a l i n elevation to the lower l i m i t of the saltmarsh i n Boundary Bay (+1.12 m Geodetic Datum, Swinbanks, 1979), and as i n Boundary Bay, l i e s very close to the lower l i m i t of the upper atmozone ( F i g . 3), which has l a i n at +1.23 ± 0.08 m (range 1.04-1.38 m) Geodetic Datum for the past ten years at Tsawwassen (Swinbanks, 1979). This i s the t i d a l e levation at which the maximum duration 136 E X P O S U R E Z O N E T R A N S E C T A B C l i I 2H UPPER ATMOZONE S a l t m a r s h Z o n a L O W E R A T M O Z O N E Lower limit of ' So l tmonh surface 1 UPPER A M P H I Z O N E O O O OH L O W E R A M P H I Z O N E UPPER A Q U A Z O N E 2H L O W E R A Q U A Z O N E S a n d f l a t Z o n a E e l g r a s s Z o n a A l g a l M a t Z o n o S a n d f l a t Z o n a Cl I E e l g r a s s Z o n a 3-" Figure 3. The e l e v a t i o n a l l i m i t s of the four major f l o r a l / s e d i m e n t o l o g i c a l zones on the inter-causeway t i d a l f l a t with respect to Geodetic Datum and the average exposure zone l i m i t s f o r observed tides between 1968-78 at the Tsawwassen t i d a l gauge (source: Swinbanks, 1979). The dotted envelopes ind i c a t e one standard deviation from the mean l e v e l of the exposure zone boundary. The exposure zone l i m i t s f o r the lower aquazone are based oh only two years of records (1976-78) because these boundaries are s i g n i f i c a n t l y modulated by an 18.6 year d e c l i n a t i o n a l cycle i n the moon (Swinbanks, 1979)• The elevation of the saltmarsh/algal mat zone boundary was determined at seven points between transects B and C (Fig. 2). 137 of exposure jumps from about 10 to 20 days (Swinbanks, 1979). The saltmarsh i s characterized by a dense growth of halophytic vegetation with T r i g l o c h i n  maritima and S a l i c o r n i a v i r g i n i c a dominating i n the lower regions (Beak— Hinton, 1977). The saltmarsh i s dissected by deeply i n c i s e d t i d a l channels 1-2 m deep, which have remained stable i n p o s i t i o n f o r decades (Beak-Hinton, 1977). The sediments of the saltmarsh consist of laminated s i l t s r i d d l e d with r o o t l e t s (Figs. 4a, 4b). The a l g a l mat zone i s lower atmozonal to upper amphizonal i n exposure and extends down to about +0.30 m Geodetic Datum (Fi g . 3), thus a t t a i n i n g a lower elevation than i n Boundary Bay, where the a l g a l mat zone i s r e s t r i c t e d to lower atmozonal elevations (Swinbanks, 1979). The sediments within the zone are muds or muddy sands. Mud contents decrease r a p i d l y seaward from about 90% to 20% ( F i g . 5). The zone i s dissected by an extremely dense network of d e n d r i t i c t i d a l channels which drain water from the impermeable muds (Fig. 6a). The channels terminate abruptly at the lower l i m i t of the zone. They are up to 1 m i n depth and are l i n e d with sand and s h e l l deposits (valves of Mya arenaria, many s t i l l i n growth p o s i t i o n ) . Blue-green a l g a l mats (mainly Phormidium sp. with minor O s c i l l a t o r i a sp. and S p i r u l i n a sp.) bloom i n summer on the plateaus between the channels. The a l g a l mats i n the upper h a l f of the zone become extensively cracked i n summer and the a l g a l mats b l i s t e r and c u r l ( F i g . 6b) . Crabs (Hemigrapsus? gregonens.is-) Infest the cracks and undermine the a l g a l mats to such an extent that a l g a l mat 'cakes' can be peeled from the substrate (Fig. 6c); Hemigrapsus burrows up into the al g a l mat cakes from below (Figs. 6c, 6d). In winter a f i l m of mud and very fine sand washed i n by winter storms buries the a l g a l mats and f i l l s the cracks and crab burrows. Thus, the plateaus between t i d a l channels consist of f i n e l y laminated sediments with black organic r i c h laminae and mud laminae cross-cut by casts of cracks and crab burrows (Fig. 6d). T i d a l pools are 138 Figure 4. a) Laminated s i l t s of the saltmarsh. The coarser laminae stand out i n r e l i e f . Pen i s 15 cm long. b) The weathered surf ace of the saltmarsh reveals that the saltmarsh deposits are r i d d l e d with r o o t l e t s , which have weathered out as holes here. Pen i s 15 cm long. 139 140 Figure 5. The mud contents of surface sediments. Numbers next to stations i n d i c a t e the percent mud at each s t a t i o n . The general trends of the contours between transects have been determined by q u a l i -t a t i v e f i e l d observations. Figure 6. a) The a l g a l mat zone. The channel i n the foreground i s about one meter wide. Taken on f l o o d t i d e , j u s t as the channels are beginning to f i l l . Mudcracked plateaus l i e between the channels, and w a t e r - f i l l e d depressions are present on the plateaus. b) Mudcracked surface of the a l g a l mat zone. The a l g a l mats b l i s t e r and c u r l under the e f f e c t s of d e s i c c a t i o n , and c r a c k i n g produces i s o l a t e d ' a l g a l mat cakes.' c) Undersurface of an ' a l g a l mat cake,' r i d d l e d by crab burrows. d) Laminated sediments of an a l g a l mat cake, cross-cut by a crab burrow (HemigrapBus oregonensis) . t-1 142 143 present i n depressions on the plateaus. Filamentous green algae (mainly Rhizoclonium sp. with minor Enteromorpha sp.) bloom around the moist edges of the pools i n summer, while the polychaete worms Abarenicola p a c i f i c a and Spio sp. are abundant i n the pools - Swinbanks (1979) provides descrip-tions of the burrows of these worms. Whereas the a l g a l mats are fi r m and easy to walk on, the t i d a l pools are underlain by s o f t d i l a t a n t muds. In the lower a l g a l mat zone blue-green algal'mats become patchy i n d i s t r i -bution, and the dominant a l g a l form i s a brown f i l m of diatoms (Plurosigma sp., Gyrosigma sp., Navicula sp. and N i t z s c h i a sp.). The sandflat zone i s predominantly lower amphizonal to upper aquazonal i n exposure.(Fig. 3). As the name implies, the zone i s f l a t , l acking i n t i d a l channels and dominated by very fine to fine sands, although a small area of muds i s present i n the middle of transect A (Fig. 5). The zone generally lacks any f l o r a l cover although a patch of Zostera americana (dwarf eelgrass) i s present between stations C H andC17 and smaller patches are present elsewhere. The c h a r a c t e r i s t i c feature of t h i s zone i s the very high density of thalassinidean burrowing shrimps which i t supports. Unlike the zones of non-vegetated sand i n Boundary Bay (Swinbanks, 1979), this zone lacks sand wave bedforms. The eelgrass zone i s e x c l u s i v e l y lower aquazonal i n exposure. I t s upper l i m i t terminates at the upper l i m i t of the lower aquazone at about -1.5 m Geodetic Datum (Fig. 3), a l e v e l below which the maximum duration of submergence begins to increase abruptly by a ser i e s of steps, the f i r s t being a jump from about 10 to 20 days of continuous submergence (Swinbanks, 1979). The eelgrass consists e n t i r e l y of the larger species Zostera marina. Z. marina beds a t t a i n higher elevations (up to -0.5 m Geodetic Datum) i n Boundary Bay by extending as fingers up the broad depressions of t i d a l channels which remain w a t e r - f i l l e d during low tide (Swinbanks, 1979). The 144 upper l i m i t of the eelgrass bed studied by Moody (1978) on the southern side of the ferry causeway i s bounded by a 'causex^ay zone.' Moody (1978) reported an anomalously low elevation f o r the upper l i m i t of the eelgrass bed i n h i s study area (-2.10 m Geodetic Datum; +0.85 m Tsawwassen Chart Datum; Moody, 1978). The sediments within the eelgrass zone are f i n e sands on transects B and C and very fin e sands and muds on transect A (Fig. 7). On the Coalport side of the t i d a l f l a t the eelgrass zone i s currently being eroded by a d e n d r i t i c drainage channel system, which extends from the head of a borrow p i t dredged during construction of the Coalport i n 1969 (Beak-Hinton, 1977). Since Coalport construction, the eelgrass zone has been advancing shorewards at the rate of about 25 m yr (Beak-Hinton, 1977) , but there i s no h i s t o r i c a l data on the density of eelgrass cover. Grain Size of Surface Sediments The general grain s i z e c h a r a c t e r i s t i c s of each zone seaward of the saltmarsh have already been described. Grain s i z e coarsens seawards on transects B and C (Fig. 7), which i s a t y p i c a l c h a r a c t e r i s t i c of t i d a l f l a t s ( Klein, 1971). The steepest gradient i n grain s i z e occurs from upper amphi-zonal elevations upwards; i . e . , above about mean sea l e v e l (just below the lower l i m i t of the a l g a l mat zone) much as reported by Swinbanks (1979) f or Boundary Bay. Transect A does not follow t h i s trend and a puzzling patch of mud i s present i n the middle of the transect. The s i x samples taken from the two?'causeway zones' i l l u s t r a t e the anomalous nature of these zones, as the sediments have-coarser median grain sizes and/or lower mud contents than the immediately adjacent t i d a l f l a t s (Figs. 5, 7). Box cores revealed l i t t l e evidence of s t r a t i f i c a t i o n except that already mentioned i n the a l g a l mat zone. However, on transect C i n the eelgrass zone, 145 Figure 7. The median grain s i z e (0) of surface sediments. The numbers next to stations i n d i c a t e median grain s i z e (0) at each s t a t i o n . 146 black organic r i c h sediments were underlain at a few centimeters depth by clean, l i g h t coloured sands probably i n d i c a t i n g the recent landward encroach-ment of the eelgrass zone (Beak-Hinton, 1977). On transect A stations A l to A4 are underlain at about 30-40 cm depth by a thin peat horizon which i n turn i s underlain by blue-grey clayey muds. Beyond s t a t i o n A4 the peat horizon was not detected, but instead the s u r f i c i a l muddy sands or muds were d i r e c t l y underlain at about 40 cm depth by the blue-grey clayey muds. This blue-grey clayey mud horizon was traced as f a r seawards as A7. S a l i n i t y and Tur b i d i t y A e r i a l photographs c l e a r l y i n d i c a t e that the Coalport causeway defle c t s the turbid, s i l t - l a d e n waters of the Fraser plume away from t h i s t i d a l f l a t and the inter-causeway waters are now cl e a r e r than they were (e.g., A37170-39 National A i r Photo Lib r a r y , Ottawa, Canada). R. Moody (1978) has monitored the s a l i n i t y of surface waters immediately south of the f e r r y terminal causeway on a monthly, bi-monthly and diurnal basis over a 10-month period. On 18 out of the 20 days sampled surface water s a l i n i t i e s were greater than or equal to 20%,.% (range 20-28%.). On two days, one i n August the other i n October, s a l i n i t y dropped to between 14 and 15.5%, (Moody, 19 78). Moody (19 78) has demonstrated that the t u r b i d i t y of surface waters increases during the summer months when the Fraser i s i n freshet, as indicated by a decrease i n secchi disc depth from about 5 m i n winter to 2 m i n summer (May-September). Levings and Coustalin (1975) took three surface substrate s a l i n i t y measure-ments i n the inter-causeway area i n February, 1974. Their values ranged between 26.5%. and 2 7%0.„ Surface substrate s a l i n i t i e s were monitored over the period July 1-7, 1977 at 64 stations on transects A, B, C and D, and on August 13, 1977 surface 147 substrate s a l i n i t i e s were measured at a l l 67 stations on transects A, B and C i n a three-hour period. Values ranged between 22 and 32%o (Appendix 5 ). Results for August 13 are t y p i c a l (Fig. 8). The r e s u l t s show l i t t l e l a t e r a l v a r i a b i l i t y i n s a l i n i t y over the t i d a l f l a t apart from a s l i g h t increase i n s a l i n i t y shorewards, probably due to evaporation (e.g., the s a l i n i t y of surface waters at C6 increased (from 30.5%\Tt;o^31i^%o/^overc;'the; three¥hour''samplijig i n t e r v a l ) . Anomalously low s a l i n i t i e s were recorded i n the v i c i n i t y of s t a t i o n A2. This i s possibly a r e s u l t of groundwater seepage from' the shore along the impermeable peat and mud horizon previously mentioned. A substrate s a l i n i t y p r o f i l e taken at s t a t i o n A2 detected low s a l i n i t y water (10%o) associated with this horizon (Appendix 5 ). Twenty-two substrate s a l i n i t y p r o f i l e s taken on transects A, B and C were almost a l l i s o h a l i n e (Appendix 5 ). Figure 9 shows a t y p i c a l example. On June 8, 1978 three s a l i n i t y p r o f i l e s of the water column, taken i n the inter-causeway area at high t i d e , were v i r t u a l l y i s o h a l i n e with s a l i n i t i e s between 22-24% 0 (Part 4B.). DISTRIBUTION OF THALASSINIDEAN SHRIMPS Thalassinidean shrimp burrows dominate the sandflat zone (Figs. 10, 11). Callianassa are abundant throughout the zone (Fig. 10) while Upogebia only -2 a t t a i n high de n s i t i e s (>20 burrow openings m ) i n the muddy sediments on the f e r r y terminal side of the sandflat zone (Fig. 11). Callianassa a t t a i n -2 a maximum density of 172 ± 22 burrow openings m (equivalent to about 70 -2 shrimps m ) i n the sandflat zone. However, the highest d e n s i t i e s of Callianassa were recorded i n the narrow 'causeway zone' next to the ferry -2 .- ...- -terminal, where burrow opening densities a t t a i n 446 i 31 m (equivalent to about 180 shrimps'mT2)'\. The presence of the saltmarsh l i m i t s C a llianassa d i s t r i b u t i o n at upper i n t e r t i d a l l e v e l s . Callianassa densities are low i n the a l g a l mat 148 L e g e n d - 3 2 - C o n t o u r o f s u b s t r a t e s a l i n i t y % o .2*0 T r a n s e c t s t a t i o n w i t h s a l i n i t y ' m e a s u r e m e n t i n d i c a t e d % o S a m p l i n g T i m e s Co - C29 1130-1200 821 — BI 1225-1305 A12 - Al 1320- 1350 C 6 - C 1 1415-1425 Figure 8. Surface substrate s a l i n i t i e s as measured on transects A, B and C August 13, 1977 at low tide between 11:30 and 14:25 i n the order indicated. Numbers next to stations i n d i c a t e s a l i n i t y . 149 E u Z h-O. UJ Q A10 J U L . 5 S A L I N I T Y 7oo K H 20-30-40-50J 10 _L_ 20 _i 30 —i Figure 9 . T y p i c a l substrate s a l i n i t y p r o f i l e from the inter-causeway area. 150 Figure 10. D i s t r i b u t i o n of Callianassa burrow openings i n contoured map form. General trends of contours between transects determined by q u a l i t a t i v e f i e l d observation. 151 500 1000 m L e g e n d »1 " C o n t o u r o f b u r r o w o p e n i n g d e n s i t y m ~ 2 f o r Upogebia §11 > 3 0 m - 2 Figure 11. D i s t r i b u t i o n of UpbjeMa burrow openings i n contoured map form. 152 zone. At low i n t e r t i d a l l e v e l s Callianassa densities decrease dramatically on transects B and C where eelgrass forms a dense and continuous mat (.Fig. 10). Upogebia has not been observed.in the a l g a l mat zone. Upogebia attains i t s highest elevations on transect A extending up to the base of the upper amphizone (about mean sea l e v e l ) . A comparison of Figure 5, showing mud d i s t r i b u t i o n , with Figure 11, showing the d i s t r i b u t i o n of Upogebia, c l e a r l y demonstrates a preference on the part of Upogebia for mud. This has been pointed out q u a l i t a t i v e l y by b i o l o g i s t s many times i n the past (Stevens, 1928; MacGinitie, 1930; L. Thompson and P r i t c h a r d , 1969; Thompson, 1972, for JJ. pugettensis; Ott et a l . , 1976, for JJ- l i t o r a l i s ) . In the following section the r e l a t i o n s h i p w i l l be analyzed q u a n t i t a t i v e l y . Influence of Grain Size on Thalassinidean Shrimp D i s t r i b u t i o n The inter-causeway t i d a l f l a t forms an almost i d e a l environment i n which to study the r e l a t i o n s h i p between grain s i z e and thalassinidean shrimp d i s t r i -bution because: (1) L a t e r a l v a r i a t i o n s i n s a l i n i t y and t u r b i d i t y have been reduced to a minimum by causeway construction. (2) The t i d a l f l a t shows considerable v a r i a t i o n i n grain si z e parameters i n a d i r e c t i o n p a r a l l e l to the shoreline, enabling comparison of stations of s i m i l a r e l e v a t i o n but d i f f e r e n t grain s i z e . (3) Thalassinidean shrimp densities are high, thus allowing easy r e s o l u t i o n of trends. Upogebia—Figure 12a compares the density of Upogebia burrow openings with .median grain si z e and Figure 12b with mud content, regardless of s t a t i o n -2 elevation. I t i s apparent that high d e n s i t i e s (:>20 m ) of Upogebia are associated with the f i n e r grained sediments (median >3.8 0) containing more (a) (b) 9 I? M i D I A N (0| •J. —1»-25 S O M U D IK) Figure 12., a) Density of Upogebia burrow openings vs median grain size (0) , regardless of elevation, b) Density of Upogebia burrow openings vs mud content (%), regardless of elevation. 154 than about 40% mud. However, Upogebia density i s also a function of . .v.i ; , elevation, and this obscures the trends i n Figure 12. To overcome t h i s problem the stations were classed into 0.25 m elev a t i o n class i n t e r v a l s from -2.20 m to +0.05 m (Geodetic Datum). The re s u l t s for mud content are .-.r presented i n Figure 13. Very s i m i l a r r e s u l t s were obtained for median grain s i z e (Appendix 6 ). For a l l class i n t e r v a l s higher de n s i t i e s of Upogebia occur i n the muddier f i n e r grained sediments. The trend can be approximated by a s t r a i g h t l i n e , the slope of which i s dependent on elevation. The slope of the l i n e i s minimized at the upper and lower l i m i t s of Upogebia's d i s t r i -bution, and maximized at the elevations of highest Upogebia density. Corre-l a t i o n c o e f f i c i e n t s are quite high, many exceeding 0.9 and 13 out of 18 are s i g n i f i c a n t at the 95% confidence l e v e l (jr t e s t ) . C a l l i a n a s s a — A s i m i l a r analysis of the data for Callianassa was c a r r i e d out. For unclassed data no trends are apparent. Callianassa attains high -2 densities (>50 burrow openings m ) i n sediments which range i n median grain s i z e from 2.57 0 to 3.94 0, and' i n mud content from 4.5% to 47% (Appendix 6 ) On breaking the data down into 0.25 m elevation class i n t e r v a l s no consistent c o r r e l a t i o n can be discerned. Figure 14 presents the. r e s u l t s for mud content Very s i m i l a r r e s u l t s were obtained for median grain s i z e (Appendix 6 ). At low elevations (e.g., F i g . 14D) Callianassa densities decrease with i n c r e a s i n mud content and decreasing grain s i z e , but as elevation increases the slope of the regression l i n e decreases and at high elevations the trend i s reversed with high Callianassa densities occurring i n the muddier f i n e r grained s e d i - . ments (e.g., F i g . 14H). Callianassa densities are very low (<0.5 burrow -2 openings m ) i n the muddiest sediments on the t i d a l f l a t , which occur high on transect C (Stations C1-C3), where mud contents range between 65% and 92%. However, i t i s d i f f i c u l t to know whether the low densities are a function of 155 -1.70/-1.45 m M u d % M u d % Figure 13. Relationship between mud content (%) and Upogebia burrow opening density, with data grouped in t o 0.25 m elevation class i n t e r v a l s . B e s t - f i t l i n e a r regression l i n e s are indicated, along with t h e i r c o r r e l a t i o n c o e f f i c i e n t s (r) and confidence l e v e l s (£ t e s t ) . Elevation (Geodetic Datum) increases from A to J . 