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Studies of the bivalve, Macoma balthica (L.) on a mudflat receiving sewage effluent and on an unpolluted… McGreer, Eric Rae 1979

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STUDIES OF THE BIVALVE, Macoma balthica (L.) ON A MUDFLAT RECEIVING SEWAGE EFFLUENT AND ON AN UNPOLLUTED MUDFLAT, FRASER'RIVER ESTUARY, BRITISH-COLUMBIA by ERIC RAE McGREER B.Sc, Trent U n i v e r s i t y , 1971 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE xn THE FACULTY OF GRADUATE STUDIES I n s t i t u t e of Oceanography We accept t h i s thesis as conforming to the required standards THE,UNIVERSITY OF BRITISH COLUMBIA February, 1979 • (c) E r i c Rae McGreer,- 1979 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f an advanced degree at the U n i v e r s i t y o f B r i t i s h C o lum b i a , I a g ree tha 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 r e f e r e n c e and s tudy . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d that c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i thout my w r i t t e n p e r m i s s i o n . Department o f OCEANOGRAPHY  The U n i v e r s i t y o f B r i t i s h Co lumbia 2075 W e s b r o o k P l a c e V a n c o u v e r , C a n a d a V6T 1WS D a t e F F . R K T T A K Y I f i . 1 Q7Q i i ABSTRACT' An i n v e s t i g a t i o n to determine the f a c t o r s responsible for the d i s t r i b u t i o n of a population of Maaoma balthica (L.) on a mudflat r e c e i v i n g sewage e f f l u e n t was c a r r i e d out i n the Fraser River estuary of B r i t i s h Columbia. The f a c t o r s examined were those suggested by previous studies to be 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 the macro-invertebrate community. They included reduced s a l i n i t y , low l e v e l s of d i s s o l v e d oxygen, e f f l u e n t t o x i c i t y , t o x i c i t y due to c h l o r i n a t i o n , substrate grain s i z e , and the e f f e c t s of metal contaminated sediments. Results of the study showed that s a l i n i t y , d i s s o l v e d oxygen l e v e l s and sediment grain s i z e did not s a t i s f a c t o r i l y explain the d i s t r i b u t i o n of M. balthica. Both unchlorinated and c h l o r i n a t e d sewage e f f l u e n t were shown to be non-toxic to the clam i n laboratory t e s t s , and these r e s u l t s were confirmed by 7 day in situ bioassays. The factor which appeared to be responsible f o r the d i s t r i b u t i o n of M. balthica was the degree of contamination of the substrate which affected the s e t t l i n g and s u r v i v a l of l a r v a l and j u v e n i l e clams. Heavy metals occurred i n high concentrations i n the contaminated sediments and were considered to be the most l i k e l y c o n t r o l l i n g f a c t o r . " •-i i i Sublethal e f f e c t s of estuarine sediments containing high l e v e l s of heavy metals were also studied using < Maooma bdtthica. Burrowing behaviour was i n h i b i t e d i n a l l contaminated sediments as demonstrated by the time required. . -i f o r 50% of the population to burrow (ET50), which ranged from 0.17 h i n the co n t r o l to 4.8 h i n the most contaminated substrate. A comparison of l i n e a r regressions of the concentrations of i n d i v i d u a l metals i n the sediments versus the burrowing response times showed the regressions f or mercury and cadmium to be s i g n i f i c a n t (p <0.05 and <0.001 r e s p e c t i v e l y ) . An a c t i v e threshold avoidance response by burrowed M. balthioa was also demonstrated as clams showed a s i g n i f i c a n t (p <0.05) avoidance of the sediment containing the highest metal l e v e l s . Both behavioural responses were considered us e f u l sublethal tests to assess the impact of polluted sediments. The ecology of Macoma balthica from an unpolluted -mudflat in"the Fraser River estuary was studied for one ~ • • year ( A p r i l , 1977 - March, 1978). The maximum number of ind i v i d u a l s (1830 m"2) occurred i n A p r i l , then s t e a d i l y declined throughout the year. Spawning occurred between mid-June and l a t e J u l y but recruitment was slow and prolonged. Peak s p a t f a l l (age 0+ in d i v i d u a l s ) did not occur u n t i l the following March when a maximum density of only 410 m - 2 for newly s e t t l e d spat was observed. The oldest animals found were 5+ years of age-. Maximum growth took place from A p r i l through J u l y and had ceased by August. The growth rate measured was one of the highest recorded f o r any M. balthica population. A d i r e c t r e l a t i o n s h i p between the high water temperatures, and the f a s t growth rate as w e l l as reduced longevity was apparent. A regression of body weight on s h e l l height was used to c a l c u l a t e a condition f a c t o r (CF) which reached i t s highest value during growth and i t s lowest value immediately a f t e r spawning. The mean annual biomass measured was 2.96 g dry wt m - 2. V PREFACE The t h e s i s presentation consists of three manuscripts which have been submitted f o r publication 1.: The'manuscripts -describe research c a r r i e d out on the estuarine bi v a l v e , Macoma balthica (L.) in•the'Fraser' River estuary, B r i t i s h Columbia. The f i r s t and second manuscripts describe the r e s u l t s of f i e l d and laboratory studies of the fa c t o r s a f f e c t i n g the d i s t r i b u t i o n of •M. balthica on Sturgeon Bank, a mudflat r e c e i v i n g sewage e f f l u e n t . The t h i r d manuscript describes the seasonal growth and reproduction of M. balthica on Roberts Bank, an unpolluted area of the Fraser River estuary. The r a t i o n a l e for the approach followed i n the research i s given below. Previous e c o l o g i c a l surveys on Sturgeon Bank i n the v i c i n i t y of the Iona Island sewage treatment plant have shown changes i n the d i s t r i b u t i o n pattern of the benthic fauna due to the sewage e f f l u e n t (Greater Vancouver Sewerage and Drainage D i s t r i c t , 1973, 1975, 1977; Otte and Levings, 1975). These surveys suggested that low l e v e l s of dissolved oxygen, the input of sewage derived organic matter, e f f l u e n t t o x i c i t y , c h l o r i n a t i o n , and e f f l u e n t derived heavy metals i n the sediments, si n g l y or i n combination, might be 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 the benthic invertebrates. M. balthica was c l a s s i f i e d as an i n d i c a t o r species, and one feature of i t s d i s t r i b u t i o n was an absence i n the area adjacent to the sewage o u t f a l l . F i s h e r i e s & Marine Service (1976) tested the J t o x i c i t y of the chlorinated sewage'effluent in situ to a number of organisms i n c l u d i n g M. balthiaa. Organisms were exposed i n a se r i e s of f l o a t i n g cages located at various s i t e s i n the Iona treatment plant sewage channel (see F i g . 1, p. 39). S u r v i v a l of M. balthiaa was low ranging from 100% m o r t a l i t y at the o u t f a l l , to 37% at s i t e c, mid way along the channel, a f t e r 96 h exposure. Deaths were also recorded as far away as s i t e E, at the seaward end of the Iona j e t t y ( F ig. 1, p. 39). These data pointed to e f f l u e n t t o x i c i t y as a major c o n t r o l l i n g f a c t o r w i t h i n the sewage channel. This was the only experimental study which had been undertaken, and no information was derived f o r the mudflat proper. C l e a r l y , an experimental study to assess more exte n s i v e l y which f a c t o r s associated with the sewage e f f l u e n t were 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 the benthic fauna was required. Such a study would also provide a better understanding of the benthic ecology of t h i s p o l l u t e d area on Sturgeon Bank. M. balthiaa was chosen as a representative organism because of i t s status as an i n d i c a t o r species, the a v a i l a b i l i t y of l i t e r a t u r e on i t s tolerance to environmental v a r i a b l e s and i t s s u i t a b i l i t y as an estuarine, bioassay t e s t animal. Because of the r e l a t i v e l y broad frame : of reference, the study was designed to be exploratory i n nature with emphasis on the observation of responses to a wide v a r i e t y of test conditions. The approach combined sampling i n the f i e l d , v i i laboratory and in situ experiments and the use of a v a i l a b l e l i t e r a t u r e on the physiology and ecology of the t e s t species as w e l l as information from previous studies. The aim of the thesis was to assess a number of f a c t o r s using the methods described above and to i d e n t i f y the f a c t o r or factors which were the most important i n determining the d i s t r i b u t i o n of M. balthica i n the area affected by the sewage e f f l u e n t . The study was not intended to be d e f i n i t i v e f o r any s i n g l e f a c t o r (eg. e f f l u e n t t o x i c i t y ) , but rather an overview of the r e l a t i o n s h i p of each f a c t o r to the others i n t h e i r a b i l i t y to e xplain the d i s t r i b u t i o n of M. balthica. The f i r s t two manuscripts i n the thesis achieved these aims and have i d e n t i f i e d areas where further research i s required. A twelve month sampling program was started on Roberts Bank in A p r i l of 1977 to obtain baseline information on aspects of the ecology of M. balthica i n an "unpolluted" area^Monthly samples were c o l l e c t e d to determine the density, biomass, age composition, depth d i s t r i b u t i o n , growth and reproductive status of M. balthica. The r e s u l t s of t h i s study are presented i n the t h i r d manuscript presented i n t h i s t h e s i s . Although there have been other studies of M. balthica on the P a c i f i c Coast of North America (Vassallo, 1969, 1971; Nichols, 1977; Dunnhill and E l l i s , 1969), the present work i s the most comprehensive to date and the f i r s t from the Fraser River estuary. v i i i TABLE OF CONTENTS Page ABSTRACT. i i PREFACE v LIST OF TABLES. x LIST OF FIGURES x i i ACKNOWLEDGEMENTS x i v FACTORS AFFECTING THE DISTRIBUTION OF THE BIVALVE, Macoma balthiaa (L.) ON A MUDFLAT RECEIVING SEWAGE EFFLUENT, FRASER.RIVER ESTUARY, BRITISH COLUMBIA . . . . 1 INTRODUCTION -2 Description of the study area 3 METHODS 5 F i e l d Methods . . 5 Laboratory Procedures . 7 RESULTS 11 Sediments 11 Density, Biomass and Condition Factor of M. balthiaa- • 13 Age Structure of M. balthiaa 15 Bioaccumulation i n M. balthiaa 16 To x i c i t y Bioassays 17 DISCUSSION 19 i x i Page : SUBLETHAL EFFECTS OF HEAVY METAL CONTAMINATED SEDIMENTS ON THE BIVALVE Macoma balthica ( L . ) . . . . . . . 44 INTRODUCTION . • .45 METHODS . ' . 46 RESULTS. . .' 48 Burrowing Behaviour 48 Avoidance Behaviour 49 DISCUSSION 50 GROWTH AND REPRODUCTION OF Macoma balthica (L.) ON A MUDFLAT IN THE FRASER RIVER ESTUARY, BRITISH COLUMBIA. . 59 INTRODUCTION 60 METHODS 62 RESULTS . 63 Density 63 Age Structure. . . . . . . . 64 Size Composition and Seasonal Growth 65 Depth D i s t r i b u t i o n 65 Seasonal Changes i n S h e l l Height and Body Weight . . . . 66' Biomass 66 Spawning Cycle 67 DISCUSSION 68 REFERENCES 87 X i L I S T OF T A B L E S • " „ Page •FACTORS APFECTING THE DISTRIBUTION OF THE BIVALVE, Macoma balthica (L.) ON A MUDFLAT RECEIVING SEWAGE EFFLUENT, FRASER RIVER ESTUARY, BRITISH COLUMBIA TABLE 1. Physical and chemical c h a r a c t e r i z a t i o n of sediments i n study area 30 TABLE 2. Summary of sediment metal data. . . . 31 TABLE 3. Relationship between sediment oxygen uptake and d i s t r i b u t i o n of M. balthica 32 TABLE 4. Density and biomass for M. balthica from s i t e s on Sturgeon Bank. 33 TABLE 5. Values for y intercept (a) and slope (b) of regression l o g l O y = a + bx where y = mean dry f l e s h weight i n mg, and x = s h e l l height i n mm for M. b a l t h i c a 34 TABLE 6. Concentration of heavy metals i n ti s s u e s of M. balthica (ug/g dry weight) . 35 TABLE 7. Bioconcentration r a t i o of metals i n M. b a l t h i c a 36 TABLE 8. Results of t o x i c i t y bioassays with M. balthica. 37 TABLE 9. In situ bioassay r e s u l t s and water q u a l i t y data, May 8, 1978 38 x i Page LIST OF TABLES (cont'd) SUBLETHAL EFFECTS OF HEAVY METAL CONTAMINATED  SEDIMENTS ON THE BIVALVE Macoma balthica (L.) TABLE 1. Characterization of contaminated sediments used i n burrowing and avoidance t e s t s 54 TABLE 2. The median e f f e c t i v e time (ET50) and 95% confidence l i m i t s for burrowing of M. balthica i n sediments with d i f f e r e n t concentrations of- heavy metals . . . . 55 TABLE 3. Values for y-intercept (a) and slope (b) of regression y = a + bx where y = ET50 f o r burrowing response i n hours and x = i n d i v i d u a l metal concentration (ppm) i n sediments. . . . . . . . . . . 56 TABLE 4. Results of sediment avoidance t e s t s with M. balthica. 57 GROWTH AND REPRODUCTION OF Macoma balthica (L.) ON A MUDFLAT IN THE FRASER RIVER ESTUARY, BRITISH COLUMBIA TABLE 1. Age class composition of M. balthica for each sampling occasion (no./m2) 74 TABLE 2. Mean s h e l l height ( i n mm ± S.E.) and range ( in brackets) f o r various age classes of M. balthica on each sampling occasion I. 75 TABLE 3. Depth d i s t r i b u t i o n of M. balthica within the substratum f o r each sampling date 77 TABLE 4. Values f o r y intercept (a) and slope (b) of regression loglO y = a + bx where y = mean dry f l e s h weight i n mg, and x = s h e l l height i n mm for M. b a l t h i c a 78 TABLE 5. Biomass (g dry weight/m 2) of M. balthica on each sampling occasion 79 TABLE 6. Percentage of immature, mature, sexually u n d i f f e r e n t i a t e d and p a r a s i t i z e d M. balthica on each/sampling occasion 80 x i i L I S T OF FIGURES Page FACTORS AFFECTING THE DISTRIBUTION OF THE BIVALVE, Macoma balthica (L.) ON A MUDFLAT RECEIVING SEWAGE EFFLUENT, FRASER RIVER ESTUARY, BRITISH COLUMBIA FIGURE 1. Location of study area and sampling s i t e s on ,Sturgeon Bank, Fraser River estuary, B r i t i s h Columbia 39 FIGURE 2. D i s t r i b u t i o n pattern f o r concentration of copper (ppm) i n sediments over study area . . . . 