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The population biology of two intertidal seagrasses, Zostera Japonica and Ruppia Maritima, at Roberts… Bigley, Richard Ernest 1981

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THE POPULATION BIOLOGY OF TWO INTERTIDAL SEAGRASSES, ZOSTERA JAPONICA AND RUPPIA MARITIMA, AT ROBERTS BANK, BRITISH COLUMBIA by RICHARD ERNEST BIGLEY B.Sc, Washington State U n i v e r s i t y , 1979 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Botany) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October, 1981 (c). Richard Ernest Bigley 1981 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. 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 copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head o f my department or by h i s o r her r e p r e s e n t a t i v e s . I t i s understood t h a t copying or 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 allowed without my w r i t t e n p e r m i s s i o n . Department of ^ (^rO^V' The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date DE-6 (2/79) i ABSTRACT Growing (vegetative) and seed components of co-occurring i n t e r t i d a l populations of Zostera japonica Aschers. and Graebn. and Ruppia maritima L. were studied i n southwestern B r i t i s h Columbia to determine what factors a f f e c t t h e i r population maintenance. Results of repeated mapping and examination of shoots (ramets) i n permanent p l o t s on a t i d a l gradient showed that shoot f l u x , age structure, survivorship, l e a f and rhizome growth, and seed production were a l l aff e c t e d by the amount of exposure of plants to the a i r . Plots having the greatest exposure had fewer shoots, a greater percentage of shoots flowering e a r l y i n the season, and s u b s t a n t i a l l y lower seed production than pl o t s with le s s exposure to the a i r . The length o f the R. maritima l i f e c ycle was the same regard-less of differences i n exposure to the a i r , while plants of Z. japonica with high exposure i n i t i a t e d and ended flowering and entered a quiescent overwintering state e a r l i e r than plants with less exposure. Seed of both species was shed innately dormant, and through enumeration of seed i n the sediment i t was found that most was exported from the s i t e of production. Experimental seed b u r i a l showed that remaining seed suffered ^50% overwinter mortality. Simultaneous germination of Z_. japonica and R. maritima seed was triggered by the warming of sediments i n March when low tides occurred during the day-time. Germination was confined to aerobic sediments; seed buried i n anaerobic sediment was maintained i n an enforced dormancy year-round. Observations that low temperatures and anaerobic conditions retard seed germination were confirmed by laboratory studies. In addition, seed was found to be non-photoblastic and was prevented from germinating by loc a l seawater s a l i n i t i e s only when emerging from innate dormancy. - i i TABLE OF CONTENTS Page ABSTRACT i LIST OF TABLES -'v LIST OF FIGURES v i ACKNOWLEDGEMENTS ,"Ix INTRODUCTION 1 GENERAL DESCRIPTION OF PLANTS 4 STUDY AREA 9 METHODS Physical environment 13 Shoot demography 14 Morphology and ovary fates 16 Buried seed enumeration 17 V i a b i l i t y and germination of seed stored i n aerobic conditions 19 The e f f e c t s of temperature and anaerobic conditions on the v i a b i l i t y of new seed 21 V i a b i l i t y of experimentally buried seed 22 Data analyses 23 Sampling i n t e r v a l s 23 SECTION 1. PHYSICAL ENVIRONMENT Transect topography 26 Tides 26 Sediments 29 S a l i n i t y 29 Sediment temperatures 33 Oxidizing p o t e n t i a l 35 SECTION 2. THE GROWING COMPONENT OF THE POPULATION Results Shoot recruitment 39 Shoot losses 47 Total shoots 48 i i i Cohort contributions 57 Survivorship 71 Leaf length 71 Rhizome internode length 79 Genet growth 84 Flower and inflorescence d e n s i t i e s 88 Ovary fates 89 Seed production 93 Discussion Shoot recruitment and death 95 Leaf and rhizome internode lengths 100 Genet growth 104 Numbers of flowers and inflorescences 104 Ovary fates 105 Seed production 107 SECTION 3. THE SEED COMPONENT OF THE POPULATION Results Buried seed 109 V i a b i l i t y of untreated seed c o l l e c t i o n s stored at 5 °C. 117 Ef f e c t s of temperature and anaerobic conditions on seed v i a b i l i t y and germination 119 V i a b i l i t y of buried seed 121 New seed germination 121 Old seed germination 127 Germination i n seed bank 141 Recruitment and death of seedlings and shoots from overwintering rhizomes 141 Discussion Buried seed numbers 148 Seed v i a b i l i t y 155 Germination and seedling recruitment 159 SUMMARY 164 LITERATURE CITED 167 APPENDICES i v APPENDICES 1. Taxonomy of Zostera japonica 176 2. Oxidizing p o t e n t i a l s of Roberts Bank sediment 188 3. Shoot demography data. 191 4. Averages and standard deviations of morphological data. 202 LIST OF TABLES Page I. Sampling and observation dates and approximate times of temperature readings at Roberts Bank, 1980 - 1981. 25 II . Percent of days of exposure per month from tides reaching low during peak heating hours of the day at or below 2.5 m CD t i d a l height, Roberts Bank. 28 I I I . Percentage of sediment i n each of f i v e s i z e classes from three stations at Roberts Bank, 1980 - 1981. 30 IV. Mean numbers of ovaries per inflorescence and percentages i n each ovary fate: ovary abortion, ovary predation, incomplete development, mature seed; f o r two age classes of Zostera japonica and Ruppia maritima from three t i d a l heights at Roberts Bank. 92 2 V. Estimated seed production per 0.1 m of Zostera japonica and Ruppia maritima at Roberts Bank, showing absolute number and percent of t o t a l at each s t a t i o n by age class at three t i d a l heights. . 94 VI. Percent v i a b i l i t y of seed stored at 5°C and 27 o/oo s a l i n i t y during 1980 - 1981. 118 VII. Percent v i a b i l i t y , death, and germination of new seed maintained i n c o n t r o l l e d laboratory environments. 120 VIII. Percent v i a b i l i t i e s of buried seed at four sediment depths. 122 IX. Shoot base depths and absolute numbers of Zostera shoots established from overwintering rhizomes i n haphazardly selected sediment cores near s t a t i o n 2 at Roberts Bank, 1981. 144 X. Relationship of seed depth i n various ecosystems. 149 XI. Estimates of buried seed numbers from various ecosystems. 152 XII. Mean character values and standard deviations of four ' species i n the Zostera subgenus, Z o s t e r e l l a . 179 LIST OF FIGURES v i Page 1. Zostera japonica and Ruppia maritima showing growth form of plants. 5 2. D i s t r i b u t i o n of Zostera japonica i n North America. 6 3. General l o c a t i o n of study s i t e . 8 4. a. General l o c a t i o n of study transect. b. Stations and p l o t configuration at each s t a t i o n . 10 5. Flow chart of b u r i a l , storage, and germination treatments of old and new seed of Zostera japonica and Ruppia maritima. 24 6. General topography of study transect showing posi t i o n s of stations at Roberts Bank. 27 7. S a l i n i t y of water near s t a t i o n 2 at Roberts Bank, 1980 - 1981, measured i n shallow pools at low t i d e . 32 8. A i r and sediment temperatures near s t a t i o n 2 at Roberts Bank, 1980 - 1981. 34 9. Selected p r o f i l e s of o x i d i z i n g p o t e n t i a l s of the sediment near s t a t i o n 2 at Roberts Bank 1980 - 1981. 36 10. Recruitment and loss of shoots between successive sampling dates at each of three stations at Roberts Bank 1980 - 1981. 40 11. Absolute contribution of cohorts r e c r u i t e d at successive i n t e r v a l s , and t o t a l shoot density of vegetative and reproductive shoots from three stations at Roberts Bank, May 1980 - May 1981. 49 12. Percentage of t o t a l shoots which were flowering at each of three stations on successive sampling dates at Roberts Bank, 1980 -1981. 55 13. Age structure of vegetative and reproductive shoot populations as percent cohort contribution to the t o t a l population f o r three stations at Roberts Bank, 1980 -, 1981. 58 14. Survivorship curves of selected cohorts from three stations at Roberts Bank, 1980 - 1981. 72 V l l 15. Mean lengths of newest rhizome internodes from three stations at Roberts Bank, 1980 - 1981. 76 16. Leaf cross sections showing lacunae. 80 17. Mean lengths of newest rhizome internodes from three stations at Roberts Bank, 1980 - 1981. 81 18. Schematic drawing of hori z o n t a l and v e r t i c a l branching of vegetative and reproductive shoots of Zostera japonica and Ruppia maritima and estimates of ramets per genet over time. 85 19. Mean number of ovaries per inflorescence at three stations at Roberts Bank, 1980. 90 20. Mean number of inflorescences per flowering shoot at three stations at Roberts Bank, 1980. " 91 21. Cumulative percent of a l l buried seed from three stations at Roberts Bank. 110 22. Percent germination rate of current year v i a b l e seed of Zostera japonica and Ruppia maritima i n 10 °C d i s t i l l e d water. 123 23. Percent germination rate of current year v i a b l e seed of Zostera japonica and Ruppia maritima i n 10 "C water at four • s a l i n i t i e s . 124 24. Percent germination rate of scored and unscored current year v i a b l e seed of Zostera japonica and Ruppia maritima i n 10 °C water at two s a l i n i t i e s . 128 25. Percent germination rate of current year v i a b l e seed of Zostera japonica and Ruppia maritima i n 20 o/oo s a l i n i t y water at temperature regimes o f constant 10°C and 18°C, and 12-hour a l t e r n a t i n g 15 - 7°C. 131 26. Percent germination rate of current year v i a b l e seed of Zostera japonica and Ruppia maritima i n 10 "C water at two s a l i n i t i e s i n the dark and exposed to white l i g h t . 134 27. Percent germination rate of previous years' v i a b l e seed of Zostera japonica and Ruppia maritima i n 10 °C d i s t i l l e d water. 137 v i i i 28. Percent germination rate of previous years' v i a b l e seed of Zostera japonica and Ruppia maritima i n 10°C water at 10 o/oo and 20 o/oo s a l i n i t y exposed to white l i g h t , and at 20 o/oo i n the dark. 138 29. Percent occurrence of seedlings i n the sediment p r o f i l e near s t a t i o n 2 at Roberts Bank, 1981. 142 30. Recruitment and l o s s , absolute and percent c o n t r i b u t i o n o f seedling cohorts of Zostera japonica and Ruppia maritima from three stations at Roberts Bank, 1981. 145 31. Characters used to d i f f e r e n t i a t e Z o s t e r e l l a taxa. 180 32. Representative spathe cross sections of Z o s t e r e l l a taxa. 183 33. P r i n c i p a l component analysis of Z o s t e r e l l a taxa; projections of characters on components I and I I . 184 ACKNOWLEDGEMENTS I would l i k e to express my sincere appreciation to Dr. Paul G. Harrison who provided d i r e c t i o n and f i n a n c i a l assistance at a l l stages of t h i s work. The other members o f my committee, Drs. Roy turkington and Peter J o l l i f f e , I would l i k e to thank f o r t h e i r advice and c r i t i c a l reading of t h i s t h e s i s . I am further indebted to those students and fa c u l t y who f r e e l y engaged i n the exchange of ideas and the lending of equipment, and Dr. Ron P h i l l i p s f o r h i s encouragement on t h i s p r o j e c t . A portion of t h i s work could not have been conducted without the f i n a n c i a l support o f the North Cascades Audubon Society. F i n a l l y , I wish to thank my wife, Jeannette, for countless hours of hearing about seagrasses, reading and typing of manuscripts, and her endless encouragement and enthusiasm. 1 INTRODUCTION The population of an annual (or s h o r t - l i v e d perennial) plant i s composed of two parts: the growing plants and the dormant seed (Watkinson 1978). U n t i l now, the approaches to the study of seagrass population growth and maintenance have been l i t t l e more than d e s c r i p t i v e and p r i m a r i l y concerned with the growing (vegetative) component. Few authors have mentioned the seed component as a means of population main-tenance or establishment (Arasaki 1950b, P h i l l i p s 1972, Thorhaug and Austin 1976, Greg and M o f f l e r 1978, Verhoeven 1979, Lewis and P h i l l i p s 1980). The population dynamics of seagrasses have been studied at various l e v e l s to answer a wide range of questions. Seagrass bed area and biomass have been studied extensively, mostly i n r e l a t i o n to seasonal p r o d u c t i v i t y (see Zieman and Wetzel 1980). On the other hand, studies dealing with the underlying changes i n population have been few. Bak (1980) counted and measured leaves o f Zostera marina L. and suggested that because of the existence of f i v e s i z e classes, an equal number of age classes might e x i s t i n the population he studied. Mukai et a l . (1979) studied the l i f e span of i n d i v i d u a l leaves i n outdoor culture, and found that leaves on vegetative shoots of Z. marina l i v e d longer than leaves on reproductive shoots. Nienhuis and DeBree (1980) used a leaf-marking technique to estimate l e a f turnover and production. These methods, however, give l i t t l e i n s i g h t into the " t a c t i c s " (Harper and Ogden 1972) employed by the plants to achieve the observed population. It i s the task of a demographer to describe, measure, and explain changes i n the numbers that compose a population. The demography of seagrass shoots would introduce a new, as yet unexplored, aspect of t h e i r population dynamics. Sarukhan and Harper (1973) state that only 2 i n studies where very frequent and d e t a i l e d observations of i n d i v i d u a l s are made can the underlying changes i n a population be observed. They continue, and state that i t i s also e s s e n t i a l to observe the performance of the i n d i v i d u a l s i n terms of s i z e and reproductive output i n order to reveal those e f f e c t s of b i o t i c and physical factors which are otherwise disguised by simply considering numbers. Studies contributing to our understanding of buried dormant seed are l a r g e l y d e s c r i p t i v e , and are p r i m a r i l y from t e r r e s t r i a l environments such as a g r i c u l t u r a l f i e l d s (e.g., Brenchley and Warington 1930, 1933, Schafer and Chilcote 1970), forests (e.g., Oosting and Humphreys 1940, Johnson 1975, Moore and Wein 1977), and natural grasslands (e.g., Major and Pyott 1966). Buried seed of marsh s o i l s has received l i t t l e a t t ention (Milton 1939, van der Valk and Davis 1976, 1978, Leek and Graveline 1979, van der Valk 1980). There have been no studies of i n t e r t i d a l or subtidal buried seed banks. There i s considerable i n t e r e s t i n using seed to r e - e s t a b l i s h disturbed seagrass beds, avoiding c o s t l y shoot transplants (Thorhaug 1976, P h i l l i p s 1980). Much progress on these e f f o r t s i s u n l i k e l y without more information on the biology of seed. Seagrass communities are widely d i s t r i b u t e d along coasts of temperate and t r o p i c a l seas and are regarded as one of the conspicuous features of the shore (den Hartog 1970). The important functions of sea-grasses as sediment s t a b i l i z e r s and primary producers, the backbone to the food web i n many areas, i s well recognized. However, e s s e n t i a l l y nothing i s known about the establishment and maintenance of seagrass populations and what factors determine f l u c t u a t i o n s i n t h e i r numbers (Bak 1980, Lewis and P h i l l i p s 1980, Nienhuis and De Bree 1980). I f , i n 3 the future we are to manage seagrass beds e f f e c t i v e l y , the nature and magnitude of factors which influence population numbers must be under-stood . It was the objective of t h i s study to provide basic information on the maintenance of co-occurring i n t e r t i d a l populations of Zostera japonica Aschers. and Graebn. and Ruppia maritima L. by examining both the dormant and growing portions of the population. Two general questions were pursued: (1) How do the dynamics of shoot populations of Z. japonica and R. maritima vary on an exposure gradient i n terms of l i f e span, growth, and seed production? (2) Where i s seed located and what are the factors which influence the numbers of seed that f i n a l l y germinate and establish? The approaches taken to answer these questions were f i r s t to map shoots r e g u l a r l y and to observe and measure morphological features of plants on an exposure gradient over a year, and the second, to examine the responses of seed to f i e l d b u r i a l , and to storage and germination under various c o n t r o l l e d laboratory conditions. The i n t e r t i d a l seagrass community lends i t s e l f well to t h i s type of study because of the combination of c l e a r l y defined shoots, easy removal of undisturbed s o i l p r o f i l e s , and access to large numbers of seed. 4 GENERAL DESCRIPTION OF PLANTS Zostera japonica and Ruppia maritima (Fig. 1) are both monocotyled-onous marine vascular hydrophytes, and members of the Potamogetonaceae. They belong to a group of organisms confined to a sin g l e subclass (Helobiae), a portion of which are generally known as the seagrasses. A l l seagrasses share the c h a r a c t e r i s t i c of being able to carry out t h e i r entire l i f e cycle submerged i n the marine environment. Both Z. japonica and R. maritima may behave as either annuals (overwintering by seed) or s h o r t - l i v e d perennials (overwintering through rhizomes). The main body of each plant consists of an underground rhizome which branches and also gives r i s e to vegetative shoots. Sympodial branching of shoots takes place as flowering proceeds ( F i g . 1). The inflorescence of Z. japonica consists of a one-sided spadix with a l t e r n a t i n g s e s s i l e staminate and p i s t i l l a t e flowers i n two rows. Seed of Z. japonica i s e l l i p s o i d , about 2 mm by 1 mm i n s i z e , and the testa i s smooth, shiny and brown with f i n e s t r i a t i o n s . In R. maritima the inflorescence consists of two flowers born on a peduncle. Each flower has two s e s s i l e anthers between generally four p i s t i l s . Seed i s ovoid to pyriform, from 1.5 to 3 mm by 1 mm. The testa i s black to brown with f i n e pores. Much confusion has surrounded the taxonomic status of Z. japonica i n North America (see Appendix 1). The ecology and d i s t r i b u t i o n of Z. japonica has been summarized by den Hartog (1970). Zostera japonica i s common on sheltered, sandy or muddy t i d a l f l a t s , but i t also occurs i n brackish coastal lagoons with a more or less f i x e d water l e v e l . It extends from 12 to 55 degrees north l a t i t u d e ; thus growing conditions vary from t o p i c a l to cold-temperate. It i s widely d i s t r i b u t e d i n East Zostera japonica Ruppia maritima 6 Asia from the coast of Vietnam to the t r o p i c of Cancer. The known d i s t r i b u t i o n of Z. japonica i n North America i s shown i n Figure 2. Some confusion has also surrounded the taxonomy of R. maritima because of i t s wide ecolo g i c a l amplitude and overlapping d i s t r i b u t i o n with other Ruppia species (see Verhoeven 1979). There appear to be two d i s t i n c t v a r i e t i e s of Ruppia maritima, the common v a r i e t y maritima L. with long podogyns (formed by f e r t i l i z e d p i s t i l s ) and a very rare v a r i e t y , b r e v i r o s t r i s (Agardh) Aschers. and Graebn., with short podogyns. A l l references made within t h i s thesis to Ruppia maritima w i l l be to the va r i e t y maritima. Ruppia maritima i s found i n coastal brackish waters and inland temporary and permanent sa l i n e habitats. Its d i s t r i b u t i o n i n North America extends from Alaska to Baja C a l i f o r n i a on the West Coast and from Newfoundland to F l o r i d a i n the East. It i s also found throughout South America and the Old World (Hitchcock et a l . 1969). 4 5° Fig. 2. D i s t r i b u t i o n of Zostera japonica i n North America. S o l i d c i r c l e s = known c o l l e c t i o n s ; open c i r c l e s = locations searched by the author during 1980 - 1981 where no Z. japonica was observed. Fig. 3. General l o c a t i o n of study s i t e . 9 STUDY AREA F i e l d studies were conducted i n January 1980 and from May 1980 to May 1981 on the southern portion of Roberts Bank, located twenty k i l o -meters south of Vancouver, B r i t i s h Columbia (Fig. 3). The study s i t e , composed of three stations, was located between the Tsawwassen f e r r y terminal and the Roberts Bank coal port causeways (49°02'N; 123°08'W). Stations were established 1.3 km seaward from shore along a t i d a l gradient perpendicular to the southeast side of the coal port causeway ( F i g . 4). The Fraser River d e l t a i s composed of Roberts Bank and Sturgeon Bank and encompasses approximately 11,300 hectares of i n t e r t i d a l area. The d e l t a averages 6.5 km i n width and slopes gently (about 8 degrees) toward the S t r a i t of Georgia. Underlying the 90 to 120 m thick unconsolidated sediments of the d e l t a are Pleistocene sediments (Mathews and Shepard 1962). The Fraser River, north of the study s i t e , i s ranked s i x t h largest i n North America and the thirty-second largest on a world scale by discharge (van der Leeden 1975). The Fraser River c a r r i e s an estimated average of 20 m i l l i o n metric tons of suspended sediment into the d e l t a annually. Peak r i v e r discharge occurs i n l a t e May to June (Beak Hinton Consultants Ltd. 1977) . Because the coal port causeway prevents most of the brackish waters from the Fraser River mouth from entering the intercauseway region, the general area i n which the study was conducted had a more marine influence than the remainder of the Fraser d e l t a ; there was however increased t u r b i d i t y of the water during peak r i v e r discharge which may have altered physical conditions at the study s i t e . Tides at Roberts Bank are of the mixed, mainly diurnal type. The mean water l e v e l at the Tsawwassen f e r r y terminal i s 2.96 m chart datum U.S.A. 1000 0 1000 2000 m e t e r s meters Fig. 4. a. General l o c a t i o n of the study transect. b. Stations and p l o t configuration at.each s t a t i o n . 