156 Figure 14. Relationship between mud content (%) and Callianassa burrow opening density, with data classed into 0.25 m elevation class intervals. Best-fit linear regression lines are indicated, along with their correlation coefficients (r) and confidence levels (r test). Elevation (Geodetic Datum) increases from A to L. 157 158 grain s i z e of elevation. Thalassinidean Shrimp Interrelationships Callianassa and Upogebia burrows occur side by side (Fig. 10, 11). Upogebia i s a suspension feeder (MacGinitie, 1930; Thompson, 1972), while Callianassa i s a deposit feeder (MacGinitie, 1934). They occupy very s i m i l a r niches within the i n t e r t i d a l environment. Do the two shrimps interact? Is there any evidence of competition or trophic group ammensalism (Rhoads and Young, 1970)? Linear regression analysis was c a r r i e d out on Callianassa and Upogebia density data which had been classed into 0.25 m elevation class i n t e r v a l s (Fig. 15). Three out of nine of the class i n t e r v a l s have c o r r e l a t i o n c o e f f i -cients wnich are s i g n i f i c a n t at the 95% confidence l e v e l (jr test) , while a fourth i s s i g n i f i c a n t at the 90% confidence l e v e l . In a l l four cases the co r r e l a t i o n c o e f f i c i e n t s are negative. Examination of Figure 15 reveals that i t i s only at elevations where both Upogebia and Callianassa have the poten-::". •• t i a l of a t t a i n i n g high densities that a negative c o r r e l a t i o n between iL.; Call i a n a s s a and Upogebia density i s apparent. BIOGENIC REWORKING OF SEDIMENT Sediment reworking rates were monitored within protected metal enclosures during the summer of 1978 (Table I ) . The rate at which Callianassa reworks sediment i s quite v a r i a b l e , ranging between 9 to 33 ml/shrimp/day (assuming there are 2.5 burrow openings per shrimp). The average rate i s 18 ± 9 ml/ shrimp/day. These rates are comparable with the rate of 25 ml/shrimp/day reported by Ott et a l . (19 76) f o r Callianassa stebbingi, but i s appreciably D UO-i -1.45/-1.20 m I— 1 Figure 15~ Relationship between Upogebia burrow opening density and Callianassa burrow opening density with the data classed into 0.25 m elevation class i n t e r v a l s . B e s t - f i t l i n e a r regression l i n e s are indicated along with t h e i r c o r r e l a t i o n c o e f f i c i e n t s (r) and confidence l e v e l s (r t e s t ) . Eleva-t i o n (Geodetic Datum) increases from A to J. 1 6 0 T A B L E I Rates of biogenic reworking of sediment by Callianassa and Upogebia, as measured i n protected metal enclosures on the surface of the substrate Date Station Temperature of substrate and sea water ( C) Exposure Duration (hours) Burrow Openings (0.25 m - 2) *C. *U-Rate (.ml/shrimp/day) *C. *U. J u l 31/Aug 1 1978 Aug 4/Aug 5 1978 Aug 30/Aug 31 1978 Sept 3/Sept 4 1978 C23 C15 C12 C23 C17 A7 C21 C17 27, 26 (2 cm)** 23, 24 (.10 cm)** 22, 22 (S.W.)*** 27, 28 (2 era) 24, 25 (10 cm) 22, 22 (S.W.) 27, 29 (2 cm) 25, 26 (10 cm) 22, 22 (S.W.) 26, 29 (2 cm) 24, 24 (10 cm) 22, 22 (S.W.) 25, 20 (10 cm) 19, 18 (S.W.) 25, 19 (10 cm) 19, 18 (S.W.) 19, 20 (10 cm) 18, 18 (S.W.) 19, 20 (10 cm) 18, 18 (S.W.) 3.5 b.5 7.0 3.3 5.4 4.2 2.0 4.0 17 13 11 10 15 27 22 33 11 14 13 Average 18 ± 9 0 * C. = Callianassa U. = Upogebia ** 2 cm = Substrate temperature at 2 cm. *** S.W. = Flooding sea water-at 50 cm depth. Note: a l „ „ . » « . date,- . . , o„ "fj^;X r\f 8"r/»/«"; f°«i™SeS^; t. 1 6 1 less than the estimate of 50 ml/shrimp/day made by MacGinitie (1934) for Callianassa c a l i f o r n i e n s i s ; MacGinitie (1934) gave no d e t a i l s of the technique used to determine rates. The population of Callianassa on the inter-causeway t i d a l f l a t was g estimated to be about 100 m i l l i o n (actual estimate 1.08 x 10 ). This was done by d i v i d i n g Figure 10 into areas of comparable shrimp density and computing shrimp populations f or each area, by determining the average burrow opening density for the area multiplying this density by the area and assuming a burrow opening to shrimp r a t i o of 2.5 to 1. During the summer (June 21-September 21), when substrate temperatures are probably comparable to those presented i n Table 1,- this population of Callianassa turns over about 0.2 -2 m i l l i o n cubic meters of sand, and i n areas of highest shrimp density (178 m ) Callianassa reworks the sediment i t l i v e s i n to a depth of 50 cm i n about f i v e months. Upogebia did not produce any measurable mounds of sediment within 25 hours (Table !)>s However, th i s does not mean that Upogebia does not rework sediment. Both Thompson (1972) and Ott et a l . (1976) have noted i n the laboratory that Upogebia constantly tends to i t s burrow and transports s e d i -ment from one part of the system to another, by excavating sediment i n one place and tamping i t into the walls i n another. Figure 5 i l l u s t r a t e s that there i s an anomalous patch of muds i n the middle of transect A, which i s hard to account f o r i n terms of p h y s i c a l processes, and i s associated with high densities of Upogebia (Fig. 11). As s h a l l be seen i n Part B, Upogebia constructs a muddy inner l i n i n g to i t s burrow, which i s 1-4 mm thick. A blue-grey clayey mud horizon has been detected at about 40 cm depth on tran--.;.u sect A and can be traced at le a s t 600 m seawards as far as A7. Upogebia burrows, which extend down to about 60 cm depth, cross into t h i s horizon (Fig. 16). Perhaps the anomalously high mud contents of the sediments on « 100-, Figure 16. The near-surface stratigraphy of the t i d a l f l a t on transect A. Upogebia burrows extend down into , a blue-grey clayey mud horizon (the v e r t i c a l dimensions of the burrows are drawn to s c a l e ) . This ^ bioturbation may account for the anomalously high.mud contents of the surface sediments. M 163 transect A are the r e s u l t of intense bioturbation by Upogebia, which can -2 a t t a i n densities of 84 burrow openings m i n t h i s area (Fig. 16). Upogebia i n constructing burrows have possibly transported mud from the blue-grey clayey mud horizon at depth and incorporated i t into the burrow walls near' the surface, thereby increasing the mud content of the surface sediments. DISCUSSION The ranges of s a l i n i t y encountered within the inter-causeway area are within the tolerance l i m i t s of both Callianass a and Upogebia, the lower l e t h a l l i m i t f o r Callianassa being 10% o and for Upogebia 3.5%a (L. Thompson and P r i t c h a r d , 19,69). Callianassa i s probably protected from any transient incursions of low s a l i n i t y water by high s a l i n i t y i n t e r s t i t i a l waters at depth which are free to enter i t s unlined burrow (L. Thompson and P r i t c h a r d , 1969). Thus, i n the inter-causeway area,, s a l i n i t y does not influence thalassinidean shrimp d i s t r i b u t i o n . Upper Limits of Thalassinidean Shrimp D i s t r i b u t i o n The presence of the saltmarsh l i m i t s C a llianassa d i s t r i b u t i o n at upper i n t e r t i d a l l e v e l s . As i n Boundary Bay the lower l i m i t of the saltmarsh l i e s at the lower l i m i t of the upper atmozone. This i s the t i d a l l e v e l at which the maximum duration of exposure jumps from about 10 to 20 days, and t h i s period of exposure occurs at the time of the spring equinox (.Swinbanks, 1979). As Swinbanks (1979) has suggested, based on the saltmarsh studies of Chapman (1974), t h i s prolonged period of exposure may be e s s e n t i a l for successful seedling germination. Apart from being excluded by the dense vegetative cover of the saltmarsh Callianassa probably cannot survive at the upper atmo-zonal elevations of the marsh because the maximum duration of exposure exceeds 164" Callianassa's tolerance of exposure. More than about 5 days of anoxia i s l e t h a l to Callianassa (R. Thompson and Pr i t c h a r d , 1969). On transect A, Upogebia extends up to the base of the upper amphizone (about mean sea l e v e l ) . Upogebia burrow waters can become anoxic within one hour of exposure (R. Thompson and Pr i t c h a r d , 1969). The maximum duration of exposure within^ the lower amphizone i s l e s s than h a l f a lunar day (Swinbanks, 1979), whereas within the upper amphizone the maximum duration of exposure l i e s between 0.7 and 1.0 lunar days ( i . e . , 17-25 hours, Swinbanks, 1979), which i s within the range of exposure to anoxia l e t h a l to postmolt Upogebia (range 12-42 hours,- R. Thompson and Pr i t c h a r d , 1969). Hence, Upogebia probably att a i n s i t s absolute p h y s i o l o g i c a l l i m i t i n elevation on transect A by extending up to the base of the upper amphizone. Lower Limits of Thalassinidean Shrimp D i s t r i b u t i o n Ott et a l . (19 76) suggest that dense meadows of seagrass (Cymodocea) l i m i t the d i s t r i b u t i o n of Upogebia l i t o r a l i s and Swinbanks (1979) suggests that Callianassa d i s t r i b u t i o n may be l i m i t e d i n Boundary Bay by the dense f l o r a l cover of Z_. marina i n the lower eelgrass zone. Further evidence f o r Z. marina l i m i t i n g C a llianassa d i s t r i b u t i o n i s found i n the inter-causeway area on transects B and C. On these transects C a l l i a n a s s a density decreases -2-dramatically to less than 0.5 m , were Z. marina coverage becomes dense and continuous. In the absence of eelgrass, Callianassa extend to lower i n t e r t i d a l l e v e l s (Part$B). The dense r o o t l e t s of Z. marina may hinder the mining a c t i v i t i e s of Callianassa and/or the dense f l o r a l cover, black organic r i c h surface sediments and dense r o o t l e t s may discourage settlement by p o s t l a r v a l C a l l i a n a s s a . .. There i s no evidence that Upogebia i s l i m i t e d by Z. marina. In Boundary Bay Upogebia i s abundant i n dense Z. marina beds 165 (Swinbanks,11979). Upper Limit of Eelgrass—Moody (1978)aattributes the anomalously low elevation (-2.10 m Geodetic Datum, 1% mean exposure) f o r the upper l i m i t of eelgrass adjacent to the 'causeway zone' south of the ferry causeway to more rapid desiccation associated with the sandy substrate. However, eelgrass attains much higher elevations on sandy substrates i n the inter-causeway area (-1.43 m Geodetic Datum, 7.8% mean exposure, Transect B; -1.56 m Geodetic Datum 6.4% mean exposure, Transect C) and attains the same elevation on muddy substrates (-1.45 m Geodetic Datum, Transect A), disproving Moody's (1978) theory that the upper l i m i t of eelgrass i s influenced by grain s i z e . Desiccation may be enhanced by the steepened p r o f i l e associated with the 'causeway zone,' but probably much more important i s the increased e f f e c t s of wave action on the steepened p r o f i l e as evidenced by the presence of sand bars (Fig. 2). The 'r i p rap' adjacent to t h i s area had to be replaced with coarser material a f t e r causeway construction, because i t was being transported by wave-induced longshore currents (A. Tamburi, Western Canada Hydraulics, o r a l commun. 1978). Eelgrass i s absent i n the presence of sand waves i n Boundary Bay (Swinbanks, 1979). Eelgrass cannot t o l e r a t e wave shock (Beak-Hinton, 1977). The upper l i m i t of eelgrass bounded by the 'causeway zones' i s probably determined by wave shock, rather than exposure. The inter-causeway area may be the only area on the Fraser Delta where the factor determining the upper l i m i t of Z. marina may be a simple function of tides and ele v a t i o n , because the area i s not subject to strong wave action (as evidenced by the lack of sand waves), lacks t i d a l channels ( i n which submergence duration i s enhanced), and has a r e l a t i v e l y stable s a l i n i t y regime. The upper l i m i t l i e s at the upper l i m i t of the lower aquazone that has l a i n at -1.51 •+ 0.14 m Geodetic Datum f o r the past two years (Swinbanks, 1979). This i s the l e v e l below which the maximum duration of submergence jumps from about 10 to 20 days, and t h i s period of prolonged submergence always occurs at the time of the spring and autumn equinoxes (Swinbanks, 1979). U n t i l now b i o l o g i s t s have suggested that the upper l i m i t to Z. marina i s .'.determined by desiccation due to exposure, rather than being any function of submergence (Keller and H a r r i s , 1966; den Hartog, 1970; Moody, 1978). However, there are no jumps i n exposure duration within the aquazone and therefore there i s no way of accounting f o r the abrupt termination of the eelgrass zone , i n terms of exposure. Perhaps spring i s a c r i t i c a l time of the year when eelgrass puts on renewed growth a f t e r winter dormancy, and seedling germination and/or vegetative reproduction requires continuous submergence without exposure i n order to succeed. Could the advance of the upper l i m i t of Z_. marina i n the inter-causeway area over the past eight years (Beak-Hinton, 1977) be a r e s u l t of the fa c t that the upper l i m i t of the lower aquazone has r i s e n about 30 cm over the past ten years from -1.80 m (Geodetic Datum) i n 1968/70 to -1.51 m (Geodetic Datum) i n 1976/78, due to an 18.6 year cycle ..in the moon's declination' (Swinbanks, . . 1979)?^-These .ideas are designed ;td provoke thought and further study of the el e v a t i o n a l l i m i t s of eelgrass, rather than to o f f e r answers. Factors Influencing Thalassinidean Shrimp Density Callianassa does not e x h i b i t a clear preference for a p a r t i c u l a r grain s i z e of sediment, and i s abundant i n sediments ranging from sandy muds to pure sands. Callianassa densities are low i n the muds and muddy sands of the a l g a l mat zone, but rather than being a r e s u l t of substrate s e l e c t i o n by Callianassa, t h i s i s probably because drainage enhanced exposure on the . -. plateaus between t i d a l channels r e s u l t s i n a mudcracked, inhospitable environ-ment that can only be colonized by blue-green algae and crabs. Upogebia shows , 167 a d i s t i n c t preference for muddy substrates igroFab'ly^.e^cause' i t l i n e s " its' burrow with>,mud' (Swinbanks , 19,79 ; Part''4B,);'; A, durable "mud-linjid burrow is"~probably e s s e n t i a l for Upogebia's suspension feeding and r e s p i r a t i o n a c t i v i t i e s . There- i s some evidence to suggest that Callianassa and Upogebia densities are negatively correlated at t i d a l elevations where densities of both shrimps are high, suggesting that they may be competing for a v a i l a b l e space. S i g n i -f i c a n t l y , the four elevation class i n t e r v a l s which show high negative corre-l a t i o n c o e f f i c i e n t s between Callianassa and Upogebia density (Figure 15C, D, E and F) are the same four which have high negative c o r r e l a t i o n c o e f f i c i e n t s between Callianassa density and mud content (Fig. 14C, D, E and F). The negative correlations between Callianassa density and mud content could be the r e s u l t of a p o s i t i v e c o r r e l a t i o n between Upogebia density and mud content, combined with a negative c o r r e l a t i o n between Callianassa density and Upogebia density, rather than being the r e s u l t of a preference on the part of Callianassa for sandy coarser grained substrates. Conversely, the negative c o r r e l a t i o n between Callianassa density and Upogebia density could be the ~ r e s u l t of a p o s i t i v e c o r r e l a t i o n between Upogebia and mud content combined with a negative c o r r e l a t i o n between Callianassa and mud content, rather than being the r e s u l t of any negative i n t e r a c t i o n between Callianassa and Upogebia. But i f we accept the explanation that Callianassa shows a p r e f e r e n c e . r f o r ' -.r sandy-coarser grained-substrates at the elevations of these four class i n t e r -v a l s , we must also accept that t h i s trend i s completely reversed at higher elevations with Callianassa e x h i b i t i n g a preference for muddier, f i n e r grained sediments (Fig. 14H). Such a r e v e r s a l of preference i s hard to explain. This contradiction does not a r i s e i f we accept the former explanation that Callianassa and Upogebia exhibit a negative i n t e r a c t i o n where densities of both shrimps can be high, because then we need only accept that Callianassa 168 may be p o s i t i v e l y correlated to mud content. This p o s i t i v e c o r r e l a t i o n i s only exhibited at high i n t e r t i d a l elevations where UpOgebia i s absent or i s only present i n low d e n s i t i e s . At i n t e r t i d a l elevations where Upogebia and Callianassa d e n s i t i e s can be equally high, Callianassa's preference f o r muddy substrates i s overridden and suppressed by a negative i n t e r a c t i o n between Callianassa and Upogebia combined with a strong preference on the part of Upogebia f o r muddy substrates. Accepting the above explanation of the correlations what form does the competition between Callianassa and Upogebia take? I t may be a form of trophic group ammensalism (.Rhoads and Young, 1970) , as Upogebia i s a suspen-sion feeder (MacGinitie, 1930;, Thompson, 19 72), while Callianassa i s a deposit feeder (MacGinitie, 1934). Perhaps the reworking a c t i v i t i e s of Callianassa are detrimental to Upogebia. In constantly p i l i n g up sediment i n mounds on the surface Callianassa may increase mortality among, p o s t l a r v a l Upogebia, which construct small 'Y' shaped burrows a few centimeters deep i n the surface sediments f o r the purposes of suspension feeding (Thompson, 1972). A l l e r and Dodge (1974) have suggested that the reworking a c t i v i t i e s of Callianassa i n a t r o p i c a l lagoon produce an unstable bottom which cannot be colonized by most kinds of suspension feeding organisms. S h i f t i n g , ii unstable bottoms cause high mortality for s e t t l e d suspension feeding larvae of bivalves (Levinton and Bambach, 1970), and high fluxes of resuspended sediment may clog f i l t e r i n g mechanisms of suspension feeders and prevent e f f i c i e n t feeding (Loosanoff, 1962). The p l a n k t i c larvae of Upogebia are carnivorous (Thompson, 1972), and perhaps the larvae of UpOgebia feed on those of Callianassa. The negative c o r r e l a t i o n between Callianassa and Upogebia could be a r e s u l t of one or both of these suggested f a c t o r s . '.16,9 > ACKNOWLEDGEMENTS This research was financed by Geological Survey of Canada contract D.S.S. No. 0SS77-08177 from the Department of Supply and Services, Ottawa, Ontario, Canada. We are indebted to Mr. W. J . Rapatz, Regional T i d a l Superintendent i n Sidney, B.C. f o r providing ten years of observed t i d a l records from the t i d a l gauge located at the Tsawwassen ferry terminal. Mr. E. Medley, Mr. J.P. Napoleoni and Ms. N. Hayakawa ably a s s i s t e d i n the f i e l d . Dr. M. Pomeroy i d e n t i f i e d the cyanophytes, chlorophytes and diatoms, and Dr. W.C. Barnes, Dr. CD. Levings and Dr. J.L. Luternauer c r i t i c a l l y read the manuscript. We thank Mrs. CM. Armstrong and Mr. G.D. Hodge for dra f t i n g the diagrams and Ms. N. Hayakawa for typing the s c r i p t . 17P, REFERENCES A l l e r , R.C. and Dodge, R.E., 1974, Animal-sediment r e l a t i o n s i n a t r o p i c a l lagoon, Discovery Bay, Jamaica: Jour. Mar. Res., v. 32, p. 209-232. Bawden, C.A., Heath, W.A., and Norton, A.B., 1973, A preliminary baseline study of Roberts and Sturgeon Banks: Westwater Research Centre Tech. Rept. No. 1, University of B r i t i s h Columbia, 54 p. Beak-Hinton, 1977, Environmental impact assessment of Roberts Bank port expansion. Vol. 4, App. B. The e x i s t i n g b i o l o g i c a l environment: Beak Consultants Ltd., Vancouver, 140 p. Chapman, V.J. (ed.), 1974, Salt marshes and s a l t deserts of the world (2nd ed.): Leonard H i l l , London, 392 p. Dewindt, J.T., 1974, C a l l i a n a s s i d burrows as indic a t o r s of subsurface beach trend, M i s s i s s i p p i River Delta P l a i n : Jour. Sed. Petrology, v. 44, p. 1136-1139. Frey, R.W. and Howard, J.D., 1975, Endobenthic adaptions of juv e n i l e thalassinidean shrimp: B u l l . Geol. Soc. Denmark, v. 24, p. 283-297. den Hartog, C. , 1970, The sear-grasses of the world: North-Holland Publishing Co., Amsterdam, 275 p. H i l l a b y , F.B. and Barrett,_D.T., 1976, Vegetation communities of a Fraser River s a l t marsh: Environment Canada, Fi s h e r i e s and Marine Service, Tech. Rept. Series No. Pac/T-76-14. K e l l e r , M. and H a r r i s , S.W., 1966, The growth of eelgrass i n r e l a t i o n to t i d a l depth: ,Jout,. W i l d l y Mgnit.yW7'30J(2)V'-'P• '280Jf285. K e l l e r h a l s , P. and Murray, J.W., 1969, T i d a l f l a t s at Boundary Bay, Fraser River Delta, B r i t i s h Columbia: B u l l . Can. Pet. Geol., v. 17, p. 67-91. K l e i n , G.D., 1971, A sedimentary model tor determining p a l e o t i d a l range: Geol. Soc. America B u l l . , v. 82, p. 2585-2592.' Levings, CD. and Coustalin, J.B., 1975, Zonation of i n t e r t i d a l biomass and relat e d benthic data from Sturgeon and Roberts Bank, Fraser River estuary, B r i t i s h Columbia: Fi s h e r i e s and Marine Service, Environment Canada, Tech. Rep. no. 458, 138 p. Levinton, J.S. and Bambach, R.K., 1970, Some e c o l o g i c a l aspects of bivalve mortality patterns: Amer. Jour. S c i . , v. 268, p. 97-112. Loosanoff, V.L., 1962, Eff e c t s of t u r b i d i t y on some l a r v a l and adult biv a l v e s : Proc. Gulf Caribb. F i s h . Inst., 14th Ses., p. 80-95. Luternauer, J.L. and Murray, J.W., 1973, Sedimentation on the Western Delta-front of the Fraser River, B r i t i s h Columbia: Can. Jour. Earth S.ci^-j ~ " ~ _~) v. 10, p. 1642-1663. 