40 FIGURE 3. Density of M. balthica along transects across study area 41 FIGURE 4. Biomass of M. balthica along transects across study area 42 FIGURE 5. Age clas s frequency d i s t r i b u t i o n of M. balthica at s i t e s within study area 43 SUBLETHAL EFFECTS OF HEAVY METAL CONTAMINATED SEDIMENTS  ON THE BIVALVE Macoma balthica (L.) FIGURE 1. Burrowing rates f o r M. balthica i n sediments A (•) , B (X), C(+), D(A) and co n t r o l (•), containing ; d i f f e r e n t l e v e l s of heavy metals. 58 GROWTH AND REPRODUCTION OF Macoma balthica (L.) ON A MUDFLAT IN THE FRASER RIVER ESTUARY, BRITISH COLUMBIA FIGURE 1. Location of study area (0) i n the Fraser River estuary. Dottled l i n e i n d i c a t e s approximate seaward edge of t i d a l f l a t s at low t i d e . . . . . 81 FIGURE 2. Density of M. balthica on each sampling occasion (± 1 S.E.) . 82 x i i i L I S T OF FIGURES ( c o n t ' d ) Page FIGURE 3. Average growth rate (length i n mm) for M. balthica on Roberts Bank. 83 FIGURE 4. Density of M. balthica expressed as changes i n d i f f e r e n t s h e l l height s i z e ranges 84 FIGURE 5. Changes i n (a) mean s h e l l height i n mm, and (b) mean dry weight per i n d i v i d u a l i n mg for each sampling date 85 FIGURE 6. A i r , water and sediment temperatures, and s a l i n i t y over study area during sampling p e r i o d , . . . . . . . 86 x i v ACKNOWLEDGEMENTS I would l i k e to thank Dr. A.G. Lewis and Dr. K.J. H a l l of the U n i v e r s i t y of B r i t i s h Columbia, and Dr. CD. Levings of the P a c i f i c Environment I n s t i t u t e for reviewing the manuscript and for t h e i r advice throughout the course of the study. I also g r a t e f u l l y acknowledge the assistance of the analysts of the Water Quality Group, B.C. Research for performing the heavy metals a n a l y s i s , and the cooperation of B.C. Research i n providing laboratory space. I e s p e c i a l l y wish to extend my appreciation to Dr. T.E. Howard for h i s patience and understanding during my tenure at B.C. Research. Special thanks must go to Ms. J u l i e Paul for assistance with the rigorous f i e l d sampling program. The assistance of Mssrs. Doug MacKay and Stan Vernon of the Greater Vancouver Sewerage and Drainage D i s t r i c t i n providing access to the Iona J e t t y , the Iona sewage treatment plant and t e c h n i c a l data regarding plant operation i s also appreciated. FACTORS AFFECTING THE DISTRIBUTION OF THE B I V A L V E , Macoma balthica ( L . ) ON A MUDFLAT R E C E I V I N G SEWAGE EFFLUENT, FRASER RIVER ESTUARY, B R I T I S H COLUMBIA 2 INTRODUCTION The Fraser River estuary i s located i n the southeastern corner of the S t r a i t of Georgia and i s char a c t e r i z e d by an expanse of i n t e r t i d a l mudflats ( F i g . 1, i n s e t ) . I n t e r s e c t i n g the t i d a l f l a t s are f i v e rock j e t t i e s designed f o r use as deepwater terminals or as breakwaters to d e f l e c t r i v e r flow. One of these s t r u c t u r e s , the Iona Island j e t t y ( F i g . 1) was b u i l t to d e f l e c t the flow of sewage e f f l u e n t from the Iona Island sewage treatment plant northwards, away from the beaches the c i t y of Vancouver. Thus, sewage e f f l u e n t i s concentrated i n an area on Sturgeon Bank south of the Iona j e t t y . Previous studies of t h i s region (Greater Vancouver Sewerage & Drainage D i s t r i c t , 1 9 7 3 , 1975, 1977; Otte and Levings, 1975) have documented the b i o l o g i c a l e f f e c t s due to the sewage discharge, p a r t i c u l a r l y on the macrobenthic community. A wide v a r i e t y of f a c t o r s (eg. sediment organic and heavy metal content, low l e v e l s of d i s s o l v e d oxygen and e f f l u e n t t o x i c i t y ) have been put forward to e x p l a i n the d i s t r i b u -t i o n patterns observed but no experimental data has been presented i n any of these studies. Other studies d e a l i n g with the e f f e c t s of sewage e f f l u e n t on estuarine mudflat macrofauna are rare (Fraser, 1932), and t h i s area of the Fraser River estuary provided an e x c e l l e n t l o c a t i o n f o r research on the r e l a t i v e importance of selected v a r i a b l e s 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 benthic invertebrates. 3 This section of the thesis describes the r e s u l t s of the study undertaken to examine mpre c l o s e l y a number of factors associated with the e f f l u e n t discharge which could explain the d i s t r i b u t i o n of the deposit-feeding b i v a l v e Macoma balthica (Linnaeus, 1758). Macoma balthica (=M. inconspicua, Broderip & Sowerby, 1829) was one of the benthic i n d i c a t o r species whose d i s t r i b u t i o n had previously been shown to be affected by the Iona Island sewage treatment plant discharge (G.V.S.D.D., 1975, 1977; Otte and Levings, 1975). The f a c t o r s con-sidered i n the present study were e f f e c t s of reduced s a l i n i t y due to d i l u t i o n , low l e v e l s of dissolved oxygen, acute l e t h a l t o x i c i t y of the sewage e f f l u e n t , t o x i c i t y due to e f f l u e n t c h l o r i n a t i o n , substrate p a r t i c l e s i z e , sublethal e f f e c t s of metal-contaminated sediments and the e f f e c t s of metal bioaccumulation. In addition to d i s t r i b u t i o n , the e f f e c t s of the e f f l u e n t on the density, biomass, age str u c t u r e , recruitment, and condition of M. balthica were also i n v e s t i g a t e d . Description of the study area The study area south of the Iona j e t t y on Sturgeon Bank ( F i g . 1), receives sewage e f f l u e n t discharged from the Iona Island sewage treatment plant (ISTP). Sewage e f f l u e n t from the plant flows seawards along a shallow (1-3 m deep at low t i d e ) , dredged channel and i s dispersed at high t i d e over the mudflat area. The ISTP provides primary treatment year round, and c h l o r i n a t i o n before discharge f o r approx-imately s i x months of the year from May u n t i l October. The plant t r e a t s domestic sewage, as we l l as i n d u s t r i a l wastewater and stormwater runoff, 4 and has an average discharge of approximately 70 Imp. MGD (318 X 10 J m3 d a y - 1 ) . Sediments near the shore are muddy, becoming sandy at lower e l e v a t i o n s . The t i d a l f l a t slopes g e n t l y from the s h o r e l i n e where there i s a marsh of sedges (Carex sp.) and b u l l r u s h e s (Scirpus sp), seaward to the edge of Sturgeon Bank. The marsh area i s about 3.5 m above chart datum and the distance from shore to the edge of Sturgeon Bank i s about 5.8 km. A major t i d a l creek and s e v e r a l minor drainage channels flow across the mudflat and empty i n t o the sewage channel about midway along i t s length. Stations on the mudflat were located at the i n t e r s e c t i o n of transect l i n e s determined from markers po s i t i o n e d along the Iona j e t t y (a-D, shown as large black c i r c l e s i n F i g . 1), and the adjacent shore-l i n e (1-8, shown as small black c i r c l e s i n F i g . 1). This g r i d system was o r i g i n a l l y contructed by B.C. Research f o r the f i r s t i n t e n s i v e study of the area (G.V.S.D.D., 1973). The sampling s t a t i o n s ( F i g . 1) were positioned by taking compass bearings from these markers. For example, the coordinate A5 represents a s i t e located at the i n t e r s e c t i o n of transects A and 5. Reference to s i t e coordinates i n t h i s manner w i l l be made throughout t h i s paper. 5 METHODS Field Methods Sampling for M. balthica was c a r r i e d out at 23 s i t e s on Sturgeon Bank ( F i g . 1) during low t i d e s i n J u l y , 1977. At each s i t e , the sediment wi t h i n a 0.06 m2 quadrat was scooped out with a trowel to a depth of 10 cm. Samples were returned to the laboratory where they were washed through a 0.5 mm sieve and a l l M. balthica sorted, enumerated and fro z e n . Sediment samples for a n a l y s i s of p a r t i c l e s i z e , heavy metals and organic content were c o l l e c t e d with a 4.8 cm diameter p l e x i g l a s s corer pushed i n t o the mud to a depth of 5 cm. Duplicate cores were taken at each s i t e concurrent with sampling f o r M. balthica. Samples were frozen i n the f i e l d and returned to the laboratory f o r l a t e r a n a l y s i s . The s a l i n i t y of sediment surface water i n t i d e pools was measured i n the f i e l d with a Yellow Springs Instrument, Model 33 S a l i n i t y -Conductivity-Temperature meter. Polyethylene tubes (4.8 cm diameter) were used to take core samples f o r the determination of sediment oxygen uptake r a t e s . Cores 10 cm i n depth were taken at s i x s i t e s (a3, A3, a5, A5, b5, B5) on three occassions i n September, 1977. Cores were transported to the laboratory w i t h i n two hours where oxygen uptake determinations were st a r t e d immediately. 6 In situ bioassays with M. balthica were c a r r i e d out during the week of May 4 - 11, 1978. P l a s t i c containers (7 X 7 X 12 cm) with large "windows" cut i n the sides to f a c i l i t a t e water flow were fastened to wooden planks and placed on the mud surface at s i t e s a, A, B, and C on the south s i d e of Iona j e t t y ( F i g . 1) and at a c o n t r o l s i t e (A) on the north s i d e of the j e t t y . The bioassay containers were f i l l e d , with mud to a depth of 5 cm and ten M. balthica i n d i v i d u a l s were added to each. A f i n e , p l a s t i c mesh n e t t i n g was f i t t e d over each container and the planks were then weighted down with two concrete blocks. At the end of the experiment (168 h), the containers were emptied and the number of dead clams recorded. Surface water samples were c o l l e c t e d , using a p l a s t i c bucket, from each s i t e at high t i d e on May 11, 1978. S a l i n i t y and temperature were determined with a YSI, Model 33, S a l i n i t y -Conductivity-Temperature meter, and d i s s o l v e d oxygen was measured with a YSI, Model 54 polarographic oxygen analyser. The concentration of t o t a l r e s i d u a l c h l o r i n e was a l s o measured on s i t e with a p o r t a b l e , Fisher and Porter Model 17T 1010 amperometric t i t r a t o r . The back t i t r a t i o n procedure recommended by Carpenter et al. (1977) f o r ch l o r i n a t e d seawater samples was employed. 7 Laboratory Procedures Sediment p a r t i c l e s i z e a n a l y s i s was c a r r i e d out by the wet sieve method according to Morgans (1956). A Wentworth s e r i e s of sieves with mesh s i z e s of 2, 1, 0.5, 0.25, 0.125 and 0.063 mm was used and r e s u l t s were expressed as the percent dry weight f o r each s i z e f r a c t i o n . The dry weight of a w e l l mixed subsample was determined f o r each sample and the s i l t / c l a y f r a c t i o n (<0.063 mm) c a l c u l a t e d by d i f f e r e n c e . The median g r a i n s i z e was c a l c u l a t e d from a cumulative percent weight curve f o r each sample. Sediment samples f o r organic carbon content were treated with 4N hydrochloric acid to remove carbonates and bicarbonates, d r i e d at 60°C under reduced pressure, and then 1 g portions were i g n i t e d i n a LECO furnace. A c o r r e c t i o n was applied to account for the weight change on acid treatment. Sediments f o r a n a l y s i s of heavy metals (except mercury) were dried at 105°C and 5 g p o r t i o n s were digested f o r 12 h i n a mixture of 50% n i t r i c a c i d / p e r c h l o r i c a c i d (5:1). The d i g e s t s were d i l u t e d with deionized water, f i l t e r e d and analysed f o r metal content by flame atomic absorption spectrophotometry (AAS) using a P e r k i n -ELmer Model 306 instrument with deuterium background c o r r e c t i o n . Mercury was determined on d u p l i c a t e (0.5 g) a i r dri e d samples digested with aqua r e g i a for two minutes at 95°C, then potassium permanganate and persulphate f o r 30 min at 95°C. The d i g e s t was treated with 8 hydroxylamine/HCl, sparged with a i r for 30 seconds to remove c h l o r i n e , and reduced with stannous c h l o r i d e . Mercury was measured by the cold vapour technique using a Pharmacia UV monitor. Metals i n the tis s u e s of M. balthica were analysed i n a s i m i l a r manner a f t e r the clams had been allowed to depurate themselves of sediments for 24 h i n clean seawater. The r a t e of sediment oxygen uptake was determined from undisturbed core samples on the same day they were c o l l e c t e d . Cores conta i n i n g Sturgeon Bank sediments were f i l l e d with seawater which had been c o l l e c t e d from the Fraser estuary and p r e v i o u s l y aerated to s a t u r a t i o n with laboratory a i r . The d i s s o l v e d oxygen content of the seawater i n each tube was measured with a Beckman Model 777 polarographic oxygen analyser and the tubes sealed with a rubber stopper. The tubes were continuously rotated i n s i d e a New Brunswick 'Gyrotory' Schaker, Model G-25 to promote water mixing without d i s t u r b i n g the sediment surface. The experiment was terminated a f t e r 3 h and the dissolved oxygen concentration measured i n each tube., Tests were run at 15 ± 1°C and with a s a l i n i t y of 15 - 1 6 ° / 0 o > corresponding to ambient conditions north of the Iona j e t t y on Sturgeon Bank. A tube containing seawater only was use"d as a c o n t r o l . The age of M. balthica was determined by counting annual growth r i n g s using the methods of Lammens (1967). Animals were placed i n year c l a s s e s from age 0+ (newly s e t t l e d spat) to 5+ year old i n d i v -i d u a l s . 