11 (CD)'. T i d a l amplitude has average ranges of 4.69 m f o r large tides and 3.05 m for mean t i d e s . The climate at Roberts Bank has a heavy maritime influence with wet, cool winters and warm summers. Few f l o r i s t i c studies have been conducted on the nearshore plants of the Fraser River d e l t a . Marsh vegetation has been described q u a l i t a t i v e l y by Forbes (1972), McLaren (19,72) and H i l l a b y and Barrett (1976), and q u a n t i t a t i v e l y by Yamamaka (1975) and A. Moody (1979). The marshes near the study s i t e have been strongly a l t e r e d by diking. Roberts Bank marshes are composed p r i m a r i l y of D i s t i c h l i s s t r i c t a (Torr.) Rydb. and G r i n d e l i a i n t e g r i f o l i a DC. i n the higher areas, and S a l i c o r n i a  v i r g i n i c a L. and T r i g l o c h i n maritima L. i n the lower areas of the marsh including the patches of marsh vegetation which occur i n areas between drainage channels i n the high i n t e r t i d a l . Other important marsh species nearshore include Carex lynbgyei Hornem., Scirpus americanus Pers., Scirpus maritimus L., and Scirpus validus Vahl. Studies of submerged vegetation on the Fraser foreshore include those of R. Moody (1979), who studied the factors which influence the standing crop of Zostera marina L. on the eastside of the Tsawwassen f e r r y terminal, and P.G. Harrison ( i n preparation), who studied the d i s t r i b u t i o n of biomass of Z. japonica (= Z. americana) and R. maritima. The d i s t r i -bution of submerged and marsh vegetation has also been studied by s a t e l l i t e imagery (G. Tomlins personal communication 1980). Much attention has been given to the f i s h populations i n the Roberts Bank area (Dept. of Environment, F i s h e r i e s and Marine Service 1975). Twenty-five species of f i s h are known to occur at Roberts Bank; the most important and abundant sport and commercial fishes are the chinook salmon (Oncorhynchus tshawgtscha) and the P a c i f i c herring (Clupea harengus p a l l a s i ) . Also common i n the lower i n t e r t i d a l and subtidal i s the dungeness crab (Cancer magister). The Fraser River estuary i s of major importance as a feeding stop for birds migrating on the P a c i f i c flyway; peak numbers occur i n November (Beak Hinton Consultants Ltd. 1977). Birds commonly observed at the study s i t e were, i n order of abundance, American Widgeon (Anas  americana), Dunlin ( A l i d r i s a l p i n a ) , and Great Blue Heron (Ardea herodius). Large numbers of ducks were often found i n the intercauseway area. Human use of the Roberts Bank area can be traced many thousands of years (Goddard 1945). The t r a d i t i o n a l Tsawwassen (members of the coast S a l i s h Indians) winter v i l l a g e s i t e was located between the causeways on what i s now the Tsawwassen Indian Reservation. Many of the abundant i n t e r t i d a l resources once harvested by the Tsawwassen are gone, l i k e l y as a r e s u l t of extensive diking along the marsh and construction of the fe r r y terminal and coal port i n 1960 and 1970, re s p e c t i v e l y . Today there i s li m i t e d use of the intercauseway area except for the occasional crab c o l l e c t o r or b i r d watcher. The gravel edges of the cause-ways are r e g u l a r l y used for beachcombing year-round. Future developments at southern Roberts Bank are scheduled to include a m u l t i - f o l d expansion of the e x i s t i n g coal terminal to f a c i l i t a t e transportation and storage of an increased volume of materials, mostly c o a l . 13 METHODS Physical environment Sediment p a r t i c l e s i z e analyses were conducted using the hydrometer method (Day 1965). P a r t i c l e s i z e categories were as defined by the U.S. Department of Agri c u l t u r e (Day 1965): gravel, greater than 2 mm, coarse sand, 2.00 - 0.25 mm, f i n e sand, 0.25 mm - 0.05 min, s i l t , 0.05 - 0.002 mm, and clay, below 0.002 mm. Subsamples (200 g) of sediment f o r analysis were obtained from cores 8 cm i n diameter by 20 cm, which had been a i r -dried and thoroughly mixed. Water samples f o r s a l i n i t y measurements were c o l l e c t e d from depres-sions near s t a t i o n 2. S a l i n i t y was estimated to the nearest part per thousand (o/oo) using a portable Yellow Springs Instrument Co. s a l i n i t y / conductivity/temperature meter model 33. Temperatures of the a i r and sediment at the surface and at 5 and 15 cm depths were measured to the nearest degree centigrade using a mercury thermometer. A l l measurements recorded took place near s t a t i o n 2. Approximate time of each measurement was also recorded. Oxidizing (redox) po t e n t i a l s of the sediment p r o f i l e s at s t a t i o n 1 were recorded i n the f i e l d . A Ple x i g l a s tube of 8 cm insi d e diameter (i.d.) was used to remove a core of the sediment p r o f i l e approximately 25 cm deep. Upon removal, the tube was corked at both ends u n t i l redox measuring began, usually less than 5 minutes from recovery. With sediment temperatures pre-set, the redox p o t e n t i a l was measured as ra p i d l y as possible to insure minimal oxidation. The redox electrode was inserted a minimum of 1 cm into the side of the core at 2 cm i n t e r -vals to the depth of 20 cm as the core was extruded into a l o n g i t u d a l l y cut tube of 8 cm i . d . f o r support. E f f o r t s were made to minimize a i r 14 leakage along the shaft of the probe during measurements. The instrument used was an Orion s p e c i f i c ion meter model 407 A/F with an Orion platinum redox electrode model 96-78. Substrate elevations along the transect were obtained by a survey on December 22, 1980. Standard l e v e l i n g techniques with a David White meridian transect were used. McElhanney surveyers of Vancouver survey datum No. 9628 (11.67 feet Geodetic el e v a t i o n ) , located on power pole #158 on the Roberts Bank coal port causeway served as the reference datum for a l l measurements. Readings were corrected to the 1980 ti d e and chart datum by adding 2.95 m to the Geodetic datum elevation. The height of a l l stations measured was checked by oversight techniques. A l l elevations are presented as meters above t i d e (and chart) datum. T i d a l heights and the duration of exposure at 2.5 m CD were predicted for Tsawwassen i n accordance with Canadian Tide and Chart Tables Volume 5. Exposure periods of tides i n which the low occurred during peak heating hours (1130 - 1400 winter, 1030 - 1600 summer P a c i f i c Standard Time, Environment Canada personal communication, 1980) were obtained f o r 2.5 m by c a l c u l a t i n g exposure duration f o r selected tides which appeared to be close to the duration c u t - o f f points; intermediate t i d e exposures were not generally predicted. Percent exposure f o r 2.5 m and 3.1 m was estimated by averaging exposure predictions at new, f u l l , and f i r s t and l a s t quarter moons for the period. Shoot demography At each of the three t i d a l heights, nine p l o t s were established at 1 m i n t e r v a l s ( F i g . 4). Row length was di c t a t e d by l o c a l topographic r e l i e f . Stations were located su b j e c t i v e l y to insure that r e p l i c a t e 15 plo t s were from the same t i d a l height and slope. This subjective s e l e c t i o n o f v i s u a l l y homogeneous p l o t s and repeated sampling of the same i n d i v i d u a l s remove some of the need for the large r e p l i c a t i o n required i n random biomass samples (Harper and White 1974). Others who have taken t h i s approach (e.g. Mack 1976) found that r e p l i c a t e p l o t v a r i a t i o n can be very small and population f l u x patterns can be accurately portrayed for s p e c i f i c environments. At each p l o t , a 30 cm length of PVC p l a s t i c pipe was inserted into the sediment to a depth of 29 cm to act as a permanent pl o t marker. A plo t frame of 20 cm X 50 cm (inside dimensions) could be relocated accurately by placing one of the four legs of the p l o t frame into the buried pipe and a l i g n i n g the edge of the frame with the remaining pipes i n the row. A pl o t was defined as the area v e r t i c a l l y projected through the inside of the frame when properly aligned. A sheet of Pl e x i g l a s f i t onto the frame to form a t a b l e - l i k e surface which, when c o r r e c t l y positioned, was 10 cm above the sediment. Plots at each t i d a l height were divided evenly into three treatments. In treatment A, a l l shoots were repeatedly mapped throughout t h e i r l i f e span. In treatments B and C, a l l shoots of Z. japonica and R. maritima, r e s p e c t i v e l y , were removed on June 11; by removal of subsequently appearing shoots the emergence and i d e n t i t y of seedlings was monitored u n t i l the following May. To map the shoots i n each p l o t , an acetate sheet was l a i d over the P l e x i g l a s . Each shoot was i d e n t i f i e d to species and then mapped by v e r t i c a l l y p r o j e c t i n g the l o c a t i o n of i t s base onto the acetate sheet. V e r t i c a l alignment was insured by the use of a pa r a l l a x tube (a 10 cm Plexiglas tube of 8 cm inside diameter with cross hairs at ei t h e r end) 16 which remained v e r t i c a l by r e s t i n g on the Pl e x i g l a s surface of the p l o t frame. Shoot locations were coded (by symbol and color) to indic a t e dates of shoot appearance, f i r s t flowering, and death. A l l shoots that appeared between any two successive sampling dates were treated as a cohort. Morphology and ovary fates' Monthly observations were made on samples of Z. japonica and R. maritima c o l l e c t e d from each t i d a l height. Samples consisted of a minimum of 15 shoots selected haphazardly from within 3 m of each s t a t i o n and preserved i n 5% (v/v) formaldehyde i n seawater. Measure-ments were taken at a l a t e r date on ten haphazardly selected shoots to determine the average length of the longest l e a f measured from the top of the l e a f sheath, the number of inflorescences per flowering shoot and the number of ovaries per inflorescence. The average length of r e c e n t l y produced rhizome internodes was found by measuring the internode length behind each of ten terminal shoots. Attempts were also made to determine the extent of clonal growth of i n d i v i d u a l genets through c a r e f u l excavation i n the f i e l d and monitoring seedlings grown under laboratory conditions. The fates of i n d i v i d u a l ovaries contained within two age classes of 25 inflorescences each were assessed for both Z. japonica and R. maritima at the three t i d a l height st a t i o n s . Inflorescences were chosen haphazardly within treatment A p l o t s (no unnatural manipulation) and were mapped and relocated using the p l o t frame previously described. The p o s i t i o n of the inflorescences observed was always on the second formed l a t e r a l branch and required no spe c i a l tagging f or r e l o c a t i o n . The f i r s t 25 inflorescences were chosen on June 11, 1980, the beginning of the flowering period, f o r each species at each s t a t i o n . Inflorescences of the second age class were selected midway through the expected flowering period, as determined by observations made at the s i t e the previous year (P.G. Harrison unpublished data). For Ruppia, the inflorescences f o r the second age class were chosen on Ju l y 10, 1980, and for Zostera, on July 30, 1980. Observations were made on ovaries of both age classes twice monthly u n t i l the inflorescence was l o s t or the ultimate fate was c e r t a i n . Ovaries were counted and evaluated i n terms of four possible f a t e s : 1. Abortion of developing ovary 2. Damage (by predation) r e s u l t i n g i n death while on inflorescence 3. Incomplete development of ovary r e s u l t i n g from death of parent shoot 4. Maturation complete Seed production was estimated f o r both species at each of the three t i d a l heights. The number of flowering shoots r e c r u i t e d was m u l t i p l i e d by the average number of inflorescences per flowering shoot, the number of female flowers per inflorescence, and the percent of ovaries produc-ing mature seed f o r a defined age c l a s s . Buried seed enumeration The number and d i s t r i b u t i o n of buried seed was assessed i n the winters of 1980 and 1981 and the summer of 1981 by examining ten duplicate samples within a 5 m radius of the three s t a t i o n s . Each sample was taken by placing a Ple x i g l a s tube of 8 cm inside diameter into the sediment to at l e a s t 23 cm depth. Cores were extracted, placed i n 18 p l a s t i c bags, and f i r m l y wrapped i n paper to support the core shape and minimize v e r t i c a l displacement of s t r a t a . Samples were then stored at 10°C i n the dark for between one day and two weeks before examination. At the time of inspection, samples were unwrapped and placed i n a l o n g i t u d i n a l l y divided tube of the same diameter as the coring tube. The sediment c y l i n d e r was then cut into one centimeter discs to the depth of 20 cm by a wire guided by horizon-t a l s l i t s i n the divided tube. Each disc was then examined i n d i v i d u a l l y by washing, with saltwater, through a series of sieves. F i r s t a coarse screen of 4 mm mesh was used to remove mollusks, the larger d e t r i t u s , and much of the root and rhizome network. Then material was passed through a sieve of 1.9 mm mesh to remove remaining roots, d e t r i t u s , and coarse p a r t i c l e s . T i g h t l y matted roots were separated to release any trapped seed. S l i t s , sand, seed, and f i n e organic p a r t i c l e s continued into a sieve of .84 mm mesh which retained a l l seeds and few inorganic materials. Several examinations of the remaining f r a c t i o n revealed no seed, so inspection was discontinued. The f r a c t i o n containing seed was examined by d i s s e c t i n g scope, illuminated magnifying lens or naked eye, and the number and type of seed were noted. The v i a b i l i t y of seed was tested i n four groups per s t a t i o n : 0 (surface) - 5 cm, 5 - 1 0 cm, 10 - 15 cm, and 15 - 20 cm depth. By species, seed from each of the ten r e p l i c a t e s was combined i n the four sections, and twenty seeds, i f a v a i l a b l e , were selected haphazardly for v i a b i l i t y t e s t i n g . V i a b i l i t y was tested by two methods used separately or i n combina-t i o n . Non-dormant seed was simply germinated i n d i s t i l l e d water at 10°C; a f t e r a period of about one week, seed which had not germinated 19 was examined with the tetrazolium t e s t . The tetrazolium t e s t consisted of soaking seed at approximately 15°C f o r between 3 and 24 hours i n a 1% (v/v) so l u t i o n of 2,3,5-triphenyl 2H-tetrazolium chloride i n d i s -t i l l e d water (Moore 1961). V i a b i l i t y was confirmed i f the seed germinated or the hypocotyl stained red with the tetrazolium t e s t . Germination has been defined f o r these studies as the emergence of the hypocotyl for a minimum of 5 mm. This d e f i n i t i o n was needed because on occasion dead seed can imbibe water and the swollen endo-sperm s p l i t s , giving the appearance of i n i t i a l stages of germination. The depth of seed germination was studied at s t a t i o n 2 on f i v e occasions from March 10 to May 1, 1981. Cores were c o l l e c t e d , near s t a t i o n 2, transported to the laboratory, and cut into 1 cm discs as' previously described f o r studies of buried seed numbers. Shoots a r i s i n g from overwintering rhizomes were also noted when encountered. V i a b i l i t y and germination of seed stored i n aerobic conditions Seed used i n the following experiments was of two age categories. Seed recovered from sediments during the summer and thus at le a s t one year old, was designated " o l d " . Seed obtained from the current year's production w i l l be ref e r r e d to as "new". New seed of R. maritima and Z. japonica was harvested on Ju l y 10, and J u l y 30, 1980, res p e c t i v e l y , from mature inflorescences on plants growing 100 m seaward from s t a t i o n 3. Seed was placed i n 27 o/oo seawater and were stored at 5°C i n the dark u n t i l used. Old seed of Z. japonica and R. maritima was recovered on Ju l y 10, 1980 by sie v i n g sediments from the v i c i n i t y of s t a t i o n 2 with seawater through a .84 mm mesh screen. Seed was separated by species and stored 20 in 27 o/oo seawater i n the dark at 10°C for one week, a f t e r which time a few seeds were observed to be germinating. Seed was then transferred to dark storage at 5°C where i t remained the duration of the storage period (9 months) with only an infrequent germination. The v i a b i l i t y of new and old seed was r e g u l a r l y tested with two or three r e p l i c a t e s of not less than 50 seeds each, with methods previously described for buried seed. A l l v i a b i l i t i e s were expressed as a percent of seed i n i t i a l l y recovered or harvested. Germination curves are expressed as the percent of v i a b l e seed i n each r e p l i c a t e at the end of the germination period. To test the e f f e c t s of s a l i n i t y on germination, new Zostera and Ruppia seed was placed between f i l t e r paper discs i n p e t r i dishes containing 15 to 20 ml of d i s t i l l e d water and/or seawater, and was held at 10°C. Four plates containing 50 seeds each were prepared per species for each of four s a l i n i t i e s : 0, 10, 20, and 27 o/oo. The e f f e c t of scoring on germination of new seed was tested by nicking seed coats with a razor blade. F i f t y - s e e d r e p l i c a t e s of scored seed of each species were placed i n 1 and 10 o/oo s a l i n i t i e s . Two to four 50-seed r e p l i c a t e s of old seed of each species were germinated i n 0 and 20 o/oo s a l i n i t i e s . Additional r e p l i c a t e s of old seed were placed under 10 and 27 o/oo s a l i n i t i e s ; t h e i r exposure to white l i g h t was prevented by sorting under a g r e e n - f i l t e r e d 25 watt incandescent lamp emitting l i g h t of 450 « 560 nm wavelength. Seed exposed to white l i g h t was placed i n sunlight (200 - 1000 microeinsteins m"2 s e c - l ) f o r ten minutes. 21 The e f f e c t s of temperature and anaerobic conditions on the v i a b i l i t y  of new seed New seed of Z. japonica and R. maritima was c o l l e c t e d and stored as previously described. On October 14, 1980, nine 100-seed r e p l i c a t e s of each species were placed i n each of four storage conditions. Temper-atures were eit h e r constant 5°C (anticipated winter sediment temperature) or a l t e r n a t i n g between 15° and 7°C (anticipated spring sediment temperatures) f o r 12 hours each. Both temperature regimes had treatments with and without oxygen. Seed was placed between two f i l t e r paper discs i n 9 cm p l a s t i c p e t r i dishes which each contained 15 to 20 ml of 27 o/oo seawater. The p e t r i dishes were stacked inside 4 - l i t e r p l a s t i c j a rs which were then closed. To assure jars were gas-tight, the outside surfaces were painted with two coats of Varathane p l a s t i c enamel and the inside surface was coated with a t h i n layer of petroleum j e l l y . Anaerobic conditions were produced by passing moist nitrogen gas through the j a r by means of two ports i n the l i d for 30 minutes every week to ten days. Several times during the 30-minute period the gas o u t l e t was closed, allowing pressure to b u i l d i n s i d e , and thus encouraging exchange under p e t r i l i d s . Sea-water placed i n anearobic treatment p e t r i dishes was bubbled with nitrogen gas for 30 minutes before i t was added to the seed. The pH of the seawater was 7.3 before bubbling with nitrogen, and 8.5 a f t e r , as measured with a Fisher Model 150 pH meter. Aerobic conditions were maintained using untreated seawater, and by p u l l i n g moist a i r through the jars at one-week to 10-day i n t e r v a l s with a vacuum pump. The a i r was exchanged to avoid excessive CO2 l e v e l s that could r e s u l t from the decomposition of ovary walls s t i l l 22 covering many of the seeds at the time the experiment began. On February 5, and A p r i l 4, 1981, four 100-seed r e p l i c a t e s were removed from each storage treatment. Each r e p l i c a t e was divided i n h a l f ; 50 seeds were placed at constant 5°C and 50 under a l t e r n a t i n g 15 and 7°C (12 hours each), a l l i n 27 o/oo s a l i n i t y , aerobic conditions. Sorting and observations involving the ninth set of 100-seed r e p l i c a t e s from each storage treatment took place i n February under a green safe-l i g h t . These seed were germinated i n aerobic, a l t e r n a t i n g 15 and 7°C conditions. V i a b i l i t y of experimentally buried seed To test the e f f e c t s of b u r i a l depth on seed v i a b i l i t y , new and old seed of Z. japonica and R. maritima was separated into 16 r e p l i c a t e s of 100 seeds each. Four r e p l i c a t e s of each species and age category were buried at each of four sediment depths: 5, 10, 15, and 20 cm near s t a t i o n 2 at Roberts Bank. Additional 100-seed r e p l i c a t e s of new Z. japonica and R_. maritima were buried at each sediment l e v e l to t e s t the germination response of seed not exposed to l i g h t . Just p r i o r to b u r i a l on October 14, 1980, 100-seed r e p l i c a t e s of each species were placed i n i n d i v i d u a l 10 x 15 cm p l a s t i c coated 2 f i b e r g l a s s bags of 1 mm mesh. The bags were p a r t i a l l y f i l l e d with sediment from the l e v e l the bag would be buried at; the f i n a l volume was about 150 ml. Attention was given not to have seed aggregate i n one l o c a t i o n . Bags were folded closed and attached to 2 X 2 X 50 cm wooden stakes with aluminum staples. Buried samples remained undisturbed u n t i l recovery, with the exception of r e b u r i a l of an occasional bag from the 5 cm l e v e l that would become p a r t i a l l y exposed. The exposure of these bags was a r e s u l t of erosion i n the immediate area around the stake. Two r e p l i c a t e s of each species and age category, and a l l of the seed which was not exposed to white l i g h t were recovered from each sediment depth on February 2; a l l others were recovered on A p r i l 9, 1981. The stakes and attached bags were kept moist and transported d i r e c t l y to the laboratory, about a one-hour t r i p . The bags were stored at 10°C u n t i l they were sieved under seawater with a .84 mm screen. Each r e p l i c a t e of 100 was divided i n h a l f ; 50 seeds were placed at 5°C, and 50 were placed under temperatures a l t e r n a t i n g from 15 to 7°C every 12 hours. Seed was contained between f i l t e r paper discs i n p e t r i dishes inside p l a s t i c j a rs as described f o r new seed i n the laboratory. Seed was checked for germination i n i t i a l l y on a d a i l y basis and then at regular i n t e r v a l s as indicated i n the r e s u l t s . Seed which was not exposed to l i g h t was handled i n the same way as pre-vi o u s l y described for new seed stored i n the laboratory. Figure 5 summarizes storage, r e b u r i a l , and germination treatments of new and old seed of Zostera and Ruppia. S a l i n i t y and scoring experiments have been omitted. Both stored and reburied seed was submitted to the same germination treatments. Data analyses One way analysis of variance for unblocked data was performed at the 5% s i g n i f i c a n c e l e v e l . Sampling i n t e r v a l s Table I summarizes i n t e r v a l s of samplings throughout the study. NEW S E E D B U R I A L I 1 1 I 5 cm 10 cm 15 cm 20 cm S T O R A G E T R E A T M E N T S 1 1 I 1 Aerobic Anaerobic Aerobic Anaerobic 5 C 5 C 15/7 C 15/7 C ^ \J/ ^ ^ G E R M I N A T I O N T R E A T M E N T S ( A E R O B I C ) f t t f 5 cm 10 cm 15 cm 20 cm B U R I A L OLD S E E D Dark 15/7 C February Recovery V Light Light 5 C 15/7 C t t A p r i l Recovery 5. Flow chart of b u r i a l , storage, and germination treatments of old and new seed of Zostera japonica and Ruppia maritima. : I. Sampling and observation dates and approximate times of temperature readings at Roberts Bank, 1980 - 1981. Codes for observations made are as follows: DI, D2, etc. = shoot demography establishing cohorts 1, 2, etc.; Rl, R2, Z l , and Z2 = Ruppia maritima and Zostera japonica flower fate age classes 1 and 2, M = morphological samples, 0 = oxidizing potential, and S = s a l i n i t y . Date Time Observations Made May 15 1200 DI M s June 2 1300 D2 s 11 1100 D3 Rl - Zl M 0 s 23 0900 D4 Rl Zl July 10 1100 D5 Rl,2 Z l M 0 S 21 0800 D6 s 30 1300 Rl,2 Zl,2 s August 6 D7 Rl,2 Zl,2 15 1400 0 s 26 1500 D8 R2 Z2 M September 5 0900 s 12 D9 12 M 0 23 1000 s October 6 1000 D10 M 14 0200 0 s 21 Dll December 3 2200 D12 M 0 s January 6 2300 D13 M 23 0100 0 s February 2 s 6 0100 D14 M 21 0 s March 3 2100 D15 10 14 30 D16 M 0 s 24 1300 D17 s Apri 1 9 1400 D18 M s 18 1200 D19 0 s May 1 1000 D20 0 s 26 SECTION 1. PHYSICAL ENVIRONMENT Transect topography Figure 6 shows a p r o f i l e of general sediment topography along the study transect. Stations 1, 2, and 3 were located at +3.17, 2.54, and 2.60 m, r e s p e c t i v e l y . There were no noticeable changes i n sediment l e v e l at stations 2 and 3 r e l a t i v e to the height of the p l a s t i c pipes marking pl o t locations during the study period. At s t a t i o n 1, however, sand waves t r a v e l l e d across the s t a t i o n i n A p r i l , temporarily covering p l o t s with an addi t i o n a l 8 cm of sandy sediment, 2 cm of which were s t i l l present on the 1st of May. The diffe r e n c e of .06 m between stations 2 and 3 did not cause a differ e n c e i n the amount of time they were covered by t i d e s . Station 2 however, was located where water draining from the higher areas of the t i d a l f l a t was channeled and plants were submerged i n about 4 cm of water i n excess of an hour a f t e r the t i d e receded while water drained from the upper areas. Water did not pool at any s t a t i o n . M i c r o r e l i e f within p l o t s varied with burrowing animal a c t i v i t y . Most small-scale v a r i a t i o n s i n sediment height (mostly from f e c a l casts) were removed by the incoming t i d e . Tides Table II shows estimates of the percent of days within each month of the year on which the t i d e exposes +2.5 m sediments at Roberts Bank during the peak heating hours of the day. It was estimated that sediments at +3.2 m, approximately the height of s t a t i o n 1, are exposed a t o t a l o f between 8 to 10% more often than are sediments at +2.5 m. On the coast of B r i t i s h Columbia during the summer, extreme Fig. 6. General topography of study transect showing positions of stations at Roberts Bank. 28 Table I I . Percent of days of exposure per month from tides reaching low during peak heating hours of the day at or below 2.5 m CD t i d a l height, Roberts Bank. Peak heating hours, determined by Environment Canada, were 11:30 - 14:00 i n winter, and 10:30 - 16:00 i n the summer. Actual t i d a l exposures may vary s l i g h t l y from predicted l e v e l s according to meteorological events (e.g. extreme or prolonged wind, r a i n , or barometric pressure). Month Total % < 3 hrs. 3 - 6 hrs.. > 6 hrs. Jan. 0 0 0 0 Feb. 16 16 0 0 Mar. 48 19 28 0 Apr. 60 10 40 10 May 61 0 32 29 Jun. 63 0 23 40 J u l . 