171 MacGinitie, G.E., 1930, The natural h i s t o r y of the mud shrimp Upogebia  pugettensis (Dana): Ann. Mag. nat. H i s t . , v. 6, p. 36-44. , 1934, The natural h i s t o r y of Callianassa c a l i f o r n i e n s i s Dana: Amer. Midi. N a t u r a l i s t , v. 15, p. 166-177. ' and MacGinitie, N., 1968, Natural h i s t o r y of marine animals: MacGraw-Hill, New York, 523 p. Moody, R., 1978, Habitat, population and l e a f c h a r a c t e r i s t i c s of Zostera  marina L . on Roberts Bank, B r i t i s h Columbia: unpub. M.Sc. t h e s i s , University of B r i t i s h Columbia, Vancouver, B.C., 104 p. O'Connell, G., 1975, F l o r a and fauna of Boundary Bay t i d a l f l a t s , B r i t i s h Columbia: unpub. report to B.C. Government P r o v i n c i a l Part Branch, V i c t o r i a , B.C. 0tt,.J.S., Fuchs, B., Fuchs, R., and Malasek, A., 1976, Observations on the biology of Callianassa stebbingi Borradalle and Upogebia l i t o r a l i s Risso and t h e i r e f f e c t upon sediment: Senckenbergiana maritima, v. 8, p. 61-79. Pemberton, S.G., 1976, Deep bioturbation by Axius serratus i n the S t r a i t of Canso, Nova Scotia: unpub. M.Sc. t h e s i s , McMaster U n i v e r s i t y , Canada, 225 p. Rhoads, D.C. and Young, D.K. 1970, The influence of deposit-feeding benthos on bottom s t a b i l i t y and community trophic structure: Jour. Marine Res., v. 28, p. 150-178. Risk, M.J., Venter, R.D., Pemberton, S.G. and Buckley, D.E. 1978, Computer simulation and sedimentological implications of burrowing by Axius  serratus: Can. Jour. Earth Sciences, v. 15, p. 1370-1374. Stevens, B., 1928, Callianassidae from the west coast of North America: Pub. Puget Sound B i o l . Sta., v. 6, p. 315-369. Swinbanks, D.D. , 1979, /Environmental factors^ coritro^.ling_floral zonation",and •. JtHe' d i s t r i b u t i o n Tof -burrowing" and tube-dwelling organisms on Fraser. Delta t i d a l f l a t s , B r i t i s h Columbia: unpub. Ph.D. t h e s i s , University of B r i t i s h ' Columbia, Vancouver, B.C., 274 p. Thompson, L.C. and P r i t c h a r d , A.W., 1969, Osmoregulatory capacities of Callianassa and Upogebia (Crustacea: Thalassinidea): B i o l . B u l l . , v. 136, p. 114-129. Thompson, R.K., 1972, Functional morphology of the hind-gut of Upogebia pugettensis (Crustacea, Thalassinidea) and i t s role i n burrow construction: unpub. Ph.D. t h e s i s , University of C a l i f o r n i a , Berkeley, 202 p. and Pritchard,. A.W., 1969, Respiratory adaptions of two burrowing crustaceans, Callianassa c a l i f o r n i e n s i s and Upogebia pugettensis (Decapoda, Thalassinidea): B i o l . B u l l . , v. 136, p. 274-287. 172 Torres, J . J . , Gluck, D.L. and Childress, J . J . , 1977, A c t i v i t y and physio-l o g i c a l s i g n i f i c a n c e of the pleopods i n the r e s p i r a t i o n of Callianassa  c a l i f o r n i e n s i s '/Dana. (Crustacea: Thalassinidea): B i o l . B u l l . , v. 152, p. 134-146. Weimer, R.J. and Hoyt, J.H., 1964, Burrows of Callianassa major Say as ind i c a t o r s of l i t t o r a l and shallow n e r i t i c environments: Jour. Paleonto-logy,- v. 38, p. 761-767. Part 4B ENVIRONMENTAL CONTROLS ON THE DISTRIBUTION OF THALASSINIDEAN BURROWING SHRIMPS ON FRASER DELTA TIDAL FLATS, BRITISH COLUMBIA The Marine to Brackish T i d a l F l a t s of Central and Northern Roberts Bank 174-ABSTRACT The thalassinidean burrowing shrimps Callianassa c a l i f o r n i e n s i s and Upogebia pugettensis are abundant on the 'marine' t i d a l f l a t s of the south-easternmost section of ce n t r a l and northern Roberts Bank on the Fraser Delta-f r o n t . Northwestward toward the d i s t r i b u t a r i e s of the Fraser River, there i s an abrupt t r a n s i t i o n from 'marine' to brackish conditions i n the v i c i n i t y of Canoe Pass, and thalassinidean shrimp densities decrease dramatically. A technique for contouring this t r a n s i t i o n i n s a l i n i t y regime, based on the per- . cent thickness of the s a l t wedge, i s outlined. Accompanying t h i s t r a n s i t i o n i s a complete r e s t r u c t u r i n g of the f l o r a l / s e d i m e n t o l o g i c a l zonation of the t i d a l f l a t s . The 'marine' t i d a l f l a t s can be divided into four f l o r a l / s e d i -mentological zones. These are, from the shoreline seawards, the saltmarsh zone, the a l g a l mat zone, the sandflat zone and the eelgrass zone. T h a l a s s i -nidean burrowing shrimps are most abundant i n the sandflat zone. On the brackish t i d a l f l a t s a brackish marsh zone displaces the saltmarsh and a l g a l mat zones and the upper h a l f of the sandflat zone, while a sandflat/mudflat zone crosscut by both active and r e l i c t channels displaces the eelgrass zone and the lower h a l f of the sandflat zone. The brackish marsh extends to much lower t i d a l elevations than the saltmarsh, almost reaching the upper l i m i t of the aquazone. In response to these changes, the peak i n Callianassa d i s t r i -bution moves to lower i n t e r t i d a l l e v e l s , because of the presence of low s a l i -n i t y water at higher t i d a l l e v e l s and because of the disappearance of eelgrass i n lower i n t e r t i d a l regions. The lower l i m i t of the brackish marsh forms the upper l i m i t to Callianassa d i s t r i b u t i o n . In close proximity to the major channels of the Fraser River, Callianassa are absent. Callianassa burrow opening density i s p o s i t i v e l y correlated to the ' : • s a l i n i t y of surface substrate waters. Upogebia, although p h y s i o l o g i c a l l y . - \ 1 7 5 better adapted to cope with f l u c t u a t i n g s a l i n i t i e s , demonstrates lower tolerance of brackish water i n i t s d i s t r i b u t i o n than Ca l l i a n a s s a , probably because the function of i t s mud-lined burrow as a conduit for suspension feeding and r e s p i r a t i o n exposes Upogebia to low s a l i n i t y surface waters, while Callianassa, i n i t s unlined burrow used for deposit feeding, i s protected from surface waters by high s a l i n i t y i n t e r s t i t i a l waters. The d i s t i n c t i o n between mud-lined, permanent dwelling burrows and unlined, temporary feeding burrows i s , therefore, considered to be of great s i g n i f i -cance from the paleoenvironmental point of view. The aoundance of each of the two types of burrow i n the geological record could be used as a quali-^."--t a t i v e i n d i c a t i o n of p a l e o s a l i n i t y , burrows of the former being more s e n s i t i v e than those of the l a t t e r . Both Upogebia and Callianassa are probably capable of producing trace f o s s i l burrows resembling e i t h e r Ophiomorpha or Thalassinoides, depending on whether the knobbly outer or smooth inner burrow w a l l , r e s p e c t i v e l y , i s accentuated during f o s s i l i z a t i o n ; therefore, the d i s t i n c t i o n between Ophiomorpha and Thalassinoides i s considered to be of l i t t l e s i g n i f i c a n c e . In the high energy environment and unstable s a l i n i t y regime of northern and cen t r a l Roberts Bank, Callianassa constructs burrows with long c o n s t r i c t e d apertural necks which extend about 30 cm down into the substrate. This i s thought to r e f l e c t a change i n feeding mode, with Callianassa abandoning near-surface feeding because of the i n s t a b i l i t y of the surface environment. 173 176 INTRODUCTION The primary aim of these two papers (Part 4Ayand B) i s to assess the e f f e c t s of various environmental factors on the d i s t r i b u t i o n of the thalas-sinidean burrowing shrimps, Callianassa c a l i f o r n i e n s i s fbana and Upogebia pugettensis (Dana) , on Fraser Delta t i d a l flats,,, i n the^,hope-.;-that..their, distinle-,tive burrows may-be_used as paleoenvironmental ind i c a t o r s in.ancient, d e l t a i c sequences. Thalassinidean shrimps are known to occur as far back as the Cretaceous (Borradaile, 1903). The studies of thalassinidean shrimp d i s t r i -bution on the inter-causeway t i d a l f l a t on southern Roberts Bank (Part 4A) and i n Boundary Bay (Swinbanks, 1979) dealt with areas experiencing r e l a t i v e l y s table, 'marine,' s a l i n i t y regimes. The aim of this paper i s to examine shrimp d i s t r i b u t i o n on the t i d a l f l a t s of c e n t r a l and northern Roberts Bank, north of the Coalport causeway (Fig. 1), where the i n f l u x of freshwater from the Fraser River system i s an added environmental f a c t o r i n f l u e n c i n g shrimp d i s t r i b u t i o n . . The following questions are considered to be the points of most i n t e r e s t : (1) Which shrimp i s most s e n s i t i v e to changes i n s a l i n i t y regime? (2) To what extent does the nature and function of the burrow system a f f e c t the shrimps' capacity to tolerate reduced s a l i n i t i e s ? (3) Could the d i s t i n c t i v e burrows of these shrimps be used as p a l e o s a l i n i t y indicators? To assess the e f f e c t s of reduced s a l i n i t i e s on thalassinidean shrimp d i s t r i b u t i o n i t i s necessary to have a sound understanding of the e f f e c t s of other parameters (e.g., elevation, grain s i z e and f l o r a l cover"K Such datahayebeen obtained i n the studies of Boundary Bay (Swinbanks ,-..1979) and of the inter-causeway t i d a l f l a t (Part 4A). Secondly, i t i s necessary to have p h y s i o l o g i c a l data on the responses of Upogebia and Callianassa to reduced s a l i n i t i e s , and knowledge of t h e i r ethology, p a r t i c u l a r i l y i n regard to feeding and r e s p i r a t i o n . Such dataare a v a i l a b l e i n the studies of L. Figure 1. Location of study area. T i d a l flats, are s t i p p l e d , land area of Recent alluvium i s blank, and older deposits cross-hatched (adapted from Luternauer and Murray, 1973). 178 Thompson and P r i t c h a r d (1969), R. Thompson and P r i t c h a r d (1969), Thompson (1972) and MacGinitie (1930, 1934). Felder (1978) has recently studied the osmoregulatory capacities of three species of Callianassidae (jC. major, _C. islagrande and _C. jamaicense)and related 1 t h i s to t h e i r d i s t r i b u t i o n on the Louisiana and M i s s i s s i p p i coast. Unfortunately Felder (1978) -'did. ; not include descriptions of the feeding or r e s p i r a t o r y a c t i v i t i e s of the three shrimps, nor :did he include descriptions of t h e i r burrows, although, of course, the burrows of C. major are w e l l known to geologists, as they are a modern equivalent of Ophiomorpha (Weimer and Hoyt, 1964). Previous studies of Roberts Bank have established that there i s a change from brackish to more marine conditions i n going from north to south. Levings and Coustalin (1975) recorded higher numbers of benthic species on a transect immediately north of the Coalport causeway ( F i g . 1) than on any transects further to the north, and cumaceans were abundant at many of the stations _2 (up to 15,872 m , Levings and Coustalin, 1975), both of which indicates that the p o r t i o n of Roberts Bank immediately north of the Coalport causeway s t i l l has the 'marine' c h a r a c t e r i s t i c s of the inter-causeway area (Part 4A). Amphithoc v a l i d a , an amphipod which prefers more saline conditions, i s also abundant i n the area immediately north of the causeway (Dr. M. Pomeroy, P a c i f i c Environment I n s t i t u t e , West Vancouver, o r a l commun. 1978). Moody (1978) has mapped and studied the marsh at Brunswick Point immediately south of Canoe Pass (Fig. 4). This i s a t y p i c a l brackish marsh. The upper marsh i s dominated by Carex lyngbyei while the lower marsh consists of Scirpus americanus and Scirpus maritimus. However, towards the Coalport causeway the a r e a l extent of the marsh decreases abruptly and only a narrow fringe of marsh i s present, with D i s t i c h l i s sp. and S a l i c o r r i i a sp. i n the upper parts and T r i g l o c h i n maritima i n the lower (Moody, 1978). These marsh plants are t y p i c a l of the saltmarshes of the inter-causeway and Boundary Bay 4.79 areas (Kellerhals:.and:'Murray:,. 19.69.; ,0' Connell;.1975; Parsons, :1975; i l i l l a b y and Barrett, 1976; Beak-Hinton, 1977; Swinbanks, 1979). Accompanying t h i s marsh t r a n s i t i o n , surface substrate s a l i n i t i e s increase from <3%o i n the brackish marsh to as high as 21%„ i n the saltmarsh (Moody, 1978). The highest s a l i n i t i e s were recorded on the transects clo s e s t to the Coalport causeway (Moody,.. 1978) . . METHODS Seventy-five stations were sampled at low tide by hovercraft and h e l i -copter on a gr i d of approximately 1 km extending from the marsh perimeter down to about 0.6 m Chart Datum on the dates indicated i n Figure 2. Nineteen stations l y i n g on the same gr i d system between Canoe Pass and the Tsawwassen ferry terminal were occupied by hovercraft on June 8, 1978 at high t i d e , and s a l i n i t y p r o f i l e s were taken with a Beckman RS5-3 portable, inductive salinometer, sampling at 0.25 m i n t e r v a l s . Eleven of these stations were reoccupied on the same day (June 8) to obtain substrate s a l i n i t y and shrimp density data at low t i d e . Hovercraft stations were located using Decca radar and are accurate to within a radius of about 50 m. Helicopter stations^ were p l o t t e d on a recent a e r i a l photograph (A37597-146, National A i r Photo Li b r a r y , June, 1978).with a scale of 1 to 72,000, a f t e r being located v i s u a l l y from the a i r , and are accurate to within a radius of about 100 m. Using a topographic map (Swan Wooster, 1967) of 0.6 m contour i n t e r v a l ( F i g . 3) the hovercraft and hel i c o p t e r stations were grouped into 0.6 m eleva t i o n class i n t e r v a l s . Some of the contours on th i s map may be inaccurate, p a r t i c u l a r l y i n the v i c i n i t y of the main channel of Canoe Pass, as t h i s channel has changed d i r e c t i o n since 1967. However, i t i s the best elevation data a v a i l a b l e . The Chart Datum for the contours on the map i s -2.63 m Geodetic Datum. The t i d a l data used i n th i s paper was derived from the tide gauge at 180 Figure 2. Locations of stations sampled by h o v e r c r a f t ^ helicopter-"and on foot, Ln -1977 and .19 78. 181 . U.S.A. Figurej3. Topographic map of Roberts Bank i n 1967. The datum f o r contours i n t h i s map i s -2.63 m Geodetic Datum. Contours i n meters , (source: Swan Wooster, 1967). the Tsawwassen fe r r y terminal, where present-day Chart Datum i s -2.95 m Geodetic Datum. To avoid confusion as to datum, throughout the remainder of t h i s paper elevations w i l l be given with respect to Geodetic Datum, which remains fixed i n time and space. Where reference i s made to Chart Datum the datum referred to i s present-day datum at Tsawwassen. At a l l s t a t i o n s , shrimp burrow opening densities were determined by 2 sampling eight times at each s t a t i o n with a 0.25 m quadrat. Surface grain s i z e samples were obtained at a l l stations using a 2 cm deep rectangular box. Unfortunately, the twenty-six grain s i z e samples c o l l e c t e d on August 17, 1978 were l o s t . Surface water s a l i n i t i e s were recorded at a l l sta t i o n s at low tide using a refractometer (Endeco type 102). The geometry of shrimp burrows was studied using a box core (15 x 20 cm by 30 cm deep), r e s i n casts (Shinn, 1968) and simple excavation with a spade. In addition to the stations mentioned above, s i x stations were located on foot on Roberts Bank between Canoe Pass and the Coalport causeway (Fig. 2) s p e c i f i c a l l y to monitor the e f f e c t s of changing substrate s a l i n i t y on shrimp d i s t r i b u t i o n at a f i x e d t i d a l l e v e l . On September 10, 1977 at slack water low tide (observed t i d a l height: +1.4 m Chart Datum or -1.55 m Geodetic Datum) surface substrate s a l i n i t i e s were monitored with a refractometer at approximately 100 m i n t e r v a l s i n close proximity to the waterline walking from the Coalport causeway towards Canoe Pass. An area showing a pronounced s a l i n i t y gradient was located where s a l i n i t i e s dropped from 24.5%. to 12.5%. over a distance of about 0.75 km. Six stations with sandy substrates showing a range i n surface s a l i n i t i e s were located within a few meters of the water- -:: l i n e and marked with wooden stakes, and mapped using a Brunton compass 7 • „ J (Fig. 2). The following day surface s a l i n i t i e s , s a l i n i t y p r o f i l e s , grain s i z e samples and shrimp burrow density data were c o l l e c t e d at these s t a t i o n s . 2 Sixteen quadrat readings were taken at each s t a t i o n , with a 0.25 m quadrat 183 } to o b t a i n accurate estimates of shrimp burrow d e n s i t y . In the l a b o r a t o r y , g r a i n s i z e samples were washed free of s a l t , wet sieved through a 63 um s i e v e to e x t r a c t the s i l t / c l a y f r a c t i o n , d r i e d and the percent mud values c a l c u l a t e d . The data from the inter-causeway area (Part 4A) has demonstrated that percent mud values are p e r f e c t l y adequate f o r assessing the r e l a t i o n s h i p s between t h a l a s s i n i d e a n shrimp d i s t r i b u t i o n and g r a i n s i z e . The d i s t i n c t i v e f l o r a l / s e d i m e n t o l o g i c a l zones of the Roberts Bank t i d a l f l a t s were mapped using a high l e v e l , c o l o u r , a e r i a l photograph taken i n June, 19 78 (A37597-146, N a t i o n a l A i r Photo L i b r a r y , Ottawa, Canada) i n con-j u n c t i o n w i t h low l e v e l , c o l o u r , a e r i a l photographs (Scale 1:12,000) taken i n J u l y , 1977 (A31164, N a t i o n a l A i r Photo L i b r a r y ) . NORTHERN AND CENTRAL ROBERTS BANK I t i s e s s e n t i a l to place the t h a l a s s i n i d e a n shrimp d i s t r i b u t i o n data w i t h i n the o v e r a l l framework of f l o r a l / s e d i m e n t o l o g i c a l zones on the t i d a l f l a t s as the two are i n t i m a t e l y i n t e r r e l a t e d ' (Part 4A). In the case of these marine to b r a c k i s h t i d a l f l a t s , i t i s also necessary to describe i n d e t a i l the l a t e r a l changes i n s a l i n i t y regime which occur on the t i d a l f l a t s , as t h i s i n f l u e n c e s shrimp d i s t r i b u t i o n . For these reasons extensive d e s c r i p t i o n s of both f o l l o w . F l o r a l / S e d i m e n t o l o g i c a l Zones Figure 4 i s a map of the f l o r a l / s e d i m e n t o l o g i c a l zones of Roberts Bank north of the Coalport causeway. Two zonation schemes have to be employed to subdivide the i n t e r t i d a l r e g i o n ; one f o r the 'marine' area i n the v i c i n i t y of the Coalport causeway and one f o r the b r a c k i s h area from Brunswick P o i n t Figure 4. Floral/sedimentological zones of Roberts Bank, prepared from a colour a e r i a l photograph of June, 1978 (A37597-146, N.A.P.L., Ottawa, Canada). 185 northwards. The t r a n s i t i o n between the two environments i s not gradual but abrupt. In the area southeast;of Brunswick Point a f o u r - f o l d zonation i s present which i s very s i m i l a r to that of the inter-causeway area (Part 4A). The zones are, from the dyke seawards, the saltmarsh zone, the a l g a l mat zone, the sandflat zone and the eelgrass zone. In ..contrast: to the inter-causeway area,ther..algat .mat zone: i s - dominated by filamentous green algae (mostly Rhizoclonium sp.) with only minor f i l a -mentous blue-green algae (.Oscillatofia sp.) . The filamentous green a l g a l mats are coated with a brown f i l m of diatoms (Navicula sp., N i t z s c h i a sp. and P i n n u l a r i a sp.). The green algae are probably better,adapted than cyanophytes to cope with the reduced l i g h t a v a i l a b i l i t y and unstable s a l i n i - : t i e s north of the causeway (Dr. M. Pomeroy, P a c i f i c Environment I n s t i t u t e , o r a l commun. 1978). In addition, t idar_"chanriels. i n the inter-causeway a l g a l . mat zone "are, more deeply.