9 A l l s o f t t i s s u e was removed from s h e l l s over 1.0 mm i n height and d r i e d at 60°C f o r 24 h. For each sample, t i s s u e s from s h e l l s of the same s i z e were pooled and a mean dry weight determined f o r each s i z e category. The s i z e (height) of the s h e l l s from the umbo to the v e n t r a l edge was measured to the nearest 0.5 mm, and a c o n d i t i o n f a c t o r (CF) was then c a l c u l a t e d as the slope (b) i n the re g r e s s i o n l o g j o y = a + bx, where y = mean dry t i s s u e weight and x = s h e l l height. S i g n i f i c a n c e f o r the r e g r e s s i o n f u n c t i o n was expressed as the p r o b a b i l i t y of the slope being equal to zero. Laboratory bioassays with M. balthica were c a r r i e d out i n polyethylene containers (35 X 15 X 14 cm) f i l l e d to a depth of 5 cm with sediment to which 2.5 I of test s o l u t i o n was added. Sediments and t e s t organisms were c o l l e c t e d from Roberts Bank, an unpolluted area of the Fraser River estuary. Sediments were sieved through a 1 mm mesh screen p r i o r to being used and te s t animals were held at 15 °/0o and 10°C one week to permit acclimation to l a b o r a t o r y c o n d i t i o n s . M. balthica used i n the t e s t s were 5-15 mm i n length. A l l bioassays were conducted i n temperature c o n t r o l l e d (±1°C) rooms with a standard photo-period of 16 h l i g h t and 8 h darkness. Treated sewage e f f l u e n t was obtained p r i o r to each test from the o u t f a l l of the Iona Island sewage treatment p l a n t . The e f f l u e n t was c o l l e c t e d i n acid-washed, 45 g a l l o n polyethylene drums and stored at 2°C. A l l bioassays were s t a r t e d w i t h i n three days of e f f l u e n t c o l l e c t i o n . S a l i n i t y -measurements were made with a YSI Model 33, S a l i n i t y - C o n d u c t i v i t y -Temperature meter and pH measurements were taken with an Instrumentation Laboratory Model 175 portable pH meter. 10 Conductivity of the treated sewage e f f l u e n t was c o n s i s t e n t l y less than 400 umhos/cm2 (<0.1°/ Oo s a l i n i t y ) and pH values ranged from 6.6 to 7.0. U n f i l t e r e d seawater for te s t d i l u t i o n s and controls was obtained from the P a c i f i c Environment I n s t i t u t e , West Vancouver, B.C. seawater intake system which draws seawater from English Bay at a depth of 19 m below low t i d e . S a l i n i t y of the seawater ranged from 24.8 to 26.2 °/00 and pH values'were between 7.6 and 7.8. D i l u t i o n s of 80, 40, 20, 10 and 5% (v/v) sewage eff l u e n t were used i n a l l t e s t s . On one occasion, a 100% concen-t r a t i o n of sewage e f f l u e n t was tested a f t e r the s a l i n i t y was i adjusted by add i t i o n of Instant Ocean (Aquarium System, Inc) , which was also used as a c o n t r o l . The co n t r o l for a l l other t e s t s was seawater d i l u t e d 50% with tap water. Aeration f or each test container was supplied by laboratory a i r through Pasteur p i p e t t e s . Pre-aeration of the sewage e f f l u e n t before d i l u t i o n was necessitated by the high BOD of the e f f l u e n t . Dissolved oxygen l e v e l s were checked d a i l y (Beckman, Model 777 polarographic oxygen analyser) and were found to be >90% of a i r saturation values at a l l times. Chlorine (12% s o l u t i o n sodium hypochlorite) was added to sewage eff l u e n t on two occasions to produce l e v e l s f o r t o t a l r e s i d u a l chlorine a f t e r 2 h r e a c t i o n time of 1.00 and 2.25 ppm i n the 80% test s o l u t i o n . Measurements of t o t a l r e s i d u a l c h l o r i n e were taken with a Fisher and Porter Model 17 T 1010 amperometric t i t r a t o r using a back t i t r a t i o n procedure with excess phenylarsine oxide s o l u t i o n . 11 The e f f e c t of e f f l u e n t c h l o r i n a t i o n - d e c h l o r i n a t i o n was tested by d e c h l o r i n a t i n g one sample of e f f l u e n t with an excess of 1 M sodium thiosulphate. Test s o l u t i o n s were replaced every 24 h during the bioassays which ran f o r 7 days (168 h). A l l solutions were adjusted to the correct test temperature before being added to the containers. R E S U L T S Sediments Data on the median p a r t i c l e s i z e , i n t e r s t i t i a l s a l i n i t y , organic and heavy metal content of sediments c o l l e c t e d from Sturgeon Bank are given i n Table 1. Sediments with the f i n e s t median grain s i z e (<63um)were generally found along the sewage channel up to s i t e B (F i g . 1), and along transects a, A, b and B on the mudflat. As these s i t e s a l s o showed high levels, of organics (Table 1), i t can be assumed that most of the f i n e p a r t i c u l a t e m a t e r i a l was derived from the sewage e f f l u e n t . This conclusion i s supported by the appearance of the sediments i n these areas, which was of a f i n e , s o f t mud, black i n colour and often smelling of hydrogen s u l f i d e . The sediments seaward of transect C were coarser i n texture, sandy and not di s c o l o u r e d . They were also lower i n organic content than sediments c l o s e r to the sewage o u t f a l l . The values f o r i n t e r s t i t i a l s a l i n i t y 12 recorded over the study area (4.1 - 9.8 °/ 0o) were not extreme despite the presence of the sewage o u t f a l l . The s a l i n i t y data r e f l e c t the r e l a t i v e l y uniform s a l i n i t y of bottom water over the area which was measured by Otte and Levings (1975). The lowest s a l i n i t y recorded (4.1 °/ 0o) was from s i t e b5, adjacent to the major t i d a l drainage channel which flows over the area. The concentration of heavy metals i n the sediments sampled are given i n Table 1. Elevated l e v e l s of most metals were recorded at s i t e s along the sewage channel and adjacent to the sewage o u t f a l l , with the highest values being found at s i t e s a and A. A summary of the sediment metal values by transect and f o r the study area (Table 2) shows that the highest l e v e l s measured were along t r a n s e c t s a and A. The general d i s t r i b u t i o n pattern observed f o r most heavy metals (and organics) i n Sturgeon Bank sediments i s i l l u s t r a t e d f o r copper i n Figure 2. Cadmium and manganese were found to be exceptions to t h i s general p a t t e r n . Cadmium occurred i n high concentrations at only four s i t e s along the sewage channel (Table 1), and manganese was not associated with the e f f l u e n t discharge as sediment concentrations did not change with distance from the sewage o u t f a l l . The conc-e n t r a t i o n s of metals w e r e s i m i l a r to those recorded by B.C. Research (G.V.S.D.D., 1977) and w e r e somewhat lower than those of Bindra and H a l l (1977) f o r t h i s area. The highest l e v e l s f or the metals 13 copper (234 ppm), lead (166 ppm) and zinc (264 ppm) are i n the range of concentrations found i n contaminated sediments from other areas of the world ( P h i l l i p s , 1977) , but are one to two orders ' -of magnitude lower than the highest concentrations published ( P h i l l i p s , 1977). Rates of sediment oxygen uptake are shown i n Table 3. The values ranged from 82 - 141 mg 02/m2/h and are s i m i l a r to the r a t e s p r e v i o u s l y recorded f o r the sediments near the ISTP o u t f a l l (G.V.S.D.D., 1977). The highest value f o r oxygen uptake was f o r the sediment at s i t e b5 which was on the. bank of the t i d a l channel d r a i n i n g the marsh on the east of the study area. The occurrence of M. balthica (Table 3) does not appear to be r e l a t e d to the l e v e l of sediment oxygen uptake and the mean uptake of sediments c o n t a i n i n g Macoma was a c t u a l l y greater than those where Macoma did not occur. Density, Biomass and Condition Factor of M. b a l t h i c a To show more c l e a r l y the e f f e c t s of the sewage e f f l u e n t on M. balthica, i t i s p r e f e r a b l e to minimize the i n f l u e n c e of n a t u r a l f a c t o r s such as t i d a l height. The data f o r Macoma yrlll t h e r e f o r e .: be compared by examining each tra n s e c t , since the e l e v a t i o n across a s i n g l e transect was r e l a t i v e l y constant. Data on the d e n s i t y and biomass of M. balthica i s given i n Table 4 and a comparison by transect i s shown i n Figures 3 and 4 r e s p e c t i v e l y . Outside the area adjacent to the o u t f a l l i n which there was a complete absence of M. balthica^ 14 the s i t e s with the greatest density of M. balthica (1038 i n d i v i d u a l s / m2) were s i t e s C5 and C8. The number of i n d i v i d u a l s along each transect was c o n s i s t e n t l y lower i n the v i c i n i t y of the sewage channel ( F i g . 3). A reduction i n the number of i n d i v i d u a l s at s i t e s f a r t h e s t from the sewage channel was a l s o observed on tran s e c t b and A. A major f a c t o r i n explaining the increase i n d e n s i t y of M. balthica from tra n s e c t b to C i s the corresponding decrease i n t i d a l height. Although tra n s e c t D i s lower i n e l e v a t i o n than C, fewer i n d i v i d u a l s were recorded from'D." \This i s most l i k e l y due to the coarse, sandy, wave-swept sediments which c h a r a c t e r i z e the area seawards of transect C. Generally, the numbers of i n d i v i d u a l s of M. balthica i n the present study were considerably lower than d e n s i t i e s found i n previous studies by Otte and Levings (1975) and the G.V.S.D.D. (1977), who recorded a maximum of 3920/m2 and 3648/m2 r e s p e c t i v e l y i n samples from the same s i t e s . The maximum biomass measured f o r M. balthica was 8.313 g dry weight/m 2 at s i t e b5. The o v e r a l l p a t t e r n f o r biomass (Figure 4) shows a maximum along transect b. This enrichment due to the a d d i t i o n a l input of organic m a t e r i a l from the sewage e f f l u e n t was f i r s t observed by Levings and Cou s t a l i n (1975) and l a t e r by Otte and Levings (1975). Increased biomass f o r M. balthica was a l s o noted at s i t e s C and D, adjacent to the e f f l u e n t channel. 15;. A r e g r e s s i o n equation ( s h e l l height on body weight of M. balthica) was c a l c u l a t e d f o r each s i t e and changes i n the slope of the l i n e served as a c o n d i t i o n f a c t o r (CF). The data (Table 5) show a general increase i n CF with distance from the o u t f a l l . The lowest CF (0.1309) was at s i t e b5 and the highest at s i t e D8 (0.4629). Values f o r CF were low f o r s i t e s along t r a n s e c t b and then p r o g r e s s i v e l y increased for transects B, C and D. However, the data must be i n t e r p r e t e d with caution. Beukema and de Bruin (1977) have shown that >CF i n M. balthica i s independent of length only for the s i z e range of 12-20 i n s h e l l length, and that both very l a r g e and very small i n d i v i d u a l s show c o n s i s t e n t l y lower values than the medium sized animals. mm Age Structure of M. balthica The age-class, frequency d i s t r i b u t i o n f o r M. balthica was p l o t t e d f o r s i t e s on Sturgeon Bank ( F i g . 5) to show any changes due to the sewage e f f l u e n t . I t was evident that f o r s i t e s c l o s e s t to the e f f l u e n t channel where. Macoma was found (c, C, and D), there~was no successful recruitment of newly s e t t l e d spat (age 0+ i n d i v i d u a l s ) . A lower proportion of newly s e t t l e d i n d i v i d u a l s was a l s o found at s i t e s b5, b7 and B8 ( F i g . 5). At s i t e A5, adjacent to the area devoid of M. balthica, there was a high percentage of age 0+ but no 1+ year old i n d i v i d u a l s . T h i s may i n d i c a t e 16 a lack of s u r v i v a l of newly s e t t l e d larvae or j u v e n i l e s at t h i s s i t e . A high proportion of age 0+ and 1+ clams were present at a l l the other s i t e s sampled, but 2+ year old i n d i v i d u a l s were never found i n large numbers. These data d i f f e r considerably from those of Roberts Bank (p. 74) where there was a high proportion of 0+ and 1+ clams at a l l s i t e s , and i n d i v i d u a l s up to 5+ years of age were found. Bioaccumulation in M. balthica The concentration of heavy metals i n the soft t i s s u e s of M. balthica are given i n Table 6. Copper (313.6 ppm), zinc (743.2 ppm) and mercury (6.76 ppm) e x h i b i t e d t h e i r highest concentrations i n clams from s i t e b5 while the highest l e v e l f o r i r o n (1452.1 ppm) was from s i t e b2. Bioaccumulation g e n e r a l l y decreased with d i s t a n c e from the o u t f a l l except f o r s i t e D8 which showed a sharp increase i n the concentration of i r o n , and to a l e s s e r extent f o r copper, zinc and mercury. S u f f -i c i e n t q u a n t i t i e s of M. balthica t i s s u e f o r a n a l y s i s could not be obtained from a l l s i t e s due to the low numbers and small s i z e of the clams present. The l e v e l s of heavy metals i n sediments and M. balthica were compared using a b i o c o n c e n t r a t i o n r a t i o (Table 7). The b i o c o n c e n t r a t i o n r a t i o gives an i n d i c a t i o n of the degree of concentration of metals i n -clam t i s s u e over those l e v e l s found i n the sediment. The greatest range i n tissue:sediment r a t i o s (18.50 - 45.07) was-that- f o r mercury, followed by the r a t i o s f o r zinc (1.27 - 10.04) and copper (1.95 - 7.98). Iron was not concentrated to the same degree having r a t i o s of l e s s than 1 (0.01 - 0.07). The wide range of values f o r 17 the bioconcentration r a t i o s observed suggests that other f a c t o r s than the t o t a l trace metal content of the sediments are important i n regulating t h i s process. A v a i l a b i l i t y w i l l depend upon the p h y s i c a l -chemical nature of the metal-sediment a s s o c i a t i o n (Luoma and Jenne, 1975) and on b i o l o g i c a l f a c t o r s such as the presence of b a c t e r i a (Patrick and L o u t i t , 1976). B a c t e r i a l methylation of mercury i s well established (Woods, 1974), but more recently P a t r i c k and L o u t i t (1976) have shown that low l e v e l s of other metals (eg. Cu, Cr, Mn, Fe, Pb, Zn) i n e f f l u e n t s and sediments are concentrated by b a c t e r i a and than passed to other organisms i n a food chain. M. balthica i s known to feed on the b a c t e r i a attached to sediment p a r t i c l e s (Newell, 1965). Metal l e v e l s i n M. balthica from t h i s area by Bindra and H a l l (1977) with the exception of i r o n , which was reported to be three orders of magnitude higher i n clam ti s s u e than found i n the present study. This d i f f e r e n c e may be due i n part to d i f f e r e n t a n a l y t i c a l techniques used i n the two studies (K. Bindra, pers. comm.). Metal concentration i n t i s s u e s were i n the same range, although s l i g h