64 6 23 35 Aug. 54 9 29 16 Sep. 43 20 23 0 Oct. 32 29 3 0 Nov. 6 6 0 0 Dec. 0 0 0 0 29 lower water i s associated with, bimonthly spring tides and occurs near midday, and i n the winter near midnight. Thus i n winter, i n t e r t i d a l sediments may have prolonged exposures to nighttime temperatures. Freezing of surface sediments to 2 - 3 cm depth was observed on several occasions at Roberts Bank during midwinter. In the summer, the longest exposures take place during the hottest part of the day. During exposure to the midday summer sun and winds, l e a f surfaces of Zostera and Ruppia i n contact with the a i r can dry to the point that they become b r i t t l e . Considerable desiccation occurred at s t a t i o n 1, with progressively less at stations 3 and 2. Sediments Sediment textures were e s s e n t i a l l y i d e n t i c a l at each t i d a l height during the winter, and showed a s l i g h t increase i n s i l t and clay fract i o n s i n the summer (Table III) . P a r t i c l e s i z e analyses of sediments at the study s i t e show well-sorted homogeneous sands. Increases i n f i n e p a r t i c l e s i n the summer could be explained by the input of s i l t from the Fraser River freshet. In addition, slowing of water movements by the vegetation would increase the deposition of f i n e suspended p a r t i c l e s from the water column. Marshall and Lukas (1970) reached the same conclusion when they found that the f i n e - p a r t i c l e f r a c t i o n of sediments was higher within beds of Zostera marina compared with adjacent bare areas. S a l i n i t y S a l i n i t y measurements recorded at the study s i t e f luctuated between 31 and 21 o/oo, reaching a low in August and highs i n October and January. Table I I I . Percentage of sediment i n each of f i v e s i z e classes from three stations (1 = 3.17 m, 2 = 2.54 m, 3 = 2.60 m CD t i d a l height) at Roberts Bank, 1980 - 1981. Jan. 1980 Aug. 1980 Jan. 1981 Size Class (mm) > 2.0 0 0 0 0 0 0 0 0 0 (Gravel) .25 - 2.0 26 24 22 21 23 24 22 24 27 (Coarse Sand) .05 - .25 68 70 73 73 70 70 72 71 68 (Fine Sand) .002 - .05 6.5 6.0 5.0 6.5 6.5 6.0 6.0 5.0 5.5 ( S i l t ) < .002 0 0 0 0 0.5 0 0 0 0 (Clay) 31 S a l i n i t i e s were t y p i c a l l y between 39 and 25 o/oo most of the year (Fig. 7). Seawater s a l i n i t i e s vary considerably i n coastal regions under the influence of the Fraser River. Swinbanks (1979) recorded surface s a l i n i t i e s at low ti d e on t i d a l f l a t s of the Fraser Delta that ranged between 1 and 33 o/oo. Waldichuck (1957), found s a l i n i t i e s t y p i c a l l y l i e i n the range of 24 to 29 o/oo, i n the southern s t r a i t of Georgia. These "normal" S t r a i t of Georgia s a l i n i t i e s are what are usually encountered i n the intercauseway area at Roberts Bank (Levings and Coustalin 1975, Swinbanks 1979). As indicated by measurements from August, there i s a reduction i n the intercauseway s a l i n i t y as a r e s u l t of the Fraser River freshet. The period during which the freshet occurs varies from mid-July to mid-August (Waldichuck 1957) . Locations immediately north and south of the intercauseway area are influenced to a much greater extent by the Fraser River plume. Swinbanks conducted an extensive survey of surface s a l i n i t i e s i n early July, 1977 and found s a l i n i t i e s to be f a i r l y constant over much of the t i d a l f l a t , between 24 and 27 o/oo. There was a marked increase towards the marsh i n the northern corner, where s a l i n i t i e s reached 30 o/oo. The increased s a l i n i t y of these areas was thought to be caused by evaporation of pooling water i n t h i s region. Water draining over s t a t i o n 2 from marsh areas at low ti d e may have resulted i n s l i g h t l y higher s a l i n i t i e s there than in the re s t of the intercauseway area. The s a l i n i t y of i n t e r s t i t i a l waters of sediment p r o f i l e s was not monitored, but i t has been studied i n the intercauseway area, i n t e r -t i d a l l y by Swinbanks (1979) and i n the strandline by A. Moody (1978) . 33 They found that subsurface s a l i n i t i e s d i d not d i f f e r unless there were dramatic fluctuations i n surface s a l i n i t i e s . At Roberts Bank, small-scale s a l i n i t y changes occurred d a i l y as a r e s u l t o f t i d a l c y c l e s , drainage patterns, amount and period of p r e c i p i t a t i o n , and temperature. These changes were l i k e l y ephemeral and affected only the f i r s t few centimeters of the sediment p r o f i l e , the subsurface s a l i n i t i e s remaining the s a l i n i t y of the t i d a l waters. Sediment temperatures Sediment temperatures roughly p a r a l l e l e d seasonal changes i n a i r temperature (Fig. 8). The magnitude of temperature fl u c t u a t i o n s decreased with increasing sediment depth. Maximum surface sediment temperatures which were well above a i r temperatures occurred i n l a t e J u l y and early August. The minimum surface temperature was recorded i n December, at which time the sediment was frozen hard to the depth of 1 to 2 cm and was s t i f f e n e d an addi t i o n a l 2 to 3 cm. In March, temperatures at a l l sediment l e v e l s measured showed a marked increase from winter values. The amelioration of temperature fl u c t u a t i o n s with depth recorded at Roberts Bank has also been observed i n most s o i l s (Russell 1973) . Levings and Coustalin (1975) recorded surface sediment temperatures throughout the Fraser foreshore and also found that sediment surface temperatures tended to follow seasonal trends; they attributed the extreme fluctuations of surface sediment temperatures to the r e f l e c t i v e and absorptive properties of the sediment (Perkins 1963) . Levings and Coustalin also observed the winter freezing o f surface sediments. General trends i n subsurface temperatures were strongly influenced by the seawater temperature. R. Moody (1978) recorded water temperatures on the east side of the Tsawwassen f e r r y terminal, 1.5 m below the May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr Fig. 8. A i r and sediment temperatures near s t a t i o n 2 (2.54 m CD t i d a l height) at Roberts Bank, 1980 - 1981. = a i r , — = surface sediment, = 5 cm depth, and = 15 cm depth of sediment. Readings were taken during low t i d e ; approximate times are l i s t e d i n Table I. 35 surface and found winter temperatures of about 8°C. A maximum tempera-ture of 16°C was reached i n August. These temperatures correspond with temperatures measured 15 cm below the sediment surface ( F i g . 8). Oxidizing p o t e n t i a l A progressive reduction of sediments (lowering of redox potential) took place from June to August, and resulted i n the anaerobic boundary i n the sediment (defined as + 100 mv redox p o t e n t i a l , Fenchel 1969) r i s i n g from 2 cm depth to only a few millimeters below the sediment surface ( Fig. 9). The aerobic layer of sediment deepened i n the autumn, t h i s trend continued throughout the winter. In the spring, the reduction of sediments again brought the anaerobic layer nearer to the surface. Typical redox p r o f i l e s i n marine sediments, as measured at Roberts Bank, are the r e s u l t of many b i o t i c and a b i o t i c factors (Fenchel 1969). Heterotrophs within the sediment can both reduce (through metabolic a c t i v i t i e s ) and oxidize t h e i r environment (through the aeration of sediments by burrowing). Photoautotrophic organisms also reduce and oxidize t h e i r environment depending on t h e i r photosynthetic a c t i v i t y (Taylor 1964) . Fenchel (1969) found that the redox influence of photo-autotrophs extend only a few millimeters below the sediment surface. Any marked diurnal f l u c t u a t i o n s i n surface redox at Roberts Bank were not sampled because the electrode t i p was inserted a minimum of 1.5 cm below the sediment surface to prevent a i r contamination. Decomposition of d e t r i t u s by microorganisms i s a major contributor to the reduction of sediments (Fenchel 1969). Temperature often d i c t a t e s the rate at which various oxidative and reductive processes proceed (Sorensen et a l . 1979). Wave action and sediment texture influence the incorporation of organic 36 Fig . 9. a - c. Selected p r o f i l e s of o x i d i z i n g (redox) po t e n t i a l s of the sediment near s t a t i o n 2 (2.54 m CD t i d a l height) at Roberts Bank 1980 - 1981. A l l redox measurements are i n Appendix 2. S e d i m e n t D e p t h ( c m ) O CD CTi NJ O CO CT> f - N J o OD cn r~ K J o o  r-cj O CD O r» M O CO (7* *•» NJ 38 material i n the sediment and also the transportation of oxygenated water down through the sediment p r o f i l e . A major problem i n gaining an accurate picture of marine i n t e r t i d a l redox p r o f i l e s i s the tremendous heterogeneity caused by infauna and pockets of organic material within the sediment. Jorgensen (1977) has given attention to the presences of reduced microsites i n otherwise oxidized environments. He calculated that a f e c a l p e l l e t of 200 micro-meters diameter could provide an anoxic core, where the pore water oxygen content of sediment was only 1% of a i r saturation. 39 SECTION 2. GROWING COMPONENT OF POPULATION Results Shoot recruitment The recruitment of shoot cohorts, showed d e f i n i t e seasonal trends (Fig. 10). Zostera shoot recruitment was s i g n i f i c a n t l y higher than that of Ruppia at a l l st a t i o n s . Shoot recruitment to the Zostera population occurred throughout the year, with a decline during the winter, whereas shoot recruitment to the Ruppia population lasted only from March through August. Recruitments of Zostera shoots at the three stations showed several r e l a t i o n s h i p s i n t h e i r s i z e and timing ( F i g . 10 a-c). Numbers of shoots found i n May were not s i g n i f i c a n t l y d i f f e r e n t between t i d a l heights. During the flowering period, June through October, cohorts were s i g n i f i c a n t l y larger at s t a t i o n 2 than at s t a t i o n 1; cohort s i z e at st a t i o n 3 was not s i g n i f i c a n t l y d i f f e r e n t from 1 or 2. Zostera at each s t a t i o n exhibited a slow increase i n s i z e of cohorts u n t i l the end of July, and then a reduction to low l e v e l s of recruitment by December. A reduction of cohort s i z e began at most stations with cohort 7 (August 6). Low rates of recruitment were maintained through February at each s t a t i o n with a s i g n i f i c a n t increase i n cohort s i z e at a l l stations i n March. Ruppia shoot numbers at st a t i o n 1 were s i g n i f i c a n t l y lower than at stations 2 or 3. The average s i z e of cohorts during the flowering period at st a t i o n 1 was smaller than at 2 and 3, but there were no s i g n i f i c a n t differences o v e r a l l between the st a t i o n s . The pattern of subsequent growth was s i m i l a r to that i n Zostera, but temporal boundaries were much t i g h t e r , and no overwintering vegetative population existed. 40 10. a - f. Recruitment and loss of shoots between successive sampling dates at each of three stations at Roberts Bank 1980 -1981 (absolute numbers). No shading = shoots with no reproductive record, s t i p p l i n g = shoots which became reproductive a f t e r f i r s t record, Crosshatch = shoots which were reproductive at f i r s t record. 75' 50 251 E o 0-QJ CL 25-(/) • « * O o JZ 50-75-Recruitment -r±r- -e- - €S B=b—[T]p, m rooH 125-1 Losses 2 3 A 5 6 7 -H 1 1 1 1 H 10 11 — I 1- 12 -+- 13 — f - 15 16 17 18 19 20 -i—1 1 1—I 1 May Jun Jul Aug Sep Oct Cohort Number and Month of Sampling F i g . 10a. Zostera japonica at s t a t i o n 1 (3.17 m CD). Nov Dec Jan Feb Mar Apr 100 75 50 H 25H CM E ,— o 0-i_ CD C L 25-CO • 4 • o o JZ CO 50-75-Recruitment -a [TTn LT 1_L l_l3 iooH 125H Losses 135 2 3 4 H 1 H 5 6 H 1 -7 8 1-/4ug 9 10 11 —l h-12 13 — f — H -+- 15 16 17 18 19 20 — I — I 1 1 — I 1 May Jun Jul Sep Oct Nov Dec Jan Feb Mar Apr Cohort Number and Month of Sampling Fig. 10b. Zostera japonica at s t a t i o n 2 (2.54 ra CD). 75-50H 25H o C D Q . 25-O O CO 50H Recruitment -4-U-i—EH—m, i i i h Losses 1 2 3 4 5 6 7 8 9 10 11 12 13. H 15 16 17 18 19 20 j, 1 1 1 1 1 1 1 1 1 1 1 • 1 h 1—I : — I 1 1 1 May Jun Jul Aug Sep Oct Nov Dec . Jan Feb Mar Apr Cohort Number and Month of Sampling F i g . 10c. Zostera japonica at s t a t i o n 3 (2.60 m CD). 44 15-1 10 0 JP Eprp. 5 -E 10 -C D 1 5 -L _ QJ C L 1 5 -U ) " o 1 0 -O - C CO 5 -0 -5 -10 -1 5 -20 -I I Recruitment H (-Losses Recruitment 4-1 H H H Losses 2 3 —I H- — I — Jun 5 6 H 1— 9 10 11 — I h -Sep Oct May n Jul Aug Cohort Number and Month of Sampling Fig. 10d. Ruppia maritima at s t a t i o n 1 (3.17 m CD). F i g . lOe. Ruppia maritima at s t a t i o n 2 (2.54 m CD). 1 1 t Recruitment V/, 1 Losses 1 2 3 4 5 6 7 8 9 10 11 1 1 — I 1 1 1 1 1 1 1 j — May Jun Jul Aug Sep Oct Cohort Number and Month of Sampling F i g . lOf. Ruppia maritima at sta t i o n 3 (2.60 m CD). 46 Ruppia cohort s i z e peaked i n the f i r s t part of J u l y at a l l s t a t i o n s . Shoot recruitment ended e a r l i e s t at s t a t i o n 1 (July 21); no new Ruppia shoots were seen a f t e r August 6 at any s t a t i o n . By p a r t i t i o n i n g cohorts into flowering and vegetative shoots, under-l y i n g patterns were revealed. The shoots that never became reproductive, and thus died (or were l o s t from the population) i n the vegetative state, had patterns of recruitment that were recognizably d i f f e r e n t from those with a recorded reproductive phase. Further d i s t i n c t i o n s can be made between shoots which had a recorded vegetative phase before they became reproductive and those which were reproductive from f i r s t record. Once reproductive, a shoot produced flowers u n t i l i t died. The recruitment of Zostera shoots that eventually became reproductive increased with cohort s i z e and then declined once peak cohort si z e was reached (Fig. 10 a-c), except f o r s t a t i o n 1 where the majority of reproductive shoots were re c r u i t e d before the peak cohort. This s h i f t represented a marked asymmetry i n the temporal boundaries of reproductive shoot recruitment between s t a t i o n 1 and the other two s t a t i o n s . Stations 2 and 3 were s i m i l a r i n t h e i r general pattern of recruitment, except f o r the high vegetative recruitment of cohort 8 at s t a t i o n 3. I n i t i a l cohorts of Zostera shoots were s i m i l a r i n size but not i n fate. About h a l f the f i r s t cohort at s t a t i o n 1 became reproductive, along with a small proportion at 2, and none at s t a t i o n 3. The end of reproductive shoot recruitment (early August at s t a t i o n 1 vs. l a t e August or early September at stations 2 and 3) again emphasized the temporal s h i f t to early reproductive shoot recruitment at s t a t i o n 1 (Fig. 10 a-c). It i s not know i f any of the overwintering shoots from 1980 - 1981 became reproductive. 47 Ruppia showed two outstanding features i n i t s pattern of shoot p a r t i t i o n i n g (Fig. 10 d - f ) . F i r s t , few shoots remained vegetative, and second, 71, 39, and 26 percent of shoots at stations 1, 2, and 3, respectively, were reproductive at f i r s t record, a phenomenon which was never observed to happen with Zostera shoots. Ruppia showed generally high reproductive recruitment i n early cohorts. At s t a t i o n 1, recruitment of Ruppia shoots was t y p i c a l l y less than at stations 2 and 3 i n the early growing season (cohorts 1 and 2), but a higher proportion was reproductive. Station 3 showed larger proportions of reproductive shoots than s t a t i o n 2 throughout the growing season, a f t e r i t s i n i t i a l t o t a l l y vegetative cohort. Reproductive shoot recruitment ended at stations 1 and 2 with cohort 6, and at s t a t i o n 3 with cohort 7. Shoot losses No Zostera shoots were l o s t from s t a t i o n 1 between May 15 and June 2, whereas there were losses at both stations 2 and 3 (Fig. 10 a-c) Losses at a l l stations were i n i t i a l l y low but increased with successive cohorts, reaching a peak one to four cohorts a f t e r maximum recruitment. Peak losses were i n August at s t a t i o n 1, and during October at stations 2 and 3. Losses balanced recruitments at a l l stations from December to May. Few or no Ruppia shoots were l o s t through June (Fig. 10 d - f ) . The same synchrony seen i n Ruppia shoot recruitment between t i d a l heights was also displayed i n shoot l o s s . Peak losses, recorded i n early August, were at stations 1 and 3. A l l shoots of Ruppia were l o s t by September 12. 48 Zostera shoots l o s t u n t i l J u l y 21 (cohort 6) were p r i m a r i l y vegetative (Fig. 10). Additional vegetative losses occurred throughout the reproductive phase, and were generally most severe following increased rates of recruitment. Reproductive losses usually' increased with successive cohorts. Ruppia showed a s i m i l a r pattern, l o s i n g mostly vegetative shoots f i r s t and then increasing proportions of reproductive shoots. Total shoots Throughout the study Ruppia stands were only one-fourth to one-ninth the si z e of Zostera stands (Fig. 11). Peak shoot densities of Zostera were twice as high at s t a t i o n 2 as at 1, although more shoots were main-tained at s t a t i o n 1 during the l a t e f a l l . Zostera shoot density at st a t i o n 3 was generally intermediate year-round. Also, as would be expected from the analysis of shoot recruitment and l o s s , there was a s h i f t i n the timing between the three stations. Flowering began and ended e a r l i e s t at s t a t i o n 1. Plants at stations 2 and 3 flowered synchronously, but almost two weeks l a t e r than at s t a t i o n 1. Numbers of both vegetative and reproductive shoots peaked l a t e s t at s t a t i o n 3. Ruppia showed some patterns i n shoot density i n r e l a t i o n to t i d a l height s i m i l a r to those of Zostera. Station 1 had the, lowest shoot numbers for Ruppia. At the time of maximum shoot density there were h a l f as many shoots at s t a t i o n 1 as at 2, but flowering shoot numbers were s i m i l a r . Flowering peaked simultaneously at a l l stations on J u l y 21 (cohort 6). Figure 12 summarizes the r e l a t i v e timing and density of flowering shoots at a l l stations. 49 11. a - f. Absolute contribution of cohorts r e c r u i t e d at successive i n t e r v a l s ( s o l i d and broken l i n e s d i f f e r e n t i a t e odd and even numbered cohorts for c l a r i t y ) , and t o t a l shoot density (bold l i n e ) of vegetative and reproductive shoots from three stations at Roberts Bank, May 1980 - May 1981. Zostera cohorts 11 - 20 are not shown fo r c l a r i t y . Data f or a l l cohorts are located i n Appendix 3. 100-1 2 3 4 5 6 7 8 9 10 11 12 13 H 15 16 17 18 19 20 I 1—| j 1 1 1 1 1 1 1 1 1 1 1—l 1 1—! 1 May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr Cohort Number and Month of Sampling F i g . 11a. Zostera japonica at s t a t i o n 1 (3.17m CD t i d a l height). on o Cohort Number and Month of Sampling F i g . l i b . Zostera japonica at s t a t i o n 2 (2.54 m CD). 53 Fi g . l i d . Ruppia maritima at s t a t i o n 1 (3.17 m CD). Fig. l i e . Ruppia maritima at s t a t i o n 2 (2.54 m CD). 54 F i g . l l f . Ruppia maritima at s t a t i o n 3 (2.60 m CD). 55 Fig. 12. Percentage of t o t a l shoots which were flowering at each of three stations ( = station 1, 3.17 m, = s t a t i o n 2, 2.54 m, = sta t i o n 3, 2.60 m CD) on successive sampling dates at Roberts Bank, 1980 - 1981. X 57 Cohort contributions Following i n d i v i d u a l cohorts through one year revealed that vege-t a t i v e and reproductive shoots of the same cohort d i f f e r e d i n longevity (Fig. 11). In both Zostera and Ruppia, the t r a n s i t i o n from the vegetative to reproductive state usually increased a shoot's l i f e span. Cohorts which were i n i t i a l l y the same s i z e at d i f f e r e n t t i d a l heights had d i f f e r e n t impacts on t h e i r respective stands, e.g. Zostera i n cohort 5 made a major contribution to reproductive shoot numbers at stations 1 and 2, but was sh o r t - l i v e d at s t a t i o n 3. The contribution of cohort 5 to numbers of Ruppia reproductive shoots was higher at s t a t i o n 3 than at st a t i o n 2. Station 1 was intermediate, receiving low vegeta-t i v e input from cohort 5, but a r e l a t i v e l y high reproductive contribution. By examining the i n d i v i d u a l cohort contribution to the t o t a l population, a deeper understanding of small-scale population changes can be obtained. The increase i n vegetative Zostera shoot numbers at st a t i o n 1 a f t e r the decline i n August can now be seen as a r e s u l t of the retention of cohort 8 shoots and the addition of cohort 9. The simultaneous depression of Ruppia vegetative shoots at a l l stations i n June was c l e a r l y not the r e s u l t of shoot loss, as might be expected, but of uniformly low shoot recruitment. To allow a clear examination of the contribution of i n d i v i d u a l cohorts to the population over time, F i g . 