-entrenched r e s u l t i n g i n desiccation on plateaus bet-we eh channels favourable to blue-green a l g a l matedeyeOiopment. The e l e v a t i o n a l l i m i t s of the saltmarsh and a l g a l mat zones are very s i m i l a r to those i n the inter-causeway area (Table I ) . These elevations were determined by overlaying the topographic map (Fig. 3) on Figure 4, and l i n e a r l y i n t e r p o l a t i n g between contours to determine the e l e v a t i o n a l ranges of each boundary. The accuracy of t h i s technique was checked by determining the e l e v a t i o n a l l i m i t s of zone boundaries i n the inter-causeway area on transects A, B and C (Part 4Aj, and then t h i s data was compared with the surveyed data presented i n Part 4A. The average discrepancy between the surveyed data and that from the topographic map i s ±11.cm (range +22 cm to -16 cm) (Table I ) . Thus the map gives a reasonable estimate of elevation. The lower l i m i t of the saltmarsh i s , i f anything, s l i g h t l y higher than i n the inter-causeway area, l y i n g at +1.02 to +1.29 m Geodetic Datum, while the lower l i m i t of the a l g a l mat zone i s p o s s i b l y s l i g h t l y lower, l y i n g close to TABLE ...I E levation ranges f o r zone boundaries on the 'marine' t i d a l f l a t s as determined from a topographic map. The accuracy of elevations obtained from the map i s checked by comparing with surveyed elevation data obtained i n the inter-causeway area (Part 4A) . . GEODETIC ELEVATION (m)  ZONE BOUNDARY North of Coalport Inter-Causeway Area (Map) (Map) (Surveyed) (Discrepancy) Saltmarsh Zone/ +1.02 +0.80 +0.83 -0.03 A l g a l Mat Zone +1.29 +1.00 +1.07 -0.07 A l g a l Mat Zone/ +0.13 B +0.16 +0.29 -0.13 Sandflat Zone -0.13 C +0.15 +0.31 -0.16 Sandflat Zone/ -1.74 A -1.34 -1.45 +0.11 Eelgrass Zone (highest B -1.21 -1.43 +0.22 elevation) C -1.52 -1.56 +0.04 Average ±0.11 Note: A, B and C (refer to transects A, B and O i n the inter-causeway area. : 187 0.0 m Geodetic Datum. The highest l e v e l that the eelgrass zone attains i s -1.74 m Geodetic Datum, which i s appreciably lower than i n the i n t e r -causeway area (Table I ) , and, unlike i t s counterpart i n the inter-causeway area, the upper l i m i t i s not delimited by elevation, but veers o f f towards low water mark i n a westerly to south westerly d i r e c t i o n as Canoe Pass i s approached. The eelgrass cover also becomes patchier i n t h i s d i r e c t i o n (Fig. 4). From Brunswick Point northwards, the i n t e r t i d a l region can be divided into two major zones, the brackish marsh zone and the sandflat/mudflat zone (which i s equivalent to the i n t e r t i d a l p ortion of the 'main platform,' Luternauer and Murray, 1973). The Brunswick Point marsh can be subdivided i n t o an upper and lower part on the basis of Moody's (1978) map. The Westham Island marsh can t e n t a t i v e l y be divided into an upper and lower part on the basis of information provided by Burgess; :(197.0) ,."who noted an abrupt change i n ele v a t i o n of between 0.15 m to 0.45 m over a distance of 1.5-3.0 m, which marks the boundary between the upper marsh, dominated by Carex lynbyei, and the lower marsh, dominated by Scirpus americanus. This break appears to be marked i n a e r i a l photographs by a l i n e of driftwood and changes i n drainage channel d i r e c t i o n . Moody (1978) determined that t h i s break occurs at about +0.10 m Geodetic Datum i n the Brunswick Point marsh, while Burgess (19 70) estimated i t to occur at 0.0 m Geodetic Datum. The boundary between upper and lower marshes thus l i e s close to the boundary between the upper and lower amphizones at -0.08 ± 0.15 m Geodetic Datum, a l e v e l above which the maximum duration of exposure jumps from les s than 0.5 lunar days to greater than or equal to 0.7 1 unar days (Swinbanks, 1979). The e l e v a t i o n of the lower l i m i t of the lower marsh was determined at several l o c a l i t i e s by overlaying the topographic map (Fig. 3) on Figure 4 (.Table I I ) . At Brunswick Point the marsh extends down to -0.20 m Geodetic Datum (Table I I ) . This TABLE I I E l e v a t i o n of the Lower L i m i t of the B r a c k i s h Marsh LOCATION ELEVATION RANGES (Geodetic Da turn,-cm) Brunswick P o i n t E/W boundary p a r a l l e l -0.08,to -0.20.0 to Canoe Pass NW/SE boundary -0.20.to +0..1.0.-SSW/NNE boundary +0.04.to +1.02. > (b r a c k i s h marsh/algal mat zone) Westham I s l a n d Canoe Pass to Main Channel -0.20,to -0.52. ..189/ compares with Moody's (1978) figure of -0.13 m Geodetic Datum. However, t h i s boundary i s not delimited by elevation, but r i s e s from Canoe Pass towards the Coalport causeway, and i n the area where the marsh transforms to a saltmarsh the boundary r i s e s abruptly towards the NNE from about +0.04 m to +1.02 m Geodetic Datum (Table I I ) . The lower l i m i t of the marsh at Westham Island ranges i r r e g u l a r l y between about -0.20 to -0.52 m Geodetic Datum j u s t above the upper l i m i t of the aquazone (-0.74 ± 0.10 m Geodetic Datum, . Swinbanks, 1979). Burgess (1970) estimated that the lower l i m i t of the marsh extends down to -0.9 m Geodetic Datum. However, Burgess (1970) did not -.:::../ specify whether he was r e f e r r i n g to the main body of the marsh or to the i s o l a t e d l i t t l e clumps of marsh plants which l i e below the boundary mapped i n Figure 4. The lower l i m i t of the brackish marsh l i e s at l e a s t 1 to 1.5 m below that of the saltmarsh, and the brackish marsh l a t e r a l l y replaces the a l g a l mat zone and the upper part of the sandflat zone of the 'marine' t i d a l f l a t s . This i s c l e a r l y i l l u s t r a t e d i n a high l e v e l a e r i a l photograph (Fig. 5) taken on a flooding t i d e , when the waterline lay at about -0.12 m Geodetic Datum (est imated from tide tables for Pt. Atkinson). The waterline i n t h i s photo has reached the lower l i m i t of the marsh at Westham Island, i s j u s t below the lower l i m i t of the marsh at Brunswick Point and i s approaching the a l g a l mat zone i n the area south of Brunswick Point and i n the inter-causeway area, while i n Boundary Bay i t l i e s at the upper l i m i t of the eelgrass zone (which consists of _Z. americana i n the upper regions; Swinbanks, 19 79). The Brunswick Point marsh has been undergoing rapid expansion since the l a t e 1940's (Moody, 1978). I t has been suggested that t h i s i s a r e s u l t of the deposition of large quantities of sediment i n front of the marsh by a major flood i n 1948, forming an elevated region which marsh plants could colonize (A. Tamburi, Western Canada Hydraulics, o r a l commun. 1978). Below the brackish marsh zone l i e s the sandflat/mudflat zone which can 190 Figure 5. High l e v e l a e r i a l photograph of the Fraser Delta. The waterline l i e s at about -0.12 m Geodetic Datum on a flooding t i d e , and l i e s above the lower l i m i t of the marsh at Westham Island (centre) i s approaching the a l g a l mat zone i n the inter-causeway area and l i e s at the upper l i m i t of the eelgrass zone i n Boundary Bay ( r e f e r to Fig. 1 f o r l o c a t i o n ). Water i s beginning to f l o o d i n t o the d i s t r i -butary channels of the Brunswick Point marsh from Canoe Pass. j . 9 r be subdivided i n t o 'muddy domains' and 'sandy domains.' The muddy domains consist of sediments containing more than about 30% mud (.range 29.4-86%) while the sediments of the sandy domains contain le s s than about 15% mud (range 0 . 21 -14.3%). This was determined by overlaying percent mud values for 48 samples from the sampling g r i d (Fig. 6 ) , over the a i r photos - a technique developed by Medley and Luternauer ( 1 9 7 6 ) . The muddy domains dominate near the fringe of the marsh, but are also present at lower elevations associated with topographic depressions and in p a r t i c u l a r with r e l i c t channels. The increase i n mud content shorewards i s c l e a r l y apparent i n Figure 6 but the coarse sampling g r i d i n most cases f a i l s to detect the muddy•domains i n lower i n t e r t i d a l regions. The main channel of Canoe Pass has changed d i r e c t i o n i n the recent past. This can be seen by comparing the Swan Wooster (.1967) topographic map (Fig. 3) with the map compiled from present-day a e r i a l photo-graphs ( F i g . 4 ) . The channel on the Coalport side of the present main channel, which was a major channel of Canoe Pass, i s r a p i d l y necking o f f i n the upstream d i r e c t i o n , and w i l l probably be abandoned and i n f i l l e d with mud in the near future. To the east of t h i s channel l i e s an elongate depression that i s probably the r e l i c of a former channel which suffered a s i m i l a r fate. S a l i n i t y The Fraser River reaches i t s peak discharge i n June and July ( Fig. 7 ) . A e r i a l photographs i n d i c a t e that the turbid water ^f'thervF'raser plume i s dispersed over a l l parts of northern and ce n t r a l Roberts Bank (e.g., F±g..<.3^ 1,. A30339-116, National A i r Photo L i b r a r y ) , but i t i s impossible to t e l l the thickness of the plume or the s a l i n i t y of i t s water from the photographs. S t r a i t of Georgia waters o f f the Fraser Delta-front are s t r a t i f i e d into a brackish surface layer and an underlying s a l t wedge (Waldichuk, 195 7 ) . The 192 Figure 6. Percent mud i n surface sediments of northern and c e n t r a l Roberts Bank. Mechanical contouring employed. 193 ure 7. Discharge curves for the Fraser River in c l u d i n g the freshet portion of the runoff for 1948, a severe flood year i n the Fraser Valley (adapted from R. Thompson, unpublished data). 194 -s a l t wedge intrudes the d i s t r i b u t a r y channels of the Fraser on flood tides by under-running r i v e r water, extending as far as 20 km upstream of the inner t i d a l f l a t s during the winter, but extends no further than the inner edge of the f l a t s during the summer freshet (Ages and Woollard, 1976). Figure 8 presents a '. t y p i c a l d i s t r i b u t i o n pattern of surface substrate s a l i n i t y on c e n t r a l and northern Roberts Bank. Similar contour patterns were obtained for data c o l l e c t e d i n August, 1977 (Appendix 8 ) and February, 1974 (Leyings and Coustalin, 1975; Appendix 8 ), although the absolute values at equivalent stations v a r i e d quite considerably,<probably due to d i f f e r i n g wind conditions and r i v e r discharge during the preceding high t i d e . There are two features which a l l the r e s u l t s have i n common: (1) there i s a general increase i n s a l i n i t y seaward as one goes to lower i n t e r t i d a l l e v e l s (2) a higher s a l i n i t y region i s always present immediately northwest of the Coalport causeway. How do substrate s a l i n i t i e s at low tide r e l a t e to s a l i n i t i e s i n the water column at high tide? I t was suspected that surface substrate s a l i n i t i e s should c l o s e l y r e f l e c t surface water s a l i n i t i e s at high tide on ebb, as sur- ". • face substrate waters should be derived from the l a s t water to drain o f f the t i d a l f l a t . To test t h i s hypothesis and to better define the t r a n s i t i o n from marine to brackish conditions between Tsawwassen and Canoe Pass, s a l i n i t y profiles'were recorded at high tide on ebb at nineteen stations ,,in t h i s area;;, ZZA within a, 3. 3 hour,, period pnC'June^ 8 , . 19.7'8.. In the' v i c i n i t y ' o f Canoe- Pass,"" pro-f i l e s were. Isohaline and almost:, pure .freshwater,,, while. immediately, north of. the (Coalport causeway '.profiles were i s o h a l i n e and- 'marine.' i n s a l i n i t y ( F ig. ,9a) . / Between these.;two extremes .lay -a region ..where a. dis.tin-et h a l o c l i n e .was •, developed (Fig. 9a). Figure 9a contours surface water s a l i n i t i e s at high tide on ebb, while Figure 9b contours surface substrate and surface shallow water (<0.7 m) s a l i n i t i e s on approaching low tide (the shallow water s a l i n i t y p r o f i l e s were 195 Figure„8. Surface substrate s a l i n i t i e s on Roberts Bank at low tide on August 17, 1978. Mechanical contouring employed. Figure 9. a) Surface-water s a l i n i t i e s at high tide on ebb between Canoe Pass and the f e r r y causeway„on June 8, 1978. S t i p p l e d area indicates region where a h a l o c l i n e i s present i n the water column.. Mecha-n i c a l contouring employed. Three representative s a l i n i t y / .'tempera-ture~profiles included. -b) Surface substrate and shallow water s a l i n i t i e s on approaching low tide on June 8, 1978. Mechanical contouring employed. 197; i s o h a l i n e (Appendix 8 ) and these waters would almost c e r t a i n l y have the same s a l i n i t y as the surface substrate waters at low t i d e , had i t been possible to continue sampling u n t i l low water). The contours i n Figures 9a and b are almost coincident, the only difference being that the two lobes of low s a l i n i t y water, one from Canoe Pass and the other from the smaller d i s t r i b u t a r y channels of the Brunswick Point marsh, have extended s l i g h t l y further to the east during the late stages of ebb. Thus, at l e a s t i n t h i s case, surface substrate s a l i n i t i e s at low t i d e are a very close r e f l e c t i o n : : of surface water s a l i n i t i e s at high tide on ebb. I t i s not possible on the basis of Figure 8 or 3 to q u a n t i t a t i v e l y define or contour the t r a n s i t i o n from marine to brackish environments, because the data does not divide n a t u r a l l y into two groups. However, i t i s possible to p r e c i s e l y define brackish and marine water masses on the basis of the s a l i n i t y p r o f i l e s , the boundary between them being defined by the h a l o c l i n e . This l i e s anywhere between 15 and 20%» (Fig. Appendix 8 ). Three d i f f e r e n t values were tested to define the boundary between the brackish and marine water masses, namely 15%o , 17.5%,, and 20% 0 . As a measure of 'marines." ness' the percent thickness of the s a l t wedge at each s t a t i o n was calculated for each of the three boundary d e f i n i t i o n s (percent thickness rather than actual thickness was calculated to eliminate v a r i a t i o n due to v a r i a b i l i t y i n water depth across the i n t e r t i d a l zone). The three sets of values were contoured. Figure 10 i s the r e s u l t f o r 17.5%„. Although the values at i n d i -v i d u a l stations v a r i e d depending on which of the three d e f i n i t i o n s was used, the contours remained very s i m i l a r i n l o c a t i o n and pattern (Appendix 8 ), and i t does not appear to be important to define the boundary between marine and brackish water masses to within c l o s e r than 5%». Figure .10, of course, only represents one part of one t i d a l cycle on one day at one p a r t i c u l a r time of year. However, the data are taken from that part of the t i d a l .cycle which 198 /Figure 10. Percent thickness of the s a l t wedge, for 17.5%. as the boundary between marine and brackish water masses. Mechanical contouring. Numbers next to stations i n d i c a t e percent thick-ness of s a l t wedge. :-J"99 determines surface substrate s a l i n i t i e s at low t i d e , which are p a r t i c u l a r l y important to benthic organisms dependent on the surface environment, and i t i s representative of a f a i r l y c r i t i c a l time of year when the Fraser i s at i t s peak discharge. Discussion of S a l i n i t y Regime Figure 10 i l l u s t r a t e s the abrupt t r a n s i t i o n from a marine to brackish s a l i n i t y regime between the Coalport causeway and Canoe Pass. The exact point of t r a n s i t i o n between 'marine' and brackish environments can a r b i t r a r i l y be defined at the 50% contour i n Figure 10. The cause of the abrupt change i s thought to be due to the presence of a topographic high between Canoe Pass and the Coalport causeway ( F i g . 3) which divides the two environments and prevents low s a l i n i t y water i n Canoe Pass from flooding over towards the causeway at the beginning of flood t i d e . At the early stages of flood t i d e , S t r a i t of Georgia water, which i s probably of high s a l i n i t y , floods i n around the Coalport from the southeast (Beak-Hinton, 1977) into the topographic depression on the immediate northwest side of the causeway (Fig. 3), estabr-. . l i s h i n g a s a l i n e wedge i n the area before low s a l i n i t y water can flood over from Canoe Pass. The southeasterly t i d a l currents on flood would also tend to prevent the low s a l i n i t y waters of Canoe Pass from reaching t h i s area. However, i n the immediate v i c i n i t y of the Brunswick Point marsh, d i s t r i b u t a r y channels from Canoe Pass cut through the marsh and discharge low s a l i n i t y water over t h i s topographic high e s t a b l i s h i n g a low s a l i n i t y plume i n front of the Brunswick Point marsh (Fig. 9, 10). These d i s t r i b u t a r y channels are unusual i n that they dry out during low t i d e , but act as r i v e r channels during high tide stages. As the ti d e floods into Canoe Pass r i v e r water i s diverted i n t o these d i s t r i b u t a r i e s and floods along them to meet the incoming sea. 200-J Strong seaward flowing currents continue during ebb tide as evidenced by the seaward o r i e n t a t i o n of bedforms (dunes and ri p p l e s ) at the mouth of the main channel at low t i d e . At low t i d e , small streams i n the channels drain water back towards Canoe Pass, i n d i c a t i n g that the seaward mouths of the channels l i e at higher elevations than t h e i r entrances at Canoe Pass. Thus, the dominant seaward flow i n the channels on both flood and ebb tides must be maintained by a difference i n elevation between the water surface i n Canoe Pass and sea l e v e l over Roberts Bank, that of Canoe Pass being s l i g h t l y higher. This i s a p e r f e c t l y reasonable suggestion to make as there must be a difference i n hydraulic head between Canoe Pass and the sea i n order to maintain r i v e r flow. These d i s t r i b u t a r y channels are probably e s s e n t i a l for the development and maintenance of the brackish marsh at Brunswick Point, as they e s t a b l i s h a buffer zone of brackish water between the marsh and the 'marine' area immediately offshore. The recent advance of the marsh at Brunswick Point (Moody, 1978)»is not simply a function of elevation, because, i f i t were, marsh would mantle the en t i r e a l g a l mat zone southeast of th i s area. The diversion of fresh water flow from Canoe Pass to :this area has probably been e s s e n t i a l f o r brackish marsh expansion. DISTRIBUTION OF THALASSINIDEAN SHRIMPS Description The d i s t r i b u t i o n of thalassinidean burrowing shrimps i s presented i n Figure 11, based on hovercraft and h e l i c o p t e r sampling surveys i n August, 1977, June, 1978, and August, 1978. Stations sampled on foot i n September, 1977 and on a transect i n September, 1976 (Swinbanks and Murray, 1977) are also included to maximize coverage. Results from the inter-causeway area are included to give a complete p i c t u r e of shrimp d i s t r i b u t i o n on Roberts Bank. Table I I I compares Callianassa and Upogebia densities at ten stati o n s sampled 201 — 3 — 1 Contour of Colllonoiwi burrow opening rjemlry m - 2 U.S.A. Figure 11. D i s t r i b u t i o n of thalassinidean shrimp burrow openings on Roberts Bank based on data c o l l e c t e d i n 1977 and 1978. ' Where stations sampled i n 19 77 were reoccupied i n 1978 the average density has been used. Data for the inter-causeway area are presented i n Part 4A. Mechanical contouring employed. TABLE III Comparison of Thalassinidean Shrimp Densities at Stations Sampled i n 1977 and Reoccupied i n 1978 August, 19 77 June, 1978 August, 1978 St. C i l l : LaiiasSan 'Upogebia St. Callianassa . Upogebia st-.-,. :'CGall±anas s ay RA1 0 0 RBI 0 0 - -RA2 0.5 ± 0.6 0 RB2 0.5 ± 0.7 0 - -RA3 0 0 RB3 0 0 - -RA4 0.5 ± 0.7 0 RB4 0.5 ± 0.7 0 - -RA5 35.5 ± 6.4 3.5 ± 3.5 RB5 20.5 ± 7.8 12.5 ± 2.1 RC12 26.0 ± 10.0 RA7 60.5 ± 2.1 0 RB6 51.5 ± 2.1 0 - -RA8 1.0 ± 0.8 0 RB7 24.0 ± 2.8 0 - -RA9 36.5 ± 4.9 0 - - - RC9 • 11.0 ± 5.1 RA10 0 0 _ _ RC6 1.0 ± 1.9 2.0 ± 2.3 0 0 Note: Stations i n June, 1978 are coincident within 50 m with those occupied i n August, 1977. Stations occupied i n August, 1978 l i e within 100 m of those occupied i n August, 1977. 