t l y lower, than those reported for M. balthica by Bryan and Humerstone (1977) i n the i n d u s t r i a l l y polluted Looe estuary (Cornwall). Toxicity Bioassays Results of the acute, l e t h a l bioassays with sewage e f f l u e n t are shown in Table 8. No m o r t a l i t i e s were observed i n any of the bioassays c a r r i e d out i n d i c a t i n g that the e f f l u e n t was non-toxic to adult M. balthica. The problem of lowered s a l i n i t y i n high .. e f f l u e n t concentrations- was- overcome by mixing e f f l u e n t with commercially prepared, Instant Ocean s a l t s , but no deaths r e s u l t e d from exposure to 100% e f f l u e n t concentration (Feb. 6 t e s t , Table 8). An increase i n temperature to 18°C also d i d not r e s u l t i n any m o r t a l i t i e s (Jan. 23, Feb. 6 t e s t s ) . Tests with l a b o r a t o r y c h l o r i n a t e d (Apr. 18, Apr. 25 t e s t s ) , i n - p l a n t chlorinated (May 3 t e s t ) and c h l o r i n a t e d - d e c h l o r i n a t e d sewage e f f l u e n t did not produce any deaths. A d d i t i o n a l observations on the burrowing and feeding behaviour of M. balthica during the bioassays were made as an i n d i c a t i o n of s u b l e t h a l responses, but no e f f e c t s were observed i n any of the c oncentrations tested. The r e s u l t s of the in situ bioassays with M. balthica and the water q u a l i t y parameters measured on the f i n a l day of the test are presented i n Table 9. No m o r t a l i t i e s had occurred a f t e r 7 days which confirmed r e s u l t s of the l a b o r a t o r y bioassays, that the sewage e f f l u e n t was not acutely toxic to M. balthica. A r e d u c t i o n i n s a l i n i t y and d i s s o l v e d oxygen values were recorded from s i t e a c l o s e s t to the o u t f a l l . Residual chlorine was not d e t e c t a b l e i n any of the water samples analysed ' 19 D I S C U S S I O N Several f a c t o r s known to a f f e c t the d i s t r i b u t i o n of benthic i n v e r t -ebrates do not appear to e x p l a i n the p a t t e r n observed f o r M. balthica i n the study area on Sturgeon Bank. The r e l a t i v e l y small range of values f o r i n t e r s t i t i a l s a l i n i t y over the study area i n d i c a t e that the s a l i n i t y of the bottom water adjacent to the sewage o u t f a l l was not lowered s i g n i f i c a n t l y by d i l u t i o n with the e f f l u e n t . T h i s f i n d i n g i s i n agreement with Otte and Levings (1975) who observed that the " f r e s h " sewage e f f l u e n t flowed over the surface of the more s a l i n e incoming t i d e and on the ebb t i d e , flowed seaward i n s i d e the sewage channel. They concluded that s a l i n i t y d i d not i n f l u e n c e the q u a l i t a t i v e d i s t r i b u t i o n of the species found on the mudflat. Some d i l u t i o n of the incoming seawater was measured at s i t e a (3.6 °/ 0 0) but M. balthica i s a strong e u r y h a l i n e species and i s frequently found i n areas with s a l i n i t i e s i n t h i s range (Clay, 1967). Dissolved oxygen values i n the water column have been shown to be lowered (1.5 - 4 mg/1) by the high BOD of the sewage e f f l u e n t i n areas adjacent to the ISTP o u t f a l l (G.V.S.D.D., 1975; Otte and Levings, 1975). However, f o r M. balthica, which extends i t s siphons a short distance i n t o the water o v e r l y i n g the sediment, the oxygen concentra-t i o n near the bottom i s a.more meaningful measure of the oxygen a v a i l a b l e f or r e s p i r a t i o n . A high rate of sediment oxygen demand may r e s u l t i n the d e p l e t i o n of oxygen i n the o v e r l y i n g water which i n turn w i l l a f f e c t the d i s t r i b u t i o n of many species of benthic i n v e r t -ebrates. High r a t e s of sediment oxygen demand were measured on Sturgeon 20 Bank i n the present study but they were not co r r e l a t e d with the d i s t -r i b u t i o n of M. balthica (Table 3). The highest values for sediment oxygen uptake were also not correlated with distance from the o u t f a l l which i s common i n areas of sewage p o l l u t i o n (Smith et al., 1973). Continuously recorded values of dissolved oxygen l e v e l s 1 cm above the sediment near the sewage channel have shown that the oxygen concentration does drop to zero for short periods of time (Otte and Levings, 1975) but only within an area very close to the sewage o u t f a l l (to s i t e A along the sewage channel). At s i t e b, further away from the o u t f a l l but within the area devoid of Macoma, oxygen values of >5 mg/1 were found to be present 97% of the time i n the same study. Laboratory studies have shown that M. balthica i s extremely r e s i s t a n t to low l e v e l s of dissolved oxygen. B r a f i e l d (1963) found M. balthica survived 2-3 days under completely anaerobic conditions, and when very low concentrations of oxygen (0.15 ml/1) are present, M. balthica has been shown to survive for over 20 days (Dries and Theede, 1974). In a d d i t i o n , M. balthica can take up oxygen from the a i r when i t i s not covered with water, by d i f f u s i o n through the mantle c a v i t y when the valves are agape (De Wilde, 1975). Thus, i t does not appear that acute exposure to low l e v e l s of dissolved oxygen can explain the d i s t r i b u t i o n of M. balthica on Sturgeon Bank. The a b i l i t y of Macoma to function and maintain i t s e l f over long periods of time i n conditions of f l u c -tuating oxygen concentrations however, i s unknown. Acute t o x i c i t y t e s t s c a r r i e d out i n the present study indicated 21 that the ISTP e f f l u e n t was :not toxic to adult M. balthica. However, i n an e a r l i e r study, the placement of M. balthica i n f l o a t i n g cages i n the sewage channel r e s u l t e d i n m o r t a l i t i e s of 100, AO and 37% at the o u t f a l l , s i t e A and s i t e C r e s p e c t i v e l y , a f t e r 96 h exposure ( F i s h e r i e s and Marine Service, 1976 unpublished). The d i f f e r e n c e i n t o x i c i t y between the in situ bioassays i n the two s t u d i e s may be explained by the very d i f f e r e n t set of c o n d i t i o n s found i n the sewage e f f l u e n t channel compared with the adjacent mudflat. As mentioned pr e v i o u s l y , on a r i s i n g t i d e the r e l a t i v e l y " f r e s h " e f f l u e n t i s f o r c e d to the surface by the incoming s a l t water and accumulates near the o u t f a l l . But on a f a l l i n g t i d e , the channel i s f i l l e d e n t i r e l y with sewage e f f l u e n t while the mudflat i s exposed. Thus Af. balthica on the mudflat would not be exposed to as high a concentration of e f f l u e n t and the exposure period would be much shorter. The t o x i c i t y of sewage ef f l u e n t i s also highly v a r i a b l e and . -depends upon the toxic c o n s t i t u e n t s present, type of treatment, q u a l i t y of the r e c e i v i n g water, d i l u t i o n at the o u t f a l l , the l e v e l of c h l o r i n a t i o n and at the ISTP the e f f e c t of storm water r u n o f f . "Slug" discharges of t o x i c constituents such as heavy metals occur f r e q u e n t l y i n sewage treatment systems, and have been shown to d r a m a t i c a l l y increase the t o x i c i t y of the e f f l u e n t (Stober et al^. ,1977). Corrosion of household piping has been i d e n t i f i e d as a majoo-rsource of copper "in wastewaters from the Vancouver area (Koch et al., "1977). 22 In a study of wastewater t o x i c i t y at the ISTP, Higgs (1977) recorded copper l e v e l s for the e f f l u e n t to be between 0.11 - .0.15 mg/1 with peak concentrations up to 0.43 mg/1. Copper concentrations measured i n sewage e f f l u e n t samples for bioassays i n the present study were between 0.05 and 0.09 mg/1. This i s well above the 168 h LC50 of 0.035 mg/1 for copper (added as CuCl 2) found f o r the s o f t s h e l l calm, Mya avenavia ( E i s l e r , 1977a). Information on the t o x i c i t y to marine bivalves of complex mixtures of metals s i m i l a r to that found i n sewage e f f l u e n t i s almost non-existent. One of the few papers which addresses t h i s subject ( E i s l e r , 1977b), has shown that the LC50 for Mya avenaria of a mixture of metals with concentrations s i m i l a r to that of the ISTP e f f l u e n t was greater than 10 days. However, i n the same study the bi v a l v e Mercenavia meTcenaria survived 16 weeks of exposure with no m o r t a l i t i e s . C l e a r l y , more research on the t o x i c i t y of complex metal mixtures to marine invertebrates needs to be c a r r i e d out. In t h i s regard, the chemical state or species of trace metal i s an important consideration when t r y i n g to predict the t o x i c i t y of an e f f l u e n t from the r e s u l t s of laboratory experiments. Most laboratory studies expose test organisms to simple so l u t i o n s of metal s a l t s which are not comparable to the s i t u a t i o n i n the n a t u r a l environment. In a review of the b i o l o g i c a l importance of copper i n the sea, Lewis and W h i t f i e l d (1974) state that the b i o l o g i c a l e f f e c t of copper changes with i t s concentration, form, and a b i l i t y to enter into complexes with organic ligands. Sewage e f f l u e n t has been shown 23 to contain s e v e r a l of the complexing agents known to bind heavy metal ions and to decrease t h e i r t o x i c i t y (Bender et at-,. ,1970). The complexing capacity of sewage e f f l u e n t and i t s e f f e c t on reducing metal t o x i c i t y has been demonstrated f o r copper by Lewis et al,. ,(1972) , who showed that a d d i t i o n of unchlorinated ISTP e f f l u e n t to copper enriched seawater increased the s u r v i v a l of the calanoid copepod, Euchaeta japonica. High m o r t a l i t i e s have been observed i n the clam, Venerupis deoussata exposed to 0.1 mg/1 of copper but no acute or s u b l e t h a l e f f e c t s were evident i n the same concentrations of copper which' had been chelated with EDTA (Stephenson and Taylor, 1975). C h l o r i n a t i o n w i l l a l s o s i g n i f i c a n t l y increase the t o x i c i t y of sewage e f f l u e n t ( E s v e l t , 1973). Although the t o x i c e f f e c t s of c h l o r i n e are w e l l documented (Brungs, 1973), very l i t t l e i s known about the t o x i c i t y of combined c h l o r i n e i n marine and estuarine eco-systems. Muchmore and Epel (1973) showed that c h l o r i n a t e d sewage e f f l u e n t i n h i b i t e d sea urchin f e r t i l i z a t i o n at 0.05 ppm t o t a l r e s i d u a l c h l o r i n e a f t e r only 5 minutes exposure. • Larvae of the clam Mercenaria mercenaria were shown to have a 48 h TL50 of <0.005 mg/1 by Roberts et al (1975) and s i m i l a r l e v e l s have 'been found to a f f e c t mollusc l a r v a e i n the James River estuary (Bellanca, 1977). The concentration of t o t a l r e s i d u a l c h l o r i n e i n the r e c e i v i n g water of the ISTP were below detectable l i m i t s (0.01 mg/1) and no acute e f f e c t s ' of the chlo r i n a t e d e f f l u e n t were observed a f t e r the 7 day in situ bioassay 24 with adult M. balthica. Chlorine can combine with a large number of the organic compounds i n sewage e f f l u e n t to form stable organo-chlorinated compounds which are toxic to aquatic b i o t a (Katz and Cohen, 1976) but these were not investigated i n the present study. S u i t a b i l i t y of the substratum has frequently been shown to be the most important factor governing the d i s t r i b u t i o n of benthic animals (Thorson, 1957; Boyden and L i t t l e , 1973; Gray, 1974; Kay and Knights, 1975). P h y s i c a l , (eg. grain s i z e ) , chemical (eg. inorganic and organic compounds) and b i o l o g i c a l (eg. presence of l i v i n g micro-organisms) properties of the sediments act to regulate the establishment and maintenance of benthic communities (Gray, 1974). The influence of the substrate on many bottom species, i n c l u d i n g M. balthica, has been demonstrated to be p a r t i c u l a r l y important i n areas affected by sewage p o l l u t i o n (Anger, 1975; Leppakoski, 1975). Very few studies, however, have investigated further to determine which c h a r a c t e r i s t i c s of the substratum influence benthic populations i n p o l l u t e d areas. P a r t i c u l a t e organic matter associated with sewage sludge has been suggested to be a major substrate r e l a t e d f a c t o r (Watling, 1975), but smaller sediment p a r t i c l e s i z e s , higher sediment metal concentrations, greater numbers of b a c t e r i a , and r i c h deposits of nitrogen and organic carbon are also associated with the sediments i n areas receiving sewage e f f l u e n t (O'Sullivan, 1971). Thus the i d e n t i f i c a t i o n of a s p e c i f i c f a c t o r of causal agent re l a t e d to the sewage p o l l u t i o n i s d i f f i c u l t . 25 Data from the present study (Fig. 5) strongly suggest that substrate properties were responsible for the d i s t r i b u t i o n of M. balthica on Sturgeon Bank. The lack of newly s e t t l e d i n d i v i d u a l s (0+ age group) i n the area where no Macoma were found and at s i t e s along the j e t t y i ndicates that the substrate was unsuitable for su c c e s s f u l recruitment of Macoma spat. If t h i s trend continues, e x i s t i n g populations at the l a t t e r s i t e s w i l l die out i n about two years. The absence of one year old (1+ age group) i n d i v i d u a l s at s i t e A5 suggests that newly s e t t l e d larvae or young j u v e n i l e clams cannot survive at t h i s l o c a t i o n , and therefore t h i s population too, w i l l die out. The apparent settlement at t h i s s i t e may have been the r e s u l t of "secondary s p a t f a l l " ( i e . t i d a l movement from the area of o r i g i n a l settlement) as described by Beukema (1973) . Several a d d i t i o n a l f a c t s support the conclusion that changes i n the substrate are c o n t r o l l i n g the M. balthica populations. A comparison of s i t e s where M. balthica was found i n the present study with those of the most recent, previous study (G.V.S.D.D., 1977), shows the complete d i s -appearance of M. balthica from s i t e a5 i n one year, i n d i c a t i n g that the area a f f e c t e d by the sewage p o l l u t i o n i s expanding. This expansion was also confirmed by the increase i n sediment metal and organic l e v e l s over the same period for s i t e s ( i e . a5, c, C i n F i g . 