13 shows the percent c o n t r i -bution of each cohort to the shoots present at the time of sampling. In the period between May and September (cohorts 1 - 12), when shoot numbers r a p i d l y increased and flowering took place, there were some members of two to four, (a mean of three) Zostera cohorts present at any given time (Fig. 13 a - f ) . Overwintering shoots, between October 58 Fig. 13. a - L. Age structure of vegetative and reproductive shoot populations as percent cohort contribution to the t o t a l population for three stations (station 1 = 3.17 m, 2 = 2.54 m, 3 = 2.60 m CD t i d a l height) at Roberts Bank, 1980 - 1981. Not a l l stations contained every cohort. Gaps between vegetative cohort numbers (on time axis) and reproductive cohort numbers shown i n the reproductive shoot figures represent time lags f o r cohorts to become reproductive. F i g . 13a. Vegetative shoots of Zostera japonica at s t a t i o n 1. F i g . 13b. Reproductive shoots of Zostera japonica at s t a t i o n 1. o Cohort Number and Month of Sampling F i g . 13c. Vegetative shoots of Zostera japonica at s t a t i o n 2. 1 2 3 4 5 6 7 8 9 10 11 12 13 U 15 16 17 18 19 20 1 — | 1 1 1 1 — 1 1 1 1 1 1 1 1 — 1 1 1 1 1 May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr Cohort Number and Month of Sampling F i g . 13d. Reproductive shoots of Zostera japonica at s t a t i o n 2. O N Cohort Number and Month of Sampling Fig. 13e. Vegetative shoots of Zostera japonica at s t a t i o n 3. a* ON Cohort Number and Month of Sampling F i g . 13g. Vegetative shoots of Ruppia maritima at s t a t i o n 1. O N Fig. 13h. Reproductive shoots of Ruppia maritima at s t a t i o n 1. ON ON 2 3 U 5 6 7 . 8 9 10 11 12 13 U 15 16 17 18 19 20 - I 1 1 1 1 1 H \ h - 1 1 — I 1 1 1 1 May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr Cohort Number and Month of Sampling F i g . 13j. Reproductive shoots of Ruppia maritima at s t a t i o n 2. O N 00 Cohort Number and Month of Sampling F i g . 13k. Vegetative shoots of Ruppia maritima at s t a t i o n 3. O N t o 100 90-8M C 70-o E 60 o c cu o cu 30H 20-10 9 10 — f - 11 —t-12 - 4 — 13 U 15 16 — I — I — 17 - 4 -18 19 20 — I — I 1 May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr Cohort Number and Month of Sampling F i g . 13£. Reproductive shoots of Ruppia maritima at s t a t i o n 3. o 71 and March (cohorts 10 - 14), had generally an increased l i f e span. This i s the opposite of what was seen i n shoots that never became reproduc-t i v e i n the r a p i d l y growing summer populations. Both stations 2 and 3 had a single cohort, 11 and 10 r e s p e c t i v e l y , that c l e a r l y dominated the winter shoot numbers; at s t a t i o n 1, cohorts contributed more uniformly throughout the winter. When more rapid recruitment of Zostera shoots resumed i n March (cohort 15), shoots generally retained the longevity seen during the winter, although heavy m o r t a l i t y occurred i n February and early March, reducing the l i f e s p a n of some cohorts present during that period. Ruppia stands usually had two to four cohorts present at once, although at a given sampling date the proportions of cohorts varied greatly (Fig. 13 g-1) . Survivorship The l i f e span of shoots varied with species, t i d a l height and time. The survivorship curves for selected cohorts ( i n F i g . 14) i l l u s t r a t e s i m i l a r i t i e s i n survivorship between Zostera and Ruppia cohorts which appeared early i n the growing season (cohort 2) and t h e i r marked departure l a t e r i n the year (cohort 5). Also, the survivorship of cohorts from the same t i d a l height can be seen at d i f f e r e n t times. The most marked change was i n the survivorship of Zostera cohorts at s t a t i o n 2 from a Deevey (1947) Type I to a Type III between cohorts 5 and 8. Leaf length When observations began, i n May, mean lengths of the longest Zostera leaves were equal at a l l stations (Fig. 15). Leaf lengths at 14. a .- £. Survivorship curves of selected cohorts from three stations .(: = s t a t i o n 1, 3.17 m, = s t a t i o n 2, 2.54m, sta t i o n 3, 2.60 m CD t i d a l height) at Roberts Bank, 1980 - 1981. 73 Zostera japonica 2 3 4 5 6 7 l—l 1 1 1 1 Jun Jul Cohort Number Fig. 14a. Cohort 2. Ruppia maritima 2 3 4 5 6 7 (—l 1 1 1 1 Jun Jul and Month of Sampling F i g . 14b. Cohort 2. 74 Fig. 14c. Cohort 5. F i g . 14d. Cohort 5. 75 Fig. 14e. Cohort 8. F i g . 14f. Cohort 12. 76 15. a, b. Mean lengths of longest leaves from three stations ( = s t a t i o n 1, 3.17 m, = s t a t i o n 2, 2.54 m, = s t a t i o n 3, 2.60 m CD t i d a l height) at Roberts Bank, 1980 - 1981. Standard deviations are i n Appendix 4. L e a f L e n g t h ( c m ) cn • o a i o 1/1 _ i l l 1 . . i LL 0 May Jun Jul Aug F i g . 15b. Ruppia maritima 79 each s t a t i o n increased to c l e a r l y defined peaks, and then decreased to r e l a t i v e l y constant l e v e l s over f a l l and winter. After leaves resumed growth i n A p r i l l e a f length at st a t i o n 1 increased.rapidly between May and June, but f e l l d r a s t i c a l l y i n July. Stations 2 and 3 had slower i n i t i a l increases i n l e a f length and reached peak lengths which were considerably greater than at s t a t i o n 1, i n July. Stations 2 and 3 maintained s i m i l a r l e a f lengths between September and A p r i l which were longer than at s t a t i o n 1. Average l e a f lengths were the same at a l l stations i n March. Leaves produced at s t a t i o n 1 between August and February, and at stations 2 and 3 from September to February, were narrower than leaves which i n i t i a t e d growth during the summer (Fig. 16). Ruppia leaves were generally shorter than those of Zostera, and mean lengths showed a comparatively uniform pattern of growth at a l l stations. I n i t i a l l e a f lengths measured i n May were longest at st a t i o n 1. Peak l e a f lengths occurred i n July at each s t a t i o n . At l a s t record in August, leaves present at a l l stations were shorter than for July, but were s i m i l a r to each other. Rhizome internode length Lengths of the newest rhizome internodes i n Zostera were i n i t i a l l y not s i g n i f i c a n t l y d i f f e r e n t (Fig. 17). Station 1 showed only declines in length from May values, a pattern c l e a r l y divergent from that of station 2. A slow increase i n mean internode length was seen at stations 2 and 3; 3 peaked i n June and station 2 peaked i n July. At stations 2 and 3 there was a slow decline i n the length of rhizome grown between nodes, beginning i n July. Newly produced internodes I a DOl o L .5 _L mi l l imete r s Fig. 16 a - c. Leaf cross sections showing lacunae.. Fiber bundles have been omitted, a. Zostera japonica winter l e a f . b. Z. japonica summer leaf . c. Ruppia maritima 00 o 17. a, b. Mean lengths of newest rhizome internodes from three stations ( = s t a t i o n 1, 3.17 m, = s t a t i o n 2, 2.54 m, = s t a t i o n 3, 2.60 m CD t i d a l height) at Roberts Bank, 1980 1981. Standard deviations are i n Appendix 4. Rhizome Internode Length (cm) 84 were equal i n length at a l l stations i n September. Uniformly short rhizome segments were produced at each s t a t i o n during the f a l l . These short rhizome segments were s l i g h t l y smaller i n diameter than summer rhizomes, 1 - 2 mm i n the summer and 1 mm i n the winter. The upper surface of the rhizome elongated i n the f a l l , r e o r i e n t i n g the meristematic t i p of the rhizome from i t s horizontal p o s i t i o n i n the summer to a v e r t i c a l l y downwards d i r e c t i o n a f t e r approximately six i n t e r -nodes. Rhizome apices continued growing downwards at a slow rate much of the winter. T y p i c a l l y , less than 2 cm of rhizome was added during the winter, burying the rhizome apex below i t s summer l e v e l of 5 cm. Rhizome internode lengths showed a marked increase i n March as they resumed horizontal growth. Ruppia rhizome internodes at each s t a t i o n increased i n length uniformly from May to a peak i n July. The length of a l l newly produced internodes at a l l stations declined to nearly i d e n t i c a l lengths i n August. Genet growth Schematic representations of Zostera and Ruppia genets over an annual cycle are presented i n Figure 18. In Zostera, horizontal rhizomes emerge from seedlings i n early May. Each node usually supports a ramet. Several short nodes were t y p i c a l l y produced before the f i r s t rhizome began branching i n late May. Genets were d i f f i c u l t to i d e n t i f y by l a t e June, even with caref u l excavation, because of rhizome fragmentation. In early July, horizontal rhizomes branched r e g u l a r l y , and v e r t i c a l branching of flowering shoots had begun. Throughout July, 85 F i g . 18. Schematic drawing of horizontal and v e r t i c a l branching of vegetative and reproductive shoots of Zostera japonica and Ruppia maritima and estimates of ramets per genet over time. = l i v e rhizome, = decayed rhizome, O = vegetative shoot, • = reproductive shoot = = v e r t i c a l branching. April Zostera japonica • 86 E s i i mated r ame t s pe r genet 1 5-10 November o 1 March 1-3 87 88 rapid horizontal branching continued. Older segments of rhizomes were b r i t t l e , e s p e c i a l l y at the nodes. Nearly a l l shoots except the most recently produced were v e r t i c a l l y branching and flowering i n August. Much of the older rhizome was severed and/or decaying. An excavation i n September revealed that genets had fragmented into units of f i v e to ten ramets. The decomposition and erosion of Zostera rhizomes increased through October. By November, only small segments of rhizomes could be found. A few longer internodes usually remained attached to a downward hooked apex through February. The resumption of rapid rhizome growth i n March was usually i n i t i a t e d from more than one l o c a t i o n on an overwintering rhizome. Horizontal rhizomes were i n i t i a t e d from Ruppia seedlings i n A p r i l . By May, horizontal rhizomes were branching, each node having a ramet. V e r t i c a l branching and flowering of non-terminal shoots began i n June. In July, horizontal branching slowed but v e r t i c a l p r o l i f e r a t i o n con-tinued. Genets were usually recognizable because much of t h e i r rhizome was only p a r t i a l l y buried by sediment. During l a t e July and early August, Ruppia rhizomes became b r i t t l e and leaves became heavily covered with epiphytes and r a p i d l y decayed. Large portions of genets were uprooted and c a r r i e d o f f by the t i d e . Some securely anchored shoots or groups of shoots remained through much of August while t h e i r v e r t i c a l branches and leaves decayed. Flower and inflorescence densities The mean numbers of Zostera ovaries per inflorescence showed no s i g n i f i c a n t differences throughout the flowering period at a given t i d a l height (Fig. 19). Station 1 had c o n s i s t e n t l y lower numbers of flowers than stations 2 or 3, which were equal. There were no s i g n i f i c a n t differences between stations i n the number of ovaries per inflorescence i n Ruppia for any sampling date and over 75% of a l l inflorescences produced 8 ovaries. The number of inflorescences per flowering shoot i n Zostera showed two d i s t i n c t patterns (Fig. 20). Numbers at s t a t i o n 1 were at f i r s t higher than at stations 2 and 3, then f e l l continuously u n t i l i t s l a s t flowering record i n August. Stations 2 and 3 shadowed one another through at f i r s t a shallow and then a rapid increase i n number of inflorescences per flowering shoot to s i m i l a r peaks i n August; the peaks for stations 2 and 3 were not s i g n i f i c a n t l y d i f f e r e n t . At l a s t record i n October, inflorescence number at stations 2 and 3 had f a l l e n to June l e v e l s . Ruppia showed a constant increase i n the average number of i n f l o r -escences per flowering shoot at each s t a t i o n . Differences between i n i t i a l values were small, but the mean value f o r station: 1 i n August was s i g n i f i c a n t l y lower than f o r s t a t i o n 2; stations 2 and 3 were not s i g n i f i c a n t l y d i f f e r e n t . Ovary fates The average number of ovaries per inflorescence and the percent of those ovaries that f e l l into each of the ovary fat e s , by age c l a s s , are shown in Table IV. Aborted ovaries, fate 1, remained consistent i n Zostera with no s i g n i f i c a n t changes between stations. Ruppia at stations 1 and 3, however, showed s i g n i f i c a n t increases of abortion i n the second age Zostera japonica Ruppia maritima 2H ; OJ Jun Jul Aug Sep Oct Jun Jul Aug F i g . 19. Mean number of ovaries (female flowers) per inflorescence at three s t a t i o n s ( = s t a t i o n 1, 3.17 m, = s t a t i o n 2, 2.54 m, = s t a t i o n 3, 2.60 m CD t i d a l height) at Roberts Bank, 1980. Standard deviations are i n Appendix 4. F i g . 20. Mean number of inflorescences per flowering shoot at three stations ( = s t a t i o n 1, 3.17 m, = s t a t i o n 2, 2.54 m, = s t a t i o n 3, 2.60 m CD t i d a l height) at Roberts Bank, 1980. Standard deviations are i n Appendix 4. Table IV. Mean numbers of ovaries per inflorescence and percentages i n each ovary fate: ovary abortion (1), ovary predation (2), incomplete development (3), mature seed (4); for two age classes of Zostera  japonica (Z) and Ruppia maritima (R) from three t i d a l heights at Roberts" Bank. For each species, 25 inflorescences i n age cl a s s 1 were selected on June 11, and 25 inflorescences i n age c l a s s 2 were selected on J u l y 10 (Ruppia) and J u l y 30, 1980 (Zostera) . Standard deviations are i n parentheses. Age Class Station 1 (3.17 m) % i n Fate Class: No. No. Station 2 (2.54 m) % i n Fate Class: 1 2 3 < No, Station 3 (2.60 m) % i n Fate Class: Z l 4.3 23.8 0.7 16.0 59.5 (1.0)(28.9) (3.5)(37.4)(37.4) Z2 4.2 35.6 0.0 49.7 14.7 (1.3M39.1) (0.0) (47.2) (28.7) 6.2 15.8 4.7 0.0 79.5 (1.2)(14.7)(10.2) (0.0)(14.4) 6.2 27.2 9.2 13.2 50.4 (1.1)(18.8)(15.1)(31.3)(32.5) 6.1 18.8 3.8 3.6 73.8 (1.3) (17.4) (9.9)(10.1)(15.4) 5.7 26.4 0.0 9.2 64.4 (1.0)(19.4) (0.0)(22.8)(20.5) Rl 7.4 31.9 2.6 0.0 65.5 (1.2)(22.6) (7.3) (0 . 0 X 2 3.6) R2 6.5 62.8 1.9 14.6 20.7 (1.5)(29.0) (5.4)(18.0)(28.2) 7.9 22.7 12.9 0.0 64.4 (0.5)(23.7)(23.8) (0.0)(32.0) 7.4 36.8 2.7 3.5 57.0 (1 . 0 X 2 7.8) (5.3) (8.5) (28.9) 7.9 29.0 11.4 0.0 59.6 (0.5)(27.2)(19.2) (0.0X31.7) 7.6 55.2 3.4 15.6 25.8 (0 . 8 X 1 7.0) ( 6 . 5 X 1 8 . 5 ) (9.3) t o 93 class to values double those i n the f i r s t age c l a s s . Loss of Zostera ovaries by predation, fate 2, was recognized when portions of the ovary were missing; predation remained low at a l l stations except f o r i t s r i s e to nearly 10 percent of age class 2 at s t a t i o n 2. Predation on Ruppia ovaries was t y p i c a l l y higher than on Zostera and showed no s i g n i f i c a n t changes over time. Numbers of developing ovaries l o s t before maturation, fate 3, were s i g n i f i c a n t l y higher i n Zostera at s t a t i o n 1 than at s t a t i o n 2 for the f i r s t age c l a s s . No Ruppia ovaries i n age class 1 were l o s t because of premature inflorescence l o s s . Stations 1 and 3, however, showed s i g n i f -icant increases to around 15% for fate 3 i n the second age c l a s s . The percentage of the Zostera ovaries i n age class 1 reaching maturity, fate 4, was lowest at s t a t i o n 1. The maturation rate f e l l s t i l l lower for the second age c l a s s . Station 2 also showed a s i g n i f -icant drop i n seed maturation between age classes. Ruppia ovary success was about equal between stations within the f i r s t age c l a s s . Seed maturation at stations 1 and 3 dropped s i g n i f -i c a n t l y i n the second age c l a s s . Seed production Estimates of seed production at each t i d a l height s t a t i o n are presented i n Table V. Zostera seed production at s t a t i o n 1 was consider-ably lower than at 3, which was i n turn lower than at s t a t i o n 2. At st a t i o n 1, most of the seeds were produced during the f i r s t age c l a s s ; the second age class yielded more seeds at stations 2 and 3. Ruppia seed production was lowest at s t a t i o n 1 and highest at sta t i o n 2. There were e s s n e t i a l l y no differences i n the seed production 94 Table V. Estimated seed production per 0.1 m of Zostera japonica and Ruppia maritima at Roberts Bank, showing absolute number and percent of t o t a l at each s t a t i o n ( i n parentheses) by age class at three t i d a l heights. Station 1 Station 2 Station 3 Species ;e Class (3.17 m CD) (2.54 m CD) (2.60 m CD) Zostera japonica 1 447 (95) 507 (15) 206 (09) 2 26 (05) 2899 (85) 1967 (91) Total 473 3406 2173 Ruppia maritima 1 81 (48) 169 (34) 127 (40) 2 87 (52) 322 (66) 188 (60) Total 168 491 315 95 between age classes at s t a t i o n 1. Both stations 2 and 3 were more successful, i n terms of seed production, during the second age c l a s s . Discussion Shoot recruitment and death In clonal plants, a si n g l e genotype (genet - i n d i v i d u a l a r i s i n g from a seed) displays i t s f i t n e s s as a more or less fragmented pheno-type (Noble et a l . 1979). These fragments make a population of d i s c r e t e modular shoots, also c a l l e d ramets - vegetatively produced units (White 1979). Ruppia and Zostera rhizomes become b r i t t l e with age and break, making i t almost impossible to i d e n t i f y i n d i v i d u a l genets. Harper and White (1974) state that the shoot i s the e f f e c t i v e unit of population f l u x i n rhizomatous plants. Their reasoning i s that recruitment of new genets i s often rare among clonal plants, and the environment influences p r i m a r i l y the recruitment and death rates of subunits once in d i v i d u a l s are established; t h i s i s also true for Zostera and Ruppia. Recruitment and death rates of shoots are the main indicators of a population's behavior; together with immigration and emigration of i n d i v i d u a l s they describe population f l u x . T i d a l height affected patterns of shoot f l u x i n Zostera more than i n Ruppia. Shoot losses and gains remained synchronous i n Ruppia at a l l three stations, whereas Zostera at s t a t i o n 1 retained early shoots longer and declined i n numbers e a r l i e r than at other stations; t h i s behavior may have been due to the greater desiccation that plants at s t a t i o n 1 experienced. Researchers studying monocotyledonous (Lamp 1952, Langer 1956, Langer et a l . 1964, Robson 1968, Nobele et a l . 1979) and dicotyledonous 96 plants (Sarukhan and Harper 1973) have found the greatest r i s k of death of ramets coincided with the greatest recruitment rate. Noble et a l . (1979) att r i b u t e d the synchrony i n recruitment and death to either the higher density due to new b i r t h s or reduced densities caused by deaths allowing more b i r t h s . My data indicate that severe shoot losses may follow large shoot recruitments, suggesting a density-dependent mortal-i t y as found by Noble et a l . However, because time lags are evident between recruitments and losses e s p e c i a l l y at the s t a t i o n with the densest population the mortality i s l i k e l y more c l o s e l y t i e d with growth phases of the shoots (vegetative or reproductive) and ecological cues such as water temperature and desiccation (see Verhoeven 1979 i n reference to Ruppia) than absolute shoot density. Antonovics (1972), working with Anthoxanthum odoratum L. found as I have with Zostera and Ruppia, a d i f f e r e n t i a l death rate between vege-t a t i v e and reproductive shoots. This phenomenon may represent a p r e f e r e n t i a l p a r t i t i o n i n g of resources to flowering i n d i v i d u a l s that reduces the survivorship of non-flowering shoots. Support for t h i s 14 hypothesis comes from Harrison (1978) who found that when C was translocated between shoots on the same rhizome of Z. americana den Hartog .(= Z. japonica), flowering and young vegetative shoots were the major sinks, and that older vegetative shoots export two to s i x times more 14 newly-fixed C than flowering shoots. The degree of epiphytism may have affected the death rate of shoots. Ruppia remained r e l a t i v e l y epiphyte-free u n t i l f r u i t develop-ment was complete and l e a f t i p s eroded; Zostera shoots accumulated increasing amounts of epiphytes with age u n t i l the shoots died. Thus shoot loss may have reduced the r e s p i r a t o r y burden of epiphyte-laden 97 leaves to the remaining plant. The t o t a l density of shoots showed mixed responses to t i d a l height; i n some instances fluxes of vegetative and reproductive shoots appeared to behave independently. Despite the great differences i n t o t a l shoot number between Zostera and Ruppia, the reduction i n vegetative shoots of each species was proportional to the greater desiccation at s t a t i o n 1 as compared to s t a t i o n 2 ( i . e . , 8 to 10% more exposure to the a i r during the day, see Section 1). Reproductive shoot numbers, however, showed a d i f f e r e n t pattern. Zostera r e f l e c t e d the pattern seen in the vegetative shoots, and Ruppia remained r e l a t i v e l y constant i n density at a l l three stations. Recruitment and death rates combined to y i e l d c h a r a c t e r i s t i c age structures at each t i d a l height (Fig. 13). Noble et a l . (1979) found death rates i n ramets , of Carex arenaria L. to be " r e l a t i v e l y independent of i t s age and of i t s season of b i r t h . " This i s obviously not the s i t u a t i o n i n either Z. japonica or R. maritima at the locations studied, presumably because of the more or less unpredictable environmental conditions in the i n t e r t i d a l (Harrison 0.979) states that f l u c t u a t i o n s i n temperature and s a l i n i t y can be large and unpredictable during the long d a i l y exposures to the a i r ) , whereas the dunes where C. arenaria grow are environmentally, stable. The v a r i a t i o n i n Zostera and Ruppia cohort survivorship curves i l l u s t r a t e how shoot l i f e spans change with environmental conditions. Survivorship curves tend to change when desiccation increases at a given s t a t i o n during the growing season (e.g. Deevey Types I to II or III at s t a t i o n 2). The pattern i s reversed i n winter; s t a t i o n 1 had the longest and most frequent exposure to low a i r temperatures, and the longest l i v e d shoots. The increased shoot longevity at s t a t i o n 1 i n the winter may have allowed growth to resume as r a p i d l y as possible when suitable conditions returned. Survivorship curves constructed by Harper (1967) from data on natural populations of established plants studied by a number of researchers (Tamm 1948, 1956, Sagar 1959, Antonovics 1972) showed steady log reductions i n numbers over time (although the rate may be s p e c i e s - s p e c i f i c ) . This led Harper to suggest that, i n the absence of d r a s t i c environmental change, the r i s k of m o r t a l i t y may be a constant throughout the l i f e of the plant. Williams (1970), i n a study of ramets of Danthonia caespitosa Gaudich. i n various habitats found that the survivorship of populations may approximate a l l three main types of survivorship curves described by Deevey (1947). Williams expanded on the ideas of Harper (1967) and suggested that "steady mortality i n populations r e f l e c t s an environ-mental regime that i s c o n s i s t e n t l y favourable, or unfavourable" or i n other words, an environment which i s stable. Several studies, other than the present one, also support t h i s hypothesis (e.g. Watt 1960, Williams 1968, Noble 1979). My r e s u l t s demonstrate that the amount of exposure to the a i r and the p h y s i o l o g i c a l state of a mature shoot (rapid growth, flowering, or winter quiescence) are features that influence shoot survivorship. The fact that several cohorts of various ages are a l i v e simultaneously, a s i t u a t i o n i n which a s i n g l e genet may have shoots i n d i f f e r e n t p h y s i o l o g i c a l states or phases, underscores the need for deomgraphic studies to understand the dynamics of seagrass populations. 