203 ;• i n August, 1977 and reoccupied i n June, 1978 and/or August, 1978. The agree-ment between the three sets of data i s reasonably close, with the exception of stations RA8 and RB7, where Callianassa d e n s i t i e s are markedly d i f f e r e n t -2 -2 (1 m at RA8, 24 m at RB7). The general s i m i l a r i t y between the three sets of data suggests that thalassinidean shrimps maintain f a i r l y stable popurr : n . ; l a t i o n s , which might be expected as they are probably l o n g - l i v e d organisms (MacGinitie, 1930 and 1934). There i s a pronounced gradient i n Callianassa density i n the v i c i n i t y of s t a t i o n RA8 and RB7 and the discrepancy between these two r e s u l t s may be the r e s u l t of sampling i n s l i g h t l y d i f f e r e n t l o c a - S . L „ tions. _2 High densities (>50 burrow openings m ) of Callianassa are r e s t r i c t e d to the marine section of Roberts Bank immediately north of the Coalport causeway and i n the inter-causeway area ( Fig. 11). The peak i n Callianassa d i s t r i b u t i o n s h i f t s to; lower i n t e r t i d a l l e v e l s as one moves northward from -2 the Coalport, and quite high densities (>20 burrow openings m ) of . :, i,,-' '.: Callianassa occur down to the -2.4 m l e v e l (Geodetic Datum) - low water mark on August 17 and 18, 1978. As Main Channel i s approached Callianassa densi-_2 t i e s decrease to le s s than 0.5 m (burrow openings) at a l l i n t e r t i d a l l e v e l s . Upogebia were only recorded at two stations on the g r i d , and they are r e s t r i c t e d to the 'marine' area between the Brunswick Point marsh and the causeway. Relationship Between Shrimp Density and Substrate Parameters Stations were grouped i n t o 0.6 m elevation class i n t e r v a l s on the basis of the Swan Wooster (1967) topographic map (Fig. 3), to analyze r e l a t i o n s h i p s between Callianassa density and the substrate parameters of percent mud and s a l i n i t y . Data on Upogebia are i n s u f f i c i e n t to carry out any s t a t i s t i c a l 204 ' a n a l y s i s . Mud content data for 1977 and ly 78 were pooled, because the general lack of laminated deposits at s t a t i o n s suggests that mud contents do not vary appreciably with time. On the other hand, substrate s a l i n i t y does show>iq.uite d r a s t i c temporal v a r i a t i o n s (Figs. 8, 9; Appendix 8 ), and for t h i s reason the data for August, 1977 were i n i t i a l l y treated separately from that f o r August, 1978. Linear regression analysis reveals no s i g n i f i c a n t c o r r e l a t i o n between percent mud and Callianassa density (Table IV; Appendix 9 ). Correlation c o e f f i c i e n t s were less than 0.6, none were s i g n i f i c a n t at the 95% confidence l e v e l (r t e s t ) , and four out of f i v e were not even s i g n i f i c a n t at the 80% l e v e l . Segregating the data into 1977 and 1978 groups did not improve corre-l a t i o n s (Table IV). On the other hand, substrate s a l i n i t y i s p o s i t i v e l y and s i g n i f i c a n t l y correlated to Callianassa density. Five out of eight class i n t e r v a l s show p o s i t i v e c o r r e l a t i o n s , which are s i g n i f i c a n t at the 95% c o n f i -dence l e v e l ( Fig. 12B, C, F, H and J ) . I n t e r e s t i n g l y , the only class i n t e r v a l which showed a negative c o r r e l a t i o n (Fig. 12G) contains a s t a t i o n from the eelgrass zone next to the Coalport, where s a l i n i t i e s are high but Callianassa are absent. Exclusion of t h i s s t a t i o n r e s u l t s i n a p o s i t i v e c o r r e l a t i o n c o e f f i c i e n t (r=0.383). Unclassed data for 1977 (Fig. 12E), 1978 (Fig. 12K) and 1977/78 (Fig. 12L) also give s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n s . Pooling the classed data for 1977/78 resulted i n s i g n i f i c a n t l y reduced c o r r e l a t i o n c o e f f i c i e n t s (Appendix 9 ) because substrate s a l i n i t i e s i n August, 1978 were s i g n i f i c a n t l y lower than those at comparable stations i n August, 1977 (Fig. 9; Appendix 8. ). . . In order to further test the hypothesis that Callianassa d i s t r i b u t i o n i s correlated to s a l i n i t y , s i x stations were located on foot at a fixed t i d a l l e v e l (-1.55 m Geodetic Datum) between Canoe Pass and the Coalport ( F i g . 2) i n an area showing a pronounced s a l i n i t y gradient. The grain s i z e of the substrate does not vary appreciably between the stations (Table V)., , and l i e s 205 1 9 7 7 - M l to - 0 8 0 40-E 30->. «• 20-C & 10-0 10 20 30 40 S a l i n i t y °fco 0 10 20 30 40 S a l i n i t y "loo 60-50-1 30H 20-\04 -0.80 ro -0.20 n o r • f'•- i 1 1 0 10 20 30 40 S a l i n i t y %o -0.20 to —0.41m a 0 10 20 30 40 S a l i n i t y %o S a l i n i t y %o 19 7 8 -2.45 to -2.02 m 7 40 E . 30H ioH 0' 10 20 30 40 S a l i n i t y % ° 50-40-E 30-X «• *5 e • a 20--2J02 to -141 m -1.41 to'-OjBOm 4 Station in Mrcrass bad 0 10 20 30 40 S a l i n i t y %o 10 20 30 40 S a l i n i t y %o -0.80 to-0.20 fe40. e 20 a 10 10 20 20 S a l i n i t y %o 0 10 20 30 40 S a l i n i t y % o 0 10 20 30 40 S a l i n i t y %o ure 12. Relationship between Callianassa burrow opening density and the s a l i n i t y of surface substrate waters. B e s t - f i t l i n e a r regression l i n e s are drawn along with t h e i r correlation:.coefficients (r) and s i g n i f i c a n c e l e v e l (r t e s t ) . TABLE I V C o m p a r i s o n o f C o r r e l a t i o n C o e f f i c i e n t s ( r ) Be tween C a l l i a n a s s a Bu r row Open ing D e n s i t y and P e r c e n t Mud U s i n g P o o l e d and U n p o o l e d P e r c e n t Mud D a t a E l e v a t i o n I n t e r v a l P o o l e d P e r c e n t Mud D a t a U n p o o l e d P e r c e n t Mud D a t a ( G e o d e t i c Datum, m) 1 9 7 7 / 7 8 1977 1978 r r t e s t (%) r r t e s t (%) r r t e s t (%) - 2 . 4 5 to - 2 . 0 2 0 . 5 7 5 80 I n s u f f i c i e n t Da ta (N=2) 0 . 886 95 - 2 . 0 2 to - 1 . 4 1 - 0 . 2 5 4 <80 - 0 . 1 6 9 <30 - 0 . 4 8 1 <80 - 1 . 4 1 to - 0 . 8 0 - 0 . 2 0 1 <80 - 0 . 2 0 1 <80 No D a t a - 0 . 8 0 to - 0 . 2 0 - 0 . 1 9 9 <80 - 0 . 2 3 0 <80 I n s u f f i c i e n t D a t a (N=l) - 0 . 2 0 t o +0 .41 0 . 0 5 3 <80 0 . 0 5 3 <80 No D a t a U n c l a s s e d D a t a - 0 . 1 7 2 <80 - 0 . 1 8 9 <80 - 0 . 3 2 2 <80 TABLE V Relationship Between Surface Substrate S a l i n i t y and Callianassa Burrow Opening Density at a Fixed T i d a l Level at Six Stations Midway Between Canoe Pass and the Coalport Causeway (Fig. 2)„ Station Median (0) Mud Content (0) Surface Substrate S a l i n i t y (%<>) Density Cm"2) September. 10 September 11 1 2.94 7.39 24.5 21 23.50 ± 4.4 2 2.73 5.41 24.5 21 19.25 ± 2.4 3 2.60 10.48 18.0 19 10.00 ± 1.4 4 2.73 6.09 17.5 19 10.50 ± 4.1 5 2.96 17.94 12.5 9 - 6.75 ± 1.7 6 3.36 35.93 12.5 9 5.00 ± 0.8 Note: Burrow opening density (m - 2) estimated by taking 16 readings with a 0.25 m' quadrat. 20:8 w e l l within the range of median grain s i z e and percent mud known to be acceptable to Callianassa i n the inter-causeway area (Part 4A). Although the s a l i n i t y values d i f f e r e d on the two days monitored there was a consistent decrease in.^.alinity.^from^stations'-!;, to '6... accompanied by a decrease i n Callianassa density ( F i g . 13a). S a l i n i t y p r o f i l e s , taken with the apparatus described i n Appendix 5 , revealed that high s a l i n i t y water (>20%o) i s present within 15 cm of the surface, even at s t a t i o n s 5 and 6, where surface s a l i n i t i e s are low (Fig. 13b). Thus, adult C a l l i a n a s s a , which l i v e at depths greater than 30 cm on Roberts Bank, encounter these high s a l i n i t y i n t e r s t i t i a l waters which are free to enter t h e i r unlined burrows (L. Thompson and * .-'• -. P r i t c h a r d , 196y), and they are not d i r e c t l y exposed to the low s a l i n i t y surface waters. Discussion of Thalassinidean Shrimp D i s t r i b u t i o n The absence of Upogebia from northern Roberts Bank cannot be accounted for i n terms of the mud contents of the substrate or t i d a l e l e v a tion. The t i d a l f l a t s immediately i n front of the Westham Island marsh are upper aqua-zonal to lower amphizonal i n exposure and the sediments contain about 50% mud. Under the same conditionsrof exposure and mud content Upogebia a t t a i n d e n s i t i e s _2 of 84 burrow openings m i n the inter-causeway area (Part 4A) , while not one sing l e Upogebia burrow has been observed i n front of the Westham Island marsh. S i m i l a r l y , C allianassa d i s t r i b u t i o n on the brackish t i d a l f l a t s of Roberts Bank cannot be explained i n terms of t i d a l elevation or grain s i z e . Underf-standably, Callianassa i s absent i n the brackish marshes where dense r o o t l e t s probably render deposit feeding and burrowing impossible for Cal l i a n a s s a , and, because these marshes extend to much lower elevations than the saltmarshes of Boundary Bay and the inter-causeway area, the upper l i m i t to Callianassa 209 (a) 25 Scllnity on September 10, 1977 Selintty on September 11, 1977 IS SALINITY (%,) — I 26 (b) STl September 11, 1977 S a l i n i t y % c 25 15 20 -1 L 0 5 !0-15-1 ST4 S«pf e m b e r 10, 1977 S a l i n i t y %© 0 / 5-10-15-10 15 —L V 25 -J > \ 30 _ J S T 2 September 10, 1977 S a l i n i t y °/oo 10 _1_ 15 _ i _ 0 5 1 15-1 $ T 3 September 11. 1977 • a l l n l t y %o 0 5 10 !5 20 25 f \ I a. s a 5-10-15- ST3 September 11. 1977 S a l i n i t y %o 15J 10 _1 \ S T 6 September 11, 1977 S a l i n i t y %o 5-4 15-J 10 15 20 _J I Figure 13. a) Relationship between Callianassa burrow opening density and . surface substrate s a l i n i t y at a fi x e d t i d a l l e v e l (-1.55 m Geodetxc Datum), September 10-11, 1977. b) Substrate s a l i n i t y p r o f i l e s at Stations 1-6, September- 10-11, 210 d i s t r i b u t i o n i s i n e v i t a b l y lowered. But Callianassa are also absent or only present i n very low densities on the unvegetated t i d a l f l a t s i n front of Westham Island, were t i d a l elevation and mud contents of the sediment l i e w e l l within the ranges known to be acceptable to Callianassa i n the i n t e r -causeway area (Part 4A).. The p o s i t i v e c o r r e l a t i o n s between Callianassa density and substrate s a l i n i t y suggest that Callianassa d i s t r i b u t i o n can be explained oy substrate s a l i n i t y . The same i s also probably true of Upogebia, although i t cannot be demonstrated s t a t i s t i c a l l y , because nearly a l l of the _2 quadrat readings registered zero Upogebja ( i . e . , <0.5 burrow openings m ). Callianassa "caTlforniensis can tolerate s a l i n i t i e s down to about 10%, but does not have any capacity to osmoregulate i t s blood chloride l e v e l i n response to reduced s a l i n i t y (L. Thompson and P r i t c h a r d , 1969); s a l i n i t i e s lower than about 10% o are l e t h a l . UpOgebia pugettensis can t o l e r a t e s a l i n i ^ t i e s down to about 3.5%» CL. Thompson and P r i t c h a r d , 1969), and shows strong osmoregulatory c a p a c i t i e s i n s a l i n i t i e s below 26% 0, maintaining i t s blood chloride l e v e l above that of the surrounding medium (R. Thompson and P r i t c h a r d , 1969).. Upogebia constructs a mud-lined burrow which opens d i r e c t l y to the surface (R.:"Thompson'.and P r i t c h a r d , 1969; Swinbanks, 1977). In contrast, Callianassa burrows lack a l i n i n g and have constricted apertural necks (R. Thompson and P r i t c h a r d , 1969; Swinbanks,.1979.) . Upogebia c i r c u l a t e s water through i t s burrow for r e s p i r a t o r y and feeding purposes (MacGinitie, 1930; Thompson, 1972), and thus the s a l i n i t y of water within the burrows i s s i m i l a r to that i n t i d a l pools at the surface CL. Thompson and P r i t c h a r d , .1969). Callianassa does not require to c i r c u l a t e surface waters through i t s burrow for the purposes of feeding as i t i s a deposit feeder (MacGinitie, 1934), nor does Callianassa need to for the purposes of r e s p i r a t i o n during exposure, because Callianassa can t o l e r a t e up to f i v e days of continuous anoxia (R. Thompson and P r i t c h a r d , 1969). Thus, because of i t s mode of l i f e , i t s 211 "% tolerance of anoxia and i t s unlined burrow, Callianassa can t o l e r a t e low s a l i n i t y surface waters (L. Thompson and P r i t c h a r d , 1969/. In contrast, Upogebia i s dependent on surface waters for suspension feeding and r e s p i r a t i o n , and i t s burrow l i n i n g i s probably impermeable to i n t e r s t i t i a l waters. Thus, despite being p h y s i o l o g i c a l l y better adapted to cope with reduced s a l i n i t y , Upogebia demonstrates lower tolerance of low s a l i n i t y water i n i t s d i s t r i -bution than Ca l l i a n a s s a , as can be seen by comparing Figure 11 with Figures 8, 9 and 10. Why then do Callianassa densities decrease i n areas of reduced surface substrate s a l i n i t y when s a l i n i t i e s at depth remain high? The answer to t h i s probably l i e s i n reproduction. Callianassa 3has a p l a n k t i c l a r v a l stage (MacGinitie, 1934). Highest shrimp m o r t a l i t y probably occurs i n the f i r s t few hours a f t e r the larvae s e t t l e onto the substrate as p o s t l a r v a l shrimps and before they manage to burrow deep into the substrate, because they are then at the mercy of predators and the environment (MacGinitie, 1934/. I f one can assume that p o s t l a r v a l C a l l i a n a s s a , l i k e the adults, cannot t o l e r a t e s a l i n i t i e s below 10%o for any length of time, then one might expect higher p o s t l a r v a l shrimp m o r t a l i t y i n areas experiencing low surface substrate s a l i n i t i e s , and hence reduced adult populations. However, t h i s assumption may not be e n t i r e l y j u s t i f i e d because Felder (.1978) has found l i m i t e d e v i -dence to suggest that j u v e n i l e ' C a l l i a n a s s a islagfaride can t o l e r a t e s a l i n i t i e s as low as 5%o, whereas adults die i n s a l i n i t i e s below about 15%«. Despite t h i s , the explanation outlined above may s t i l l be v a l i d even i f p o s t l a r v a l Callianassa c a l i f o r n i e n s i s )haye a l e t h a l l i m i t lower than 10%.. P o s t l a r v a l Upogebia form small 'Y' burrows i n the surface sediments for suspension feeding (Thompson, 1972). In areas where surface substrate s a l i n i t y can drop below 3.5%o mortality among p o s t l a r v a l Upogebia, i s probably high, as they must be e n t i r e l y dependent on surface substrate waters for suspension feeding ' 212-and r e s p i r a t i o n . The possibly must be entertained that the c o r r e l a t i o n between t h a l a s s i -nidean shrimp d i s t r i b u t i o n and substrate s a l i n i t y i s caused by some other factor c l o s e l y i n t e r r e l a t e d to s a l i n i t y , f o r example t u r b i d i t y , the concent-r a t i o n of suspended sediments and/or gross sedimentation rates., S y v i t s k i (19 78) has demonstrated that suspended sediment concentration and sedimen-t a t i o n rates are negatively correlated with surface s a l i n i t y i n Howe Sound, B.C. The decrease i n Cal l i a n a s s a and Upogebia densities accompanying decreases i n s a l i n i t y could perhaps be caused by increases i n any or a l l three of the factors mentioned above. High t u r b i d i t y l e v e l s might r e s u l t i n low primary pr o d u c t i v i t y due to reduced l i g h t a v a i l a b i l i t y , r e s u l t i n g d i r e c t l y or i n d i r e c t l y i n a reduced food supply f o r l a r v a l shrimps. Or l a r v a l or p o s t l a r v a l shrimp mortality might be high i n areas where suspended sediment concentrations are excessively high, because, for example, high fluxes of suspended sediments may clog the f i l t e r i n g mechanisms of p o s t l a r v a l Upogebia preventing e f f i c i e n t feeding. Or p o s t l a r v a l shrimp m o r t a l i t y may be high i n areas where high gross sedimentation rates may r e s u l t i n j u v e n i l e shrimps being smothered to death. Callianassa are absent from the Z_. marina bed adjacent to the Coalport, much as reported for the Z. marina beds i n Boundary Bay (Swinbanks, 1979) and the inter-causeway area (Part 4A). This r e s u l t s i n an anomalous negative c o r r e l a t i o n between Callianassa density and substrate s a l i n i t y i n Figure 12G. On the brackish t i d a l f l a t s , i n the absence of eelgrass, Callianassa extend to lower i n t e r t i d a l l e v e l s . ,213" BURROW GEOMETRY Tn: Boundary^BayJ C a l l i a n a s s a constructs u n l i n e d feeding burrows which extend 20 to 30 cm down i n t o the s u b s t r a t e , and then branch h o r i z o n t a l l y f o r distances of up to a meter (Swinbanks, 1979; Fig.,014)•'.(< Each'system has two, -three or r a r e l y four e x i t s w i t h c o n s t r i c t e d a p e r t u r a l necks which meet as a bulbous chamber at 5 cm to 10 cm depth. Branching i s dichotomous, and there are bulbous turnarounds w i t h i n the system. The Upogebia burrow i s a 'Y' tube„extending down to depths of 50 to 60 cm (Swinbanks, 1979;-'Fig. 14). Thompson (1972) reports burrow depths as great as 90 cm. In cont r a s t to C a l l i a n a s s a burrows, Upogebia burrows are predominantly v e r t i c a l l y o r i e n t e d , do not have c o n s t r i c t e d entrances, and lack bulbous turnarounds ( F i g . 14). The i n t e r n a l w a l l s of the burrow are smooth and l i n e d w i t h mud. Upogebia burrows appear to be permanent d w e l l i n g burrows (Thompsohy.,1972; Swinbanks, 1979). Thompson (1972) has demonstrated that Upogebia secretes mucus from i t s hind-gut gland to cement the w a l l s of the burrow. However, Thompson s u r p r i s i n g l y a s s e r t s that "no p a r t i c u l a r s i z e c l a s s of sediment p a r t i c l e i s se l e c t e d by the shrimp to b u i l d the burrow," (1972,- p . i i i ) d e s p i t e the f a c t that h i s own g r a i n s i z e data demonstrates that Upogebia burrow l i n i n g s contain between one and a h a l f to f i v e times as much mud (>4.0 0) as the surrounding s u b s t r a t e . In a d d i t i o n , the S.E.M. micrographs presented by Thompson (1972) c l e a r l y i l l u s t r a t e a high con c e n t r a t i o n of p l a t y c l a y minerals i n the inner burrow l i n i n g , and equant sand gr a i n s i n the outer l i n i n g . The cohesive mud almost c e r t a i n l y aids i n w a l l support. The sand grains form a knobbly e x t e r i o r to the burrows ( F i g . 14), resembling the trac e f o s s i l Ophiomorpha (Thompson, 1972). The value of d i s t i n g u i s h i n g between the trac e f o s s i l genera Ophiomorpha and Thalassinoides has r e c e n t l y been questioned ( F u r s i c h , 1973) because there C a l l i a n a s s a Burrow Uppgfibiq Burrow c o n s t r i c t e d u n c o n s t r i c t e d a p e r t u r a l B o u n d a r y B a y a n d I n t e r c a u s e w a y T i d a l F l a t s e n t r a n c e n e c k Figure 14. Typ i c a l geometry of Callianassa and Upogebia burrows on the Fraser Delta. Dimensions for cross-section of Upogebia burrow l i n i n g obtained from Thompson (1972). 215 i s trace f o s s i l evidence that the same organism can produce both Thalassinoides and Ophiomorpha wit h i n the same burrow (Kennedy and MacDougall, 1969; Kennedy and Sellwood, 1970). We have found evidence that Callianassa c a l i f o r n i e n s i s can produce both. A l l , but one, of the Cal l i a n a s s a casts r e t r i e v e d from the Fraser Delta have the c h a r a c t e r i s t i c smooth burrow surfaces of Thalassinoides. However, one cast taken x^ith a slower s e t t i n g r e s i n mixture impregnated the sandy walls of the burrow with r e s i n , and the r e s u l t i n g cast had a thick coating of sand, which has a knobbly surface reminiscent of Ophiomorpha "• ." . (Fig . 15a). This may be an a r t i f a c t of r e s i n casting or the r e s i n may have picked out subtle differences i n permeability.* i n the burrow walls whieh"could conceivably be Highlighted by cementation during diagenesis. On scraping o f f the sand coating the smooth walls, t y p i c a l of the other casts, i s revealed underneath (Fig. 16a). I f one of these burrows were i n f i l l e d with sediment of a d i f f e r e n t grain size from that of the walls then the r e s u l t i n g trace f o s s i l would be c l a s s i f i e d as Thalassinoides. A f o s s i l example was found i n a box core from Boundary Bay (Fig. 15b). Here a c a i l i a n a s s i d burrow enters a mud layer and has been i n f i l l e d with sand. However, i f a burrow such as i n Figure 15a was i n f i l l e d with sand of the same grain s i z e as the burrow walls, the sandy burrow walls with knobbly e x t e r i o r might form the out l i n e of the trace f o s s i l , and i t would be c l a s s i f i e d as Ophiomorpha. S i m i l a r l y the burrows of Upogebia could have the appearance of Thalassinoides or Ophiomorpha depending on whether the smooth inner or knobbly outer burrow wa l l i s accentuated by diagenesis. As Bromley and Frey put i t "inside every Ophiomorpha there i s a Thalassinoides i n the guise of a burrow cast" (1974, p. 330). To t h i s we would add that outside every Thalassinoides may l i e the ghost of an; Ophiomorpha (Fig..15a).; .Bromley and Frey (1974), however, s t i l l advocate d i s t i n g u i s h i n g the two and i n a subsequent paper Frey and Howard (1975) equate Upogebia a f f i n i s burrows with Thalassinoides and those of 216, Figure 15. a) Resin cast of a Callianassa burrow coated i n sand with a knobbly surface reminiscent of Ophiomorpha. Note that at: top l e f t the sand l i n i n g has been worn away by the s t r i n g and the inner burrow l i n i n g i s smooth. b) F o s s i l i z e d C a l l i a n a s s a burrow having the appearance of Thalassinoides. Sand has i n f i l l e d a burrow i n mud. 217 2 1 8 Figure 16. Cast of a Callianassa burrow fallen from an area of high burrow density (446 burrow openings m ). Two c o n s t r i c t e d entrances meet as a bulbous chamber at about 10 cm depth, and a v e r t i c a l stem extends from t h i s to about 50 cm depth. Cryptomya c a l i f o r - n i c a c l u s t e r around the bulbous chamber at the junction of the two e x i t s . About t h i r t y of these commensal bivalves are attached to the cast. The burrow i s occupied by one shrimp. A burrow system of smaller diameter branches o f f from t h i s system. I t i s joined to the main burrow system by a narrow c o n s t r i c t e d neck and i s occupied by a small juvenile shrimp. Scale i n centimeters. 