2) adjacent to the area previously i d e n t i f i e d as having the highest metal concentration. It i s apparent 26 that M. balthica does not occur i n the regions of highest contam-i n a t i o n and that t h i s area i s growing i n s i z e . Based on the data f o r age c l a s s d i s t r i b u t i o n described above, one would p r e d i c t that M. balthica would next be eliminated from s i t e c, and A5. This predicted pattern matches that of the two major contamination g r a d i e n t s , along the j e t t y and south-west along the marsh area as shown by the d i s -t r i b u t i o n pattern f o r copper (Fig. 2). Changes i n the d i s t r i b u t i o n of other i n d i c a t o r organisms (eg. Etone longa) and s e v e r a l species d i v e r s i t y i n d i c e s a p p l i e d to the benthic community have shown t h i s same pattern of expansion which can be explained by changes i n the substrate but not by the d i s p e r s a l pattern of the sewage e f f l u e n t (G.V.S.D.D., 1977). Which property or properties of the substratum are d i r e c t l y r e s p o n s i b l e f o r the absence of M. balthica are not as easy to define. Sediment grain s i z e i s u n l i k e l y to c o n t r o l the d i s t r i b u t i o n of M. balthica as t h i s species has been found to occur i n unpolluted s i l t and c l a y sediments which are as f i n e or f i n e r than those on Sturgeon Bank (Beanland, 1940; Kay and Knights, 1975). Extremely high inputs of organic m a t e r i a l from sewage have been shown to r e s u l t i n the disappearance of the Macoma populations through recruitment f a i l u r e or by non s e l e c t i v e i n f l u e n c e on a l l age groups (Leppakoski, 1975)> hcwever, the area with the highest sediment organic content on Sturgeon Bank i s s i g n i f i c a n t l y - c o r r e l a t e d (p <0.01) with the elevated concentrations 27. of heavy metals (G.V.S.D.D., 1975). The heavy metals i n sediments are u s u a l l y not i n a form to be s u f f i c i e n t l y , b i o l o g i c a l l y a v a i l a b l e to be t o x i c (Bryan, 1976), but evidence i s a v a i l a b l e to suggest that they may be responsible f o r s u b l e t h a l e f f e c t s observed i n marine b i v a l v e s exposed to contaminated sediments ( S t i r l i n g , 1975). S t i r l i n g (1975) noted, -an impaired burrowing response..-_ f o r the b i v a l v e Tellina tenuis when placed on copper -contaminated sediment . An impaired rate of burrowing has a l s o been f o u n d for M. balthica exposed to contaminated sediments from Sturgeon Bank, and mercury and cadmium were . . s t a t i s t i c a l l y s i g n i f i c a n t (p <0.05 and p <0.001 r e s p e c t i v e l y ) i n e x p l a i n i n g the response observed .( t h i s study; pp. 44-5S). An a c t i v e avoidance response by burrowed M. balthica was also demonstrated f o r the most contaminated sediment ( s i t e b) tested. No m o r t a l i t i e s of a d u l t clams i n any of the sediments tested were recorded. What s u b l e t h a l or acute e f f e c t s these high sediment metal l e v e l s could have on r e c e n t l y s e t t l e d Macoma l a r v a e or on young, j u v e n i l e M. balthica are not known. Bioaccumulation of metals has a l s o been shown to be t o x i c i n aquatic i n v e r t e b r a t e s (Spehar et aZ.,1978), but M. balthicahas some pr o t e c t i o n i n the form of a d e t o x i f i c a t i o n system i n v o l v i n g the p r o t e i n m e t a l l o t h i o n e i n (Brown et al . ,1977). However, i f the load or r a t e of loading of heavy metals i s s u f f i c i e n t to overcome the amount of 28 metallothionein produced, ' s p i l l o v e r ' of heavy metals to e s s e n t i a l enzymes w i l l occur r e s u l t i n g i n p a t h o l o g i c a l e f f e c t s . The point at which the metal loading becomes too great for M. b a l t h i c a i s unknown. M o r t a l i t y of s e t t l i n g larvae on a polluted substrate can r e s u l t from d i r e c t t o x i c i t y to the larvae by any number of contaminants i n the sediment. The post-larvae of the majority of bottom dwelling species, s e t t l i n g on a heavily p o l l u t e d substrate, die out very q u i c k l y compared to the larvae of p o l l u t i o n i n d i c a t o r s such as Capitetla aapitata (Mileikovsky, 1970). Leppakoski (1975) noted that i n s p i t e of an immediate improvement i n water q u a l i t y a f t e r p o l l u t i o n abatement i n a Norwegian f j o r d , the harmful e f f e c t s of contaminated sediments i n h i b i t e d benthic r e c o l o n i z a t i o n for several years. Data on the nature and concentrations of organics, metals or other p o l l u t a n t s i n sediments as they a f f e c t s e t t l i n g , metamorphosis and s u r v i v a l of the larvae and j u v e n i l e s of benthic invertebrates are v i r t u a l l y non-existent. In addition to t o x i c i t y , inorganic and organic compounds i n the substrate may i n t e r f e r e with or mask the chemo-reception a b i l i t y of the larvae' used to perceive a s u i t a b l e substrate and induce settlement (Scheltema, 1961). The presence of dead organisms and n o n - l i v i n g organic matter i s also known to be r e p e l l e n t to the larvae of Ophelia bieornis (Wilson, 1955). The presence of b a c t e r i a , " i n numbers neither too many nor too few" (Wilson, 1955), or the surface f i l m produced by c e r t a i n types of b a c t e r i a (Gray, 1966) have been considered c r i t i c a l to induce settlement i n many marine invertebrate larvae. Bacteria i n the sediments on Sturgeon Bank occur i n extremely high numbers due to the influence of the sewage e f f l u e n t (G.V.S.D.D., 29 1977), but the e f f e c t of these abnormally abundant b a c t e r i a on the s u i t a b i l i t y of the substrate f o r l a r v a l settlement i s a matter f o r specul a t i o n . In conclusion, the d i s t r i b u t i o n of M. balthica on Sturgeon Bank appears to be p r i m a r i l y determined by contamination of the s u b s t r a t e . The p o l l u t e d substrate a f f e c t e d the settlement and s u r v i v a l of l a r v a l and j u v e n i l e M. balthica, and the g r o s s l y contaminated area was ;showh to be moving seawards, p r o g r e s s i v e l y excluding Macoma populations. Since heavy metals from the nearby sewage treatment plant occur i n high concentrations i n the sediments, they have been implicated as a po s s i b l e c o n t r o l l i n g f a c t o r . However, the quantity and q u a l i t y of organic matter and b a c t e r i a may a l s o be important. Research i n t o the e f f e c t s of contaminated sediments on the settlement and s u r v i v a l of the larvae of benthic invertebrates i s l a c k i n g and more data on the s p e c i f i c nature of these i n t e r a c t i o n s i s req u i r e d . TABLE 1 PHYSICAL AND CHEMICAL CHARACTERIZATION OF SEDIMENTS IN STUDY A R E A * - J U L Y , 1977 Median T . . , Organic Heavy metals (ug/g dry wt) . ., I n t e r s t i t i a l ° , P a r t i c l e 0 ,. . ^  Carbon S i t e S a l i n i t y „ ~ I1" co«r *« M C ° - CU S/KS HS M- BI PT ZN a <63 8. 1 4. 28 1. 0 3. 0 14 124 234 33.5 0.89 322 46 166 264 a3 <63 5. 9 1. 52 3. 2 0. 4 13 72 95. 0 30.0 0.27 296 41 48 134 a5 <63 6. 1 1. 13 2. 6 <0. 4 13 60 67. 0 30.0 0.18 300 41 32 109 A <63 5. 1 3. 12 1. 4 2. 0 15 108 180 34.5 0.57 332 46 134 212 A3 <63 5. 9 1. 62 4. 4 0. 6 14 79 106 33.5 0.28 330 41 56 145 A5 <63 5. 6. 0. 87 2. 0 <0. 4 13 58 • 47. 0 30.0 0.16 310 40 26 93 b <63 5. 9 3. 09 1. 0 1. 4 15 90 150 36.0 0.46 340 49 74 172 b2 110 4. 2 0. 47 1. 0 <0. 4 10 44 27. 2 21.0 0.12 220 32 16 64 b5 70 4: .1 0. 56 1. 4 <0. 4 11 55 39. 6 26.5 0.15 274 38 20 74 b7 <63 5. 0 0. 83 1. 4 <0. 4 13 56 43. 0 29.5 0.16 300 41 22 84. B <63 5. 0 2. 01 1. 2 0. 8 14 78 110 33.5 0.32 318 47 62 150 B2 127 4. 5 0. 41 0. 6 <0. 4 10 38 21. ,4 20.5 0.08 228 31 14 54 B5 <63 5. ,8 1. ,12 1. ,6 <0. ,4 13 61 64. ,0 32.0 0.16 316 42 32 104 B8 <63 4. ,5 - 0. ,74 1. ,0 <0. ,4 12 48 35. .2 27.5 0.11 330 37 20 80 c 135 6, ,1 0, .92 1, .4 0.4 11 44 38, .0 21.0 0.15 226 35 20 82 C 120 5, .1 o, .59 1, .0 0, .4 11 40 30, .0 20.0 0.13 214 32 18 73 C2 168 9 .8 0, .23 <0 .4 <0 .4 11 32 13 .8 20.6 0.05 282 31 6 50 C5 168 6 .9 0 .20 <0 .4 <0 .4 12 33 13 .4 21.0 0.04 330 32 6 50 C8 130 9 .2 0 .14 <0 .4 <0 .4 11 33 13 .8 20.0 0.04 342 33 6 54 D 172 7 .0 0 .49 <0 .4 <0 .4 10 35 15 .6 19.0 0.08 232 32 6 52 D2 170 9 .8 0 .17 <0 .4 <0 .4 10 34 12 .4 19.6 0.04 310 30 6 45 D5 250 9 .0 0 .13 <0 .4 <0 .4 10 28 12 .6 19.0 0.04 340 28 3 41 D8 168 6 .0 0 .12 <0 .4 <0 .4 10 32 12 .8 19.0 0.03 330 31 4 47 *see Figure 1 TABLE 2 SUMMARY OF SEDIMENT METAL DATA (Mean ± S . D . , y g / g d r y w e i g h t ) Transect Metal Study Area Fe (g/Kg) D Ag <1.3 ± 1.0 2.3 ± 1.1 2.6 ± 1.6 1.2 ± 0.2 1.1 ± 0.4 <0.6 ± 0.3 <0.4 ± 0.0 Cd <0.6 ± 0.6 1.3 ± 1.5 <1.0 ± 0.9 <0.6 ± 0.5 <0.5 ± 0.2 <0.4. ± 0.0 <0.4 ± 0.0 Co 12.0 ± 1.7 13.3 ± 0.6 14.0 ± 1.0 12.2 ± 2.2 12.2 ± 1.7 11.2 ± 0.5 10.0 ± 0.0 Cr 55.7 ± 25.8 85.3 ± 34.0 81.7 ± 25,1 61.2 + 19.9 56.2 ± 17.3 34.5 ± 3.7 32.2 ± 3.1 Cu 60.1 ± 6 0 . 0 132 ± 89.4 111 ± 66.7 65 ± 57 57.6 ± 39.2 17.8 ± 8.2 13.4 ± 1.5 26 ± 6.2 31.2 ± 2.0 32.7 ± 2.4 28.2 ± 6.2 28.4 ± 5.8 20.4 ± 4.5 19.2 ± 0.3 Hg 0.20 ± 0.20 0.45 ± 0.39 0.34 ± 0.21 0.22 ± 0.16 0.17 ± 0.11 0.06 ± 0.04 0.05 ± 0.02 Mn 297 ± 43 306 ± 14 324 ± 12 284 ± 50 298 ± 47 292 ± 58 303 ± :49 Ni 37.2 ± 6.2 42.7 ± 2.9 42.3 ± 3.2 40.0 ± 7.1 39.2 + 6.8 32.0 ±'0.8" 30.2 ± 1.7 Pb 35 ± 4 2 82 ± 73 72 ± 5 6 33 ± 27 32 ± 21 9 ± 6 5 ± 2 Zn 97.0 ± 58.0 169 ± 83.2 150 ± 59.6 98.5 ± 49.7 97.0 ± 40.8 56.8 ± 11.0 46.2 ± 4.6 TABLE 3 32 SEDIMENT OXYGEN .UPTAKE RATE AND PRESENCE/ABSENCE OF M. balthica AT SEVERAL S I T E S OVER STUDY AREA - SEPTEMBER, 1977 Mean (±S.E.) Oxygen Uptake Values f o r Areas of M. balthica Absence and Presence a3 • 82 ± 9 A A3 108 ± 4 A x = 96 ± 8 a5 100 ± 13 A A5 107 ± 16 P b5 141 ± 19 P x = 125 ± 10 B5 126 ± 4 P Oxygen Uptake Rate mg 02/m2/h M. balthica S i t e (±S.E.) Present/Absent TABLE 4 DENSITY AND BIOMASS FOR M. balthica FROM S I T E S ON STURGEON BANK Density Biomass (No./m2 ± S.E.) (g dry weight/m 2) a absent a3 n -a5 t i -A I I -A3 I I -A5 128 + 21 1.061 b absent — b2 391 + 51 4.529 b5 405 + 54 8.313 b7 256 + 54 2.880 B absent — B2 697 + 56 1.640 B5 476 + 188 3. 615 B8 384 + 49 2.875 c 121 + 19 0.388 C 199 + 68 6.486 C2 1010 + 82 1.662 C5 1038 + 130 1.855 C8 1038 + 179 2.369 D 412 + 14 3.862 D2 544 + 140 1.037 D5 576 + 12 1.415 D8 491 + 54 0.462 34 TABLE 5 VALUES FOR y INTERCEPT ( a ) AND SLOPE ( b ) OF REGRESSION l o g 1 0 y = a + bx WHERE y = MEAN DRY FLESH WEIGHT IN MG, AND x = SHELL HEIGHT IN MM FOR M. b a l t h i c a Sample S i t e Size • a b A5 4 -1.2100 0.1855* b2 15 -0.5331 0.1511* b5 15 -0.4524 0.1309* b7 9 -0.6447 • 0.1536* B2 4 -1.5030 0.2877* B5 8 -0.6591 0.1702* B8 12 -0.7533 0.1637* c 5 -1.4600 0.3256* C 9 -0.7364 0.1834* C2 9 -1.5470 0.2538* C5 13 -1.2280 0.2353* C8 13 -1.0790 0.2140* D 4 -0.9281 0.1688* D2 7 -1.1210 0.2403* D5 9 -1.2100 0.2249* D8 6 -2.2640 0.4269* * S i g n i f i c a n t at p <0.05 l e v e l TABLE 6 35 CONCENTRATION OF HEAVY METALS IN T I S S U E S OF M. balthica ( u g / g d r y w e i g h t ) S i t e Cd* Cu Fe Hg Pb** Zn b5 313.6 475.6 6.76 743.2 (27.4) (41.6) (0.60) (65.0) B2 170.7 1452.1 2.43 — 392.2 (15.4) (131.1) (0.22) (35.4) C5 _ 74.6 699-6 0.76 - 531.7 (6.6) ' (62.3) (0.07) (47.3) D5 49.1 368.6 0.74 - 399.3 (4.8) (36.2) (0.07) (39.2) D8 51.4 797.3 0.89 - 406.4 (4.8) (74.5) (0.08) (40.0) Numbers i n brackets = ug/g wet weij ;ht. * Concentration i n ** Concentration i n 25 ml digest l e s s than detectable l i m i t of 0.02 ug/1. 25 ml digest l e s s than detectable l i m i t of 0.2 ug/1. TABLE 7 BIOCONCENTRATION RATIO OF METALS IN M. balthica 36 Tissue/Sediment Ratio S i t e Cu Fe Hg Zn b5 7.92 0.02 45.07 10.04 B2 7.98 0.07 30.38 7.26 C5 5.57 0.03 18.50 10.63 D5 1.95 0.01 18.50 4.27 D8 4.02 0.04 29.67 8.64 TABLE 8 RESULTS OF LABORATORY TOXICITY BIOASSAYS WITH M.- balthica T W of n ^ p T e s t ^est e f f l u e n t . S a " " " y % M o r t a l i t y Type of Date t e m p e r a t u r e concentration i n h i S h e s t i n highest sewage e f f l u e n t tested ' , concentration e f f l u e n t concentration ( C ) U V / V ) tested (°/oo) (168 h) Unchlorinated Jan. 23/78 Jan. 23/78 Feb. 6/78 Feb. 6/78 Apr. 18/78 May 3/78 10 18 10 18 10 10 80 80 1001 1001 80 80 5.2 5.2 12.2 12.2 5.1 5.1 0 0 0 0 0 0 Chlorinated Apr. 18/78 Apr. 25/78 May 3/78 10 10 10 80 2 80 3 80 4 5.1 5.1 5.1 0 0 0 Dechlorinated Apr. 18/78 10 80 5.1 l R S a l i n i t y adjusted with Instant Ocean 2 I n i t i a l l e v e l of r e s i d u a l c h l o r i n e i n 80% e f f l u e n t concentration was 1.00 ppm 3 I n i t i a l l e v e l of r e s i d u a l c h l o r i n e i n 80% e f f l u e n t concentration was 2.25 ppm ^ I n i t i a l l e v e l of r e s i d u a l c h lorine i n 100% ef f l u e n t concentration was 0.5 ppm T A B L E 9 38 IN SITU BIOASSAY RESULTS AND WATER QUALITY D A T A , MAY 8 , 1978 % M o r t a l i t y S a l i n i t y Temperature Dissolved T o t a l Residual S i t e (168 h) (°/oo) (°C) Oxygen (ppm) Chl o r i n e (ppm) a 0 3-6 17.5 1.9 N.D.* A 0 14.1 17.0 7.2 N.D. B 0 15.2 13.0 7.6 N.D. C 0 15.2 17.5 7.0 N.D. A-control (North Side 0 16.8 13.5 7.1 N.D. of J e t t y ) E f f l u e n t - <0.1 20.0 3.1 0.50 *N.D. = none detected D C c B b A a Iona Jetty + + + + • + + + „ Sewage Effluent FIGURE 1 . L o c a t i o n o f s t u d y a r e a and s a m p l i n g s i t e s on S t u r g e o n B a n k , F r a s e r R i v e r e s t u a r y , B r i t i s h C o l u m b i a . FIGURE 2 . D i s t r i b u t i o n p a t t e r n f o r c o n c e n t r a t i o n o f c o p p e r (ppm) i n s e d i m e n t s o v e r s t u d y a r e a . 41 1 2 0 0 i iona 2 4 6 8 Jetty S i t e n u m b e r a long t r ansec t FIGURE 3. D e n s i t y o f M. balthica a l o n g t r a n s e c t s a c r o s s s t u d y a r e a . 