99 From these analyses of vegetative populations, i t i s evident that Z. japonica and R. maritima employ generally the same l i f e h i s t o r y strategies; both plants would be considered " r " selected species (Gadgil and Solbrig 1972, Harrison 1979). Both species behave as annuals or sh o r t - l i v e d perennials having indeterminate growth systems. (Ruppia flowered year-round i n the lab when maintained at a constant 10°C.) The observed differences i n the f i e l d may be explained from two approaches. F i r s t , s e l e c t i o n has acted to maintain l i f e cycle p l a s t i -c i t y (as i n Baker's (1965) all-purpose genotype) to d i f f e r e n t degrees; and secondly, plants perceive or respond to desiccation d i f f e r e n t l y . Ruppia species p r i m a r i l y inhabit inland permanent or temporary waters which have marked environmental fl u c t u a t i o n s from season to season (Verhoeven 1979). The re-occurrence of d i s t i n c t p e r i o d i c environmental hazards which do not permit the s u r v i v a l of long-lived or slowly reproducing adults w i l l select i n favor of a short seed-to-seed l i f e c ycle, as has occurred i n many annuals of the deserts and c u l t i v a t e d habitats (Harper and White 1974, Antonovics 1976). The very short time required for Ruppia to complete i t s l i f e cycle at Roberts Bank, as compared to Zostera, i s due to s e l e c t i o n pressures on t h i s species when i t inhabited predictable temporal habitats inland. Both Ruppia and Zostera have the p l a s t i c i t y to exploit environments i n which, i f the season or s p e c i f i c l o c a t i o n i s unfavorable, flowering can quickly proceed. On the other hand, i f the environment remains con-genial (as observed i n the laboratory), growth and flowering can continue, although Ruppia's performance r a p i d l y declines, possibly because of heavy epiphyte loads. 100 Differences i n the way Zostera and Ruppia respond to the environment can be att r i b u t e d to t h e i r morphologies. Ruppia generally grows poorly i n the continually f l u c t u a t i n g water l e v e l s and turbulence of the i n t e r -t i d a l . Ruppia leaves are t h i n (see F i g . 16) and lack the well-developed c u t i c l e of Z. japonica. These features increase the sev e r i t y of desiccation during periods of exposure. The upright stems of Ruppia are b r i t t l e and may not withstand the turbulence o f the i n t e r t i d a l as well as the f l e x i b l e , fiber-supported leaves of Zostera. Ruppia i s also a shallowly rooted species. In t h i s discussion, r e l a t i o n s h i p s have been drawn between shoot flux and the resultant population i n terms of absolute number, age structure and survivorship; the growth phases of plants; and the environment as alte r e d by t i d a l exposure. Many differences and close p a r a l l e l s have been observed between Zostera and Ruppia concerning t h e i r shoot dynamics. In the next discussion comparisons are continued between the species at the three d i f f e r e n t t i d a l heights by examining differences i n the components of the shoots (e.g., leaves, rhizomes and flowers). Leaf and rhizome internode lengths S i t e - s p e c i f i c comparisons of l e a f lengths and rhizome internode lengths indicate morphogenic responses of Zostera and Ruppia to environ-mental differences and also r e f l e c t aspects of a plant's growth phase. The appearance of thi n leaves, such as was observed by Arasaki (1950b), corresponded d i r e c t l y to the onset of quiescence i n rhizome growth. Ti d a l height dictated when quiescence would occur (Fig. 17). 101 Fluctuations i n winter l e a f lengths at a l l stations may be r e l a t e d to shoot-removal by storms, e.g. at stations 1 and 2 i n February. It i s not known why s t a t i o n 3 did not show a s i m i l a r decrease i n February; perhaps i t s distance from shore spared i t from the turbulence encountered closer to shore. Vari a t i o n i n l e a f dimensions of Zostera japonica with environmental factors was recognized by Miki (1933). At the time of his investiga-tio n s , there were two species of Z o s t e r e l l a reported from Japan, Z. nana Roth (Z. japonica, den Hartog 1970) and Z. japonica. Zostera nana, with leaves 4 - 6 cm long, grew i n shallow areas along the seashore, and what Miki considered a form of Z. nana, Z. japonica, grew only i n brackish waters and had narrower leaves, 50 - 65 cm long. Differences i n l e a f length of t h i s magnitude are also observed i n Z_. japonica i n North America. Miki (1933) also found differences i n l e a f width, l e a f sheath length, rhizome thickness, and internode length between Z. japonica on the open coast and i n estuaries. Phenotypic p l a s t i c i t y , e s p e c i a l l y of l e a f characters, i s a recog-nized phenomenon i n many seagrasses. P h i l l i p s (1960) found l e a f and rhizome dimensions of Diplanthera w r i g h t i i du Petit-thouars (= Halodule w r i g h t i i Aschers) to be r e l a t e d to amount of t i d a l inundation. McMillan and Mosely (1967) found leaves of Thalassia testudinum Bank ex Kb'nig to be shorter under hypersaline conditions. Water depth (Straw 1961) and sediment depth (Zieman 1974) have also been corre-lated with changes i n l e a f length of T. testudinum. McMillan (1978) studied the v a r i a t i o n of f i v e seagrass species, each c o l l e c t e d from several locations and grown under c o n t r o l l e d conditions. He found that seagrass l e a f widths varied with t h e i r 102 immediate environmental surroundings, and that the degree of p l a s t i c i t y could vary geographically, depending on the genotype. It i s not known whether the differences Miki (1933) observed were the r e s u l t of l o c a l genotypic differences or whether v a r i a t i o n s seen i n North American populations of Z_. japonica are ecotypic, r e f l e c t i n g the high degree of phenotypic p l a s t i c i t y exhibited by t h i s plant. P h i l l i p s (1972) found- through r e c i p r o c a l transplants that Z. marina exhibited phenotypic p l a s t i c i t y ( in terms of l e a f length) across t i d a l zones as marked as that which I observed i n Z. japonica at Roberts Bank. North American populations of Z. japonica may have diverged from t h e i r parentage i n the warmer waters of Japan i n a s i m i l a r manner to the diver-gence described by McMillan (1978). Ruppia leaves showed the same general response i n length to the increased exposure at s t a t i o n 1 as did Zostera, i . e . an early burst of growth and lower maximum, but the magnitude of the difference was much less i n Ruppia. Laboratory and f i e l d observations and herbarium records confirm that Ruppia can exhibit great v a r i a t i o n i n l e a f lengths. Leaves are arranged h i e r a r c h i c a l l y on shoots of both species. The majority of Zostera leaves were usually within centimeters of the length of the dominant l e a f , whereas the longest p a i r of Ruppia leaves was usually about twice as long as the remaining leaves. Thus, mean maximum l e a f lengths give a more accurate representation of Zostera than Ruppia l e a f lengths. A more de t a i l e d study of Ruppia leaves may reveal more marked responses to t i d a l height than those reported here. Zostera and Ruppia rhizome internode lengths showed patterns that corresponded to those of t h e i r l e a f lengths at d i f f e r e n t t i d a l heights. This suggests a phasic development of plant growth, the timing of which 103 i s i n part r e l a t e d to t i d a l exposure. Patterns of shoot recruitment, retention, and loss further support such an idea (see F i g . 11). During the early part of the growing season long internodes were produced as each Zostera genet expanded through ramet production. Later, during . v e r t i c a l branching, flowering, and seed development, l a t e r a l expansion was de-emphasized and internode length decreased. Ruppia rhizome internode lengths, however, peaked at each s t a t i o n the same time as l e a f lengths and flowering shoot numbers (Figs. 15 and 17). These r e s u l t s support observations made on shoot fluctuations (Fig. 11) i . e . that the growth of Ruppia exhibits a phasic development which has l i t t l e p l a s t i c i t y . Verhoeven (1979) pl o t t e d the t o t a l (horizontal and v e r t i c a l ) length of stems of three Ruppia species, including R.. maritima, and found exponential growth i n each case. Similar curves would be obtained by p l o t t i n g the t o t a l length of a stem i n a genet of Z. japonica because of t h e i r s i m i l a r i t i e s i n branching and growth form. The type of substrate on which Zostera and Ruppia grow may d i c t a t e aspects of t h e i r rhizome growth. Zostera japonica rhizome internodes observed i n areas of g r a v e l l y mud were s u b s t a n t i a l l y shorter than those i n the adjacent sandy mud. Verhoeven (1979) found that the time at which exponential growth was i n i t i a t e d i n Ruppia varied with substrate and water s a l i n i t y ; plants grew best i n muddy sediment i n low s a l i n i t y water. The slow downward growth of Z. japonica rhizome apices i n the winter, as seen at Roberts Bank, has also been observed i n Japan (Miki 1933, Arasaki 1950a) and may provide protection from occasional freezing i n the upper sediment during the winter. 104 In a study of Ruppia rhizomes, Verhoeven (1979) found that the t r a n s i t i o n from slow to rapid growth was always abrupt and completely temperature-dependent. Budding of rhizomes began a f t e r the mean d a i l y maximum temperature had been above 15°C for 10 days with a minimum temperature above 10°C. A s i m i l a r mechanism i s l i k e l y involved i n the i n i t i a t i o n of rapid growth from quiescent rhizomes of Z. japonica (Arasaki 1950a). Genet growth The schematic representations of Zostera and Ruppia genets help i n comprehending some of the phases and geometry of t h e i r growth. The estimations of ramets per genet for both species are poor because rhizomes fragment e a s i l y ; they do provide an idea of r e l a t i v e s i z e , and how an in d i v i d u a l expands and fragments into a colony of clonal parts. It i s also c l e a r that i t would take a large disturbance to remove an entire genet from the population once branching occurred. Mapping the rhizomes of in d i v i d u a l s would also be important i n demographic studies focusing on meristems as suggested by Tomlinson (1974). Numbers of flowers and inflorescences The number of Zostera flowers per inflorescence varies from one loc a t i o n to another. It i s my observation that spadices of plants growing i n protected pools grow consistently to f i l l t h e i r e n t i r e spathe and may have as many as 11 functional ovaries. Plants growing i n areas that are repeatedly exposed to the a i r f o r extended periods may produce spadices that are only one-half or one-third the s i z e of the spathe and contain few ovaries. The i d e n t i f i c a t i o n of s p e c i f i c factors which control 105 the amount of spaclix growth and thus ovary production i s beyond the scope of t h i s study, but as indicated by these observations, spadix growth i s r e l a t e d to the vigor of plant growth as c o n t r o l l e d by the p r e v a i l i n g environmental conditions. The majority of Ruppia inflorescences produce eight ovaries (Verhoeven 1979). However, according to Hitchcock et a l . (1973), occasionally there can be as many as eight ovaries per flower and since each inflorescence has two flowers, p o t e n t i a l l y 16 ovaries could form per inflorescence; the most I have observed i s 12. Inflorescences with fewer than eight ovaries at Roberts Bank may have had some phys-i c a l l y removed by wave action; however the s i g n i f i c a n t drop i n ovary number at s t a t i o n 1 i n age class 2 suggests that desiccation may play a r o l e i n reducing ovary number. The number of inflorescences they produce i s a function of shoot longevity. The rapid reduction of Zostera inflorescences at s t a t i o n 1 was a r e s u l t of the loss of flowering shoots produced early i n the season during vigorous growth. These shoots were not only maintained longer at the other stations, but when l o s t were replaced by other r e l a t i v e l y long-lived shoots (Fig. 11a). Low mortality i n Ruppia flowering shoots resulted i n continually growing numbers of inflorescences. Ovary fates The proportion of non-developing ovaries (fate 1) appears to be an ever-present factor l i m i t i n g seed production i n Zostera and Ruppia at Roberts Bank. Because there could be several causes for what I have c a l l e d ovary abortion, e.g. malformation or non-pollination, i t i s impossible with the observations made here to determine i f one or a l l 106 of these factors remain constant or i f they vary through time and space to a common r e s u l t . Arasaki (1950b) reported that Z. japonica had a poor rate of p o l l i n a t i o n . From one to f i v e f r u i t s ( i n the majority of cases, two to four) would develop on a s i n g l e spathe. Nothing i s known of the s e l f - c o m p a t i b i l i t y of Z. japonica p o l l e n . Dispersal of p o l l e n takes place through the water column and may be i n e f f i c i e n t . Verhoeven (1979) describes how submarine s e l f - p o l l i n a t i o n of Ruppia takes place by an a i r bubble that forms on the inflorescence and acts as a surface on which po l l e n t r a v e l s to the p i s t i l s . When flowers reach the water surface, the p o l l e n t r a v e l s on the surface. In the undisturbed laboratory environment, each p i s t i l i s usually successful i n producing a f r u i t (Verhoeven 1979) . The lower p o l l i n a t i o n success observed at Roberts Bank may be because po l l e n i s c a r r i e d away from the p i s t i l s by wave and t i d e action. The higher ovary predation (fate 2) seen on Ruppia than on Zostera could be a t t r i b u t e d to Ruppia's morphology. Zostera ovaries are en-closed and protected by the spathe. Developing f r u i t s of Ruppia are enclosed only by a t h i n pericarp. The generally low predation at s t a t i o n 1 may be a r e s u l t of poor habitat conditions for the predator(s). No e f f o r t was made to i d e n t i f y the animal(s) that would remove various portions of the f r u i t pericarp and usually the entire contents of seed of both species. Fungal or b a c t e r i a l i n f e c t i o n s often r e s u l t e d on f r u i t s , once damaged by a predator. The proportion of developing f r u i t s l o s t before they were mature (fate 3) r e f l e c t s early losses of infruitescences with increased desiccation, e.g. at s t a t i o n 1, and also the presence of asynchronous 107 f r u i t development within an infruitescence. Table IV shows how complex sets of variables integrate to y i e l d a given ovary success rate (fate 4). Increased desiccation experienced at s t a t i o n 1 over that at 2 (see section one) does not just cause an increase i n p a r t i c u l a r fates, but a l t e r s proportions of s p e c i f i c fates, e.g. the percentage l o s t to predation. Seed production Because seed production i s the product of four y i e l d components: the p r o b a b i l i t y of i n d i v i d u a l ovary success i n producing a mature seed, and the numbers of ovaries per inflorescence, inflorescences per flower-ing shoot, and flowering shoots per area, i t i s important to consider the components separately when i n t e r p r e t i n g differences i n seed production. For Zostera, a l l y i e l d components acted together to create the discrepancies i n the estimated seed production between stations. Y i e l d components at s t a t i o n 1 were generally lower than at the other two stations; the combined e f f e c t of these components on seed production was augmented by the shortened reproductive period at s t a t i o n 1. In Ruppia, however, the length of the reproductive period, the numbers of inflorescences per flowering shoot, and to a l e s s e r extent ovaries per inflorescence remained r e l a t i v e l y constant at a l l s t a t i o n s . Seed production fluctuated p r i m a r i l y with the success of i n d i v i d u a l ovaries i n producing mature seed and with flowering shoot density. These differences i n the behavior of y i e l d components i n r e l a t i o n to the environment r e f l e c t the generally more p l a s t i c responses of Zostera to changing environmental conditions compared to Ruppia (as seen previously i n shoot demography and l e a f and rhizome internode 108 lengths). Because Zostera l i v e s i n a wide range of marine habitats (Miki 1933), great f l e x i b i l i t y may be required to regulate seed production at several l e v e l s i n the developmental process to p r e c i s e l y a l l o c a t e a v a i l -able resources. The r e l a t i o n s h i p s of y i e l d components have been studied extensively i n crop plants (Adams 1967). However, Primack (1978) cautions against comparisons of natural and a g r i c u l t u r a l populations because of pronounced differences i n the amount of environmental heterogeneity, differences which can occur between reproductively mature plants, and the phenotypic uniformity between inbred l i n e s of crop plants and natural populations. SECTION 3. THE SEED COMPONENT OF THE POPULATION Results Buried seed Buried seed at Roberts Bank was almost e n t i r e l y Z. japonica and R. maritima, exceptions being seed-of Z. marina which occurred i n 2 densities only of a few per m and the occasional f r u i t of T r i g l o c h i n maritima L. During the winter Zostera and Ruppia seed was densest i n the upper 10 cm of sediment and i n the summer, i n the 10 - 20 cm depth range (Fig. 21). Ruppia seed t y p i c a l l y outnumbered that of Zostera. Both species showed a s i m i l a r pattern i n the d i s t r i b u t i o n of t h e i r v i a b l e seed. The d i s t r i b u t i o n of winter Zostera seed was uniformly deeper i n 1981 compared with the previous year as determined by a cumulative percent curve. Both winters Zostera at s t a t i o n 2 had the highest number of buried 2 seed, averaging 65 per .1 m . Stations 1 and 3 had about equal numbers of buried seed both winters; i n 1981 they were closer i n number to those at s t a t i o n 2. In the f i r s t 5 cm of sediment, the t o t a l percent of v i a b l e Zostera seed during the winter decreased from s t a t i o n 1 to 3. From 0 to 20 cm, a l l stations i n 1980 contained approximately the same percent of t h e i r v i a b l e seed (95%); i n 1981, however, the number s t i l l tended to decrease with distance from shore. Few, i f any, v i a b l e seeds were found below 15 cm i n the winter. General p r o f i l e s of Ruppia seed numbers were very s i m i l a r f o r both winters. The number of buried v i a b l e seed constantly increased from 2 st a t i o n 1 to 3, averaging 77 per .1 m at s t a t i o n 2; the range of seed 110 Fi g . 21 a - f. Cumulative .percent of a l l buried seed from three stations (station 1 = 3.17 m, 2 = 2.54 m, 3 = 2.60 m CD t i d a l height) at Roberts Bank. Bars show cumulative percent of v i a b l e seed. Numbers at the bottom of each seed p r o f i l e from l e f t to r i g h t : t o t a l seed from ten cores (20 X 8 cm diameter), t o t a l estimated viable seed 2 from ten cores, and vi a b l e seed per 0.1 m . The c o e f f i c i e n t of v a r i a t i o n of cores at each s i t e and sampling date varied between 80 and 6%, with a mean of 21%. Station 1 Station 2 Cumulative Percent 20 LO i 60 i 80 100 —i 20 LO 60 80 100 - A - 1 1 1 1 2-L —• 6-E u 2-4-£ 10-a ~ 12-c aj .§ CO CO 1 6 . 18-20 1.09 74 34.0 10-12-14-16-18-20- 181 120 60.1 Fig. 21a. Zostera japonica, winter 1980. 2-4-Station 3 20 L0 60 80 100 I , . I 1 ' I 130 65 32.5 Station 1 2 0 40 6 0 80 100 _i i i i i 103 76 38.0 Station 2 Cumulative Percent 2 0 4 0 60 80 100 J 1 I I I 2- t • » -A 314 174 87.5 Fig. 21b. Ruppia maritima, winter 1980. 21 4. Station 3 2 0 40 60 80 100 • ' J I i i 296 196 98.4 Stat ion 1 2-4 • ~ 61 10 ' 12H u 1 6 -1 8 -20 2 0 4 0 60 80 100 i i i ' i 46 32 16.0 2 4 -6 -8 10 12 14 H 16 18 20 J Station 2 Cumulative Percent 2 0 4 0 60 80 100 —I 1 I I I 66 34 17.0 Fig . 21c. Zostera japonica, summer 1980. Station 3 2 0 40 6 0 80 100 i i i i i 6^ 56 22 11.0 S t a t i o n 1 20 CO 60 80 1QQ 6 8 10 4 12 U 16-18-20 . Station 2 Cumulative Percent 20 CO 60 80 100 • ' 1 1 • i .g. 21d. Ruppia maritima, summer 1980. Station 3 20 CO 60 80 100 ' ' 1 i I 2--C . 6-8-10 12-U-16-18-20 J . Station 1 Station 2 Cumulative Percent a. cu a 2H 4. — 6-E 10 ~ 12H c ca CD tO 16. 18 • 20 J 20 i 14 7 40 60 80 100 — i 1 109 55.0 10 12 U 4 1 6 H 18 20 20 40 60 80 100 — i 1 1 1 j 177 137 69.0 Fig. 21e. Zostera japonica, winter 1981. Station 3 20 40 60 80 100 i i i i i 141 109 55.0 Station 1 Station 2 Cumulative Percent 2-4-1 — 6-E u 10' _ 12H 20 40 60 80 100 __i i 1 i j 44 6 8 10-12' 20 40 60 80 100 _ i i i i i 1 H 1 5 H 16H 18 20 141 113 57.0 18H 20 205 134 67.2 Fig. 21f. Ruppia maritima, winter 1981. Station 3 20 40 60 ' 80 100 _j i i i i 2-4-] 273 167 84.0 117 numbers was smaller i n 1981. A higher proportion o f v i a b l e seed was i n the upper layer ( 0 - 5 cm) at s t a t i o n 1 than at stations 2 and 3 i n both winters, but i f the top 10 cm are considered c o l l e c t i v e l y , a l l stations i n the winter of 1981 were about equal with approximately 80 - 85 percent of the vi a b l e seed i n the winter occurring i n that layer. Eight percent or less of the vi a b l e seed i n a l l January samples was found below 15 cm. The d i s t r i b u t i o n s of Zostera and Ruppia seed i n the summer were s i m i l a r . The major difference was i n the mean number of buried v i a b l e 2 seed; Zostera averaged 15, and Ruppia 21 per .1 m . No v i a b l e seed was found above the 5 cm sediment l e v e l . Both species showed a decrease i n the percent of v i a b l e seed located i n the top 10 cm of sediment. Over 30 percent of vi a b l e Zostera and Ruppia seed at a l l stations was located below 15 cm. V i a b i l i t y of untreated seed c o l l e c t i o n s stored at 5°C. I n i t i a l values of new Zostera and Ruppia seed v i a b i l i t y i n July averaged above 95% (Table VI). Zostera seed i n September showed a s i g n i f i c a n t decrease i n v i a b i l i t y from July's estimate. Increases i n mortality of Zostera seed were not s i g n i f i c a n t again u n t i l A p r i l , when i t was estimated that over two-thirds of the seed o r i g i n a l l y c o l l e c t e d was a l i v e . V i a b i l i t y of Ruppia seed did not change s i g n i f i c a n t l y from o r i g i n a l c o l l e c t i o n values u n t i l February. By A p r i l , s l i g h t l y less than three-quarters of the o r i g i n a l seed was v i a b l e . Old Zostera seed showed no s i g n i f i c a n t decreases i n v i a b i l i t y throughout the study period although the mean value dropped about 15, percent. Old Ruppia seed, however, showed s i g n i f i c a n t mortality a f t e r 118 Table VI. Percent v i a b i l i t y of seed stored at 5°C and 27 o/oo s a l i n i t y during 1980 - 1981. New seed of Zostera japonica and Ruppia  maritima was c o l l e c t e d from current year shoots and old seed was co l l e c t e d from the sediment at Roberts Bank, both i n July, 1980. Standard deviations are i n parentheses. New Seed Old Seed Month Zostera Ruppia Zostera Ruppia J u l . 