219 U. pugettensis with Ophiomorpha. Apart from lacking knobs the burrows of these two species of Upogebia are very s i m i l a r . Both have smooth durable burrow l i n i n g s . We f e e l that the presence of knobs i s not s i g n i f i c a n t . 0 . A. far more important d i s t i n c t i o n , i s . that between, permanent dwelling :: - , burrows with f i r m impermeable l i n i n g s and open or short constricted e x i t s used for suspension feeding and r e s p i r a t i o n , such as those of JJ. pugettensis and JJ. a f f i n i s , and temporary feeding burrows with permeable l i n i n g s and c o n s t r i c t e d entrances, used for deposit feeding, l i k e those of Callianassa  c a l i f o r n i e n s i s , because, as has been demonstrated, burrow function has a profound bearing on the surface s a l i n i t y which the organism can t o l e r a t e . Upogebia, although p h y s i o l o g i c a l l y better adapted than Callianassa to t o l e r a t e reduced s a l i n i t i e s , i n f a c t demonstrates lower tolerance of brackish water because of the function of i t s burrow as a conduit for suspension > . feeding and r e s p i r a t i o n . Such d i s t i n c t i o n s form the basis of Seilacher's (1964) e t h o l o g i c a l c l a s s i f i c a t i o n of trace f o s s i l s . The continued use an ichnotaxonomic system for trace f o s s i l c l a s s i f i c a t i o n , based on morphology without regard for function, l i m i t s the usefulness of trace f o s s i l s i n paleoenvironmental i n t e r p r e t a t i o n s . Farrow (.1971) has demonstrated that c a l l i a n a s s i d burrows e x h i b i t markedly d i f f e r e n t geometries i n the d i f f e r e n t sedimentary environments of an a t o l l . C allianassa burrows show s i g n i f i c a n t v a r i a b i l i t y i n geometry on d i f f e r e n t parts of the Fraser Delta. In most areas Ca l l i a n a s s a burrows are predominantly h o r i z o n t a l l y oriented. However, i n areas of very high burrow density.on the inter-causeway t i d a l f l a t C a l lianassa constructs burrows which are v e r t i c a l l y oriented (.Fig. 16). This i s probably a r e s u l t of population pressure f o r c i n g the shrimps to mine v e r t i c a l l y . On c e n t r a l and northern Roberts Bank Callianassa construct burrows with unusually long, c o n s t r i c t e d apertural necks, which extend about 30 cm down into the substrate (Fig. 14). Beyond -220 th i s point box cores have revealed that the burrows appear to have s i m i l a r geometry to those found i n Boundary Bay. However, the long c o n s t r i c t e d apertural necks have proved to be an insurmountable b a r r i e r to r e s i n , and casts have not been obtained from t h i s area. There are a number of possible reasons for the construction of long c o n s t r i c t e d apertural necks. Callianassa may be avoiding the upper 5-10 cm of the substrate, which i s constantly reworked by currents and waves i n the r e l a t i v e l y high energy environment of t h i s part of Robert's Bank, and/or i t may be burrowing deeper to reach regions of stable s a l i n i t y . The long c o n s t r i c t e d necks may also r e f l e c t a change i n feeding behaviour on the part of C a l l i a n a s s a . In the Boundary Bay and inter-causeway areas the bulbous chamber close to the surface may perhaps be used for suspension feeding, at high t i d e , as a supplement to Callianassa's deposit feeding a c t i v i t i e s (Swinbanks, 1979). On northern and c e n t r a l Roberts Bank, where the environmental energy i s high and the s a l i n i t y regime unstable, Callianassa probably abandons near surface feeding, and establishes i t s main burrow complex at greater than 30 cm depth. Upogebia burrows e x h i b i t s i m i l a r geometry on a l l parts of the Fraser Delta (Fig. 14). However, i n areas where currents rework the surface of the substrate, Upogebia construct short constricted apertural necks to t h e i r burrows. Thompson (1972) reports that Upogebia burrows taper towards the surface. The presence or absence of c o n s t r i c t i o n i s probably a function of environmental energy, c o n s t r i c t i o n s being formed when currents rework the surface. The degree of c o n s t r i c t i o n and the length of co n s t r i c t e d sections, however, never approaches that of Callianassa burrows, and i t seems that when conditions are favourable, Upogebia prefers no c o n s t r i c t i o n s . i n its.burrow. • 22.1; REVIEW AND CONCLUSIONS Figure 17 summarizes a l l the ava i l a b l e data on thalassinidean shrimp d i s t r i b u t i o n by i l l u s t r a t i n g the s t r a t i g r a p h i c succession of thalassinidean burrow density and fl o r a l / s e d i m e n t o l o g i c a l zones to be expected i f the Fraser Delta progrades seawards without subsidence. The contours of thalassinidean burrow density i n Figure 17, based on l i v i n g population denr;( \} sity,iar§ intended to be used as a q u a l i t a t i v e guide to the density of thalassinidean trace f o s s i l s to be expected i n the geological record. The d i s t r i b u t i o n of fl o r a l / s e d i m e n t o l o g i c a l zones i s c r i t i c a l to t h a l a s s i -nidean shrimp d i s t r i b u t i o n . In terms of both Geodetic elevation and t i d a l elevation (as defined by exposure zones) the a l g a l mat and eelgrass zones on the 'marine' t i d a l f l a t s of Roberts Bank l i e at lower elevations than t h e i r equivalents i n Boundary Bay. This i s probably due to differences i n the nature of t i d a l channel drainage and substrate i n the two areas. On the Boundary Bay t i d a l fiatsefehe substrate consists e n t i r e l y of sand and the main t i d a l channel system i s confined to the region below mean s.ea l e v e l , extending into s u b t i d a l regions. The upper reaches of the channels are broad and s h a l -low, producing depressions i n which submergence duration i s enhanced and i n which Z. marina beds can a t t a i n higher t i d a l elevations than on Roberts Bank (Swinbanks, 1979) . On Roberts Bank the main t i d a l channel system occurs above mean sea l e v e l as a d e n d r i t i c system of small channels draining water from the muds of the upper t i d a l f l a t s . Plateaus between the drainage channels experience enhanced exposure favourable to the development of blue-green a l g a l mats and,.probably as a r e s u l t , the a l g a l mat zone extends to lower i n t e r t i d a l l e v e l s . On Roberts Bank the saltmarsh zone sets the upper l i m i t to Cal l i a n a s s a d i s t r i b u t i o n , and dense Z_. marina,, growth appears to l i m i t t h e i r population at low i n t e r t i d a l l e v e l s , while the highest densities of Callianassa occur i n the non-yegetated sandflat and causeway zones. In Boundary Bay Upogebia Figure 17. Summary of the d i s t r i b u t i o n of thalassinidean burrows and floral/sedimentological zones on a l l the t i d a l f l a t s of the Fraser Delta south of Main Channel, i n the form a s t r a t i g r a p h i c succession, constructed by projecting a l l the density data i n Figure 11 onto a v e r t i c a l plane passing through points A, B and C i n Figure 11. The data on percent thickness of the s a l t wedge i n Figure 10 have; also been included to demonstrate the r e l a t i o n s h i p between thalassinidean shrimp d i s t r i b u t i o n and s a l i n i t y regime. No data on the s a l t wedge'are available NW of Canoe Pass. Data from Boundary Bay are based on Transect A alone (Swinbanks, 1979). Exposure zones allow cross c o r r e l a t i o n between Roberts Bank and Boundary Bay. Winter t i d a l data are not available for Boundary Bay and as a r e s u l t the upper l i m i t of the atmozone cannot be defined, but the spring t i d a l l e v e l s for June indicated allow cross c o r r e l a t i o n i n the uppermost i n t e r t i d a l regions. to rO N O R T H E R N A N D C E N T R A L R O B E R T S B A N K I N T E R C A U S E W A Y T I D A L F L A T E X P O S U R E B O U N D A R Y Z O N E S B A Y U p o a e b l q p r e s e n t 0 5 - 2 5 b u r r o w o p e n i n g s m -U p o p e b l a b u r r o w o p e n i n g d e n s i t y > 25 m ~ 2 C o n t o u r o f C o l l l o n o s s o b u r r o w o p e n i n g d e n s i t y m ~ 2 • 2 5 1 - P e r c e n t t h i c k n e s s o f t h e s a l t w e d g e ( > 1 7 . 5 % o . *' L e v e l o f S p r i n g H i g h e r H i g h W a t e r i n D e c e m b e r a t T s a w w a s s e n L e v e l o f S p r i n g H i g h e r H i g h W a t e r i n J u n o a t T s a w w a s s e n L e v e l o f S p r i n g H i g h e r H i g h W a t e r i n J u n e a t B o u n d a r y B a y N o w i n t e r T i d a l D a t a a v a i l a b l e f o r B o u n d a r y B a y 224 are r e s t r i c t e d to the beds of dense Z. marina growth where the mud contents of the sediments are s u f f i c i e n t l y high (greater than about 2% mud). On Roberts Bank where mud contents of the sediments are an order of magnitude higher Upogebia span a wider elevation range extending up to the base of the upper amphizone, a l e v e l above which the maximum duration of anoxia due to exposure probably exceeds the l e t h a l l i m i t for postmolt Upogebia. In Boundary nay where the upper l i m i t of the a l g a l mat zone extends to a s l i g h t l y higher elevation than on Roberts Bank, Callianassa attains i t s p h y s i o l o g i c a l l i m i t i n elevation by extending up to, but not beyond, elevations experiencing a maximum of about f i v e days of continuous anoxia due to exposure. Between the Coalport and Canoe Pass on Roberts Bank a major t r a n s i t i o n i n f l o r a l / s e d i m e n t o l o g i c a l zonation occurs as a r e s u l t of an abrupt t r a n s i t i o n from a 'marine' to brackish s a l i n i t y regime. The a l g a l mat zone i s replaced by the lower part of the upper brackish marsh, while the lower brackish marsh, which consists predominantly of Scirpus americanus, becomes the l a t e r a l equi-valent of the upper h a l f of the sandflat zone. The eelgrass zone i s replaced by a sandflat/mudflat zone, which i s devoid of f l o r a l cover and crosscut by r i v e r channels, both a c t i v e and r e l i c t . In response to these changes, the peak i n Callianassa d i s t r i b u t i o n moves to lower i n t e r t i d a l l e v e l s , because of the presence of low s a l i n i t y water at higher t i d a l l e v e l s and because of the disappearance of eelgrass i n lower i n t e r t i d a l regions. The lower l i m i t of the brackish marsh forms the upper l i m i t to Callianassa d i s t r i b u t i o n . In close proximity to the major channels of the Fraser River, C a l l i a n a s s a are absent. Upogebia i s apparently more s e n s i t i v e to the environmental factors of grain s i z e and s a l i n i t y than Cal l i a n a s s a . Upogebia shows a d i s t i n c t prefe-rence for muddier substrates and cannot t o l e r a t e low s a l i n i t y , surface sub-s t r a t e waters. The presence of s u f f i c i e n t mud i n the substrate i s probably 225 e s s e n t i a l t o U p o g e b i a f o r t h e c o n s t r u c t i o n o f i t s m u d - l i n e d b u r r o w , a n d : t h e c i r c u l a t i o n o f s u r f a c e w a t e r s t h r o u g h i t s i m p e r m e a b l e m u d - l i n e d b u r r o w f o r s u s p e n s i o n f e e d i n g and r e s p i r a t i o n makes U p o g e b i a more s e n s i t i v e t h a n i: '. C a l l i a n a s s a t o changes i n s a l i n i t y r e g i m e , d e s p i t e t h e f a c t t h a t U p o g e b i a i s p h y s i o l o g i c a l l y b e t t e r a d a p t e d t o cope w i t h f l u c t u a t i n g s a l i n i t i e s . The p o s i t i v e c o r r e l a t i o n b e t w e e n C a l l i a n a s s a d e n s i t y and s u r f a c e s u b ^ s t r a t e s a l i n i t y , and t he d i s t r i b u t i o n p a t t e r n s o f C a l l i a n a s s a and U p o g e b i a s u g g e s t t h a t t he d e n s i t y o f f o s s i l b u r r o w s o f b o t h s h r i m p s i n t h e g e o l o g i c a l r e c o r d c o u l d be used as a q u a l i t a t i v e i n d i c a t i o n o f p a l e o s a l i n i t y , U p o g e b i a b u r r o w s b e i n g a more s e n s i t i v e i n d i c a t o r t h a n t h o s e o f C a l l i a n a s s a . The two t y p e s o f b u r r o w , u s e d i n c o n j u n c t i o n , s h o u l d f o r m a p o w e r f u l t o o l i n p a l e o e n v i r o n m e n t a l r e c o n s t r u c t i o n s . I n t h i s r e g a r d i t i s c o n s i d e r e d v e r y s i g n i f i c a n t t o d i s t i n g u i s h b e t w e e n m u d - l i n e d , i m p e r m e a b l e , pe rmanen t d w e l l i n g b u r r o w s w i t h open e n t r a n c e s u s e d f o r s u s p e n s i o n f e e d i n g ( e . g . , U p O g e b i a b u r r o w s ) and u n l i n e d , p e r m e a b l e , t e m p o r a r y f e e d i n g b u r r o w s w i t h l o n g c o n s t - -r i c t e d e n t r a n c e s , u s e d p r i m a r i l y f o r d e p o s i t - f e e d i n g ( e l g . , C a l I i a n a ' s s a ~ K u r r b w s F e c a u s e " these'" f a c t o r s i ' ^ d e t e r m i n e r w h e t h e r t he o c c u p a n t s h r i m p i s e x p o s e d t o o x y g e n a t e d s u r f a c e w a t e r s s u s c e p t i b l e t o f r e s h w a t e r i n c u r s i o n , o r w h e t h e r i t i s e x p o s e d t o h y p o x i c , h i g h s a l i n i t y i n t e r s t i t i a l w a t e r s . The d i s t i n c t i o n b e t w e e n T h a l a s s i n o i d e s and O p h i o m o r p h a , on t he o t h e r h a n d , i s c o n s i d e r e d t o be o f l i t t l e s i g n i f i c a n c e f r om a p a l e o e n v i r o n m e n t a l p o i n t o f v i e w , as b o t h U p o g e b i a and C a l l i a n a s s a c a n p r o b a b l y p r o d u c e b o t h t r a c e f o s s i l s unde r t h e r i g h t c i r c u m s t a n c e s , and t h e d i s t i n c t i o n may s i m p l y r e f l e c t t h e g r a i n s i z e o f t he b u r r o w f i l l i n g s e d i m e n t a n d / o r t h e mode o f d i a g e n e t i c enhancement o f t h e f o s s i l b u r r o w . On n o r t h e r n and c e n t r a l R o b e r t s Bank C a l l i a n a s s a c o n s t r u c t s b u r r o w s w i t h l o n g e r , more c o n s t r i c t e d a p e r t u r a l n e c k s t h a n i n t h e i n t e r -causeway o r B o u n d a r y Bay a r e a s . T h i s i s t h o u g h t t o r e f l e c t a change i n .> .• f e e d i n g mode, w i t h C a l l i a n a s s a a b a n d o n i n g n e a r s u r f a c e f e e d i n g , b e c a u s e o f 2 2 6 the i n s t a b i l i t y of the surface, which i s reworked by currents and exposed to fl u c t u a t i n g s a l i n i t i e s . ACKNOWLEDGEMENT S This research was financed by Geological Survey of Canada Contract D.S.S. No. 0SS77-08177 from the Department of Supply and Services, Ottawa, Ontario, Canada. We are indebted to Captain John MacGrath and a l l crew members at the Canadian Coastguard Hovercraft Unit at Vancouver International A i r p o r t for providing and operating the Hovercraft f o r f i e l d sampling. We thank Mr. W.J. Rapatz, Regional T i d a l Superintendent at Sidney, B.C. for providing observed t i d a l data from the t i d a l gauge at the Tsawwassen f e r r y terminal. Dr. J.P. Syvitski,.Mr. G.D. Hodge and Ms. N. Hayakawa ably as s i s t e d i n the f i e l d . Dr. M. Pomeroy i d e n t i f i e d the chlorophytes, cyanophytes and diatoms. Dr. W.C. Barnes and Dr. CD. Levings c r i t i c a l l y the manuscript. We thank Mrs. CM. Armstrong for d r a f t i n g the diagrams and Ms. N. Hayakawa for typing the s c r i p t . 227 REFERENCES Ages, A. Woollard, A., 1976, The tides i n the Fraser Estuary: Pac. Mar. ^ ScT. Rept. 76-5, 100 p. Beak-Hinton, 1977, Environmental impact assessment of Roberts Bank port expansion. Vol. 3, App. A and Vol. 4, App. B. The e x i s t i n g p h y s i c a l and b i o l o g i c a l environments: Beak Consultants Ltd., Vancouver. Borradaile, L.A., 1903, On the c l a s s i f i c a t i o n of the Thalassinidea: Ann. Mag. Nat. H i s t . , s e r i e s 7, 12, p. 534-551. Bromley, R.G. and Frey, R.W., 1974, Redescription of the trace f o s s i l Gyrolithes and taxonomic evaluation of Thalassinoides, Ophiomorpha and Spongeliomorpha: B u l l . geol. Soc. Denmark, v. 23, p. 311-335. Burgess, T.E., 1970, Foods and habitat of four anatinids wintering on the Fraser Delta t i d a l marshes: M.Sc. Thesis, Dept. of Zoology, University of B r i t i s h Columbia, 124 p. Farrow, G.E., J.971, Back-reef and lagoonal environments of Aldabra A t o l l distinguished by t h e i r crustacean burrows: Symp. Zool. Soc. London, v. 28, p. 455-500. Felder, D.L., 1978, Osmotic and i o n i c regulation i n several western A t l a n t i c Callianassidae (.Crustacea, Decapoda, Thalassinidea): B i o l . B u l l . , v. 154, p. 409-429. Frey, R.W. and Howard, J.D., 1975, Endobenthic adaptions of j u v e n i l e t h a l a s s i -nidean shrimp: B u l l . Geol. Soc. Denmark, v. 24, p. 283-297. Fursich, F.T., 1973, A r e v i s i o n of the trace f o s s i l s Spongeliomorpha, Ophiomorpha and Thalassinoides: N. Jb. Geol. Palaont. Mh., v. 12, p. 719-735. H i l l a b y , F.B. and Barrett, D.T., 1976, Vegetation communities of a Fraser River s a l t marsh: Environment Canada, Fi s h e r i e s and Marine Service, Tech. Rept. Series No. Pac/T-76-14. K e l l e r h a l s , P. and Murray, J.W., 1969, T i d a l f l a t s at Boundary Bay, Fraser River Delta, B r i t i s h Columbia: B u l l . Can. Pet. Geol., v. 17, p. 67-91. Kennedy, W.J. and MacDougall, J . , 1969, Crustacean burrows i n the Weald Clay of southeast England and t h e i r environmental s i g n i f i c a n c e : Palaeontology, /v. 12,-.p. 459. and Sellwood, B.W., 1970, Ophiomorpha nodosa, a marine i n d i c a t o r from the Sparnacian of southwest England: Geol. Assoc. P r o c , v. 8-1, p. 99-110. Levings, CD. and Coustalin, J.B., 1975, Zonation of i n t e r t i d a l biomass and rela t e d benthic data from Sturgeon and Roberts Bank, Fraser River estuary, B r i t i s h Columbia: Fisheries and Marine Service, Environment Canada, Tech. Rept. no. 458, 138 p. 228 Luternauer, J.L. and Murray, J.W., 1973, Sedimentation on the Western Delta-front of the Fraser River, B r i t i s h Columbia: Can. Jour. Earth .Sci., v. 10, p. 1642-1663. MacGinitie, G.E., 1930, The natural h i s t o r y of the mud shrimp Upogebia  pugettensis (Dana): Ann. Mag. nat. H i s t . , v. 6, p. 36-44. , 1934, The natural h i s t o r y of C a l l i a n a s s a c a l i f o r n i e n s i s Dana: Amer. Midi. N a t u r a l i s t , v. 15, p. 166-177. Medley, E. and Luternauer, J.L., 1976, Use of a e r i a l photographs to map sediment d i s t r i b u t i o n and to i d e n t i f y h i s t o r i c a l changes on a t i d a l f l a t : In Report of A c t i v i t i e s , Part C, Geol. Surv. Can., Paper 76-1C, p. 293-304. Moody, A.I., 1978, Growth and d i s t r i b u t i o n of the vegetation of a southern Fraser Delta marsh: ..unpub. M.Sc. t h e s i s , University of B r i t i s h Columbia, Vancouver, B.C., 153 p. O'Connell, G., 1975, F l o r a and fauna of Boundary Bay t i d a l f l a t s , B r i t i s h Columbia: unpub. report to B.C. Government P r o v i n c i a l Part Branch, V i c t o r i a , B.C. Parsons, CO., 1975, Vegetation pattern i n a s a l t marsh at Boundary Bay, B.C.: Lamda, v. 1(2); p. 45-52. Seilacher, A., 1964, Biogenic sedimentary structures: In Imbrie, J . , and Newell, N.D. (eds), Approaches to paleoecology, John Wiley, New York, p. 296-316. Shinn, E.A., 1968, Burrowing i n recent lime sediments of F l o r i d a and the Bahamas: Jour. Paleontology, v. 42, p. 879-894. Swan Wooster, 1967, Roberts Bank and Sturgeon Bank harbour study — topo-graphic maps. Prepared for Swan Wooster Engineering Co. Ltd. by Lockwood Survey Corporation, A p r i l , 1967. National Harbours Board, Vancouver, B.C. Swinbanks, D.D., 1979, Environmental factors c o n t r o l l i n g f l o r a l zonation and the d i s t r i b u t i o n of burrowing and tube-dwelling organisms on Fraser Delta t i d a l f l a t s , B r i t i s h Columbia: unpub. Ph.D. t h e s i s , University of B r i t i s h Columbia, Vancouver, B.C., 274 p. and Murray, J.W., 1977, Animal-sediment r e l a t i o n s h i p s of Boundary Bay and Roberts Bank t i d a l f l a t s , Fraser River Delta, B.C.: unpub. report to Dept. of Supply and Services, Ottawa, Ontario, Canada, 118 p. Thompson, L.C. and P r i t c h a r d , A.W., 1969, Osmoregulatory capacities of Callianassa and Upogebia (Crustacea: Thalassinidea): B i o l . B u l l . , v. 136, p. 114-129. Thompson, R.K., 1972, Functional morphology of the hind-gut of Upogebia  pugettensis (Crustacea, Thalassinidea) and i t s r o l e i n burrow construc-t i o n : unpub. Ph.D. t h e s i s , University of C a l i f o r n i a , Berkeley, 202 p. 229 Thompson, R.K. and P r i t c h a r d , A.W., 1969, Respiratory adaptions of two burrowing crustaceans, Callianassa c a l i f o r n i e n s i s and Upogebia pugettensis (Decapoda, Thalassinidea): B i o l . B u l l . , v. 136, p. 274-287. Waldichuk, M., 1957, Physical oceanography of the S t r a i t of Georgia, B r i t i s h Columbia: Jour. F i s h . Res. Brd. Canada, v. 14, p. 321-486. Weimer, R.J. and Hoyt, J.H., 1964, Burrows of Callianassa major Say as i n d i c a t o r s of l i t t o r a l and shallow n e r i t i c environments: Jour. Pa.laont.o-. logy, v. 38, p. 761-767. 