42 Jetty S i t e n u m b e r a l o n g t r a n s e c t F I G U R E 4 . B i o m a s s o f M. balthica a l o n g t r a n s e c t s a c r o s s s t u d y a r e a . FIGURE 5 . Age c l a s s f r e q u e n c y d i s t r i b u t i o n o f M. balthica a t s i t e s w i t h i n s t u d y a r e a . S u b l e t h a l E f f e c t s o f H e a v y M e t a l C o n t a m i n a t e d S e d i m e n t s on t h e B i v a l v e Macoma balthica ( L ) . I n t r o d u c t i o n Increasing a t t e n t i o n i s being paid to the s u b l e t h a l e f f e c t s of p o l l u t a n t s on marine organisms. The importance of measuring changes i n behaviour, physiology and metabolism has been recognized as e s s e n t i a l to the assessment of chronic e f f e c t s of low l e v e l s of contaminants i n the oceans. The use of such t e s t s as routine bioassays was suggested by S t i r l i n g (1975) who reported e f f e c t s on the burrowing behaviour of the b i v a l v e Tellina tenuis a f t e r exposure to s u b l e t h a l concentrations of copper and phenol. More r e c e n t l y , behavioural studies with molluscs have been used to measure the su b l e t h a l e f f e c t s of copper (Stephenson and T a y l o r , 1975), o i l (Stainken, 1976; Linden, 1977; G i l f i l l a n and Vandermuelen, 1978) and s o l i d wastes from ocean dumping (Chang and Levings, 1976). The present paper describes the r e s u l t s of a set of experiments designed to i n v e s t i g a t e the e f f e c t of heavy metal contaminated sediments on the burrowing and avoidance behaviour of the b i v a l v e , Macoma balthica ( L . ) . 46 The taxonomy of Macoma balthica (Linnaeus, 1758) [= M. inconspicua (Broderip and Sowerby, -1829)] on the west coast of North America i s i n need of review. The fundamental question of i n t e r e s t i s whether the A t l a n t i c and P a c i f i c specimens of t h i s v a r i a b l e bivalve are separate (although c l o s e l y related) species or a s i n g l e species. In the most recent paper to date, Coan (1971) has suggested that u n t i l a major pr o j e c t addressing t h i s subject i s undertaken, i t should be considered as one circum-arctic species. This recommendation w i l l be followed here as i t has i n other studies from the P a c i f i c coast (eg. Shaw et al. , 1976). M e t h o d s Test containers were polyethylene tubs approximately 35 x 15 x 14 cm i n size f i l l e d to 5 cm depth with, sediment and covered by 2.5 I of seawater. Aeration was through a Pasteur p i p e t t e placed i n the centre of the tank and care was taken not to disturb the sediment. A l l t e s t s were c a r r i e d out at 10 ± 1°C. .M: balthica (5-10 mm s h e l l length) used i n the tests were c o l l e c t e d from an unpolluted mudlfat (Roberts Bank) i n the Fraser River estuary, B r i t i s h Columbia and acclimated to 47 laboratory conditions f o r 48 h. Sediments used as c o n t r o l s were also taken from t h i s area. Contaminated t e s t sediments were c o l l e c t e d at varying distances from a sewage treatment plant o u t f a l l which discharges onto a t i d a l f l a t i n the Fraser River estuary. E f f e c t on burrowing behaviour was measured by f i l l i n g each of f i v e containers with a d i f f e r e n t sediment, p l a c i n g ten M. balthica on each sediment and observing the number of animals which had burrowed at i n t e r v a l s of 15, 30, 45, 60, 90, 120 min. and 6, 12, 24 and 48 h. The avoidance t e s t s were set up with c o n t r o l (non-polluted) mud i n one ha l f of each container and a d i f f e r e n t contaminated sediment in the other h a l f . Ten M. balthica were added with four i n d i v i d u a l s being placed on each side and two on the boundary between the two sediment types. A f t e r 24 h, the water was drained o f f , and sediments from each h a l f of the container were, removed.and sieved to count the number of M. balthica. The numbers burrowed i n each p a i r of substrates at the end of the t e s t were compared using the binomial t e s t f o r n <25 with p = 0.05. Analyses were 1 - t a i l e d i n favour of the c o n t r o l substrate (Larkin, 1976). R e s u l t s The nature of the t e s t sediments used have been described and characterised i n Table 1 by t h e i r organic and heavy metal content. The data are those reported by McGreer (1978). Contaminated sediments (A-D) were c o l l e c t e d at increasing distances from a major sewage e f f l u e n t discharge and r e f l e c t e d the concentration of contaminants along a p o l l u t i o n gradient which decreased c o n s i s t e n t l y with distance from the source. The metals which showed elevated l e v e l s c l o s e to the o u t f a l l (Site A, Table 1) were copper, lead, z i n c , chromium, mercury, cadmium and i r o n . Burrowing Behaviour Slower burrowing rates f o r M. balthica were observed i n a l l sediments when compared to the c o n t r o l ( F i g . 1). A l l ten i n d i v i d u a l s burrowed i n the c o n t r o l , B, and D sediments compared to 90% i n C and only 60% i n A. The median e f f e c t i v e time for burrowing (ET50) and 95% confidence l i m i t s are shown i n Table 2. The values were ca l c u l a t e d from a log-probit p l o t ( F ig. 1) according to the methods of L i t c h f i e l d and Wilcoxon (1949). The ET50 increased from 0.17 h for the c o n t r o l and D to i 8 h i n sediment A, the most heavily contaminated. A l i n e a r regression of the concentration of each i n d i v i d u a l metal i n the sediments versus the burrowing response time was c a l c u l a t e d and•the r e s u l t s are given i n Table 3. The only s i g n i f i c a n t (p <0.05) regression l i n e s were f o r the metals mercury and cadmium. Of these, the regression f or cadmium was h i g h l y s i g n i f i c a n t (p <0.001) and explained 98.5% of the t o t a l variance i n the model. Avoidance Behaviour D i s t r i b u t i o n of burrowed M. • balthica a f t e r 24 h i n t e s t tanks containing c o n t r o l and contaminated sediment i s shown i n Table 4. None of the animals were found buried at the boundary between the two sediment types. A s t a t i s t i c a l l y s i g n i f i c a n t (p <0.05) avoidance response was found f o r the most contaminated sediment (A). Although 70% of the animals were found i n the c o n t r o l side i n the test with sediment B, t h i s was not s t a t i s t i c a l l y s i g n i f i c a n t at the small sample size used. No avoidance was evident i n t e s t s with sediments C, D, or from one c o n t r o l to another. The threshold f o r the avoidance response thus occurred between sediments B and C. This response was consistent with the increase i n heavy metal concentrations i n these sediments. D i s c u s s i o n The e f f e c t s of contaminated sediments on the burrowing response of M. balthica are s i m i l a r to those observed f o r bi v a l v e s i n other studies ( S t i r l i n g , 1975; Linden, 1977). A mechanism to account f o r t h i s response has not been described; however, i t should be noted that more than one explanation i s possible f o r the observations previously recorded. S t i r l i n g (1975) measured the burrowing response of animals exposed to varying concentrations of copper i n s t a t i c bioassays, then recorded t h e i r recovery when placed i n clean seawater i n the same containers. There was l i t t l e recovery of burrowing a b i l i t y with copper but i t was noted that t h i s could have been due to the e f f e c t s of exposure on metabolism, or r e s i d u a l copper i n the sand. In contrast, an experiment using a flow-through system to test copper e f f e c t s (Stephenson and Taylor, 1975), found i n h i b i t i o n of burrowing and f u l l recovery i n the clam, Venevupis decussata. S t i r l i n g (1975) also found that recovery from phenol was fa s t e r i n flow-through than i n s t a t i c t e s t s . Inadvertent contamination of clean sediments f o r use i n recovery t e s t s was not considered i n these experiments. Contamination of clean sediments i s more l i k e l y to occur i n s t a t i c rather than flow-through condi t i o n s . Contamination of test sediments could also explain the lack of recovery from o i l for M. balthica observed by Linden (1977). Results of the present study confirmed that contaminated sediments a f f e c t burrowing behaviour and demonstrated the need to use clean sediments i n behavioural t e s t s . This need i s c r i t i c a l i n studies designed to te s t whether the toxicant i n s o l u t i o n or i n a previously contaminated sediment i s causing t h e . i n h i b i t e d burrowing response. Studies c a r r i e d out to date suggest that both types of exposure may be important. Results of the present study also i n d i c a t e d that the burrowing behaviour of larger i n d i v i d u a l s was af f e c t e d more than smaller i n d i v i d u a l s . Generally, the smaller animals burrowed f i r s t and i n the test with sediment C, the one i n d i v i d u a l which never burrowed ( F i g . 1) was the l a r g e s t i n s i z e . These observations are consistent with the burrowing behaviour for d i f f e r e n t s i z e s of b i v a l v e s reported by S t i r l i n g (1975) and Linden (1977), but an explanation f o r these observations has not been apparent. These observations are contrary to the e f f e c t s of toxic a n t s on p e l a g i c species which generally show an increasing tolerance to p o l l u t a n t s with increasing s i z e and age (Sprague 1969, 1970). Avoidance of metal-contaminated sediments was r e c e n t l y demonstrated f o r midge larvae (Chironomus tentans) by Wentsel et al. (1977). A d i f f e r e n t threshold response l e v e l f or avoidance was observed f o r each metal. The threshold values were high (eg. 213-422 ppm cadmium) and a l i n e a r r e l a t i o n s h i p between sediment metal l e v e l and avoidance was found for two of the metals tested, namely cadmium and z i n c . The avoidance data from the present i n v e s t i g a t i o n supports the hypothesis of a c t i v e avoidance of metal contaminated sediments by benthic invertebrates. In both the present study and the one by Wentsel et al. (1977), the concentration of cadmium i n the sediment was most s i g n i f i c a n t i n exp l a i n i n g the behavioural responses observed. Laboratory experiments using c o n t r o l l e d concentrations and combinations of i n d i v i d u a l metals are required to test t h i s hypothesis f u r t h e r . An examination of the chemical forms of heavy metals i n the sediments used i n the chironomid avoidance t e s t s (Shepard c i t e d i n Wentsel et al., 1977), showed that l e s s than 1% of the cadmium and zinc was i n the "adsorbed or exchangeable form" ( s i c . ) . The f a c t that an avoidance 53 response was observed i s of i n t e r e s t i n l i g h t of the observations by Stephenson and Taylor (1975), who found no i n h i b i t i o n •of burrowing response with clams (Venerup'Ls deaussata)' exposed to copper i n s o l u t i o n which had been chelated with EDTA. Both the burrowing and avoidance responses described above may be u s e f u l f o r marine benthic bioassays, i n conjunction with standard chemical t e s t s , t o assess the environmental impact of contaminated^sediments. The e c o l o g i c a l s i g n i f i c a n c e of an i n h i b i t e d burrowing response i s obvious (eg. exposure to predators and wave action) but the s i g n i f i c a n c e of the avoidance response remains to be tested i n the f i e l d . Benthic invertebrates have always been considered to be sedentary and thus good i n d i c a t o r s of p o l l u t e d c o n d i t i o n s . However, as suggested by the r e s u l t s of t h i s study, these organisms may a c t i v e l y »vr>i_d sediments which become contaminated. This f a c t may cc important i n explaining the d i s t r i b u t i o n of organisms i n p o l l u t e d areas and avoidance t e s t s should be c a r r i e d out as part of future studies assessing environmental" ; impacts as they have, been shown to be a s e n s i t i v e i n d i c a t o r . TABLE 1 CHARACTERIZATION OF CONTAMINATED SEDIMENTS USED IN BURROWING AND AVOIDANCE TESTS O r g a n i c M e t a l s (ppm d r y we i g h t ) Sediment S u b s t r a t e C o n t e n t Sample Type (%) Cu Pb Zn Cr Ag Hg Cd Fe (g/Kg) A mud 3.09 150 74 172 90 1.0 0.46 1.40 36.0 B mud 1.52 95 48 134 72 3.2 0.27 0.40 30.0 C mud 1.13 67 32 109 60 2.6 0.18 <0.40 30.0 D muddy-sand 0.59 30 18 73 40 1.4 0.13 0.40 20.0 C o n t r o l mud 0.13 13 3 41 34 0.4 0.04 <0.40 19.0 55 TABLE 2 THE MEDIAN E F F E C T I V E TIME ( E T 5 0 ) AND 95% CONFIDENCE L I M I T S FOR BURROWING OF M. balthica IN SEDIMENTS WITH DIFFERENT CONCENTRATIONS OF HEAVY METALS Sample ET50 (h) 95% Confidence I n t e r v a l s A 4.8 2.4 - 9. 6 B 0.76 0.46 - 1. 27 C 0.28 0.24 - 0. 33 D 0.17 0.15 - 0. 19 Control 0.17 0.15 - 0. 19 TABLE 3 VALUES FOR y - I N T E R C E P T ( a ) AND SLOPE ( b ) OF REGRESSION y = a + bx WHERE y = ET50 FOR BURROWING RESPONSE IN HOURS AND x = I N D I V I D U A L METAL CONCENTRATION (ppm) IN SEDIMENTS Metal a b Percent of explained t o t a l v a r i a n c e by r e g r e s s i o n Cu -1.0320 0.0319 75.2 Pb -0.9475 0.0624 72.7 Zn -2.035 0.0309 62.0 Cr -2.966 0.0710 66.1 Ag 1.999 -0.4436 <1.0 Fe -4.338 0.2065 56.0 Hg -1.213 11.34* 81.4 Cd -1.437 4.455** 98.5 * S i g n i f i c a n t at p <0.05 * * S i g n i f i c a n t at p <0.001. TABLE 4 RESULTS OF SEDIMENT AVOIDANCE TESTS WITH M. balthi Sample P a i r o n t r o l v s , v s , v s . v s , v s , T e s t Sediment B C D C o n t r o l No. and l o c a t i o n o f T o t a l no. S i g n i f i c a n t a t clams a f t e r 24 h burrowed 95% c o n f i d e n c e l e v e l C o n t r o l •'9 7 4 4 5 T e s t Sediment 3 6 6 5 10 10 10 10 10 Yes No No No No T3 CD O L_ D JO c o Z5 a o a o o 95 r 9 0 70 5 0 ro 20 10 5 2 1 0.1 control / & / D d 1 0 0 % control 100 °/o 100 % * x D B FIGURE 1 J I I i i i J 1 1 1 I 1 L 0.5 1.0 4 6 12 T i m e (h) 24 4 8 B u r r o w i n g r a t e s f o r M. balthica i n s e d i m e n t s A ( . ) , „ ( X ) , C ( + ) , D ( A ) and c o n t r o l ( • ) , c o n t a i n i n g d i f f e r e n t l e v e l s o f h e a v y m e t a l s . 