96.7 95.3 87.3 82.0 (2.3) (3.1) (6.1) (7.2) Sep. 91.0 96.0 90.7 75.0 (2.6) (2.0) (7.0) (5.6) Oct. 88.0 93.3 83.3 73.7 (6.0) (4.2) (5.0) (4.5) Nov. 90.7 94.0 81.3 70.7 (4.2) (2.0) (9.9) (6.1) Jan. 89.3 87.3 76.0 66.0 (7.0) (4.2) (6.0) (5.3) Feb. 87.3 80.7 74.0 68.7 (3.1) (4.2) (8.0) (3.1) Apr. 68.7 73.3 76.0 64.7 (6.4) (8.1) (11.1) (7.0) 119 six months of storage. F i n a l v i a b i l i t i e s of a l l seeds were not s i g n i f i c a n t l y d i f f e r e n t . E f f e c t s of temperature and anaerobic conditions on seed v i a b i l i t y  and germination New Zostera seed maintained i n 5°C aerobic conditions i n the labora-tory showed s i g n i f i c a n t l y higher v i a b i l i t y and s i g n i f i c a n t l y lower mortality i n February than i n A p r i l (Table VII). Ruppia seed maintained at these conditions showed no s i g n i f i c a n t differences between the two test dates. Few seed of either species germinated during the study period. Seed death was autonomic; no fungal or b a c t e r i a l growth was ever evident i n the p e t r i dishes. Values f o r Zostera and Ruppia seed maintained i n 5°C anaerobic conditions were not s i g n i f i c a n t l y d i f f e r e n t between species or te s t dates for percentages v i a b l e , dead, or germinated. These values were also not s i g n i f i c a n t l y d i f f e r e n t from values f o r 5°C aerobic conditions. The majority of seed maintained i n the aerobic, a l t e r n a t i n g 7 - 15°C temperature regime germinated during the test period. Ten percent of Zostera seed was vi a b l e i n February, but none were v i a b l e i n A p r i l . There was no vi a b l e Ruppia seed i n either month. S i g n i f i c a n t l y larger percentages of Zostera and Ruppia seed were via b l e i n both February and A p r i l when maintained i n anaerobic 7 - 15°C conditions as compared with the aerobic a l t e r n a t i n g temperature regime. The percentage of dead Ruppia seed was s i g n f i c i a n t l y greater i n A p r i l anaerobic than i n aerobic 15-7°C conditions. The percentages of seed germinating during storage under the a l t e r n a t i n g temprature regime were always s i g n i f i c a n t l y lower under anaerobic conditions. Table VII. Percent v i a b i l i t y , death, and germination of new seed maintained in controlled laboratory environments. Eight 100-seed replicates of both Zostera japonica and Ruppia maritima seed from 1980 shoots were stored in 27 o/oo s a l i n i t y seawater under each of four combinations of temperature and atmosphere: constant 5'C, or 12 h at 7*C alternating with 12 h at 15 "C, with or without oxygen. The^seed was held under test conditions from October, 1980 u n t i l February (half the seed) or A p r i l , 1981. "Germinated" refers to germination during storage. Standard deviations are given in parentheses. Recovery 5 C 7 - 15 'C G e n u s D a t e Aerobic Anaerobic Aerobic Anaerobic Zostera Viable Feb. 85 .0 81 .0 10.3 50 .0 (4 •2) (5 •2) (6.7) (7 .8) Apr. 67 .5 77 .0 0.0 25 .0 (5 .0) (8 •1) (0.0) (5 .3) Dead Feb. 11 .8 14 .0 17.5 6 .5 (3 •9) (3 • 7) (9.8) (4 • 4) Apr. 24 .0 19 .0 15.0 . 19 .5 (5 • 2) (6 .8) (6.8) (6 • 0) Germinated Feb. 1 .5 2 .5 72.0 46 .0 (1 •9) (1 (9.9) (12. .3) Apr. 7. .5 4. .0 84.5 51 .0 (6, .9) (3, .6) (6.8) (10 .9) Viable Feb. 81. .3 78, .3 0.0 63. .3 (6. •9) (5. .0) (0.0) (3. .8) Apr. 71. ,0 80. .0 0.0 7. .5 C6. 2) (7. .8) (0.0) (3. .8) Dead Feb. 15. 5 16. ,5 9.0 13. .0 (5. 7) (3. •0} (5.0) (4. 2) Apr. 22. 0 15. 0 9.0 20. 5 (4. 3) (5. •3) (7.5) (6. 6) Germinated Feb. 1. 5 3. 0 91.0 25. 0 (1. 9) (2. 6) (5.0) (7. 6) Apr. 7. 0 5. 0 91.0 72. 0 (5. 0) (2. 6) (7.5) (9. 7) 121 V i a b i l i t y of buried seed The longevity of experimentally buried seed generally increased with sediment depth to 15 cm i n February (Table VIII). In A p r i l , few i f any vi a b l e seed remained i n the upper 10 cm. New seed of both Zostera and Ruppia followed t h i s pattern, but Ruppia retained more vi a b l e seed at each sediment l e v e l i n February. Although t h e i r percent v i a b i l i t i e s at the 20 cm depth i n A p r i l were s i m i l a r , seed v i a b i l i t i e s of Zostera at the 15 cm l e v e l were s i g n i f i c a n t l y higher than Ruppia 1s. Old seed of both species generally suffered higher and/or more rapid mortality than new seed at each sediment l e v e l . Old Zostera seed at the 20 cm depth i n February were s i g n i f i c a n t l y less v i a b l e than old Ruppia seed. The majority of dead seed consisted of only the testa upon recovery. Several of the remaining dead seeds i n each r e p l i c a t e supported fungal growth during v i a b i l i t y t e s t s . New seed germination From the time of c o l l e c t i o n i n July u n t i l October, no germination of new Zostera or Ruppia seed occurred (when seed was placed i n d i s t i l l e d water). A small proportion of seed from both species germinated i n November (Fig. 22), and by January, a l l v i a b l e seed was able to germinate. The c o e f f i c i e n t of v a r i a t i o n of r e p l i c a t e s i n a l l germination tests varied between 120 and 1.4% with a mean of 10. V i a b i l i t y averaged 86% i n r e p l i c a t e s used i n germination t e s t s . Increasing s a l i n i t i e s decreased the rate of germination of both species (Fig. 23), but e s p e c i a l l y of Zostera. More than 65% of new seed was unable to germinate i n s a l i n i t i e s of 20 and 27 o/oo; 59% of 122 Table VIII. Percent v i a b i l i t i e s of buried seed at four sediment depths. New (from 1980 shoots) and old (from sediment) seed of Zostera  japonica and Ruppia maritima were c o l l e c t e d from Roberts Bank i n July, 1980. Seed was stored i n 27 o/oo seawater at 5 °C u n t i l b u r i a l near s t a t i o n 2 (2.54 m CD t i d a l height) at Roberts Bank on October 14, 1980. Seed was recovered i n February and A p r i l , 1981, and was tested for v i a b i l i t y . Sediment Depth Recovery Genus Age Date 5 cm 10 cm 15 cm 20 cm Zostera New Feb. 18.0 47.5 69.5 66.0 Apr. 0.0 1.0 31.0 45.0 Old Feb. 10.0 24.0 28.5 29.5 Apr. 1.0 4.5 27.0 31.5 Ruppia New Feb. 26.0 63.0 82.5 80.0 Apr. 0.0 2.0 14.0 46.0 Old Feb. 5.3 12.0 41.0 52.0 Apr. 0.0 1.5 19.5 25.5 5 10 15 Days 2 0 Fig. 22. Percent germination rate of current year viable seed of Zostera japonica and Ruppia maritima i n 10°C d i s t i l l e d water. 124 Fig. 23 a, b. Percent germination rate of current year v i a b l e seed of Zostera japonica and Ruppia maritima i n 10 'C water at four s a l i n i t i e s . At 30 days, ungerminated seeds were placed i n d i s t i l l e d water. 125 126 i i i i i i i i i i O O O O O O O O O O o c n c o r ^ i o L O - J m f N - r -uoipuiujjeg 127 Ruppia would not germinate i n 27 o/oo. Germination resumed when ungermi-nated seed was transferred from high s a l i n i t i e s to d i s t i l l e d water. The scoring of seed coats increased rates of germination under low and high s a l i n i t i e s , but did not increase the f i n a l percentage of germination under high s a l i n i t i e s (Fig- 24). Increased temperatures promoted germination rates ( Fig. 25), but seed of both species germinated most r a p i d l y i n a f l u c t u a t i n g regime of 7 -. 15°C. Germination diminished to zero when seed was held at 5°C. The lack of exposure to white l i g h t had no e f f e c t on the germination of new seed of either species (Fig. 26). Regardless of the temperature and atmosphere conditions during storage, the germination rates of seed placed i n 27 o/oo s a l i n i t y sea-water at al t e r n a t i n g 7 - 15°C were equivalent. The germination of new seed i n February took place as r a p i d l y as old seed germination, with a l l v i a b l e seed germinating within f i v e to ten days. Old seed germination The germination rates of old Zostera and Ruppia seed stored at 5°C and germinated at 10°C i n d i s t i l l e d water remained constant from the time of c o l l e c t i o n i n Ju l y u n t i l the end of the study i n A p r i l . Ruppia seed germinated sooner than Zostera seed (Fig. 27). Rate, but not the amount, of germination of old seed was slowed by increasing s a l i n i t y (Fig. 28). The germination of old seed was not affected by lack of exposure to white l i g h t . (Fig. 28). 128 . 24. a, b. Percent germination rate of scored ( s o l i d l i n e ) and unscored (dashed l i n e ) current year v i a b l e seed of Zostera japonica and Ruppia maritima i n 10 "C water at two s a l i n i t i e s . At 30 days, ungerminated seeds were placed i n d i s t i l l e d water. Percent Germination 6ZI Percent Germination 131 Fig. 25. a, b. Percent germination rate of current year viable seed of Zostera japonica and Ruppia maritima i n 20 o/oo s a l i n i t y water at temperature regimes of constant 10 °C and 18 *C and 12-hour alt e r n a t i n g 15 - 7 "C. 5 10 15 20 Days F i g . 25a. Zostera japonica. No germination occurred at 5"C. Percent Germination O 134 F i g . 26. Percent germination rate of current year viable seed of Zostera  japonica and Ruppia maritima i n 10 °C water at two s a l i n i t i e s i n the dark ( s o l i d l i n e ) and exposed to white l i g h t (dashed l i n e ) . At 30 days, ungerminated seeds were placed i n d i s t i l l e d water. I 5 10 15 20 Days Fig. 27. Percent germination rate of previous years' v i a b l e seed of Zostera japonica and Ruppia maritima i n 10'C d i s t i l l e d water. 138 F i g . 28. a, b. Percent germination rate of previous years' v i a b l e seed of Zostera japonica and Ruppia maritima i n 10 °C water at 10 o/oo and 20 o/oo s a l i n i t i e s exposed to white l i g h t (dashed line) and at 20 o/oo s a l i n i t y i n the dark ( s o l i d l i n e ) . 139 uoipuiuLijeo o uoipuiuujaQ j u aD j a j 141 Germination i n seed bank Germination of Zostera and Ruppia seed at Roberts Bank was f i r s t observed i n 1981, on March 10. Except for A p r i l 9 when equal numbers of seedlings from each species were found, germinated seed of Zostera was twice as numerous as Ruppia (Fig. 29). The depth of a l l germinated seed of both species averaged between 4 and 7 cm; no germinated seed was found below 12 cm. Seedling counts within p l o t s of treatments.B and C indicated that few seedlings emerged a f t e r mid-June. From June 11 to June 23, an average of less than .3 Zostera and .6 Ruppia seedlings 2 emerged per .1 m at each s t a t i o n . On J u l y 10, no new Zostera seedlings were recorded and only two Ruppia seedlings were found at s t a t i o n 2. Recruitment and death of seedlings and shoots from overwintering rhizomes Shoots a r i s i n g from overwintering rhizomes became increasingly rare from March to May (Table IX). Their depth averaged between 4 and 5 cm. Recruitment and death rates were observed for seedlings as a whole from March to May (Fig. 30a). , The most severe losses generally followed increased rates of recruitment. The t o t a l number of seedlings present at each sampling date, along with the absolute contribution of each cohort at successive sampling dates i s shown i n Figure 30b. Figure 30c, . shows the percentage contributions of cohorts to the t o t a l shoot population at each s t a t i o n . 142 F i g . 29. Percent occurrence of seedlings i n the sediment p r o f i l e near st a t i o n 2 (2.54 m CD t i d a l height) at Roberts Bank, 1981. Absolute number of seedlings i s indicated i n the lower r i g h t of each graph. Ruppia maritima 143 Zostera japonica March 10 March 2U April 9 April 18 May 1 Pe r c en t of Seed l i n g s 0 10 20 30 40 0-1 1 1 1 5' 10' 15J 19 0 10 20 30 40 "c 0 C L CU a 0 5 10 15 0 5 10 • t 13 c cu E I  15 0-5-10' 15 10 20 30 40 —1 1 1 1 26 10 20 30 40 5' 10 15 19 0 10 20 30 40 OH l-13 Pe r c en t of Seed l i n g s 0 10 20 30 40 1 0 — 15-0 5 10 1 5 J 0 5' 10-15 0 1 0 - -15-0 5 10 1 5 J 0 10 20 47 0 10 20 30 40 27 10 20 30 40 25 0 10 20 30 40 01 ^ t _ 47 30 . _40 29 144 Table IX. Shoot base depths and absolute numbers of Zostera shoots established from overwintering rhizomes i n haphazardly selected sediment cores near s t a t i o n 2 at Roberts Bank, 1981. Depth (cm) March 10 March 24 A p r i l 9 A p r i l 10 May 1 3 0 0 1 0 0 4 2 4 0 0 1 5 6 0 2 3 0 6 0 0 2 2 0 145 30. a - c . Recruitment and l o s s , absolute and percent contribution of seedling cohorts of Zostera japonica and Ruppia maritima from three stations ( s t a t i o n 1 = 3.17 m, 2 = 2.54 m, 3 = 2.60 m CD t i d a l height) at Roberts Bank, 1981. 146 Station 1 15-20-J 3 16 17 18 19 20 — j 1 1 1 1 Mar 30-i 25-20-15-10-5-5-10" 15 20 J Station 2 Rec ru i tment Loss Apr 16 17 18 19 20 —i ! 1 1 1 Mar Apr 30-1 25" 20-15-10-5H 51 10 i 15H 20 J Station 3, 16 17 18 19 20 —1 1 1 — I 1 Mar ^pr Cohort Number and Month of Sampling F i g . 30a. Recruitment and loss o f seedlings between successive sampling dates. S t i p p l i n g indicates seedlings present May 1, 1981. 147 Cohort Number and Month of Sampling — F i g . 30b. Absolute contribution of seedling cohorts to t o t a l seedling population, and t o t a l seedling density. • « 1 1 — i 1 i i 1 1 — i 1 i 1 1 — i 1 C Mar Apr . Mar Apr Mar Apr Cohort Number and Month of Sampling F i g . 30c. Percent contribution of seedling cohorts to t o t a l seedling population. 148 Discussion Buried seed numbers Buried v i a b l e seed of Zostera and Ruppia exist year-round i n the sediments of Roberts Bank. These pe r s i s t e n t seed banks appear to be replenished to the previous year's l e v e l each f a l l and depleted to very low numbers by the summer. The d i s t r i b u t i o n of Zostera seed was deeper i n the winter of 1981 as compared with 1980 and may indicate a diff e r e n c e i n sediment disturbance. Ruppia seed, however, showed a mixed pattern, 1981 buried seed being either higher or lower i n the p r o f i l e than the previous year. These v a r i a t i o n s i n b u r i a l depths may have resulted from differences i n the time of b u r i a l and/or b u r i a l processes, e.g. sediment sorting conditions acting on the d i f f e r e n t sizes and shapes of the seed (see Harper et a l . 1970 for a discussion of seed morphology and behavior i n the s o i l ) . Most data on buried seed numbers have been based on entire p r o f i l e counts. Researchers who have examined the d i s t r i b u t i o n of seed through the seed p r o f i l e have been few (e.g. Major and Pyott 1966, Kellman 1970, S t r i c k l e r and Edgerton 1976, Moore and Wein 1977, Leek and Graveline 1979) . A comparison of the d i s t r i b u t i o n of buried seed at Roberts Bank with buried seed of other ecosystems (Table X) shows that a higher propor-t i o n of seed i n i n t e r t i d a l sediments i s buried deeper. Leek and Graveline (1979) suggested that the reason that surface layers of the marsh they were studying did not accumulate as high a proportion of seed as other t e r r e s t r i a l ecosystems was because of the considerable water f l u x i n the marsh which may r a f t away surface debris containing seed. This could also p a r t i a l l y explain why surface layers at Roberts Bank had r e l a t i v e l y few seed. Table X. Relationship of seed depth i n various ecosystems.* Ecosystem Depth r a t i o Seed r a t i o Reference Coniferous f o r e s t Litter:0-2:2-4 cm 12:5:1 S t r i c k l e r and Edgerton 1976 Deciduous-dominated f o r e s t Organic layer-0-2:2-4:4-6 cm 6.5:3:1 Moore and Wein 1977 Freshwater t i d a l marsh 0-2:4-6:8-10 cm 3:2:1 Leek and Graveline 1979 Marine i n t e r t i d a l 0-2:4-6:8-10 cm 1:6.5:6** present study winter 1981 * a f t e r Leek and Graveline 1979. ** s t a t i o n 2 at Roberts Bank (2.54 m CD t i d a l height). 150 The depth of b u r i a l i s also r e l a t e d to sediment texture and the frequency and severity of disturbances to the sediment s t r a t a . Tide-driven erosional and depositional cycles may be important i n the i n t e r -t i d a l b u r i a l process. Animals within the sediment also turn over tremendous amounts of sediment on a regular basis at Roberts Bank, and are l i k e l y the most important force a l t e r i n g sediment s t r a t a and t h e i r contents. Burrowing errant and sedentary polychaetes, as well as various prosobranchs and bivalves transport sediments both v e r t i c a l l y and h o r i z o n t a l l y . At Roberts Bank, Swinbanks (1979) found that where 2 the polychaete Abarenicola had a density of 200 per m , the animals would completely turn over the sediment to the depth of 10 cm i n one hundred days. Buried mats of roots and rhizomes may aid sediment s t a b i l i z i a t i o n (Arasaki 1950b) and create b a r r i e r s to seed movement. However, i n t e r -t i d a l seagrass roots are present only during the growing season and any s t a b i l i z i n g e f f ects quickly diminish i n the f a l l . Thus newly deposited Zostera and Ruppia seeds could be worked down through the sediment by the s h i f t i n g of sediments with l i t t l e impedance by roots. The high proportions of seed i n surface layers of forested environ-ments r e f l e c t the slow rate of b u r i a l as compared to the marine i n t e r t i d a l and the short longevity of many woodland seeds (Oosting and Humphreys 1940, Livingston and A l l e s s i o 1968). Thus, barring major disturbance, the time i t takes to become buried i n a forest may exceed the l i f e span of seed from l a t e successional species. It i s generally thought i n t e r r e s t r i a l ecosystems that the deeper seed i s the oldest (Major and Pyott 1966). Because of the rapid turn-over of sediment i n the i n t e r t i d a l , any r e l a t i o n s h i p between age and 151 depth would be i n c i d e n t a l . Another diffe r e n c e between i n t e r t i d a l seed banks and those i n other habitats i s the amount of s p e c i e s - s p e c i f i c r e f l e c t i o n of the surface vegetation. Buried seed banks i n t e r r e s t r i a l (e.g. Major and Pyott, 1966, Livingston and A l l e s s i o 1968, Kellman 1970, Whipple 1978) and s a l t marsh habitats (Milton 1939) show l i t t l e or no resemblance to the growing f l o r a . I n t e r t i d a l seed banks, l i k e those of p r a i r i e g l a c i a l marshes which undergo drawdown during periods of drought (van der Valk and Davis 1978) and freshwater t i d a l marshes (Leek and Graveline 1979), do mirror the surface vegetation. This difference i n r e l a t i o n s h i p to the surface vegetation may be due to the d i f f e r e n t rates of seed b u r i a l , seed longevity and the importance of seedling re-estab-lishment a f t e r a disturbance i n an ecosystem. Thompson (1978) discussed two t h e o r e t i c a l considerations which may influence the density of buried seed. F i r s t , he proposed that buried seed densities should r e l a t e to the frequency of disturbance because "species subjected to frequent and severe disturbance should invest a high proportion of t h e i r resources i n methods of surviving the disturbance," e.g. buried seed banks. Second, he contends that because of the high mortality of seed (Roberts and Dawkins 1967), only vegetation with s u f f i c i e n t p r o d u c t i v i t y to support large seed outputs can maintain seed banks. Thus, because stress l i m i t s production of biomass of a l l or part of the vegetation, the accumulation of buried seed i s favoured by low l e v e l s of stress. Table XI compiles data on buried seed densities i n a number of vegetation types. Caution i s needed when i n t e r p r e t i n g these data because experimental methods vary; however a l l except the present study involved enumeration of germinated seed only. Table XI. Estimates of buried seed numbers from various ecosystems. Ecosystem Depth of core (m) Seeds 2 per m Reference Pasture, formerly arable 0.30 12,259 - 69,903 Chippendale § M i l t o n (1934) Grass cut f o r hay 0.30 28,310 Chippendale § Mil t o n (1934) Desert 0.005 2,900 Went (1949) Upland Eriphorum moor 0.18 2,483 Chippendale t> Morris (1948) Salt marsh 0.18 877 Milton (1939) Freshwater t i d a l marsh 0.10 6,405 • - 32,400 Leek and Graveline (1979) Marine i n t e r t i d a l 0.20 2,580 present study (winter 1981) 153 As would be predicted by Thompson, large seed banks are associated with highly disturbed vegetation, e.g. the freshwater t i d a l marsh which i s subject to annual flooding and r i v e r scouring, the desert (note core depth) with long r a i n l e s s periods, and a g r i c u l t u r a l areas with harvest and grazing pressures. Conversely, areas of low disturbance have small seed banks, e.g. s a l t marsh and moor vegetation types. The second part of Thompson's hypothesis concerned l e v e l s of p r o d u c t i v i t y of vegetation. Although i d e a l i s t i c a l l y sound, i t lacks p r e d i c t i v e power of seed bank s i z e because of i t s extreme generality. For example, subarctic and desert habitats share high stresses on p r o d u c t i v i t y i n t h e i r respective extreme temperatures and short growing seasons. As might be predicted from Thompson's hypothesis, Johnson (1975) found no v i a b l e seed i n subarctic forest f l o o r s , but i n apparent contradiction, Went (1949) found large stores of dormant seed i n desert s o i l s . Our knowledge of buried seed outside temperate t e r r e s t r i a l areas i s very poor and perhaps Johnson (1975) was correct i n saying the time has not come for a synthesis of what i s known of buried seed; however, Thompson's ideas provide l i m i t e d p r e d i c t i o n and a d i r e c t i o n for future research. The number of seagrass seed i n sediments at Roberts Bank grossly underestimates seed production on the s i t e . At f i r s t examination i t appears that buried seed i n i n t e r t i d a l habitats (high disturbance and seasonal productivity) are s i m i l a r i n number to that found i n an upland moor (low disturbance and p r o d u c t i v i t y ) . But i f s i t e - s p e c i f i c v i a b l e seed production i s taken into account (Table V), i t i s revealed that po t e n t i a l i n t e r t i d a l seed banks could reach l e v e l s which would be intermediate for a g r i c u l t u r a l areas, i f seed remained i n s i t u . 