230 SUMMARY AND CONCLUSION The subdivision of the i n t e r t i d a l region into exposure zones at extreme c r i t i c a l t i d a l l e v e l s i s advocated because 1) i t allows cross c o r r e l a t i o n between d i f f e r e n t t i d a l regions experiencing d i f f e r e n t types of astronomically c o n t r o l l e d tide 2) The c r i t i c a l t i d a l l e v e l s on which i t i s based may be causally r e l a t e d to i n t e r t i d a l zonation. Boundary Bay t i d a l f l a t s are unusual .because they demonstrate very l i m i t e d v a r i a b i l i t y i n substrate grain s i z e . As a r e s u l t a c l e a r f l o r a l / faunal zonation i s developed, c o n t r o l l e d p r i m a r i l y by elevation and exposure. There are f i v e f l o r a l / s e d i m e n t o l o g i c a l zones on the t i d a l f l a t s characterized by d i s t i n c t i v e macrofaunal assemblages. These are from the shoreline seaward: the saltmarsh, a l g a l mat, upper sand wave, eelgrass and lower sand wave zones. Topography of small and large scale of both p h y s i c a l and biogenic o r i g i n creates l a t e r a l heterogeneity within the b i o f a c i e s of each zone. Abarenicola p a c i f i c a i s abundant i n the upper sand wave zone and 6 3 t h i s polychaete annually reworks about 10 m of sand. Abarenicola has the capacity to s i z e - s o r t a sand/clay mixture by f l o a t i n g the clay out i n the head shaft i r r i g a t i o n current. The marine t i d a l f l a t s of southeastern Roberts Bank have comparable f l o r a l zones to those of Boundary Bay, but only four major floral/sedimento-l o g i c a l zones are present: saltmarsh, a l g a l mat, sandflat and eelgrass zones. The lower l i m i t of the saltmarsh l i e s at the same elevation as i n Boundary Bay; j u s t below the lower l i m i t of the upper atmozone. However, compared with Boundary Bay, the a l g a l mat zone extends to lower i n t e r t i d a l l e v e l s while the iZoster'ainarina beds do/not a t t a i n ^uch; high elevations.. , _This i s a t t r i b u t e d to differences i n the s t y l e of t i d a l channel drainage i n the two. areas,. which i n turn i s a .function of the grain s i z e of the substrate. 231 On the inter-causeway t i d a l f l a t l a t e r a l v a r i a t i o n s i n grain s i z e are pronounced and greatly influence thalassinidean shrimp d i s t r i b u t i o n . Thalassinidean shrimps a t t a i n t h e i r highest densities on these t i d a l f l a t s and there i s some evidence of a negative i n t e r a c t i o n between Callianassa and Upogebia which may be a form of trophic group ammensalism. In the area -2 of t h e i r peak density. (446 burrow openings m ) Cal l i a n a s s a rework the substrate they l i v e i n to a depth of 50 cm i n about f i v e months. • ' On c e n t r a l and northern Roberts Bank the t i d a l f l a t s of the Fraser Delta undergo.a major t r a n s i t i o n from a marine to a brackish environment due to the i n f l u x of freshwater from the Fraser River. The floral/sedimento-l o g i c a l zones of the t i d a l f l a t s are completely restructured. A brackish marsh zone l a t e r a l l y replaces the saltmarsh zone, the a l g a l mat zone and the upper h a l f of the sandflat zone while a sandflat/mudflat zone cross-cut by both active and r e l i c channels displaces the eelgrass zone and the lower h a l f of the sandflat zone. The brackish marsh extends well below mean sea l e v e l to about the upper l i m i t of the aquazone. In response to these changes the peak i n Callianassa d i s t r i b u t i o n shifts--to lower i n t e r t i d a l l e v e l s and Upogebia disappear altogether. Upogebia constructs a mud-lined permanent dwelling burrow f o r suspen-sion feeding and r e s p i r a t i o n purposes, whereas Ca l l i a n a s s a builds an unlined temporary feeding burrow f o r deposit feeding. Upogebia i s much more s e n s i t i v e to the environmental factors of grain s i z e , exposure and s a l i n i t y than Callianassa due l a r g e l y to the .nature and function of i t s burrow. Upogebia shows a d i s t i n c t preference f o r muddy substrates. I t probably requires mud for constructing i t s burrow. Upogebia does not extend above the upper l i m i t of the lower amphizone because immediately above t h i s l e v e l the maximum duration of exposure probably r e s u l t s i n a duration of anoxia l e t h a l to postmolt Upogebia. In contrast C a l l i a n a s s a extends up to lower atmozonal 232 elevations, because of i t s greater capacity to to l e r a t e anoxia. Anoxia tolerance i s an adaption probably e s s e n t i a l for l i f e i n an unlined burrow. Upogebia although p h y s i o l o g i c a l l y better adapted to cope with reduced s a l i n i t y i n fact demonstrates lower tolerance of low s a l i n i t y water i n i t s d i s t r i b u t i o n than Ca l l i a n a s s a , probably because the function of i t s burrow as a conduit f o r r e s p i r a t i o n and feeding subjects the shrimp to low s a l i n i t y surface waters, whereas Ca l l i a n a s s a i n i t s unlined burrow used for deposit feeding i s protected from low s a l i n i t y surface waters by high s a l i n i t y i n t e r s t i t i a l waters which are free to enter i t s unlined burrow. These two types of burrow with t h e i r d i f f e r r i n g s e n s i t i v i t y to the environmental factors of exposure duration and s a l i n i t y should form a powerful tool i n paleoenvironmental reconstructions when used i n conjunction. Tables I and II summarize the factors l i m i t i n g and i n f l u e n c i n g thalassinidean shrimp d i s t r i b u t i o n on Fraser Delta t i d a l f l a t s . Table I I I summarizes a l l nine f l o r a l and faunal d i s t r i b u t i o n a l l i m i t s on the Fraser Delta t i d a l f l a t s which have been found to l i e w i t h i n 15 cm or l e s s of an exposure zone boundary or any other extreme c r i t i c a l t i d a l l e v e l . I t i s only possible to o f f e r a convincing argument for t h e i r being a causal r e l a t i o n s h i p between these f a u n a l / f l o r a l l i m i t s and extreme c r i t i c a l t i d a l l e v e l s i n the case of Callianassa and Upogebia because pertinent physio-l o g i c a l data are only a v a i l a b l e for these organisms. However, the fact that so many d i f f e r e n t f l o r a l and faunal l i m i t s i n d i f f e r e n t environments of the del t a coincide with extreme c r i t i c a l t i d a l l e v e l s should at l e a s t warrant reconsideration of the tide factor hypothesis. LIMIT Upper l i m i t of Callianassa Lower l i m i t of Callianassa Upper l i m i t of Upogebia Lower l i m i t of Upogebia TABLE I SUMMARY OF ENVIRONMENTAL FACTORS LIMITING THALASSINIDEAN SHRIMP DISTRIBUTION ON FRASER DELTA TIDAL FLATS BOUNDARY BAY ELEVATION (Geodetic EXPOSURE ZONE Datum, m) +0.9 -0.6 -0.5 -1.3 Near upper l i m i t of Lower Atmozone Lower h a l f of Lower Amphizone Lower Amphizone Upper Aquazone LIMITING FACTOR >5 days anoxia due to exposure Dense Z. marina I n s u f f i c i e n t mud* In s u f f i c i e n t mud INTER-CAUSEWAY AREA AND AREA S.E. OF BRUNSWICK PT. ELEVATION (Geodetic Datum, m) +0.9 -1.8 to -1.9 0.0 <-2.2 EXPOSURE ZONE Lower Atmozone Near upper l i m i t of Lower Aquazone Upper l i m i t of Lower Amphizone Lower Aquazone LIMITING FACTOR Presence of saltmarsh Dense Z. marina Anoxia due to Level 2 Exposure Limit • unknown NORTHERN AND CENTRAL ROBERTS BANK ELEVATION (Geodetic Datum, m) -0.2 to -0.5 <-2A EXPOSURE ZONE Lower Amphizone Near base of Lower Aquazone LIMITING FACTOR(S) Brackish marsh and low s a l i n i t y water Limit unknown NOT PRESENT DUE TO PRESENCE OF LOW SALINITY WATER Upogebia r e s t r i c t e d to beds of dense Z. marina where mud contents of sediment-are higher (>2%) , Callianassa Upogebia TABLE II SUMMARY OF EFFECTS OF VARIOUS ENVIRONMENTAL FACTORS ON THALASSINIDEAN SHRIMP DENSITY t t t t Callianassa Upogebia % Mud S a l i n i t y * * ** S a l i n i t y in the range of 0 to 30%„ t Positive correlation + Negative c o r r e l a t i o n t t t t t t Weak positive correlation Strong positive correlation N3 CO OJ 234 TABLE. I l l Schematic Summary of A l l F l o r a l and Faunal D i s t r i b u t i o n a l Limits on Fraser Delta T i d a l F l a t s Which L i e Within .15 cm Or Less of an Exposure Zone Boundary Or Other Extreme C r i t i c a l T i d a l Level UPPER MARINE TIDAL FLATS Boundary Bay Inter-causeway Area BRACKISH TIDAL FLATS Northern & Central Roberts Bank ATMOZONE LOWER ATMOZONE Lower Limit of Saltmarsh Zone Lower Limit of Saltmarsh Zone Upper Limit of Callianassa** A l g a l Mat Zone Lower Limit  Abarenicola Upper Limit UPPER AMPHIZONE M.S.L.* LOWER AMPHIZONE Upper Limit of Z. Americana Upper Limit of Upogebia Lower Limit of Upper Brackish Marsh UPPER AQUAZONE LOWER Upper Limit of Z. marina AQUAZONE * M.S.L = Mean Sea Level ** Upper l i m i t of Callianassa l i e s at l e v e l at which the maximum duration of continuous exposure jumps from 4 to 9 days. APPENDIX 1 — SURVEY DATA FOR BOUNDARY BAY (PART 2) 236 Survey Data for Transect A Boundary Bay Location of S t a r t i n g Point The s t a r t i n g point (Tl) i s located approximately where 72nd Avenue meets the dyke. The s t a r t i n g point l i e s on the d i r t track entrance to the gate i n t o the f i e l d on the west side of 72nd Avenue. Its precise l o c a t i o n i s on the axis of the d i t c h , which p a r a l l e l s the west side of 72nd Avenue, 6.49 m from the bench mark (marked by a metal sign with B.M. w r i t t e n i n yellow on a red background), which l i e s at the corner of the fence facing the dyke. Orientation of Transect The transect runs north/south ( i . e . , a continuation of 72nd Avenue). Station A l l i e s at the edge of the saltmarsh and i s 513.6 m (± 0.7 m) south of the s t a r t i n g point ( T l ) . Stations A l to A22 were spaced at 91.4 m (300 f t ) i n t e r v a l s by taping with a 30.5 m (100 f t ) tape. The spacings of stations T l to T6 were determined by s t a d i a surveying and are as follows: T1/T2 85.03 m ± 0.30 m T2/T4 155.74 m ± 0.43 m T4/T5 65.55 m ± U.30 m T5/T6 99.36 m ± 0.30 m T6/A1 107.89 m ± 0.30 m Accuracy Elevation differences were measured using an alidade which can measure -4 the sine of the angle of e l e v a t i o n d i r e c t l y to ±0.5 x 10 . At every s t a t i o n a foresight to next the s t a t i o n and backsight to the previous-station .were - o 237 ~> taken. I f the instrument was p e r f e c t l y leveled and no errors were made, then the foresight at a given s t a t i o n would exactly equal the backsight from the next. I f the alidade was not i n perfect adjustment, such that when i t was levele d i t was i n fact t i l t e d , then this would introduce a constant systematic error between foresight and backsight readings. Such an error was detected. There was a constant discrepancy between foresight > -4 and backsight readings of about 12 x 10 . P o s i t i v e readings ( i . e . , eleva-t i o n increasing towards observed station)'always exceeded negative readings by this constant amount, i n d i c a t i n g that when the instrument registered l e v e l i t was i n fact t i l t e d downwards at about 0.03°. To overcome this problem the discrepancy between foresight and backsight readings was halved to obtain an estimate of true zero and the foresight reading used i n calcu-l a t i o n s of elevation corrected using t h i s estimated zero. By averaging a l l the estimated zeros f o r transects A and B (67 stations) a better estimate of true zero was obtained and t h i s was determined to be when instrument -4 registered an elevation with sine +5.9 ± 0.7 x 10 .„ This assumes that the true zero of the instrument did not vary with time and that deviations of estimated zeros from t h i s value were due to accidental errors. Carrying t h i s assumption one step further an estimate of accidental errors i n surveying could be made by determining the average deviation of the estimated zeros from true zero, i . e . , ' n : : E I True zero - estimated zero I average error of sine value = — 1 L-N where N = 67. -4 This e r r o r was determined to be ±0.6 x 10 , which over the s t a t i o n to s t a t i o n to s t a t i o n distance of 91.4 m i s equivalent to an error of ±5.5 mm. Another possible source of error i s i n the measurement of distance between s t a t i o n s . The e f f e c t s of this error are very small unless the error i s (238;3 systematic (.e.g., tape was not exactly 30.5 m long). Assuming the worst and estimating a systematic er r o r of 0.3 m i n the measurement of distance between each s t a t i o n , then t h i s introduces an average elevation e r r o r of ±0.35 mm for the average slope of transect A. This error i s cumulative. • For s t a t i o n s T l to A l slope distances were determined by s t a d i a surveying, introducing an accidental error of ±0.3 m i n slope distance. The errors estimated i n the table are the sum of both systematic and accidental e r r o r s . By the theory of Least Squares accidental errors increase i n proportion to the square root of the number of observations. Systematic errors are cumulative. In combining systematic and accidental errors to a r r i v e at an o v e r a l l estimate of error the worst was assumed, namely that the two sources of error are cumulative. Table of Survey Data Transect A Estimated Corrected _„ „, Sine of Elevation Zero for Sine for Elevation st a t i o n , 1ri-i4v „. . . , _ Difference Angle (xlO H) Sine foresight , . (m; Sine (xlO - 1*) (xlCT k) Geodetic Elevation (m) B.M. TI T2 T4 T5 T6 Al A2 A3 A4 A5 A6 A7 A8 A9 Tl/B.M. T1/T2 T2/T4 T4/T5 T5/T6 T6/A1 A1/A2 A2/A3 A3/A4 A4/A5 A5/A6 A6/A7 A7/A8 A8/A9 -266.0 120.5* -110.0** - 68.0 79.0 76.5 - 61.0 - 80.0 88.5 -10.0 21.0 3.5 11.5 - 3.5 17.0 8.0 4.0 - 6.5 19.0 5.5 5.0 - 8.0 21.0 - 3.0 14.0 - 4.0 15.0 A9 1/3 A9/A9 1/3 - 18.0 A9 1/3 A10/A9 1/3 27.0 A10 A l l A9/A10 A10/A11 14.5 28.0 6.0 19.0 5.25 5.25 5.50 7.75 4.25 5.50 7.50 6.75 6.00 6.25 5.25 6.50 5.50 5.50 (6.75; (6.75) 6.75 6.50 -271.25 115.25 - 73.50 68.75 - 84.25 - 15.50 - 4.00 - 10.25 2.00 - 12.75 0.25 - 14.50 - 8.50 - 9.50 - 24.75 20.25 - 21.25 -12.50 -0.177 0.978 -1.143 0.451 -0.838 -0.168 -0.037 -0.094 0.018 -0.116 0.003 -0.131 -0.076 -0.085 -0.076 0.125 -0.195 -0.113 B.M. TI T2 T4 T5 T6 A l A2 A3 A4 A5 A6 A7 A8 A9 1.524 1.701 ± 0.005 2.679 + 0.011 1.536 ± 0.020 1.987 + 0.022 1.149 + 0.025 0.981 ± 0.026 0.945 + 0.028 0.850 ± 0.030 0.869 ± 0.031 0.753 ± 0.031 0.756 ± 0.033 0.625 + 0.034 0.549 + 0.035 0.463 ± 0.037 A9 1/3 0.387 ± 0.037 A9 1/3 0.390 ± 0.037 A10 A l l 0.268 + 0.037 0.155 + 0.039 * foresight ** backsight Station Sine of Angle Elevation (xlO -") .Estimated Zero for Sine (xlO - 1*) Corrected Sine f o r foresight (xlO - 1*) Elevation Difference (m) Geodetic Elevation (m) A12 A11/A12 -17.0* 31.0** 7.00 -24.00 -0.219 A12 -0.064 + 0.040 A13 A12/A13 2.5 10:0 6.25 - 3.75 -0.034 A13 -0.098 + 0.040 A14 A13/A14 - 0.5 11.5 5.50 - 6.00 -0.055 A14 -0.152 + 0.042 A15 A14/A15 - 4.5 17.5 6.50 -11.00. -0.101 A15 -0.253 + 0.043 A16 A15/A16 - 7.0 17.5 5.25 -12.25 -0.113 A16 -0.366 + 0.043 A17 A16/A17 - 8.0 18.0 5.00 -13.00 -0.119 A17 -0.485 + 0.045 A18 A17/A18 - 9.5 22.5 6.50 -16.00 -0.146 A18 -0.631 + 0.045 A19 A18/A19 -14.0 25.0 5.25 -19.75 -0.180 A19 -0.811 0.046 A20 A19/A20 -14.0 26.0 6.00 -20.00 -0.183 i A20 -0.994 + 0.048 A21 A20/A21 2.0 19.5 10.75 - 8.75 -0.079 A21 -1.073 + 0.048 A22 A21/A22 " 9 -° (10.75) -19.75 -0.180 A22 -1.253 + 0.049 * foresight ** backsight ' 24 i ; -Survey Data for Transect B Boundary Bay Location of S t a r t i n g Point The s t a r t i n g point (1) i s located on the south side of the dyke where 96th Avenue meets the dyke. Its exact l o c a t i o n i s on the north/south l i n e which p a r a l l e l s the western side of 96th Avenue, and passes through the western metal dyke gatepost at the end of 96th Avenue. I t i s on t h i s l i n e 25.41 m (uncorrected for elevation difference) from the bench mark at the end of 96th Avenue. The bench mark i s located j u s t outside the S.E. corner of the garden of the house on the western side of the end of 96th Avenue. Orientation of Transect The transect runs north/south ( i . e . , a continuation of 96th Avenue). Station B l l i e s at the southern edge of the saltmarsh. Stations B l to B38 are spaced at 91.4 m (300 f t ) i n t e r v a l s . Survey s t a t i o n 2 i s 28.37 m (uncorrected for elevation difference) south of 1, and 48.21 m north of B l . These distances were taped. Accuracy The average accidental error i n e l e v a t i o n between successive stations at 91.-.4 m i n t e r v a l s i s ±5.5 mm, as i n the case of transect A. The average systematic error'assuming a distance error of 0.3 m between successive stations i s s l i g h t l y lower than transect A at ±0.21 mm, because the average slope of transect B i s l e s s than that of transect A. The errors i n the table are the sum of systematic and accidental e r r o r s . . 24 2 Table of Survey Data Transect B Station Sine of Elevation Angle (xlO - 1*) Estimated Zero for Sine (xlO -") Corrected Sine for foresight ( x l O - 4 ) Elevation Difference (m) Geodetic El e v a t i o n (m) B.M. 1 2 Bl B2 B3 B4 B5 B6 B7 B8 B9 BIO B l l B12 B13 B14 B15 B16 B17 B.M./l 1/2 2/B1 B1/B2 B2/B3 B3/B4 B4/B5 B5/B6 B6/B7 B7/B8 B8/B9 B9/B10 B10/B11 B11/B12 B12/B13 B13/B14 B14/B15 B15/B16 B16/B17 foresight 794* -784** -846 860 - 8 17 - 2.0 .14.0 0.0 13.0 - 2.0 15.5 - 8.0 22.0 - 2.0 13.5 - 2.5 15.5 - 5.0 17.0 - 7.0 20.5 - 1.5 13.5 - 7.0 18.5 - 8.0 20.0 - 29.0 41.5 8.5 4.U - 4.5 13.5 - 10.0 19.0 - 6.5 16.0 5.00 7.00 4.50 6.00 6.50 6.75 7.00 5.74 6.50 6.00 6.75 6.00 5.75 6.00 6.25 6.25 4.50 4.50 4.75 789.00 -853.00 - 12.50 - 8.00 - 6.50 - 8.75 - 15.00 - 7.75 - 9.00 - 11.00 - 13.75 - 7.50 - 12.75 - 14.00 - 35.25 2.25 - 9.00 - 14.50 - 11.25 2.002 -2.420 -0.061 -0.073 -0.058 -0.079 -U.137 -0.070 -0.082 -0.101 -0.125 -0.070 -0.116 -0.128 -0.323 0.021 -0.082 -0.131 -0.104 B.M. 1.521 1 3.523 + 0.005 2 1.103 ± 0.010 B l 1.042 ± 0.012 B2 . 0.969 + 0.014 B3 0.011 ± 0.016 B4 0.832 ± O.018 B5 0.695 ± 0.020 B6 0.625 ± 0.021 B7 0.543 ± 0.023 B8 0.442 ± 0.024 B9 0.317 + 0.025 B10 0.247 ± 0.026 B l l 0.131 ± 0.028 B12 0.0O3 ± 0.029 B13 -0.320 + 0.030 B14 -0.299 ± 0.031 B15 -0.381 ±0.032 B16 -0.512: ± 0.033 B17 -0.616 ± 0.033 ** backsight 243 Station Sine of Angle Elevation (xlO-1*) Estimated Zero for Sine txlO" u) Corrected Sine for foresight (xlO - 4) Elevation Difference (m) Geodetic Elevation B18 B17/B18 - 1.0* 12.0** 5.50 - 6.50 -0.058 B18 -0.674 + .0.034 Biy B18/B19 - 8.5 ly .o 5.25 -13.75 -0.125 B19 -0.799 + U.035 B20 B19/B20 1.5 11.5 6.50 - 5.00 -0.046 B20 -0.844 0.036 B21 B20/B21 - 7.5 20.0 6.25 -13.75 -0.125 B21 -0.969 + 0.037 B22 B21/B22 11.5 U.5 6.00 5.50 0.049 B22 -0.920 + 0.038 B23 B22/B23 5.5 5.0 5.25 0.25 0.003 B23 -0.017 + 0.039 B24 B23/B24 9.5 3.0 6.25 3.25 0.030 B24. -0.887 + 0.039 B25 B24/B25 13.0 - 1.5 5.75 7.25 0.067 B25 -0.820 + u.041 B26 B25/B26 13.5 - 1«5 6.00 7.50 0.067 B26 -0.753 + 0.041 B27 B26/B27 - 3.5 14.5 5.50 - 9.00 -0.082 B27 -0.835 + 0.042 B28 B27/B28 0.0 11.0 5.50 - 5.50 -0.049 B28 -0.884 + 0.042 B29 B28/B29 3.5 11.0 7.25 - 3.75 -0.034 B29 -0.917 + 0.043 B30 B29/B30 1.0 10.0 5.50 - 4.50 -0.040 B30 -0.057 + 0.044 B31 B30/B31 8.U 4.0 6.00 2.00 0.018 B31 -0.939 + 0.045 B32 B31/B32 - 7.5 18.5 5.50 -13.00 -0.119 B32 -1.058 + 0.045 B33 B32/B33 4.5 8.5 6.50 - 2.00 -0.018 B33 -1.076 + 0.046 B34 B33/B34 5.0 6.5 5.75 - 0.75 -U.006 B34 -1.082 0.047 B35 B34/B35 0.0 10.0 5.00 - 5.00 -0.046 B35 -1.128 + 0.047 B3b B35/B36 - 3.0 14.0 5.50 - 8.50 -0.076 B36 -1.204 + 0.048 B37 B36/B37 1.0 10.0 5.50 - 4.50 -0.040 B37 -1.244 + 0.049 B38 B37/B38 - 7.0 19.5 6.25 -13.25 -0.122 B38 -1.365 ± 0.049 foresight ** backsight APPENDIX 2 — FAUNAL DENSITIES AND GRAIN SIZE DATA ON TRANSECTS A AND B, BOUNDARY BAY (PART 2) Transect A Station B a t i l l a r i a Nassarius Callianassa &/or Upogebia Mya Abarenicola P r a x i l l e l a Spio A l P 0 0 0 0 0 7000 A2 16 0 P P 0 0 4400 A3 25 0 P • P 0 0 4400 A4 31 0 P P 0 • 0 4400 A5 32 0 P P 19 0 600 A6 48 0 P P 25 0 2500 A7 40 0 P P 134 0 1200 A8 28 0 0. .5 (C) P 28 0 600 A9 12 0 P P 25 0 600 A9 1/3 51 0 P P 133 0 <600 A10 28 0 1 (C) 1 65 0 1900 A l l 16 0 4 (c) 0.5 36 0 1900 A12 18 0 7 (C) 3.5 20 0 36000 A13 36 0 14 (C) 2.5 24 0 22000 A14 17 0 22 (C) 0 24 0 33000 A15 6 1 24 (c; 0 9 40 48000 A16 5 1.5 10 (c/u) 0 13 110 27000 A17 2 4.5 10 (c/u) 0 1 110 37000 A18 P 1 44 (c/u) 0 P 260 20000 A19 0 1 28 (u) 0 P 410 12000 A20 0 1 18 (u, 0 P 650 4000 A21 0 1 10 (U) 0 P 320 7000 A22 0 1 6 (U) 0 P 390 <600^  P = present (<0.5 m~2) CC) = Callianassa only (C/U) = Callianassa and Upogebia (U) = Upogebia only N.B. : a l l densities i n numbers per square meter. Spio densities rounded to nearest hundred. (Cont'd ) Transect B Station B a t i l l a r i a Nassarius Callianassa Abarenicola P r a x i l l e l a , Spio BI 1.5 0 0 0 0 12500 B2 6 0 0 0 0 12500 B3 7 0 0 0 0 5000 B4 28 0 P 0 0 7500 B5 29 0 P 20 0 2500 B6 57 0 3 5 0 2500 B7 18 0 4 18 0 <600 B8 21 0 1 15 0 <600 B9 13 0 7 42 0 600 BIO 4.5 0 6 51 0 600 B l l 7 0 14 61 0 <600 B12 5 0 15 23 0 600 B13 2.5 0 5 5 0 600 B14 1.5 0 7 31 0 <600 B15 1.5 0 12 14 0 <600 B16 1 p 0 6 0, 1200 B17 0 p 0 3 3.5 P.N.D. B18 0 0.5 0 4.5 4.5 P.N.D. B19 2 P 0 0.5 56 P.N.D. B20 4 1 0 0.5 91 P.N.D. B21 13 2 0 0.5 73 P.N.D. B22 26 5 0 P 54 P.N.D. B23 32 0.5 0 0.5 44 P.N.D. B24 8 P 0 0 12 25000 B25 0 P 0 1 0 8100 B26 0 0.5 0 3.5 0 7500 B27 0 1 0 1.5 0 4400 B28 0 P 0 5 0 5000 B29 0 P 0 4.5 0 2500 B30 0 P 0 P 0 3800 B31 0 P 0 . P 10 3800 B32 0 P 0 1.5 10 1200 B33 0 5 0 P 10 1200 B34 0 1 0 P 10 3100 B35 0 0.5 0 5 100 600 B36 0 . 