00 GROWTH AND REPRODUCTION OF Macoma balthica ( L . ) ON A MUDFLAT IN THE FRASER RIVER ESTUARY, B R I T I S H COLUMBIA 60 INTRODUCTION The Fraser River estuary i s located i n the southeastern corner of the S t r a i t of Georgia and i s characterized by an expanse of i n t e r t i d a l mud-f l a t s (Fig. 1). Seaward of the Main Arm of the Fraser River i s Roberts Bank, an a c t i v e , high energy t i d a l f l a t with a v a r i e t y of substrate types. Surface (to 1 m) s a l i n i t y values for the intermediate and upper i n t e r t i d a l h a b i t a t s range from fresh water to over 2 6 ° / 0 0 (Levings and Coustalin, 1975). Two rock j e t t i e s extend i n t o deep water at the edge of the Bank for use as a co a l port f a c i l i t y (Westshore Terminals) and a f e r r y terminal (Tswwassen). The present study was p r i m a r i l y concerned with seasonal changes i n growth and reproduction of M. balthica on the Roberts Bank mudflat. Although b i o l o g i c a l research on estuarine mudflats has been extensive i n Europe (eg. Anderson, 1972; Beanland, 1940; Chambers and Milne, 1975; Lammens, 1967) and the east coast of North America ( G i l b e r t , 1973; Burke and Mann,-1974; T u n n i c l i f f e and Risk, 1977), there have been r e l a t i v e l y few d e t a i l e d studies from the P a c i f i c coast. Studies on M. balthica have included research on v e r t i c a l d i s t r i b u t i o n ( Vassallo, 1969), s t r a t i f i c a t i o n within the substrate (Vassalo, 1971), general ecology and d i s t r i b u t i o n (Dunnill and E l l i s , 1969), biomass and production (Nichols, 1977) and the e f f e c t s of o i l p o l l u t i o n (Myren and P e l l a , 1977). This paper i s the f i r s t published report of the seasonal changes i n reproduction, age-class structure and depth d i s t r i b u t i o n of an i n t e r t i d a l population of M. balthica from the P a c i f i c coast. 61 The study area -was located to the north of the Westshore Terminals causeway i n the upper i n t e r t i d a l zone ( F i g . 1). Despite the proximity of i n d u s t r i a l development, t h i s area of the mudflat has not been shown to be affected by p o l l u t i o n s t r e s s . Due to the large area of gently sloping t i d a l f l a t s , access to the region i s d i f f i c u l t and few studies have been previously undertaken. A preliminary baseline study of the macrobenthos was c a r r i e d out by Bawden et al. (1973) which described the major species composition of the area. Tellina carpenteri was found to be a dominant b i v a l v e i n the area hut a more recent examin-a t i o n of t h i s m a terial has shown these specimens to be, i n f a c t , M. balthica (L.). The most comprehensive study of the Fraser River estuary benthos has been by Levings and Coustalin (1975). Data on the species composition, biomass, density and sediment type were c o l l e c t e d f o r 116 quadrat s t a t i o n s throughout the estuary. More r e c e n t l y , the r o l e of t i d a l f l a t benthos, i n c l u d i n g Macoma balthica, i n animal-sediment r e l a t i o n s h i p s has been i n v e s t i g a t e d (Swinbanks and Murray, 1977). The taxonomy of Macoma balthica (Linnaeus, 1758) [= M. inconspicua (Broderip and Sowerby, 1829)] on the west coast of North America i s i n need of review. The fundamental question of i n t e r e s t i s whether the A t l a n t i c and P a c i f i c specimens are separate (although c l o s e l y r e l a t e d ) species or a s i n g l e species. In the most recent paper, Coan (1971) has suggested that u n t i l a major pr o j e c t addressing t h i s subject i s undertaken, i t should be considered as one circ u m - a r c t i c species. This recommendation w i l l be followed here and as i t has i n other studies from the P a c i f i c coast (eg. N i c h o l s , 1977; Vassalo, 1969). 62 METHODS Sampling of M. balthica was c a r r i e d out approximately at monthly i n t e r v a l s between A p r i l 7, 1977 and March 16, 1978 on Roberts Bank at a t i d a l height of approximately 3 m above chart datum, as estimated from l o c a l t i d e s . Monthly samples were taken at 5 s t a t i o n s marked at 5 meter i n t e r v a l s along a transect projected perpendicular from the Westshore Terminals causeway. The f i r s t s t a t i o n was located 200 meters north, outside the immediate v i c i n i t y of the causeway. On each occasion f i v e random samples were taken with an aluminum box corer (12.5 cm x 12.5 cm x 25 cm deep) from one of the s t a t i o n s . The cores were sectioned at 5 cm i n t e r v a l s to determine depth d i s t r i b u t i o n and sieved through a 500 um mesh screen. Sieved samples were returned to the laboratory where a l l M. balthica were sorted, enumerated, and allowed to depurate i n clean seawater f o r 24 hours before being frozen f o r l a t e r a n a l y s i s . The s i z e (height) of the s h e l l s from the umbo to the v e n t r a l edge was measured to the nearest 0.5 mm, and age was determined by counting annual growth r i n g s using the methods of Lammens (1967). Animals were placed i n t o year classes from age 0+ (newly s e t t l e d spat) to 5+ year old i n d i v i d u a l s . A l l t i s s u e was removed from s h e l l s over 1.0 mm i n height and d r i e d at 60°C for 24 hours. Tissues from s h e l l s of the same s i z e were pooled and a mean dry weight determined f o r each s i z e category. A c o n d i t i o n f a c t o r (CF) was then c a l c u l a t e d as the slope (b) i n the reg r e s s i o n l°8l0 y = a + bx, where y = mean dry t i s s u e weight and x = s h e l l height. 63 S i g n i f i c a n c e for the regression f u n c t i o n was calculated using the explained and unexplained variance, and expressed as the p r o b a b i l i t y of the slope being equal to zero (Table 3). The spawning c y c l e of the M. balthica population was studied by examining the state of gonad development each month. Animals were placed i n one of four categories as described by Caddy (1967): a) immature -gonads developing but not reaching the point of g i l l and palp attachment; b) mature - gonads developed past point of g i l l and palp attachment; c) u n d i f f e r e n t i a t e d - no gonad t i s s u e ; and d) p a r a s i t i z e d - p a r a s i t e s developing i n place of gonads. On each sampling date, temperature of the a i r , sediment (at a depth of 5 cm), and seawater (1 m depth) was measured. Surface seawater s a l i n i t y was measured i n the laboratory with a Yellow Springs Instrument, Model 33, Salinity-Conductivity-Temperature Meter. D i l u t i o n due to rainwater was not a f a c t o r on any of the sampling occasions. RESULTS Density The number of M. balthica .1+ and older reached a peak (1817/m2) i n l a t e A p r i l , 1977, then slowly declined throughout the year ( F i g . 2). Settled spat (age 0+ years) were present i n the A p r i l samples but disappeared l a t e r as these i n d i v i d u a l s formed a growth r i n g and became 1+ year o l d . Low numbers of newly s e t t l e d spat were present again from l a t e J u l y 64 through the f a l l and winter months u n t i l reaching t h e i r peak number of 410/m2 i n March (Table 1). This density of s p a t f a l l i s exceedingly low compared to the over 6000/m2 0+ i n d i v i d u a l s recorded by Chambers and Milne (1975) i n the Ythan estuary, using s i m i l a r sampling techniques. However, the number of 1+ and older i n d i v i d u a l s f o r the Roberts Bank (1817/m2) and Ythan estuary (1940/m2) Macoma populations were very s i m i l a r . The t o t a l numbers of i n d i v i d u a l s from a l l age groups i n summer (1971/m2) was comparable to that recorded by V a s s a l l o (1969) from San Francisco Bay (1380/m 2); but, much smaller than the maximum density recorded by Nichols (1977) of 3900/m2 or T u n n i c l i f f e and Risk (1977) who found 35007m2 on mudflats i n San Francisco Bay and the Minas Basin r e s p e c t i v e l y . Age structure The age clas s composition of M. balthica on Roberts Bank i s shown i n Table 1. The oldest i n d i v i d u a l s recovered were 5+ years old which corresponds to the longevity recorded f o r M. balthica from s i m i l a r t i d a l f l a t areas elsewhere (Chambers and Milne, 1975; Lammens, 1967). I n d i v i d u a l s i n the 3+ to 5+ year age groups were present i n the spring and ea r l y summer but they slowly died out over the summer and f a l l . Thus the two main adult year classes were the 1-2+ year olds which were also found to be dominant in populations from San Francisco Bay (Nichols, 1975) and Nova S c o t i a (Burke and Mann, 1975). U n t i l the period of maximum recruitment, the most abundant year cl a s s was the 1+ age group which comprised up to 73% of the 65 population (October 14). Newly s e t t l e d spat (0+ yr) occurred i n greatest numbers during February and March when they comprised 65-76% of the population. Size Composition and Seasonal Growth Data on changes i n the s h e l l height (Table 2) and s h e l l length (Fig. 3) f o r each age c l a s s of M. balthica (Table. 2) were used to determine the growth of the population. Growth f o r a l l ages started i n A p r i l and stopped by the end of June for 2+ animals and the end of J u l y f o r the 1+ group. This i s very s i m i l a r to the timing shown i n the population studied by Lammens (1967) i n the Wadden Sea. Growth for 0+ i n d i v i d u a l s ceased i n early September. The changing density of d i f f e r e n t s i z e groups of M. balthica i s shown i n Figure 4". Depth D i s t r i b u t i o n The percent of the M. balthica population buried at d i f f e r e n t depths within the substratum i s given i n Table 3. The majority of animals resided within the top 10 cm and p a r t i c u l a r l y within the top 5 cm of sediment. Some i n d i v i d u a l s burrowed to the maximum depth sampled of 25 cm. Generally, nearly a l l s i z e s of animals were found at each depth, except f o r the newly s e t t l e d spat which were only encountered near the surface. 66 Seasonal changes -in shell height and body weight Mean i n d i v i d u a l s h e l l height and body weight f o r 1+ and 2+ year i n d i v i d u a l s were p l o t t e d to show seasonal changes i n t h e i r r e l a t i o n s h i p ( Fig. 5). The rapid increase i n s h e l l height of approx-imately 4-5 mm during spring and early summer (F i g . 5a) coincides with an equally rapid increase i n body weight (Fig. 5b). The s h e l l height remains constant f o r the remainder of the year while the dry t i s s u e weight decreases over the same period of time. The 2+ year olds began the period of weight l o s s p r i o r to that of the 1+ year group. By February of the f o l l o w i n g year, some growth i n s h e l l height was apparent accompanied by a g r e a t l y accelerated increase i n body weight. A regression equation ( s h e l l height on body weight) was also c a l c u l a t e d and monthly changes i n the slope of the l i n e served as a c o n d i t i o n f a c t o r (CF) (Table 4). The condition f a c t o r was highest during the period of rapid growth then declined to i t s lowest value at the end of the spawning period. CF then fl u c t u a t e d s l i g h t l y u n t i l February and March when values again showed a rapid increase coincident with the accelerated increase i n i n d i v i d u a l body weight. The seasonal c y c l e of CF follows a pattern s i m i l a r to that observed i n other studies (Chambers and Milne, 1975; Beukema and De Bruin, 1977) except f o r an e a r l i e r d e c l i n e . Biomass The biomass of M. balthica was highest (6.25 - 7.30 g dry wt/m2) from A p r i l to June (Table 5). This period corresponded to the time of highest 67 d e n s i t i e s ( F i g . 2) and CF values (Table 4). As d e n s i t i e s continued to decline, the biomass a l s o f e l l and was low throughout most of the study period. T o t a l biomass values increased again i n February and March i n response to the accelerated increase i n i n d i v i d u a l body weight. The mean annual biomass was 2.96 g dry wt per m2 which i s wi t h i n the range described by Nichols (1977) of 1.2 - 10.3 g dry wt per m2 ~for M. balthica from d i f f e r e n t l o c a t i o n s i n San Francisco Bay. The mean annual biomass recorded from Roberts Bank was lower than the 4.86 g dry wt per m2 f o r M. balthica from the Ythan estuary (Chambers and Milne, 1975) but was more than double the value (1.26 g dry wt per m2) measured by Burke and Mann (1974) from Nova Scotia. Spawning Cycle The spawning cy c l e f o r M. balthica i s evident from the changes i n the sexual states shown i n Table 6. The animals matured i n May and June, then underwent a prolonged spawning period which las t e d from l a t e June through most of J u l y . This was considerably l a t e r than observed i n Scotland (Chambers and Milne, 1975) or in the Thames estuary (Caddy, 1967) but s i m i l a r to conditions in the Wadden Sea (Lammens, 1967). The prolonged spawning period, l a s t i n g for se v e r a l months, was also observed by Lammens . (1967). The end of the spawning period also coincided with the lowest value observed f o r the CF. 68 After spawning, gonad development did not commence again u n t i l December and by March only 10% of the population were sex u a l l y mature. Parasitism did not appear to play a r o l e i n suppressing gonad development as cysts did not develop i n place of gonads i n any of the animals examined (Table 6). DISCUSSION Factors which have been c i t e d as regu l a t i n g the density of Macoma populations have included t i d a l height (Beanland, 1940; V a s s a l l o , 1969), sediment g r a i n siz e (Anderson, 1972), and carbon and nitrogen content of the sediment (Newell, 1965). The environmental v a r i a b l e s recorded over the study period on Roberts Bank ( F i g . 6) were well within the tolerance l i m i t s f o r M. balthica (McGreer, 1978), and the f i n e sandy sediments of t h i s area have been shown to be r e a d i l y colonized by adults of'.this species i n laboratory experiments (Chang and Levings, 1976). A v a i l a b i l i t y of a s u i t a b l e food supply was suggested as a p o s s i b l e explanation f o r the d i f f e r e n c e s i n abundance of Macoma by Beanland (1940), and Newell (1965) l a t e r showed the increased d e n s i t i e s of M. balthica to be a t t r i b u t a b l e to increases i n the numbers of sediment microorganisms. Densities of M. balthica were shown to be p o s i t i v e l y c o r r e l a t e d with bacteria but not organic carbon on mudflats i n the Minas Basin ( T u n n i c l i f f e and Risk, 1977). However, the r o l e played by n u t r i t i o n i n determining the de n s i t i e s of M. balthica was not investigated i n the present study. 