154 The reason for the large discrepancies between seed produced and the number deposited at a s i t e i s the dis p e r s a l that takes place as seeds mature. During August as the bases of flowering shoots decay or are uprooted, extensive f l o a t i n g mats of shoots form. These mats advance and r e t r e a t with the t i d e , apparently r e s u l t i n g i n the evenness of seed deposition seen at the study s i t e despite the large differences i n l o c a l seed production. Mature seed of Zostera and Ruppia sink immediately i n seawater ( s p e c i f i c gravity of v i a b l e Z. japonica (= Z. nana) i s 1.9 or greater (Arasaki 1950b)) upon separation from the parent shoot. The cl o s e r agreement between s i t e - s p e c i f i c seed production and deposition of Ruppia than of Zostera i s explained by observations that much of the Ruppia seed i s shed before the reproductive shoot i s detached, unlike Zostera where mature seed usually remains enclosed by the spadix and i s probably removed either by wave action or with decay of the spadix. In either s i t u a t i o n , the majority of seed i s r a f t e d with vegetative shoots and i s not buried where produced. Arasaki (1950b) noted that Z_. japonica seed i n Japanese waters i s dispersed over 2 to 3 months by the f l o a t i n g shoots. He also hypothe-sized that because of the .: sinking of seed, most was deposited around the parent plant. It i s possible that i n the Japanese waters where Arasaki worked, shoots were retained longer a f t e r seed matured thus having a longer time to drop seed at that l o c a t i o n before the few remain-ing seeds were c a r r i e d away. The increases of seed numbers from s t a t i o n 1 to 3 may indicate i n part the r e l a t i v e time that unattached mats containing seed spent at various t i d a l heights. Because the d i s t r i b u t i o n of f l o a t i n g materials can be e a s i l y a l t e r e d , apparently minor changes i n water movement could 155 r a d i c a l l y a l t e r seed deposition and the resultant seed bank. My r e s u l t s show that vast numbers of seed are exported from the study s i t e ; t h i s f a c t helps to explain the rapid spread of Z. japonica and the wide occurrence of R. maritima. Seed v i a b i l i t y "There are many factors acting between seed dissemination and seed establishment which can a f f e c t the number of in d i v i d u a l s surviving i n plant populations" (Watkinson 1978). Determining what these factors are and t h e i r e f f e c t s on v i a b l e seed numbers has been the major obstacle to the use of buried seed information i n the study of population dynamics. The declines i n v i a b i l i t y of new and old seed of Zostera and Ruppia maintained at 5°C were s i m i l a r ; however, o ld seed buried i n the f i e l d died or was lo s t through germination at a much greater rate than new seed was. New seed was innately dormant and unable to germinate during part of the buried period (see following discussion), a fact which may have resulted i n i t s lower mortality. Caution i s needed when r e l a t i n g laboratory r e s u l t s to those i n the f i e l d . The rate of seed death i n the laboratory r e f l e c t e d apparently autonomous causes of death which may have been d i f f e r e n t from causes under f i e l d conditions. These data lend support to the observations of Johnson (1975) who found that the decrease i n seed s u r v i v a l with age i s not due to the loss of v i a b i l i t y , but to external factors r e s u l t i n g i n germination or death by predation. There are two environmental gradients i n the sediment p r o f i l e which appear to a f f e c t seed persistence. F i r s t , there i s a differe n c e i n temperature f l u c t u a t i o n between surface and subsurface sediments (see section 1). Turner (1933) suggested that low stable temperatures, 156 which are c h a r a c t e r i s t i c of deeper s o i l depths i n many environments, increase the persistence of seed. Several studies besides t h i s one have supported t h i s statement (Toole and Brown 1946, Darlington and Steinbauer 1961, Schafer and Chilcote 1970, Roberts and Feast 1972, Lewis 1973). More recently, Weaver and Cavers (1979) found temperature f l u c t u -ations near the s o i l surface to be the main cause of buried Rumex seed germination and thus depletion. As w i l l be elaborated on i n the follow-ing section, fluctuations i n surface sediment temperature may play a major r o l e i n the rapid depletion, through germination of Zostera and Ruppia seed from near-surface sediments. Secondly, there i s a gradient i n redox p o t e n t i a l i n the sediment which implies a decreasing oxygen a v a i l a b i l i t y i n the sediment below the surface (see Section 1). My r e s u l t s show that anaerobic conditions slow the rate of Zostera and Ruppia seed germination. Turner (1933), Bibbly (1948), and Roberts (1972) have found that low oxygen a v a i l a b i l i t y d i s -couraged precocious germination of seed and thus aided i n i t s persistence. The enforced dormancy of seed i n anaerobic environments w i l l be discussed l a t e r within t h i s section. It i s d i f f i c u l t to determine how my r e s u l t s r e l a t e to buried seed populations i n other wetlands where there exist marked gradients i n redox p o t e n t i a l , because of lack of information from these areas. It has been known for some time that there i s a postponement of germination of seed i n aquatic sediments. Guppy (1897) showed that germination of some aquatic seed can be delayed four or f i v e years without impairment to i t s v i t a l i t y , while Shull (1914) proved experimentally that seed of many land and water plants w i l l germinate a f t e r being kept i n mud and water for periods of four to seven years. 157 Few attempts have been made to determine the reasons f o r seed persistence i n wet areas. It i s known, however, that reduced oxygen pressures do not impede the germination of a l l seed. Morigaga (1926) found that by replacing the a i r around Typha l a t i f o l i a L. seed with nitrogen, germination percentages increased. Even i f , as admitted by Morigaga, seed of few species i s encouraged to germinate i n t h i s way, Typha provides example enough not to make generalizations about the factors which enforce dormancy i n wetland seed. The s p e c i f i c causes of Zostera and Ruppia seed mortality within the sediment were d i f f i c u l t to-determine, i n part because of the rapid d e t e r i o r a t i o n of soft t i s s u e . Because germination i s u n l i k e l y at winter sediment temperatures (around 5°C, see following discussion), germination probably had l i t t l e to do with the nearly complete mortality of seed buried near the surface by February. A more f e a s i b l e explanation would be that the mortality resulted from physical damage to the seed by the repeated freezing observed i n surface sediments at Roberts Bank i n the winter. Germination occurs n a t u r a l l y i n the f i e l d between February and A p r i l (see following discussion), and thus was a major means of buried seed depletion i n that period. S p e c i f i c seed predators have not been i d e n t i f i e d , although as found in t e r r e s t r i a l s i t u a t i o n s , fungal pathogens may play a r o l e i n the deple-t i o n of seed (Harper et a l . 1961, Lawrence and Rediske 1962, Taylorson 1970). Fungal degradation of seagrass seed may be aided by mechanical in j u r y to seed r e s u l t i n g from s h i f t i n g of sediments. It i s not known i f the apparent predation that took place while seed was attached to the vegetative plants occurred within the sediment as we l l . 158 It i s possible that causes of Zostera seed depletion are d i f f e r e n t than those for Ruppia. Sarukhan (1974) found dramatic differences i n the fates of seed from three Ranunculus species i n terms of predation and decay. There i s now considerable evidence that the conditions under which some seed ripen play a r o l e i n determining t h e i r longevity (Taylorson 1970). Genetic v a r i a t i o n within a population can also a f f e c t seed v i a b i l i t y (Steiner 1968, Sawhney and Naylor 1979). Harper (1965) points out that i t may be incorrect to assume that the dormancy and germination c h a r a c t e r i s t i c s of seed from the same plant are a l i k e . Patterns of seed s u r v i v a l i n the s o i l are not only of ecological i n t e r e s t , but of great p r a c t i c a l importance to a g r i c u l t u r a l i s t s , but to date few attempts have been made to analyze the parameters of buried seed persistence and depletion. Schafer and Chilcote (1969) presented a model which provides a conceptual framework for problems involving buried seed. It simply states that the t o t a l buried seed population of a species at a point i n time i s equal to the number of seed i n a state of enforced dormancy, plus the number i n innate and induced dormancies, the number undergoing germination, and the number which have l o s t v i a b i l i t y . Roberts (1972) expanded Schafer's and Chilcote's model to d i f f e r e n t i a t e persistence by innate vs. induced dormancies, depletion through germination of seed from deep vs. shallow depths, and death by aging of seed vs. death by predation. To date, neither model has advanced our knowledge of the means of seed depletion i n the sediment p a r t l y because i t i s d i f f i c u l t to d i s t i n g u i s h between loss of seed by predation, p h y s i o l o g i c a l aging or germination, thus defeating a major objective of the model (e.g., Rampton and Ching 1970). 159 Because germination has repeatedly been found to be a major cause of buried seed depletion (Roberts 1972, Watkinson 1978, Weaver and Cavers 1979), the key to understanding and pr e d i c t i n g patterns of buried seed depletion l i e s with obtaining information on how seed in t e r a c t with factors which can lead to various periods and degrees of dormancies. On the other hand, attempts to quantify predation on seed may be u n r e a l i s t i c , for i t l i k e l y varies greatly i n both time and space as predator populations and food supplies change. Unfortunately, because seed b u r i a l experiments were conducted during only part of the year, extended predictions of Zostera and Ruppia buried seed depletion cannot be made. M o r t a l i t y during the summer i s possibly quite d i f f e r e n t than that experienced i n the winter. It i s i n t e r e s t i n g to consider, however, that a small portion of the more deeply buried Zostera and Ruppia seed does survive i n excess of a year and may contribute i n part to future generations or act as a means of population re-establishment i n the case of seed production f a i l u r e by the most current generation. Germination and seedling recruitment "One of the most c r i t i c a l stages i n the l i f e cycle of halophytes (or any plant) i s the period of germination and establishment. The behavior of halophytes at t h i s stage of development i s rather poorly understood" (Waisel 1972, quoted by Ungar 1978) . The germination of Zostera and Ruppia requires the removal of at least one, and possibly two, types of dormancy. Because much confusion h i s t o r i c a l l y surrounds dormancy terminology (Roberts 1972), i t i s advantageous, i f not necessary, to define what i s meant by various types of dormancy. Three types of 160 dormancy (innate, induced, and enforced) are now recognized (Harper 1957) . Innate dormancy i s that which i s present when the new embryo ceases to grow while i t i s s t i l l attached to the parent plant (Roberts 1972). Such a dormancy prevents the seed from germinating u n t i l i t i s dispersed. After a seed has l o s t i t s innate dormancy, a s i m i l a r type of dormancy may be induced. Induced dormancy i s usually the r e s u l t of the seed being supplied with water, but i n an environment where some other factor i s unfavorable for germination. It i s the persistence of induced dormancy a f t e r i n h i b i t o r y factors have been removed which d i s -tinguishes i t from enforced dormancy. Seed of Zostera and Ruppia are shed innately dormant. The breaking of innate dormancy may involve further development of the embryo or surrounding tissues (Harper 1957). This " a f t e r - r i p e n i n g " process was manifested i n seed of Zostera and Ruppia, as i t i s i n most plants (Koller et a l . 1962), as a gradual r e l a x a t i o n i n the s t r i c t n e s s of requirements for subsequent germination. Figure 23 shows a reduction of germination of new Zostera and Ruppia seed by high s a l i n i t i e s i n January; by February, germination could take place i n a l l v i a b l e seed at 27 o/oo as indicated by the r e s u l t s of Table VII. Arasaki (1950b) also noted that seed of Z. japonica c o l l e c t e d i n the summer would not germinate u n t i l the following January to March. Old seed recovered from sediments at Roberts Bank germinated r e a d i l y , i n d i c a t i n g that t h e i r means of persistence was an enforced rather than an induced dormancy. Kidd (1941), Bibbey (1948), Thornton (1945), and K e l l e r et a l . (1962) have suggested that the raised carbon dioxide and lowered oxygen concentrations i n the s o i l atmosphere are l a r g e l y responsible for maintaining the enforced dormancy of buried seed. Although carbon dioxide l e v e l s were not monitored, my r e s u l t s show the rapid germina-t i o n of Zostera and Ruppia seed i n a favorable temperature regime can be slowed, and the amount possibly reduced, by replacing the atmosphere with nitrogen. This suggests that anaerobic conditions alone adversely a f f e c t the germination of Zostera and Ruppia seed. The lowest depth of seed germination at Roberts Bank corresponded approximately with the boundary between aerobic and anaerobic sediments. This lends support to the hypothesis that within anaerobic sediments, seed of Zostera and Ruppia were enforced into dormancy. Although experimental b u r i a l s of seed indicated high winter mortality at 5 cm below the surface o f the sediment, Figure 29 shows that some seed at that depth survived to contribute to the f l u s h of germination i n the l a t e winter and spring. Several mechanisms for anaerobic i n h i b i t i o n of germination have been suggested, including inadequate supplies o f oxygen f o r embryo r e s p i r a t i o n (Thurston 1960), and more recently the stimulatory e f f e c t s of oxygen have been at t r i b u t e d to i t s p a r t i c i p a t i o n i n the i n a c t i v a t i o n of some endogenous i n h i b i t o r (Koller et a l . 1962). It i s well recognized that abnormally high atmospheric concentra-tions of carbon dioxide or other u n i d e n t i f i e d gaseous i n h i b i t o r s which are metabolic products from the seed or from microorganisms (Wesson and Wareing 1969) are involved i n the dormancy of many species of seed (Kozlowski 1972). Harper (1957) suggested that e s p e c i a l l y where s o i l pores are small and s o i l water content i s high, the l o c a l micro-atmospheres ( l i k e those surrounding a seed i n the s o i l ) may quickly become exhausted, of oxygen and replaced by carbon dioxide. 162 The replacement of oxygen with carbon dioxide increases with depth and can be very marked i n wet s o i l s (Russell 1973). Thus as seed moves down through the Roberts Bank sediment p r o f i l e , by animal a c t i v i t y or other disturbance, from aerobic to anaerobic atmospheres, the dormancy ef f e c t s described by Kidd (1941), Thorton (1945), Bibbey (1948) and Roller et a l . (1962) would become increasingly pronounced. The seed germination of a great number of species i s influenced by i r r a d i a t i o n with white l i g h t (Karrssen 1970) and absence of l i g h t i s now a recognized form of dormancy control i n many species (McDonough 1977). Seed of Zostera and Ruppia were found to be t o t a l l y non-photoblastic (germination took place i n l i g h t as well as i n darkness) as was seed of Zostera marina (R.C. P h i l l i p s personal communication). A p o s i t i v e photoblastic response i n seagrass seed would be maladaptive because i t would r e s u l t i n seedlings at the sediment surface where establishment may be prevented by a poorly anchored root system, and the r i s k of drying would be great. The increased rates of germination caused by the scoring of seed coats could be due to a release from water uptake r e s t r i c t i o n s or to a lessening of the mechanical resistance to embryo expansion imposed by the seed coat. The recovery of germination i n d i s t i l l e d water a f t e r seed was soaked i n seawater with i n h i b i t o r y s a l t concentrations i n d i -cates that no permanent s p e c i f i c ion t o x i c i t y occurs and that the c h i e f influence of excess s a l t s may be osmotic. Temperature exerted a powerful influence, as d i d s a l i n i t y , on the a b i l i t y to germinate and the rate of germination of both Zostera and Ruppia seed. Verhoeven (1979) found that germination of R. maritima seed took place simultaneously when water temperatures exceeded the d a i l y minimum-maximum i n t e r v a l of 10 to 15°C. Set c h e l l (1924) also established that temperature was the c o n t r o l l i n g factor i n R. maritima seed germination, the optimum occurring between 10 and 18°C. Arasaki (1950b) found that seed of Z. japonica would germinate i n water temper-atures from 1 to 10°C; however, based on my observations of Z. japonica, I would not expect germination to take place at or below 5°C. Perhaps the seed Arasaki used d i f f e r e d from mine genotypically, or since d i f f e r -ing environmental conditions during development and innate dormancy can influence the germination requirements of seed (Kozlowski 1972), perhaps the environment that the seed Arasaki used was exposed to allow them to germinate at 1°C. Sediment temperatures underwent a marked increase when the lowest low tides switched from night to day i n early March (see Section 1). The natural germination of Zostera and Ruppia seed at Roberts Bank coincided with, and was probably a d i r e c t r e s u l t of, the increase i n sediment temperature. Poor seedling establishment at s t a t i o n 1 as compared to stations 2 and 3 i n March was l i k e l y a r e s u l t o f b u r i a l by sand waves (see Section 1). Several seedlings were found f r e e - f l o a t i n g or l y i n g on the sediment surface, apparently dislodged from the sediment a f t e r germination; th i s could be a major source of mortality of seeds germinating near or on the sediment surface. The seedlings which appeared a f t e r the major f l u s h of germination (March to May(?)) may represent seed which were under enforced dormancy at a deep, anaerobic sediment depth and were released from dormancy by transport to aerobic sediments through animal a c t i v i t y . 164 SUMMARY The objective of t h i s study was not to make a precise inventory of components i n the populations, but to i d e n t i f y some of the processes which a f f e c t population numbers, so that changes or differences i n i n t e r t i d a l seagrass populations can be better predicted and explained. The processes that control the numbers i n a population are t i g h t l y integrated and i n v a r i a b l y complex. The following are some general conclusions concerning the maintenance of Zostera japonica and Ruppia maritima populations at Roberts Bank. The amount of exposure to the a i r was a major influence determining the behavior of shoot f l u x . Peak shoot d e n s i t i e s were smaller with increased time of exposure. Zostera with the greatest exposure to the a i r retained a larger proportion of shoots produced early i n the growing season than did Zostera at l e s s e r exposures. Ruppia retained a large proportion of early shoots regardless of exposure l e v e l . Zostera and Ruppia with the greatest exposure also had a la r g e r propor-t i o n of shoots which flowered. Shoots that flowered u s u a l l y l i v e d longer than vegetative shoots. Phases of growth, vegetative, reproductive, and quiescent (Zostera), could be recognized i n shoot survivorship and morphology of both species. Deevey Type I survivorship curves were c h a r a c t e r i s t i c of both Zostera and Ruppia shoots (primarily vegetative) early i n the growing season. During the reproductive phase, survivorship curves were of Deevey Types II or I I I ; shoots produced as Zostera entered the quiescent phase showed a Deevey Type I pattern of survivorship. Leaves and newly produced rhizome internodes were longer i n the early, p r i m a r i l y vegetative, phase; soon a f t e r plants began to flower, new l e a f and 165 rhizome internode lengths declined. Zostera shoots entering quiescence developed short, t h i n leaves. Each phase of growth occurred i n Ruppia simultaneously regardless of the amount of exposure; i n Zostera, however, phases began e a r l i e r at the greatest exposure. Seed production of Zostera and Ruppia decreased with increasing exposure to the a i r . S p e c i f i c f a c tors i n f l u e n c i n g seed production v a r i e d between species and with time. In general, seed production fl u c t u a t e d with the success of i n d i v i d u a l ovaries i n producing mature seed and with flowering shoot density. Most seed was exported from the s i t e of production by the d r i f t i n g of detached flowering shoots. Zostera and Ruppia seed dispersed through the summer and f a l l was prevented from germinating f i r s t by innate dormancy, a condition which expired by January, and then p r i m a r i l y by low winter temperatures. Seed buried near the surface of the sediment suffered high overwinter mortali-. t i e s p ossibly linked with the freezing of sediments. M o r t a l i t y was generally reduced with depth, owing most l i k e l y to a combination of fac-tors which included an i n c r e a s i n g l y anaerobic environment or unknown carbon dioxide e f f e c t r e s u l t i n g i n an enforced dormancy. The germination of seed was prevented by l o c a l seawater s a l i n i t i e s only when seed was emerging from innate dormancy. The natural germination of seed was stimulated by the warming of sediment i n March which occurred a f t e r a predictable a l t e r a t i o n i n ti d e s exposed the sediment to the midday sun. Germination was confined to aerobic sediments. Seed remained under enforced dormancy i n anaerobic sediment year-round. New seedlings emerged through A p r i l and p o s s i b l y as l a t e as June, but only a small portion became established. Zostera shoots a r i s i n g from overwintering rhizomes contributed l i t t l e to the re-establishment of the growing population. 166 This work began with the recognition that populations of Z. japonica and R. maritima consist of growing and seed components, and that i n order to understand how populations a r i s e and are maintained, both components must be considered. The demographic approach to the study of shoots provided information on the underlying fluxes which produced the observed population. U n t i l such information was gathered, there was l i t t l e hope i n beginning to explain changes i n the numbers i n a population. Shoot demography, despite i t s time-consuming nature, w i l l l i k e l y play a prominant r o l e i n future investigations into the dynamics of seagrass populations. Laboratory studies provided much a i d i n i n t e r p r e t i n g f i e l d investigations into the persistence and depletion of seed. Future studies aimed at a more comprehensive understanding of the seed i n the population maintenance of Z. japonica and R. maritima should focus on long-term seed b u r i a l experiments and investigations into how s p e c i f i c environmental conditions influence seed longevity. LITERATURE CITED 167 Antonovics, J . 1972. Population dynamics of the grass Anthoxanthum  odoratum on a zinc mine. J . Ec o l . 60:351-365. Antonovics, J . 1976. The nature of l i m i t s to natural s e l e c t i o n . Ann. of the Missouri Bot. Garden 63:224-247. 