0.5 0 P 100 <600 B37 0 0 0 P 150 <600 B38 0 0 0 P 100 <600 P = present (<0.5 m~ ) P.N.D. N.B.: present but not determined. a l l densities i n numbers per square meter. 247 Grain Size Data Transects A and B Boundary Bay Station Graphic Mean I n c l . Graphic % Mud Station G r a P h * c ^ e a n I n c l . Graphic X Mud (0) Std. Dev. (0) (0; Std. Dev. (0) A l 3.21 0.41 4.20 B24 2.60 0.33 0.25 A2: 3.15 0.40 4.88 B25 2.59 0.31 0.56 A3: 3.22 0.43 5.98 B26 2.50 0.31 0.26 A4 3.31 0.44 4.70 B27 2.55 0.33 0.63 A5 3.26 0.36 3.99 B28 2.53 0.32 0.31 A6 3.14 0.36 4.14 B29 2.52 0.32 0.44 A7 2.99 0.34 1.97 B30 2.48 0.31 0.26 A8 2.90 0.33 1.14 B31 . 2.43 0.32 0.28 A9 2.85 0.32 1.24 B32 2.42 0.33 0.25 A9 1/3 2.88 0.35 0.92 B33 2.41 0.29 0.21 AlO 2.78 0.36 0.81 B34 2.38 0.32 0.14 A l l 2.78 0.38 2.09 B35 — — A12 2.70 0.39 1.32 B36 2.47 0.32 0.31 A13 2.75 0.39 1.23 A14 2.63 0.40 0.95 A15 2.69 0.42 1.49 A16 2.69 0.39 0.82 A17 2.75 0.43 2.51 A18 2.77 0.52 2.65 A19 2.88 0.60 7.00 A20 2.97 0.55 6.07 A21 2.97 0.53 5.70 A22 2.77 0.45 2.78 BI 2.87 0.50 3.56 B2 3.07 0.43 3.86 B3 3.20 0.45 8.03 B4 3.15 0.40 4.53 B5 3.09 0.37 3.84 B6 3.10 0.38 2.81 B7 2.98 0.33 1.71 B8 2.94 0.32 0.87 B9 2.87 0.35 0.95 BIO 2.80 0.34 0.56 B l l 2.80 0.33 0.78 B12 2.67 0.33 0.36 B13 2.81 0.37 1.36 B14 2.76 0.33 0.45 B15 2.71 0.34 0.72 B16 2.73 0.34 0.41 B17 2.71 0.35 0.46 B18 2.71 0.34 0.59 B19 2.70 0.33 0.51 B20 2.74 0.34 0.79 B21 2.73 0.36 1.77 B22 2.65 0.36 0.62 B23 2.65 0.34 0.37 7.48. APPENDIX 3 ~ SURVEY DATA FOR STATIONS ON THE INTER-CAUSEWAY TIDAL FLAT (PART 4A) 249 Survey Data f or Transects A, B, C and D on the Inter-Causeway T i d a l F l a t  Transect A Transect A p a r a l l e l s the Tsawwassen ferry terminal causeway and l i e s 150 ± 1 m from the raised concrete edge of Highway 17. Station A l l i e s on the sandflat about 10 m from the edge of the cobbles on the shore. Stations were taped at 100 m i n t e r v a l s . Station numbers increase seawards. Methods— Elevations were determined by l e v e l i n g from bench mark 'A' (Fig. 2 i n back cover) with a T2 t r a n s i t . The instrument was set up at every fourth s t a t i o n . At each set-up a backsight was taken to the preceding two stations and a foresight to the following two stat i o n s ( i . e . , maximum si g h t i n g distance 200 m). The s t a d i a rod was mounted on a 0.5 m wooden spike, and : pushed into the substrate at each s t a t i o n u n t i l i t s base was f l u s h with the substrate,and held v e r t i c a l by the rodman. This eliminated errors due to the rod sinking into the substrate while being held, and by simply r o t a t i n g i t on i t s axis the st a d i a rod could be t i e d i n to the next set-up of the t r a n s i t , by backsighting. The i n t e r s e c t i o n of the cross h a i r on the s t a d i a rod could be read to an accuracy of ±5 mm over the 200 m si g h t i n g distance employed. Readings were constantly double checked by f l i p p i n g the telescope through 180°. This eliminates the p o s s i b i l i t y of introducing an error due to in c o r r e c t l e v e l i n g . The estimated error, therefore, i s an accidental e r r o r of ±5 mm. By the theory of l e a s t squares th i s e r r o r increases i n proportion to the square root of the number of obser-vations and t h i s i s how the errors were estimated i n the following tables of data. The same surveying techniques were used from transects B, C and D. •250 Transect B Transect B s t a r t s from the promontory immediately southeast of the breach i n the dyke. I t p a r a l l e l s transect A at a distance of 1 km (±3 m). Station B l l i e s 57 m from the edge of the marsh, which forms a c l i f f about 70 cm high. Station numbers increase seawards. TransectvC Transect C i s located 258 m from the southeastern edge of the causeway road, and p a r a l l e l s the causeway. Stat i o n C l i s located 100 m from the base of the dyke. Station numbers increase seawards. Transect D Transect D s t a r t s at A2 and ends at B7. DI i s adjacent to A2 and D9 i s adjacent to B7. Transect E Transect E s t a r t s at A l l and ends at B16. E l i s adjacent to A l l and E9 i s adjacent to B16. Elevation of A l g a l Mat Zone/Saltmarsh Zone Boundary The elevation of the upper l i m i t of the a l g a l mat zone was determined at seven points along the perimeter of the marsh between transects C and B. Station 1 l i e s next to B.M. '„C' and stations were consecutively numbered going towards transect B. The elevation of the upper l i m i t of the .algal mat zone was determined on the t i d a l f l a t immediately adjacent to the edge of the marsh. The elevation of the lower l i m i t of the saltmarsh surface was deter-mined on the tops of saltmarsh clumps at the edge of the saltmarsh at stations 1 and 7. Bench Marks The elevations and locations of bench marks were obtained from the -251; municipal engineer of Delta M u n i c i p a l i t y . The bench marks were established for dyking purposes. Bench mark'A'has reference number 66. I t i s a concrete block with a metal p l a t e set on top with 'Legal Survey B r i t i s h Columbia' stamped on i t . I t marks the corner of a D i s t r i c t Lot and l i e s between Highway 17 and the gravel road p a r a l l e l l i n g the causeway about 100 m south-west of the fence of the B.C. Hydro s t a t i o n . Bench mark'B'has reference number 70. I t i s an i r o n p i n , and l i e s about 0.3 m from the edge of the marsh next to a wooden post. I t l i e s about 100 m southeast of a major breach i n the dyke. Bench mark 'c'has reference number 74. I t i s a n a i l driven h o r i z o n t a l l y into the base of a telegraph pole about 0.5 m above ground l e v e l . I t i s marked with flagging tape. The telegraph pole i s the twelth pole encountered walking southeast from the dyke gate at the Coalport causeway. The n a i l i s on the seaward side of the pole. Independent Check of Survey Data The survey data was independently checked by comparing observed t i d a l heights for ten low waters at the Tsawwassen fe r r y terminal t i d a l gauge with the observed height of the waterline on each transect at the time of low t i d e . The l o c a t i o n of the waterline between surveyed stations was estimated to the nearest 10 m and i t s el e v a t i o n determined by l i n e a r i n t e r p o l a t i o n between the known elevations of the s t a t i o n s . The table of comparisons, follows the tables of survey data. 252 Table of Station Elevations on the Inter-Causeway Tidal Flat Transect A Transect B Transect C Station Elevation Station Elevation Station Elevation (Geodetic Datum, m) (Geodetic Datum, m) (Geodetic Datum, m) B.M. 2.728 B.M. 1.594 B.M. 3.266 Al 0.140 ± 0.005 Bl 0.433 ± 0.002 Cl 0.536 ± 0.006 A2 -0.062 ± 0.005 B2 0.396 ± 0.005 C2 0.416 ±0.007 A3 -0.277 ± 0.005 B3 0.256 ± 0.005 C3 0.426 ± 0.008 A4 -0.492 ± 0.007 B4 0.081 ± 0.006 C4 0.376 ± 0.009 A5 -0.722 ± 0.008 B5 -0.059 ± 0.007 C5 0.371 ± 0.009 A6 -0.902 ± 0.009 B6 -0.224 ± 0.008 C6 0.251 ± 0.010 A7 -1.032 ± 0.009 B7 -0.389 + 0.009 C7 0.096 ± 0.010 A8 -1.192 ± 0.010 B8 -0.519 ± 0.009 C8 -0.009 ± 0.011 A9 -1.322 ± 0.010 B9 -0.669 ± 0.010 C9 -0.089 + 0.011 A10 -1.452 ± 0.011 B10 -0.839 ± 0.010 C10 -0.189 ± 0.012 All -1.597 ± 0.011 Bl l -0.954 ± 0.010 Cll, -0.294 ± 0.012 A12 -1.732 ± 0.012 B12 -1.024 ± 0.011 C12 -0.404 ± 0.013 A13 -1.807 ± 0.012 B13 -1.149 ± 0.012 C13 -0.469 ±0.013 A14 -1.907 ± 0.013 B14 -1.264 ± 0.012 C14 -0.514 ± 0.014 A15 -2.047 ± 0.013 B15 -1.379 + 0.013 C15 -0.579 ± 0.014 A16 -2.182 ± 0.014 B16 -1.469 ± 0.013 C16 -0.689 ± 0.015 B17 -1.544 ± 0.014 C17 -0.774 + 0.015 B18 -1.719 ± 0.014 C18 -0.889 + 0.016 B19 -1.874 ± 0.015 C19 -0.974 ± 0.016 B20 -1.889 ± 0.015 C20 -1.074 ± 0.017 B21 -2.004 ± 0.016 C21 -1.189 ± 0.017 C22 -1.294 ± 0.017 C23 -1.409 ± 0.017 C24 -1.499 ± 0.018 C25 -1.559 ± 0.018 C26 -1.759 ± 0.019 C27 -1.782 ± 0.019 C28 -1.892 ±0.019 C29 -1.987 ± 0.019 Upper Limit of Lower Limit of Transect D Station Al(*al Mat Zone Saltmarsh Surface Station Elevation Elevation Station Elevation (Geodetic Datum, m; (Geodetic Datum, m) (Geodetic Datum, m) D2 -0.266 + 0.011 1 0.916 ± 0.005 1 1.066 ± 0.005 D4 -0.376 ± 0.010 2 0.893 ± 0.007 7 1.071 + 0.009 D6 -0.364 i 0.009 3 0.823 ± 0.007 D8 -0.364 i 0.007 4 0.893 ± 0.007 5 0.926 ± 0.007 6 0.926 ± 0.009 7 0.826 ± 0.009 Comparison of Survey Data with Observed T i d a l Data at the Tsawwassen Ferry Terminal T i d a l Gauge Time Observed Low Water Po s i t i o n of Surveyed Elevation Discrepancy Date (P.S.T.J *Chart Datum (m) Waterline '''Chart Datum (m) (cm) Sept. 22/77 0655 1.42 (LLW) A11.0 1.358 - 6.2 II II 1.42 ( " ) C24.5 1.425 + 0.5 Sept. 24/77 0856 1.56 ( " ) A10.0 1.503 - 5.7 n II 1.56 ( " ) B16.25 1.466 - 9.4 II II 1.56 ( " ) C23.75 1.477 - 8.3 Sept. 26/77 1036 1.78 c " ) A 8.2 1.737 - 4.3 II II 1.78 ( " ) C20.9 1.776 - 0.4 Oct. 2/77 1451 2.90 (HLW) C 8.0 2.945 + 4.5 Oct. 31/77 1456 3.24 ( " ) C 5.3 3.289 + 4.9 J u l . 24/78 1413 1.55 (LLW) A 9.5 1.568 + 1.8 I f II 1.55 ( " ) B15.25 1.552 + 0.2 I t II 1.55 ( " ) C23.5 1.500 - 5.0 J u l . 25/78 1501 2.03 ( " ) A 6.5 1.988 - 4.2 II II 2.03 ( " ) B11.7 1.951 - 7.9 II II 2.03 ( " ) C18.6 2.014 - 1.6 J u l . 28/78 1800 3.19 (HLW) B 3.0 3.210 + 2.0 I I II 3.19 ( " ) C 5.5 3.265 + 7.5 Aug. 6/78 1248 1.26 (LLW) B17.5 1.322 + 6.2 Aug. 7/78 1311 1.44 ( " ) C24.5 1.425 - 1.5 Average Discrepancy ± 4.3 * Tsawwassen Chart Datum = - 2.954 m Geodetic Datum P.S.T. = P a c i f i c Standard Time Note: The waterline was observed f o r one hour around the time of low water and i t s lowest p o s i t i o n recorded. Wave action was n e g l i g i b l e at the time of low t i d e . APPENDIX 4 — GRAIN SIZE AND THALASSINIDEAN SHRIMP DENSITY DATA FOR STATIONS ON THE INTER-CAUSEWAY TIDAL FLAT (PART 4A) .255" Grain size and thalassinidean shrimp density data for Stations on Inter-Causeway Tidal Flat _^ ^. Median „. , Upogebia „ Callianassa _ Station A Mud ,, r g — -2, : -2. (0) (burrow openings m ) (burrow openings m ; Al 3.18 20.43 0 0 70 90 A2 3.61 38.95 15 3 38 36 A3 3.80 43.33 28 11 95 113 A4 3.94 47.17 4 29 56 57 A5 3.81 44.19 82 44 35 76 A6 3.99 49.51 29 80 59 17 A7 4.04 50.95 81 67 30 24 A8 4.02 50.23 60 26 18 30 A9 4.06 51.71 114 55 15 22 A10 4.15 53.53 32 34 10 12 A l l 3.85 47.27 8 14 21 15 A12 4.06 51.12 9 9 24 15 A13 3.65 43.33 0 14 10 24 A14 3.36 33.16 1 4 31 51 A15 3.59 44.13 10 3 7 8 A16 3.26 23.74 0 2 9 14 Bl 3.88 44.80 0 0 1 0 B2 3.75 38.98 0 0 1 1 B3 3.45 12.31 0 0 10 14 B4 3.34 11.78 0 0 20 30 B5 3.30 9.93 0 0 32 12 B6 3.30 10.53 0 0 55 45 B7 3.30 13.83 5. ' 0 49 53 B8 3.24 10.99 4 3 36 46 B9 3.24 10.58 10 4 53 60 BIO 3.23 10.16 5 8 45 42 Bll 3.26 12.49 0 0 51 49 B12 3.08 6.64 6 22 83 115 B13 2.98 6.14 1 6 82 128 B14 3.02 9.90 0 7 112 111 B15 3.17 13.81 7 9 97 106 B16 3.19 7.47 0 8 12 18 B17 3.17 17.07 4 0 108 72 B18 3.07 6.96 2 4 116 120 B19 3.05 8.32 0 1 67 74 B20 3.03 12.82 2 6 0 2 B21 3.02 12.10 0 0 0 0 Cl »4.00 91.84 0 0 0 0 C2 »4.00 88.09 0 0 0 0 C3 >4.00 65.49 0 0 0 0 C4 3.40 19.18 0 0 2 0 C5 3.38 18.27 0 0 0 0 C6 3.28 9.39 0 0 2 2 C7 3.25 11.58 0 0 5 7 C8 3.17 8.83 . 0 0 . 5 6 C9 3.15 8.79 0 0 4 10 CIO 3.07 7.62 3 0 5 6 256 c > „ f J Median Upogebia _ Callianassa „ Station L Mud r p — r — -2. -2. (V) (burrow openings m ) (burrow openings tn ) Cll 3.02 7.51 8 7 9 12 C12 2.98 7.03 13 7 8 18 C13 2.92 5.90 2 3 16 23 C14 2.90 6.27 10 12 6 14 C15 2.94 11.05 6 10 23 26 C16 2.89 8.22 0 4 43 62 C17 2.92 9.24 4 7 63 70 C18 2.90 8.75 7 6 68 87 C19 2.88 8.65 6 4 84 100 C20 2.90 9.86 7 2 86 101 C21 2.88 8.65 9 24 123 84 C22 2.82 8.90 0 5 96 107 C23 2.85 12.76 1 4 133 125 C24 2.88 10.99 8 0 95 82 C25 2.75 7.32 0 0 22 55 C26 2.76 8.66 0 0 13 11 C27 2.91 20.74 0 0 2 0 C28 2.86 15.78 0 0 0 0 C29 2.93 25.78 0 0 0 0 DI 3.46 29.74 2 2 180 164 D2 3.46 27.16 16 4 142 144 D3 3.44 25.94 22 18 92 120 D4 3.41 22.16 8 18 62 38 D5 3.36 15.41 0 0 104 94 D6 3.27 10.29 0 0 106 96 D7 3.32 13.34 0 0 64 102 D8 3.26 8.67 0 0 ' 62 50 D9 3.26 8.78 0 0 42 76 El 4.05 51.07 22 20 8 20 E2 3.75 46.06 6 12 6 4 E3 3.37 30.41 18 12 20 22 E4 3.38 29.56 26 10 22 30 E5 3.34 25.30 10 6 48 28 E6 3.30 20.40 2 10 44 44 E7 3.25 19.18 18 4 28 20 E8 3.13 11.21 4 8 44 42 E9 3.10 10.64 4 0 32 46 *CP12 2.48 7.79 0 10 *CP18 2.57 4.55 0 83 *CP24 2.82 8.40 0 172 **FT4 3.41 38.19 0 70 **FT8 2.65 10.48 2.5 446 **FT11 2.92 13.13 3 354 * CP12 is in 'causeway zone' next to CP18 is in 'causeway zone' next to CP24 is in 'causeway zone' next to ** FT4 is in 'causeway zone' next to FT8 is in 'causeway zone' next to FT11 is-in .'causeway zone' next to Coalport causeway adjacent to C12. Coalport causeway adjacent to C18. Coalport causeway adjacent to C24. ferry causeway adjacent to A4. ferry causeway adjacent to A8. ferry causeway adjacent to A l l . Note: There are two thalassinidean burrow density readings for each station. Every 4 of the 8 quadrats (0.25 m2) taken at each station were summed at the time of data collection. APPENDIX 5 — SUPPLEMENTAL INFORMATION REGARDING SURFACE SUBSTRATE SALINITY AND SUBSTRATE SALINITY PROFILES ON INTER-CAUSEWAY TIDAL FLAT, ROBERTS BANK (PART 4A) 258 L I Apparatus-used to take s a l i n i t y p r o f i l e s of the sediment column. A metal plug at the end of each tube, held i n p o s i t i o n by nylon l i n e under tension, prevents"water from'ehtering the sampling tubeseas the apparatus i s pushed i n t o "the substrate. "A metal rod i s then pushed i n t o each lube to release the plug and allow i n t e r s t i t i a l waters to flow i n at the required sampling depth. Water samples were drawn o f f with a glass tube and s a l i n i t y determined d i r e c t l y with a ref ractometer. - — — , 259 Lagend - 2 8 - C o n t o u r o f s u b s t r a t e s a l i n i t y %< • Jio T r a n s a c t s t a t i o n w i t h s u b s t r a t e s a l i n i t y i n d i c a t e d %° S a m p l i n g D a t e s A l - A l l A/Bl - A / B 9 8 1 - 6 1 4 C 1 - C 1 7 JULY 5. 1 9 7 7 JULY . 6 . 1 9 7 7 JULY 6 . 1 9 7 7 JULY 7 ; 1 9 7 7 n i c a l contour ing. 260 o 1 20-1 All JUL. 1 SALINITY %o 0 10 20 30 _ l l_ I t 30 a 40 50J B 0 0 i(H 20 30-40-50-Al JUL. 1 SALINITY "/« 10 20 30 Al JUL. 3 SALINITY %o 0 10 20 30 IOH S 20 x | 30-) 40 50 _1_ ii / Duplicate. A2 JUL. 5 SALINITY %o 0 io H .« 20 30-, 40 50 J 10 20 30 A3 j y i . 5 SALINITY %o 10 20 30 20 1 50 J i 0 10-20-30 40-1 50 AT J U L . 5 S A L I N I T Y %O 0 K> 20 30 J I . I 0 10 20-1 30 1 50 J A4 J U L . 5 S A L I N I T Y 7oo 0 10 20 30 I I ., I Ag JUL. 5 SALINITY %o I 0 10 20 30 10 20 30 40-1 50 AS JUL. S SALINITY %o 0-10-I 20-E 30 a 40-| 50 10 20 30 A? JUL. 5 SALINITY %o 0 10 20 30 I 20H z S 30' a 40-| 50 .6 JUL. 5 SALINITY 7oo 10 20 30 10-l 20-x £ 30-a 40-50 0-10-20-| 30 40-| 50 AlO JUL. S SALINITY °/oo I 10 20 30 0 10-1 20 30-j 40H All JUL. S I SALINITY %o 0 10 20 30 - J I . I BI JUL. t SALINITY %o 0 10 20 30 / V B3 JUL. 6 SALINITY %o 10 20 30 'OH 20 0-10-20-» B9 JUL. 6 SALINITY %o 10 20 30 BI4 JUL. 6 SALINITY 7oo 10 20 30 I ,0^ a 20 0-10-.1 20-x £ 30-m a 40-| 50 C6 JUL. 7 SALINITY °/oo 10 20 30 U 0 0-10-E 20-X a. Ui 30-O 40-50-C7 JUL. 7 SALINITY "/oo 10 20 30 I I L_ C I S J U L . 7 SALINITY °/oo 0 10 20 30 ' 1 . 1 10-20-30-40-50' Substrate s a l i n i t y p r o f i l e s of the sediment column at low tide on transects A, B and C. taken over the period July 1-7, 1977. A P P E N D I X 6 — SUPPLEMENTAL DATA ON CORRELAT IONS BETWEEN T H A L A S S I N I D E A N SHRIMP D E N S I T I E S AND GRAIN S I Z E ON T H E INTER-CAUSEWAY T I D A L F L A T (PART 4A) 2JJ 10 Relationship m e l e v a t i o n c o e f f i c i e n t s between median grain s i z e (0) and Upogebia burrow opening density, with data grouped into 0.25 class i n t e r v a l s . B e s t - f i t l i n e a r regression l i n e s are indicated, along with t h e i r c o r r e l a t i o n (r) and confidence l e v e l s Or t e s t ) . Elevation (Geodetic Datum) of increases from Ato J . (a) (b) s >oo-i z •v • • c • ... .7 ( M l J.S M U D M l iJO M C O M N (01 (a) (b) Density Density of C a l l i a n a s s a burrow openings vs median grain s i z e ( 0 ) , regardlessoof-elevatibn. of Callianassa burrow openings vs mud content (%), regardless of elevation. N3 264 I AUa-lan 0 Mad Jon 0 Relationship between median grain s i z e (0) and Ca l l i a n a s s a burrow opening density, with data grouped i n t o 0.25 m elevation class i n t e r v a l s . B e s t - f i t l i n e a r regression l i n e s are indi c a t e d , along with t h e i r c o r r e l a t i o n c o e f f i -cients (r) and confidence l e v e l s (r_ t e s t ) . Elevation (Geodetic Datum) increases A to L. APPENDIX 7 Ch a r a c t e r i s t i c s of the Stations Used to Determine Reworking Rates by Thalassinidean Shrimps Station Burrow Openings m 2 Median Mud Content Geodetic Elevation (m) Exposure Callianassa Upogebia (0) (%) Zone C2 3 129 2 2.85 13 -1.41 Upper Aquazone C21 103 16 2.88 9 -1.19 Upper Aquazone C17 66 5 2.92 9 . -0.77 Upper Aquazone C15 20 8 2.94 11 -0.58 Lower Amphizone C12 13 10 2.98 7 -0.40 Lower Amphizone A7 27 74 4.04 51 -1.03 Upper Aquazone APPENDIX 8 — SUPPLEMENTAL DATA ON SALINITY FOR NORTHERN AND CENTRAL ROBERTS BANK (PART 4B) 267 Surface substrate s a l i n i t i e s on Roberts Bank at low tide on February 5-7, 1974. Data from Levings and Coustalin (1975), recontoured by mechanical contouring. 268 Surface substrate s a l i n i t i e s on Roberts Bank at low tide on''August 25 and 26, 1977. Mechanical contouring employed. L 5 J 7 7 7 7 7 7 7 r 7 7 7 7 7 7 7 7 7 7 7 7 i . T 25 S T J I E 1 05. T T 25 S i i '//////>/)>//>>>>I >) S «H l » T 25 S '////>/>)>////>/>>> 20 5 2X>J"/"""' III > I " 10 15 20 18 T 25 S R B I O 0 9 2 0 3 14 15 16 17 18 T 10 1.5 20 25 S 7777777777777777777 10 15 20 i s T 25 S 17 18 T 20 25 S £ ">• 7DV7TT7T7TTT7TTT7T7-? t B I S 1020 13 14 15 16 17 T E x 1JO-J 10 15 20 S \ 77-7-7-7-7-7-7-77-7-7 7 7 ^ 7 - 7 7 7 - 7 7 - 7 - 7 7 - 7 - 7 -, , 14 15 16 17 I f 1? T 0 5 10 15 20 25 30 S n I • • r • U - — I E S 05-17 T 20 S 4L ' 1050 f 15 T ! 10 S TTVT 13 K B I S 10SS 15 T 10 S -7 16 T 15 S 77/77 OISERVED TIDE AT TSAWWASSEN JUNE 8, 1976 0400 0*00 tooo 1100 1400 DAYLIGHT SAVING TIME L O W T I D E 30 S - °jr—'—£—1-r' I " 5 777-77^777777V L « g * n d x % T T « m p « r a t w r « . . S S a l i n i t y 7<*> 77777 t u b i t r a t * 13 14 15 16 17 T I 20 5 14 15 16 17 1? T 30 S 16 17 18 19 20 T 35 S I I J asi" / 11 > 111 11 1 1 I 11 S a l i n i t y and temperature p r o f i l e s of the water column taken on June 8, 1978 at high tide on ebb, between Canoe Pass and the Tsawwassen fe r r y terminal. Four p r o f i l e s were also taken i n the lower i n t e r t i d a l regions on approaching low t i d e . Refer to Figure .2. < for s t a t i o n l o c a t i o n s . Percent thickness of the s a l t wedge, for 20%o as the boundary between marine and bra water masses. Mechanical contouring. ckish APPENDIX 9 — SUPPLEMENTAL DATA ON CORRELATIONS BETWEEN CALLIANASSA DENSITY!. AND SUBSTRATE PARAMETERS FOR NORTHERN AND CENTRAL ROBERTS BANK (PART 4B) 273 C E «H e O 20 E is-le-s' U 20 30 40 Mud % -ui to -aeoir -0.20 ro +CUIm 3 > = 30H e O 20-1, r»0.053 20 40 60 80 Mud % -0.80 to -0.20 m (LlNCLASSEO DATA) Relationship between Callianassa burrow opening density and the mud content of the substrate. B e s t - f i t l i n e a r regression l i n e s are drawn along with t h e i r c o r r e l a t i o n c o e f f i c i e n t s (r) and s i g n i f i c a n c e l e v e l (r t e s t ) . ;APFE fS IX Comparison of Correlation Coefficients (r) Between Callianassa Burrow Opening Density and Substrate. S a l i n i t y Using Pooled and Unpooled S a l i n i t y Data Elevation I n t e r v a l Pooled S a l i n i t y Data Unpooled S a l i n i t y Data (Geodetic Datum, m) 1977/78 1977 197 r r test (%) r r test (.%) r r test (%) -2.45 to -2.02 0. 341 <80.0 In s u f f i c i e n t Data (N=2) 0.963 99 -2.02 to -1.41 0. 333 <80.0 0.682 90 -0.158 <80 -1.41 to -0.80 0. 106 <80.0 0.750 99 0.944 99 -0.80 to -0.20 0. 904 99.9 0.812 99 0.981 99 -0.20 to +0.41 0. 619 80.0 0.577 <80 I n s u f f i c i e n t Data (N=2) 

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