69 S e l e c t i v e predation by shore b i r d s on a Macoma population i n Morecambe Bay was discussed by Anderson (1972). She found that the s i z e range of M. balthica taken by most birds (eg. Dunlin) was between 2-13 mm. Large f l o c k s of shorebirds have been seen to feed on Roberts Bank (own observa-tions) and t h i s s i z e of M. balthica disappeared most r a p i d l y during the study (Fig. 4). F l a t f i s h also feed upon M. balthica (Risk and C r a i g , 1976) but t h e i r r o l e i n reducing the population i n the Fraser River estuary i s unknown. The annual growth rate of 6-8 mm y r - 1 measured on Roberts Bank (Fig. 3) was one of the highest ever recorded when compared to the data on growth rates f or populations of M. balthica summarized by G i l b e r t (1973). By comparison, the growth rate f o r t h i s area of the Fraser River estuary was slower than that observed in Rand Harbour, Massachusetts (Gilbert,1973) but s l i g h t l y f a s t e r than a population i n the Wadden Sea, Netherlands (Lammens, 1967). G i l b e r t (1973) concluded that both the growth rate and longevity i n M. balthica were a f u n c t i o n of temperature, with warmer temperatures producing a f a s t e r growth rate but a shorter l i f e span. This observation i s borne out by data obtained i n t h i s study. The reason for the inverse r e l a t i o n s h i p between growth rate and l o n g e v i t y i s not known f o r c e r t a i n but may be due to a change i n energy balance. De Wilde (1975) demonstrated that the balance between energy uptake through feeding and energy expended becomes negative above 15°C f o r M. balthica. This 70 resulted i n emaciation and increased m o r t a l i t y i n the larger (older) specimens. High temperatures experienced over a mudflat i n summer were considered to be p o t e n t i a l l y l e t h a l to M. balthica by Nichols (1977). S a l i n i t y was ruled out as having a profound e f f e c t on growth and t h i s i s supported by the recent experiments of McLusky and A l l a n (1976) who found growth d i f f e r e n c e s f o r M. balthica could not be explained by d i f f e r e n c e s i n s a l i n i t y . Data to explain the r e l a t i o n s h i p between s h e l l s i z e and depth buried for Macoma has been contradictory. V a s s a l l o (1971) showed that l a r g e r animals burrowed deeper than smaller ones whereas Hulscher (1973) showed . the opposite i n the Wadden Sea. A recent study from England (Reading and McGrorty, 1978)suggested that the r e l a t i o n s h i p between s i z e and depth of burrowing va r i e d seasonally as animals responded to changing daylengths. However, l e s s than one percent of t h e i r study animals were found below 7.5 cm. Results of the present study (Table 3) shows evidence of seasonal "movement with more animals r e s i d i n g near the sediment surface i n summer. Seasonal migration within the sediment was o r i g i n a l l y ascribed by Swennen (1969) to the presence of a p a r a s i t i c i n f e c t i o n (Parvatrema a f f i n i s ; Swennen and Ching, 1974), but the study by Hulscher (1973) found no s i g n i f i c a n t d i f f e r e n c e between p a r a s i t i z e d and non-parasitized Macoma. None of the animals i n t h i s study had t h i s i n f e c t i o n (see Table 5), but other forms of parasitism are known to i n f e c t Macoma on nearby t i d a l f l a t s (Ching, 1973). Hulscher (1973) suggested that the l a r g e r Macoma 71 might move to the surface at periods of high temperature to meet better oxygen c o n d i t i o n s . This seems the most probable explanation as i t would al s o explain why some populations of Macoma are never found below c e r t a i n l i m i t e d depths. whether the depth d i s t r i b u t i o n observed during low t i d e changes when the t i d e covers the area i s not known. The Roberts Bank Macoma population compares favourably with that of one i n San Francisco Bay (Nichols, 1977). Both groups-had a predominance of 0+ i n d i v i d u a l s a f t e r s p a t f a l l , which l a s t e d s e v e r a l months. There were few specimens over 2+ years old and the range i n biomass, as already mentioned, overlapped. It i s , therefore, reasonable to assume that the production to biomass r a t i o f o r the Fraser River population would also be s i m i l a r , that i s , about 4.5, which would y i e l d an estimated p r o d u c t i v i t y for t h i s population of 13.3 g m - 2 y r _ 1 . This i s i n contrast to the lower r a t i o of 1.53 found by Burke and Mann (1974) i n M. balthica from the A t l a n t i c . Newly s e t t l e d spat only accounted f o r 15% of the population at any time and 2+ year old i n d i v i d u a l s were always dominant. The annual biomass (1.26 g/m2) was also much lower than i n the present study. Temperature i s also an important f a c t o r i n the timing and duration c,?. spawning i n M. balthica and other b i v a l v e s . Lammens (1967) noted that 10°C was the c r i t i c a l temperature required to i n i t i a t e spawning i n the Wadden Sea and that as spring warming of water temperature v a r i e d from one year to another, so did the s t a r t of spawning. As gonad t i s s u e u s u a l l y developed over the f a l l and winter months, a severe, cold winter 72 was found to h a l t t h i s development and again postpone the act of spawning. In h i s study of the Thames estuary, Caddy (1967) observed spawning i n M. balthica i n temperatures between 7 and 14°C and suggested that i f temperatures remained i n t h i s range, spawning would occur a l l year round. If t h i s were the only c r i t i c a l f a c t o r , one would have expected spawning twice i n the present study; once as water temperature rose i n the spring, and again as i t f e l l below 14°C i n the f a l l ( F i g. 6). Obviously, there are other f a c t o r s (eg. food supply, energy budget) involved. A f t e r reviewing the l i t e r a t u r e , Caddy (1967) found a number of d i f f e r e n c e s i n the spawning habits of M. balthica which i l l u s t r a t e d an a b i l i t y to adapt t h e i r breeding c y c l e to d i f f e r e n t environmental c o n d i t i o n s . Another i n t e r e s t i n g aspect of spawning i n t h i s species noted by Lammens (1967), concerned t h e i r increased oxygen consumption during t h i s p e riod. This may be an a d d i t i o n a l f a c t o r i n explaining the migration of Macoma towards the sediment surface i n summer. The length of time which Macoma larvae remain i n the plankton before s e t t l i n g out v a r i e s considerably but i s an aspect which has received very I l L e l e a t t e n t i o n . A range from two to f i v e weeks (Caddy, 1969), to upwards of f i v e months (Chambers and Milne, 1975) are common values appearing i n the l i t e r a t u r e . These are i n contrast to the eight months period between spawning (Junt and peak s p a t f a l l (February) evident from t h i s study (Table 1). A major s p a t f a l l i n February, 1978 was a l s o recorded by Chapman (1978) who sampled a s u b t i d a l population of M. balthica i n 73 the Fraser River north arm (see F i g . .1); but, i n J u l y of the previous year, larger numbers of spat were already present over large areas of Sturgeon Bank (t h i s study; p. 43). Which populations .spawned these species i s not known and, indeed, i d e n t i f i c a t i o n and t r a c i n g of the l a r v a l stage from a s i n g l e parent population i s at present impossible. The settlement of larvae i n any area w i l l a lso vary from one year to the next with changes i n p a r t i c u l a r environmental v a r i a b l e s . For example, the r a t e of r i v e r flow and abnormally high temperatures have been shown to a f f e c t recruitment i n some estuarine b i v a l v e populations ( M i t c h e l l , 1974). 74 TABLE 1 AGE CLASS COMPOSITION OF M. balthica FOR EACH SAMPLING OCCASION ( N o . / m 2 ) Date 5+ 4+ Age 3+ c l a s s 2+ 1+ 0+ 1977 Apr. 7 13 64 64 666 410 77 Apr. 29 0 38 307 474 998 154 May 19 0 51 166 282 486 0 June 8 0 26 26 358 410 0 June 28 13 13 115 422 371 0 Ju l y 23 0 0 38 179 371 26 Aug. 17 0 0 13 179 358 13 Sept. 7 0 0 26 65 192 26 Oct. 14 0 0 0 51 205 26 Nov. 8 0 0 0 166 154 51 Dec. 5 0 0 0 0 77 51 1978 Jan. 13 0 0 0 51 115 64 Feb. 10 0 13 0 . 77 77 282 Mar. 16 0 0 0 77 51 410 75 T A B L E 2 MEAN SHELL HEIGHT ( i n mm ± S . E . ) AND RANGE ( i n b r a c k e t s ) FOR VARIOUS AGE C L A S S E S OF M. balthica ON EACH SAMPLING OCCASION Age c l a s s Date 5+ 4+ 3+ 2+ 1+ 0+ 1977 Apr. 7 10. (10. 0* 0) 9.5±0.3 :• (8.5-10.0) 8.6±0.2 . (8.0-9.0) 6. 310.1 5-7.5) 2.6±0.2 (1.5-4.0) 1.210.1 (1.0-1.5) Apr. 29 - . 9.8±0.2 (9.5-10.0) 8.6+.0.2 (6.0-10.5) 7. (5. 510.2 5-9.5) 3.0±0.1 (1.5-6.5) 1.510.0 (1.5) May '. 19 - 11.1±0.7 (10.0-13.0) 11.5±0.2 (10.0-12.5) 10. (7. 0±0.2 5-12.0) 4.6±0.3 (2.0-7.5) -June 8 - 12.5±0.0 (12.5) 11.8±0.2 (11.5-12.0) 10. (8. 2±0.2 0-12.0) 5.910.2 (4.0-8.0) -June 28 12. (12. 5* 5) 12.0* (12.0) 12.0±0.1 (11.5-12.5) 10. (9. 2±0.1 0-12.0) 5.910.3 (3.0-8.0) -July 23 - - 12.0±0.0 (12.0) 11. (10. 1±0.2 0-12.0) 6.510.2 (3.5-8.5) 1.010.0 (1.0) Aug. 17 - - 11.0* (11.0) 10. «9. 8±0.2 5-12.0) 6.210.2 (3-0-9.0) 3.5* (3.5) Sept . 7 - - 11.2+0.2 (11.0-11.5) 10. (9. 3±0.6 0-12.0) 6.210.3 (4.5-8.0) 2.Oil.0 (1.0-3.0) Oct. t /i - - • - 9. .(9. 4±0.2 0-10.0) 6.210.3 (3.0-7.5) 0.710.2 (0.5-3.0) Nov. 8 - - - 10. (8. 2±0.9 0-12.0) 5.810.3 (3.0-7.5) 1.910.8 (0.5-3.0) Dec. 5 - - - • - '6.610.4 (5.0-7.5) 1.910.1 (1.5-2.0) *Data from one i n d i v i d u a l only Cont. 76 TABLE 2 ( c o n t ' d ) MEAN SHELL HEIGHT ( i n mm ± S . E . ) AND RANGE ( i n b r a c k e t s ) FOR VARIOUS AGE CLASSES OF M. balthica ON EACH SAMPLING OCCASION Date 5+ 4+ Age c l a s s 3+ 2+ 1+ 0+ 1978 Jan. 13 Feb. 10 Mar. 16 13.0* (13.0) 10.6±0.2 (10.0-11.0) 10.110.4 (9.0-11.5) 6.5±0.4 (5.0-8.5) 6.1±0.9 (4.5-8.5) 10.4±0.5 6.810.6 (9.5-12.5) (5.0-7.5) 1.710.1 (1.5-2.0) 1.310.2 (1.0-3.5) 1.510.2 (1.0-4.0) *Data from one i n d i v i d u a l ^only (Sample s i z e s given i n Table 3). 77 TABLE 3 DEPTH D I S T R I B U T I O N OF M. balthica WITHIN THE SUBSTRATUM FOR EACH SAMPLING DATE Date Number Examined % 0-5 cm of popu 5-10 cm l a t i o n at each depth 10-15 cm 15-20 cm 20-25 cm 1977 • Apr. 7 • 99 82.2 11.9 - 5.9 Apr. 29 153 64.5 17.4 7.1 7.1 3.9 May 19 76 70.1 27.2 - 2.7 June 8 64 73.5 26.5 - -June 28 73 93.2 5.4 - 1.4 :T"ly 23 48 87.8 12.2 - -Aug. 17 45 100.0 - - -Sept. 7 24 96.0 - _ 4.0 Oct. 14 22 90.9 9.1 - -j . Nov. 8 29 86.2 10.3 3.5 -Dec. 5 10 100.0 T - -1.978 Jan. 13 18 83.3 16.7 _ -Feb. 10 35 85.7 14.3 - -Mar. 16 42 97.6 - - - 2.4 TABLE 4 VALUES FOR y INTERCEPT ( a ) AND SLOPE ( b ) OF REGRESSION 1-0910 y = a + bx WHERE y = MEAN DRY FLESH WEIGHT IN MG, AND x = SHELL HEIGHT IN MM FOR M. balthica Date Number of data points a b 1977 Apr. 7 15 -0.5764 0.1641* Apr. 29 19 -1.2680 0.2598* May 19 21 -0.8744 0.1872 June 8 17 -0.5626 0.1563* June 28 18 -0.6193 0.1564* Ju l y 23 14 -0.3980 0.1204* Aug. 17 15 0.4279 0.0466 Sept. 7 11 -0.2423 0.1182* Oct. 14 12 -0.3931 0.1211* Nov. 8 14 -0.1818 0.0998* Dec. 5 6 -0.7123 0.1598* 1978 Jan. 13 11 -0.3241 0.1226* Feb. 10 15 -0.4859 0.1572* Mar. 16 10 -0.9388 0.2211* * S i g n i f i c a n t at p <0.05 l e v e l T A B L E 5 BIOMASS (g d r y w e i g h t / m 2 ) OF M. balth ON EACH SAMPLING OCCASION Date Biomass (g dry wt/m2) 1977 Apr. 7 -Apr. 29 6.70 May 19 6.94 June 8 6.25 June 28 7.30 Jul y 23 2.56 Aug. 17 3-03 Sept. 7 1.79 Oct. 14 0.74 Nov. 8 1.25 Dec. 5 0. 22 1978 ' Jan. 13 0.86 Feb. 10 1.58 Mar. 16 2.17 80 T A B L E 6 PERCENTAGE OF IMMATURE, MATURE, SEXUALLY UNDIFFERENTIATED AND P A R A S I T I Z E D M. balthica ON EACH SAMPLING OCCASION Date Number Immature Mature U n d i f f e r e n t i a t e d P a r a s i t i z e d examined (%) (%) (%) (%) 1977 Apr. 7 99 65 2 33 0 Apr. 29 153 55 1 44 0 May 19 76 14 57 29 0 June 8 64 16 54 30 0 June 28 73 10 35 ' 5 5 0 July 23 48 0 6 94 0 Aug. 17 45 0 2 98 0 Sept. 7 24 0 0 100 0 Oct. 14 22 0 0 100 0 Nov. 8 29 0 0 100 0 Dec. 5 10 0 101 90 0 1978 Jan. 13 18 0 0 100 0 Feb. 10 35 0 3 97 0 Mar. 16 42 2 10' 88 0 81 FIGURE 1 . L o c a t i o n o f s t u d y a r e a ( I ) i n t h e F r a s e r R i v e r e s t u a r y . D o t t e d l i n e i n d i c a t e s a p p r o x i m a t e s e a w a r d e d g e o f t i d a l f l a t s a t l o w t i d e . CM CO 2 2 0 0 r 2 0 0 0 1 8 0 0 1 6 0 0 1 4 0 0 E 6 c 1200 1 0 0 0 +-> IT) c CD 8 0 0 Q 6 0 0 4 0 0 2 0 0 © © 1+year and older © — © 0+ year A M J J A S O N D J F M 1977 1978 Time (months) FIGURE 2 . D e n s i t y o f M. balthiaa on e a c h s a m p l i n g o c c a s i o n ( ± 1 S . E . ) 83 0 2 4 5 Age ( years) FIGURE 3 . A v e r a g e g r o w t h r a t e ( l e n g t h i n mm) f o r M. balthica o n R o b e r t s Bank ( ± 1 S . E . ) . 84 1500 r 1200 h £ 800 n E 13 Z 400 h Shell height o—@ 0.5-5.0 mm o—o 5.5-10.0 mm A - - A 10.0-15.0 mm Time (months) FIGURE 4 . D e n s i t y o f M. balthica e x p r e s s e d a s c h a n g e s i n d i f f e r e n t s h e l l h e i g h t s i z e r a n g e s . 12 r (a) E 10 E X 8 "C9 J Z 6 LO C nj 4 2 2 / ,-o-cr ex. / / 9 / cn E 25 20 cn 1 15 Q 10 c 2 5 (b) o — o 2+ Year o — a 1 + Year / .Q —-O / ^ © c / L^i_j , j 1 j 1 j 1 1 1 J 1 A M J J A S O N D J F M Time (months) FIGURE 5 . C h a n g e s i n ( a ) mean s h e l l h e i g h t i n mm, a n d ( b ) mean d r y w e i g h t p e r i n d i v i d u a l i n mg f o r e a c h s a m p l i n g d a t e . 86 ®—® Air Temperature o—o Sea Water Temperature A M J J A S O N D J F M Time ( months) FIGURE 6 . A i r , w a t e r a n d s e d i m e n t t e m p e r a t u r e s , a n d s a l i n i t y o v e r s t u d y a r e a d u r i n g s a m p l i n g p e r i o d . 87 REFERENCES Anderson, S.S. 1972. The ecology of Morecambe Bay. I I . I n t e r t i d a l i nvertebrates and f a c t o r s a f f e c t i n g t h e i r d i s t r i b u t i o n . . J . Appl. E c o l . 9: 161-178. 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