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Primary p r o d u c t i v i t y of the Fraser River d e l t a f o r e -shore: Y i e l d estimates of emergent vegetation. M.Sc. Thesis. University of B r i t i s h Columbia, Vancouver, B.C. Zieman, J.C. 1974. Methods for the study of the growth and production of t u r t l e grass, Thalassia i testudinum Ko'nig. Aquaculture 4:139-142. Zieman, J.C. and R.G. Wetzel. 1980. Pr o d u c t i v i t y i n seagrasses: Methods and rates, p.87-116. Iri_ R.C. P h i l l i p s and C P . McRoy, eds. Handbook of seagrass biology: An ecosystem perspective. Garland STPM Press, New York. Appendix 1. Taxonomy of Zostera japonica 177 INTRODUCTION In 1957, a thin-leaved, i n t e r t i d a l seagrass was c o l l e c t e d by N. Hotchkiss i n Willapa Bay, Washington (Hitchcock et a l . 1969). This was the f i r s t record of a member of the Zostera subgenus Z o s t e r e l l a growing i n North America. Since then, i t has been found on mud and sand f l a t s ranging as f a r south as Netarts Bay, Oregon (45°24'N; 124°53'W) and as f a r north as Burrard I n l e t , Vancouver, B r i t i s h Columbia (49°17'N; 123°13'W). Hitchcock et a l . (1969) i d e n t i f i e d the Hotchkiss c o l l e c t i o n as Zostera nana Roth, a now i n v a l i d name which at the time included a l l of the Z o s t e r e l l a species used i n t h i s study. In a monograph of world sea-grasses, den Hartog (1970), described the Hotchkiss specimens as a new species, based p r i m a r i l y on r e t i n a c u l a ( s t e r i l e f laps on the flowering spadix) and l e a f t i p morphology. He named the species Zostera americana den Hartog. P h i l l i p s and Shaw (1976) compared specimens of Z. americana from four Washington c o l l e c t i o n s with a vegetative c o l l e c t i o n of Zostera  n o l t i i Hornem. from England, and den Hartog's i l l u s t r a t i o n s of Z. n o l t i i . They found that l e a f t i p s and r e t i n a c u l a i n Z. americana were too v a r i -able to d i f f e r e n t i a t e i t from the European species, Z. n o l t i i . They challenged den Hartog's determination of Z. americana as a new species, and concluded Z. americana should be c a l l e d Z. n o l t i i u n t i l further investigations could be conducted. Harrison (1976), also questioning den Hartog's determination, compared r e t i n a c u l a and l e a f t i p characters from B r i t i s h Columbia c o l l e c t i o n s of Z. americana with den Hartog's i l l u s t r a t i o n s of Z. japonica 178 Aschers § Graebn. Harrison concluded that the North American species was Z. japonica, probably introduced from Japan by oyster growers. B i o l o g i s t s have c a l l e d the North American Z o s t e r e l l a species any of four names, and are often unsure i f other researchers are r e f e r r i n g to the same species. Previous workers, t r y i n g to show the r e l a t i o n s h i p between the North American Z o s t e r e l l a and i t s close r e l a t i v e s , have neither quantified species c h a r a c t e r i s t i c s nor examined enough material to resolve the issue. The purpose of t h i s study was to c l a r i f y the taxonomic p o s i t i o n of Zostera americana, using numerical taxonomy. METHODS Flowering Z o s t e r e l l a specimens were c o l l e c t e d i n 1980, and preserved i n 5% (v/v) formaldehyde seawater. Zostera americana was c o l l e c t e d from Willapa Bay on the southwest coast of Washington (46°24'N, 123°57'W), and from Roberts Bank i n southern B r i t i s h Columbia (49°02*N, 123°08'W). Zostera n o l t i i was c o l l e c t e d from Krabben Kreek, the Netherlands (51°37'N, 04°03'E), and from the Tay Estuary, on the east coast of Scotland (56°27'N, 02°53'W). Zostera japonica was c o l l e c t e d from Yamada Bay (39°29'N, 141°51'W) and Odawa Bay (35°12'N, 139°39'W) i n central Japan. A c o l l e c t i o n of. Z. c a p r i c o r n i Aschers. from Smiths Bay, New South Wales (35°36'S, 137°27'E), was also examined. Characters of the Z o s t e r e l l a subgenus used i n den Hartog's monograph which appeared to d i f f e r e n t i a t e taxa were chosen for q u a n t i f i c a t i o n (see Table XII and F i g . 31). Three new characters which varied between species were also measured (see Table XII, characters 4, 5 and 16). To minimize character differences due to age, only specimens i n which po l l e n had dehisced but ovules had not elongated were selected. The f i r s t 12 Table XII: Mean character values and standard deviations of four species in the Zostera subgenus, Z o s t e r e l l a . Characters Z. n o l t i i X S Z. X americana " S Z. j X aponica S Z. X capricorni ; s 1. No. of female flowers within spathe 3.13 (1.00) 6. 67 (1.34) 5.67 (1.78) 6. 58 (1.00) 2. Retinaculum length, mm .94 (.17) 74 (.42) .97 (.10) 1. 31 (.11) 3. Retinaculum width, mm .33 (.06) 57 (.08) .59 (.06) 83 (.25) 4. Thickness of spathal corners, mm .10 (.02) 17 (.04) .21 (.03) 37 (.02) 5. Thickness of spathe at midrib, mm .11 (.02) 14 (.02) .15 (.02) 10 (.01) 6. Spathe width, mm 1.58 (.23) 1. 96 (.39) 2.21 (.30) 2. 79 (.34) 7. Spathe length, mm 14.58 (7.07) 23. ,54 (2.39) 18.29 (5.27) 22. 17 (3.56) 8. Prophyllum length, mm 22.79 (33.96) 20. .04 (3.67) 15.42 (4.88) 19. ,17 (3.95) 9. Generative leaf length, mm 11.87 (5.93) 8. .25 (2.60) 8.17 (3.37) . 6. ,17 (1.53) 10. Generative leaf width, mm 1.64 (.49) 1.35 (.28) 1.33 (.26) 2. .42 (.26) 11. Thickness of gen. le a f at midrib, mm .11 (.02) .13 (.03) .14 (.02) .12 (.03) 12. No. of accessory fibre bundles between 4.71 (.90) 4 .17 (.86) 5.42 (1.21) 5, .00 (1.12) mid and last l a t e r a l nerves, gen. leaf 13. No. of accessory fibre bundles ext. to 1.25 (.44) 3 .42 (1.02) 3.50 (.83) 1 .33 (.65) marginal nerves, gen. leaf 14. No. of accessory f i b r e bundles between 5.33 (.96) 4 .58 (.97) 6.37 (.44) 6 .25 (.14) mid and last l a t . nerves, spathal sheath 15. No. accessory fibre bundles ext. to 9.71 (2.91) 4 .21 (1.18) 9.96 (2.53) 3 .25 ' (.96) marginal nerves, spathal sheath (.30) 16. Length to width ratio of retinaculum 2.94 (.60) 1 .80 (.26) 1.65 (.25) 1.44 31. Characters used to d i f f e r e n t i a t e Z o s t e r e l l a taxa. A. Flowering shoot showing generative l e a f , spathe, and prophyllum. B. Spadix showing the p o s i t i o n s of r e t i n a c u l a and female flowers; male flowers have been omitted. C. Retinaculum. D. Spathe cross section. E. Generative leaf cross section. Numbers re f e r to character numbers from Table I. Drawings not to scale. 181 specimens encountered from each c o l l e c t i o n which were complete, non-deformed, and met the age c r i t e r i o n , were used for microscopic and macroscopic sets of character measurements. To obtain measurements on anatomical characters, specimens were embedded i n p a r a f f i n (Feder and O'Brien, 1968) and sectioned at 12 micrometers. From prepared s l i d e s , seven characters (4, 5, and 11 through 15) were measured under a c a l i b r a t e d compound microscope. Bulked specimens were grouped by c o l l e c t i o n s i t e , creating operational taxonomic u n i t s (otu's). Characters from each species were compared using analysis of v a r i -ance with a s i g n i f i c a n c e l e v e l of 5%. Character data f o r a l l specimens were evaluated using p r i n c i p a l components an a l y s i s . RESULTS Table XII shows the mean values of characters f o r the four species. Zostera n o l t i i was s i g n i f i c a n t l y d i f f e r e n t from the other species f o r 9 of the 16 characters; Zostera americana and Z. japonica were s i g n i f i c a n t l y d i f f e r e n t from the other species, but not from each other, for characters 3, 10, and 13. In the p r i n c i p a l components an a l y s i s , component 1 accounted f o r 28% of the t o t a l variance. The main contributors to component 1 i n order of decreasing importance were retinaculum width, the retinaculum length to width r a t i o , and the thickness of the spathal corners. These were followed by spathal sheath width and number of female flowers. The three characters with the heaviest loadings on component 1 separated Z. japonica and Z. americana from the other species. Zostera n o l t i i r e t i n a c u l a were long and t h i n , g i v ing them a high length to width 182 r a t i o (Table XII).Zostera c a p r i c o r n i r e t i n a c u l a d i d not have a s i g n i f -i c a n t l y smaller length to width r a t i o than Z. japonica r e t i n a c u l a , but they were s i g n i f i c a n t l y l a rger. The mean thicknesses of spathal corners were s i g n i f i c a n t l y d i f f e r e n t f o r a l l four species, but the mean values for Z. americana and Z. japonica spathes were most s i m i l a r (see Table XII, character 4, and F i g . 32). P r i n c i p a l component 1 values were p l o t t e d against component 2 values f o r i n d i v i d u a l plants (Fig. 33). Together, these components accounted f o r 45% of the t o t a l variance. Zostera n o l t i i and Z. c a p r i -corni c l e a r l y separated from the others; Z. americana and Z. japonica overlapped. DISCUSSION It i s my judgment, based on PCA r e s u l t s and s i m i l a r i t i e s of major characters, that Zostera americana and Zostera japonica are the same species. The wide spread of points i n the Z. japonica-Z. americana c l u s t e r (Fig. 33) i l l u s t r a t e s the p l a s t i c i t y of t h i s taxon; i t has also been shown that other seagrass taxa vary morphologically when grown i n areas which d i f f e r i n climate (McMillan 1978). Zostera n o l t i i displayed a s i m i l a r range of points between i t s two c o l l e c t i n g s i t e s . In h i s monograph, den Hartog (1970) emphasized species differences i n l e a f t i p and r e t i n a c u l a r morphology i n a key to Z o s t e r e l l a species and through i l l u s t r a t i o n . I found l e a f t i p morphologies were too v a r i -able within each of my c o l l e c t i o n s to use as d i s t i n g u i s h i n g character-i s t i c s . P h i l l i p s (1960) found that l e a f t i p s , used to d i s t i n g u i s h species of the seagrass Diplanthera, varied with the environment. Harrison (personal communication, 1981) observed that r e t i n a c u l a r 183 Z. nolfii Z. ame Z. japonica Z. Capricorni 32. Representative spathe cross sections of Z o s t e r e l l a taxa. Nerve bundles have been omitted. 184 -2 •+- e t-8 o • • • •o • + -2 A A + - 4 F i g . 33. P r i n c i p a l component analysis of Z o s t e r e l l a taxa; projections of characters on components I and I I . • Zostera  n o l t i i from Tay Estuary, Scotland, v Z. n o l t i i from Krabben Kreek, The Netherlands, • Z. americana from Willapa Bay, WA, U.S.A., • Z. americana from Roberts Bank B.C., Canada, • Z. japonica from Yamada Bay, Japan, o Z. japonica from Odawa Bay, Japan, A Z_. c a p r i c o r n i from Smiths Bay, A u s t r a l i a . Characters are l i s t e d i n Table XII. 185 morphology varied with age of the specimen. I c o n t r o l l e d the age influence by s e l e c t i n g plants of the same stage of reproduction; l e a f t i p characters were not used. Because P h i l l i p s and Shaw (1976) lacked flowering specimens of Z. n o l t i i , they did not see cross sections of the spathes, which are d i s t i n c t l y d i f f e r e n t from those of Z. americana (Fig. 32). Lack of age control could explain the v a r i a b i l i t y they found between Z. americana r e t i n a c u l a from d i f f e r e n t c o l l e c t i o n s . Immature Z. americana r e t i n a c u l a are as narrow as mature Z. n o l t i i r e t i n a c u l a . Harrison (1976) suggested that Z. japonica had been introduced from Japan through the oyster industry. Oyster growers i n Washington and B r i t i s h Columbia began importing P a c i f i c Oysters from Japan i n the early 1900s. By the 1930s, successful industries had developed i n Puget Sound, and e s p e c i a l l y i n Willapa Bay on the Washington Coast (Elsey 1933) . Sayce (1976) l i s t e d 8 animal species, a c c i d e n t a l l y introduced into Willapa Bay and Puget Sound from Japan. Most of those l i s t e d are parasites or preda-tors of the P a c i f i c Oyster. Scagel (1956) reported that Sargassum muticum (Yendo) Fensholt had been introduced from Japan to Washington, Oregon, and southern B r i t i s h Columbia waters. Sargassum, a common i n t e r t i d a l alga i n Japan, appeared in the Northeast P a c i f i c by the mid 1940s, often i n or near P a c i f i c Oyster beds. By 1954, Sargassum was abundant i n northern Willapa Bay, attached to oyster s h e l l s . Zostera japonica was probably introduced to the Northeast P a c i f i c as dormant seed, either i n sediment associated with P a c i f i c Oysters, or attached to adult plants. There i s anecdotal evidence that Japanese oysters were packed and shipped i n some species of eelgrass (Harrison 186 1976). Uprooted plants can r e - e s t a b l i s h themselves, but would probably not have survived the two-week long voyage from Japan. In t h i s study, species c h a r a c t e r i s t i c s which have previously only been examined sub j e c t i v e l y have been qu a n t i f i e d . I agree with P h i l l i p s and Shaw (1976) that den Hartog based the Z. americana designation more on a geographical than a morphological basis. After evaluating PCA re s u l t s and observing the s i m i l a r i t i e s of major c h a r a c t e r i s t i c s , I conclude, as did Harrison (1976), that Z. americana i n the Northeast . P a c i f i c are dis j u n c t populations of Z. japonica. ACKNOWLEDGEMENTS I thank E.A. Drew of the Gatty Marine Lab., St. Andrews, Scotland, R.P.M.W. Jacobs, of the Centraalbureau voor Schimmelcultures, Baarn, The Netherlands, V. Holland of the Univ. of New South Wales, Dept. of Botany, A u s t r a l i a , and K. A i o i , of the Ocean Res. Inst., Univ. of Tokyo, Japan, for c o l l e c t i n g and sending Zostera specimens. LITERATURE CITED Elsey, C.R. 1933. Oysters i n B r i t i s h Columbia. B i o l . Bd. Can. B u l l . , No. 34. Feder, N. and T.P. O'Brien. 1968. Plant microtechnique: some p r i n c i p l e s and new methods. Amer. J . Bot. 55:112-142. Harrison, P.G. 1976. Zostera japonica (Aschers § Graebn.) i n B r i t i s h Columbia, Canada. Syesis 9:359-360. den Hartog, C. 1970. The Sea-grasses of the World. North-Holland Publ. Co., Amsterdam. Hitchcock, C.L., A. Cronquist, M. Ownbey, and J.W. Thompson. 1969. Vascular Plants of the P a c i f i c Northwest. Univ. of Washington Press, Seattle. 187 McMillan, C. 1978. Morphogeographic v a r i a t i o n under c o n t r o l l e d con-d i t i o n s i n f i v e seagrasses. T h a l l a s s i a testudinum. Halodule w r i g h t i i . Syringondium f i l i f o r m e , Halophila engelmannii, and Zostera marina. Aquatic Bot. 4:169-189. P h i l l i p s , R.C. 1960. Environmental e f f e c t on leaves of Diplanthera du petit-thouars. B u l l , of Mar. S c i . of the Gulf and Caribbean 10:346-353. P h i l l i p s , R.C. and R.F. Shaw. 1976. Zostera n o l t i i i n Washington, U.S.A. Syesis 9:355-358. Sayce, C.S. 1976. The oyster industry of Willapa Bay. Proc. of the Symposium on T e r r e s t r i a l and Aquatic E c o l . Studies of the Northwest. E. Wash. State College Press, Cheney, WA., pp.347-356. Scagel, R.F. 1956. Introduction of a Japanese alga, Sargassum muticum, int o the Northeast P a c i f i c . F i s h . Res. Papers, Wash. Dept. F i s h . V ol. 1 No. 4. Appendix 2. Oxidizing p o t e n t i a l s of Roberts Bank sediment. 189 Appendix 2. Oxidizing (redox) po t e n t i a l s of the sediment to 20 cm depth near s t a t i o n 2 (2.54 m CD t i d a l height) at Roberts Bank, 1980 - 1981. SEDIMENT DEPTH Surface 2 4 6 8 10 12 14 16 18 20 11 JUNE 155 110 90 10 -40 -60 -85 -105 -130 -150 -155 10 JULY 115 65 -5 -35 -100 -120 -120 -120 -135 -140 -140 15 AUG. 100 60 40 -10 -120 -130 -140 -140 -145 -145 -155 12 SEPT. 140 80 45 -20 -80 -110 -130 -135 -135 -135 -135 14 OCT. 160 100 60 10 -20 -60 -100 -110 -110 -125 -125 3 DEC. 200 125 100 -30 -60 -100 -100 -100 -100 -100 -100 23 JAN. 230 165 140 120 80 -10 -50 -100 -120 -110 -130 21 FEB. 185 170 180 165 125 0 -40 -120 -100 -130 -130 10 MAR. 200 185 150 130 105 -40 -70 -110 120 -140 -140 18 APR. 190 120 90 45 -15 -45 -100 -120 -105 -140 -140 1 MAY 185 100 60 -10 -40 -80 -110 -120 -120 -130 -150 Appendix 3. Shoot demography data. 192 Appendix 3. Demographic data showing mapping dates which established cohorts, cohorts present at each sampling date, the average number of vegetative and reproductive shoots present i n plots at each st a t i o n , and the fate of shoots i n each cohort i n terms of when they were r e c r u i t e d into the vegetative or reproductive phase of the population. The number of new shoots found at each sampling date = G, the number of shoots retained from past sampling dates = R, and the number of shoots l o s t = L. 193 Station Zostera 1 VEGETATIVE COHORT REPRODUCTIVE MAPPING DATE NUMBER May 12 1 T 19 R G L T R G L June 2 1 2 35 19 16 June 11 1 2 3 51.7 9.7 14 28 5 .2 4.3 4.3 June 23 1 2 3 4 81.7 2.7 9.7 23.7 45.7 3 2.3 4.3 10.3 4.3 4 2 July 10 1 2 3 4 5 97 2 13.7 37.3 44 2.7 1 3 8.3 22 8.3 6.7 7 2 Jul y 21 1 2 3 4 5 6 89 2.3 7.7 29 50 2 3 3.7 5 45 .7 8.3 26 10 8.3 6.7 6.3 August 6 1 2 3 4 •5 6 7 34.3 3.3 15 16 2.3 7.7 12 35 34.3 10.7 10 13.7 9 15.3 August 26 1 2 3 4 5 6 7 8 16 16 3.3 4 8 39.7 10.7 10 11 8 13.7 September 12 4 5 6 7 8 9 27.7 11.7 16 4.3 39.7 10.7 10 11 8 October 6 8 9 10 8.7 4.7 4 11.7 11.3 Zostera 1 (cnt'd) VEGETATIVE COHORT MAPPING DATE NUMBER October 21 8 9 10 11 T 7 R 3 4 G 0 L 1.7 T R G L December 3 8 9 10 11 12 6.3 2 4.3 3 2 January 6 10 12 13 5 3 2 2 1.3 February 6 12 13 14 5.3 1.7 2 1.7 1.3 March 3 12 13 14 15 4.7 1.7 3 1.7 .3 1.7 March 10 13 15 16 4.7 1.7 2 1 1 March 24 13 15 16 17 4.3 .3 1 3 1.3 2 A p r i l 9 13 16 17 18 3.7 1 2.7 .3 3 A p r i l 18 16 18 2 1 1 1.7 May 1 16 18 2 1 1 REPRODUCTIVE 195 Zostera 2 MAPPING DATE May 12 June 2 June 11 June 23 July 10 July 21 August 6 August 26 September 12 October 6 COHORT NUMBER 1 1 2 1 2 3 1 2 3 4 1 2 3 4 5 1 2 3 4 5 6 2 3 4 5 6 7 3 4 5 6 7 8 4 5 6 7 8 9 4 5 6 7 8 9 10 22.3 64.3 86.7 122.3 145.7 187.7 200.7 119 89.3 27 VEGETATIVE 22.3 16 11 46.3 2 36.3 27.3 20 26.3 53.7 4 42.7 42.7 7.3 28.7 86.2 3.7 42.7 52.7 19 30.3 17.3 48.3 29.3 56.7 6.3 4.7 10 2 45.7 98.3 78.3 20 22.7 27 2 2.3 1 3 20 2.7 3 3 4 14.3 6 12 1.7 3 24.7 19 6.7 2.7 19 25.3 8 17.7 REPRODUCTIVE 4.3 18.3 34.3 67 95.3 81.7 36 4.3 6.7 13 37.3 2 4 11.3 16 16 4.3 14 19.7 35.3 6 22 19 6.7 3.7 23.7 22.3 5 10 5 4.3 7.3 6.7 6.7 13 6 6 39.3 12.7 3 4 15 39.7 6.3 196 Zostera 2 (cnt'd) VEGETATIVE REPRODUCTIVE MAPPING DATE COHORT NUMBER T R G L T R G L October 21 7 8 2.3 1 21 10 9 10 11 2.3 27 1 4 December 3 9 10 11 12 3.3 1.3 2 1 1 January 6 11 12 13 3.7 1 1 1.7 .3 1 February 6 11 12 13 14 4.7 .7 1.3 2.7 .3 1 .3 March 10 11 13 14 15 5.7 1.3 4.3 .7 1.3 1.3 March 24 14 15 16 4.7 .3 1.3 3 1 3 A p r i l 9 14 15 16 17 8 3 5 .3 1.3 A p r i l 18 16 17 18 5 3 2 3 2 May 1 17 18 19 4 3 1 0 1 197 Zostera 3 VEGETATIVE MAPPING DATE May 12 June 2 June 11 June 23 July 10 July 21 August 6 August 26 September 12 October 6 COHORT NUMBER 1 1 2 1 2 3 1 2 3 4 1 2 3 4 5 2 3 4 5 6 3 4 5 6 7 2 3 4 5 6 7 5 6 7 8 9 6 7 8 9 10 18.3 25.7 49.7 70.7 79.3 116 124 134 72.7 26 R 14 3 9.3 3 4 26.7 4 10.7 18.7 3 4 41.7 12.7 59.3 9.7 5 31.7 49 11.7 18.3 11.7 37.3 37 46 67.3 52 87.7 23.7 4.3 11 2.3 2.7 10.3 10.3 18.3 4 2.3 1 4.3 3 3.7 3 5.3 12 9.7 5 4.7 38.7 29 12 REPRODUCTIVE 2.7 8.3 24.7 33.7 61.3 61.7 25 2.7 5.7 1.3 3 26.3 8.3 13.3 2.7 5.7 5.3 13.7 4 25.3 49 8.3 27 11.7 2.7 5.3 9.7 10.7 1.7| 7 21.31 4 22.71 26.3| 22 198 Zostera 3 (cnt'd) VEGETATIVE REPRODUCTIVE MAPPING DATE COHORT NUMBER T R G L T R G L October 21 7 8 9 10 11 2 2 0 8.3 11.7 3 1 1 13.3 11.7 December 3 10 11 12 4 2 2 1 January 6 10 12 13 4 1.7 1 1.3 .3 1 February 6 10 12 13 14 1 .3 .7 0 1.3 1 .7 March 3 10 13 15 4.7 .3 4.3 .3 .3 March 10 13 15 16 3.7 3 .3 .7 March 24 15 16 17 2.3 3 2 1.3 A p r i l 9 15 16 17 18 2 3 1 .3 2 A p r i l 18 16 18 19 1 1 2 May 1 16 18 20 1 1 Ruppia 1 199 MAPPING DATE May 12 June 2 June 11 June 23 July 10 July 21 August 6 August 26 COHORT NUMBER 1 1 2 1 2 3 1 2 3 4 1 2 3 4 5 1 2 3 4 5 (3 1 2 3 4 5 6 7 5 6 T 1.3 2.3 VEGETATIVE G 1.3 1.3 REPRODUCTIVE T _0 0 ~T7T 8.3 14 19.3 1.7 1.3 .3 .7 .3 4.3 7 1.7 1.3 .3 1.3 3 3.7 .71 1 1.71 5.7| 10 2 1.7 200 Ruppia 2 MAPPING DATE May 12 June 2 June 11 June 23 July 10 Ju l y 21 August 6 August 26 COHORT NUMBER 1 1 2 1 2 3 1 2 3 4 1 2 3 4 5 1 2 3 4 5 (3 2 4 5 6 7 2 4 5 6 7 13.7 11.3 15 6.7 VEGETATIVE G 3 3 4.3 ~3 4.3 .3 4.3 .3 2 1 3.7 10.7 1.3 3.7 8.3 1.3 .3 1.7 1 3.7 1 6.3 12.3 17 19.3 12 REPRODUCTIVE 6.3 4.7 6_ 2.3 6.3 3.7 3 4 3 1 6.3 1.7 3.7 1 3 1 10.7 1.7 1 3.3 1.3 1.3 3.3! 3.7 3 4 3 2 27.6 201 Ruppia 3 COHORT MAPPING DATE NUMBER May 12 June 2 June 11 June 23 July 10 July 21 August 6 August 26 September 12 1_ 1 2_ 1 2 3_ 1 2 3 4_ 1 2 3 4 5_ 1 2 3 4 5 6_ 2 3 4 5 6 7_ 5 6 7 6 7 VEGETATIVE 12.7 10.3 11.3 7.3 4.7 2.7 R 6 2.7 5.7 G 6 6.7 1.7 5.7 1.7 7.3 2.7 5.7 1.7 1 2.3 . .3 REPRODUCTIVE 7.7 18 24 9.3 1.7 6 1.7 3.7 1.7 5.7 4.7 4.3 2.7 1.3 .3 2 1.7 0 5.7 4.7 5.7 277 1.7 2.3 3.7 1.7 5.7 6 4.3 3.3 1.3 .3 Appendix 4. Averages and standard deviations of morphological data 203 Appendix 4. Averages and standard deviations of longest l e a f length (L.L.L.), rhizome internode length (R.I.L.), number of flowering 2 2 shoots per 0.1 m (#fl.sh./.lm ), and number of ovaries per inflorescence (#0./infl.) of Zostera japonica and Ruppia maritima at three stations ( s t a t i o n 1 = 3.17 m, 2 = 2.54 m, and 3 = 2.60 m CD t i d a l height) at Roberts Bank, 1980 - 1981. Zostera Date STATION 1 L.L. X L. S R.I. X .L. S //fl.sh. X /.lm 2 S iHnfl X . / f l . s sh • #0./infl X S • 15 May 10.5 4 .1 2.5 1 0 0 0 0 0 0 11 June 15.4 4.4 2.0 .6 7.3 2.5 3.2 .8 4. 3 1 10 July 8.4 2.7 1.2 .4 33.5 3.9 1.8 .8 4. 5 • 6 26 Aug. 3.8 1.3 . 3 .14 37 9.5 1.3 .6 1 4. .1 • 8 12 Sept. 3.0 1.2 .4 .1 0 0 0 0 0 0 6 Oct. 3.9 2.1 ' .4 .23 0 0 0 0 0 0 3 Dec. 3.0 1.5 .2 0 0 0 0 0 0 0 6 Jan. 5.1 2,2 . 3 .2 0 0 0 0 0 0 6 Feb. 3.4 1.5 .5 .2 0 0 0 0 0 0 10 Mar. 6.4 1.6 .9 .6 0 0 0 0 0 0 9 Apr. 8.1 2.3 1 .8 0 0 0 0 0 0 STATION 2 15 May 10.6 4.3 2.0 1.2 0 0 0 0 0 0 11 June 11.2 3.9 2.1 .6 2.2 1 2 .8 6.2 1. 2 10 July 23.4 6.4 2.6 .4 26.3 6.8 2. 3 2 6.3 1. 1 26 August 15.8 1.5 1.5 .5 81.2 19.7 5. 5 2 7.3 1. 1 12 Sept. 6.2 .5 .5 .2 82.3 16.5 4. 6 2. 2 6.5 8 6 Oct. 6.6 .7 .6 .1 18.2 4.1 2 • 6 6.1 2 3 Dec. 8.2 1.2 .3 .1 0 0 0 0 0 0 6 Jan. 6.9 2.0 .3 .5 0 0 0 0 0 0 6 Feb. 4.1 2.2 .3 .1 0 0 0 0 0 0 10 Mar. 6.5 2.6 1.2 1.5 0 0 0 0 0 0 ? Apr. 11.2 4.8 1.3 .9 0 0 0 0 0 0 STATION 3 15 May 11.8 1.6 2.2 1.4 0 0 0 0 0 0 11 June 12.6 3 2.4 .8 1.3 .8 1. .6 6 6.1 1. .3 10 July 18.3 6.2 2.2 .3 13.7 3.4 1. .8 96 6.0 .6 26 Aug. 12.3 1.7 1 .2 47.5 6.5 4. .1 1. 1 6.5 1 12 Sept. 8.2 3.4 .5 .3 61.6 10.2 3. .6 1 5.9 1. .1 6 Oct. 7.4 1.2 .5 .1 13 6.9 1. .6 4 6.2 1 .6 3 Dec. 6.6 2.1 .2 0 0 0 0 0 0 0 6 Jan. 6.3 2.5 .3 .2 0 0 0 0 0 0 6 Feb. 6.8 l.S .4 .3 0 0 0 0 0 0 10 Mar. 6.4 2.5 1.2 1.8 0 0 0 0 0 0 9 Apr. 10.4 6.1 1.5 1.1 0 0 0 0 0 0 205 Ruppia Date L. ,L.L. R.I. , L. #fl. ,sh./, ,1m2 #infl . / f l . s h . #0./infl. STATION 1 X S X s X S X S X s 15 May 6.3 1.3 .3 .1 0 0 0 0 0 0 11 June 6.8 2.1 1. .2 4.7 1. .6 2 .6 7.4 1.2 10 July 13.5 3.7 1.7 .3 16 3, .1 ' 4.3 1.4 6.5 1.5 26 Aug. 9 4.3 1 1 1.5 ,9 5.5 2 6.6 1.2 STATION 2 15 May 4.2 1.2 .3 .2 0 0 0 0 0 0 11 June 6.5 1.9 1.2 .2 9.3 2. .9 2.7 1.4 7.9 .5 10 July 16.7 2 1.7 .6 18.2 3. .1 5.3 1.8 7.4 .1 26 Aug. 8 3.1 1 .4 6 • 1. ,5 8.0 2.1 7.7 1.2 STATION 3 15 May 4.6 .8 . .3 .1 0 0 0 0 0 0 11 June 5.8 2 1 .3 5.9 1. ,6 3.5 1.4 7.9 .5 10 July 16 1.8 1.5 .3 21.9 6. . 3 4.7 2.4 7.6 .8 26 Aug. 7.8 2.3 1 .2 10.2 2. 3 7.5 3.2 7.6 1.2 

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