FISHERY BULLETIN; VOL. 74, NO. 4 



in FLTS and statistically valid estimates of means 

 and variances on a stratum found. Furthermore, 

 no greater physical effort is required in SSS than 

 in FLTS. SSS also provides a method to quantify 

 species-presence gradients. Hence, SSS is free of 

 some of the disadvantages of FLTS while main- 

 taining the advantages often ascribed to FLTS 

 plans. 



There is a sediment gradient in the bay in the 

 sense of a gradual increase in coarseness (silt to 

 sand) south to north over the four areas for all 

 seasons. Over all seasons, subarea IB generally 

 contained the largest number of bivalve species 

 and, were it not for the abundance of Transen- 

 nella tantilla (which is discussed later), subarea IB 

 would have the largest number of bivalves also. In 

 addition, in almost all seasons subarea IB con- 

 tained the largest number of individuals and 

 species of polychaetes. Thus, there is a distribution 

 in bivalve and polychaete presence, from high 

 density and species numbers to low as the 

 sediment becomes more coarse. The sediment 

 composition, as measured by average percentage 

 composition by weight of various grain sizes, is a 

 necessary factor to consider in predicting 

 macrofauna population dynamics, but it is not a 

 sufficient predictor by itself. This viewpoint is 

 based on the necessity of employing qualitative 

 information concerning the types of material 

 retained by the L981-mm sieve (see Results sec- 

 tion), and the role we attribute to the algae 

 Enteromorpha sp. in the population dynamics of T. 

 tantilla (see later discussion). 



Newell (1965) found a higher number of 

 microorganisms in areas composed of finer grades 

 of sediment and an associated higher number of 

 the deposit feeders {Hydrohia ulvae and Macoma 

 balthica). He concluded that the large number of 

 microorganisms was a result of the greater sur- 

 face area of fine sediment grades which is related 

 to the amount of organic nitrogen (protein) 

 available to deposit feeders. The polychaete data 

 from Garrison Bay, and subsequent statistical 

 analyses, suggest that Newell's (1965) hypothesis 

 can be extended to incorporate a statement about 

 the biological effects of different sediment compo- 

 sitions in the presence of temporal heterogeneity. 

 Recall that the sediment data show that subarea 

 4A, the most exposed subarea, experiences greater 

 interseason fluctuations than does subarea IB, the 

 most sheltered subarea. Furthermore, the poly- 

 chaete assemblage in subarea IB shows the small- 

 est seasonal fluctuation with regard to both total 



numbers of individuals and species as compared 

 with subarea 4A. Subareas 2A and 3A also show 

 smaller seasonal variations in both polychaete 

 assemblage and sediment composition than does 

 subarea 4A. All of this suggests that mixed fine 

 sediment grades (silty areas) may act as insulators 

 for certain infauna against seasonal stresses. That 

 is, fine sediments with their larger total surface 

 area to volume ratio retain larger quantities of 

 nutrients (organic nitrogen) and hold more inter- 

 stitial water. If the areas composed chiefly of fine 

 sediment grades occur in the cul de sac of an 

 embayment, where wave action is minimal, then 

 these areas are more likely to retain larger 

 numbers of individuals and species than other 

 areas within the embayment. Thus, despite the 

 periodic fluctuations in many environmental pa- 

 rameters of the intertidal zone, a constant 

 sediment particle composition contributes to a 

 high degree of environmental predictability. 

 Slobodkin and Sanders (1969), Levinton (1972), 

 and Gray (1974) considered aspects of the con- 

 sequences of temporal predictability for deposit 

 and suspension feeders. 



The bivalves and polychaetes listed in Tables 2 

 and 3 represent both suspension and deposit 

 feeders. Rhoads and Young (1970) advanced the 

 hypothesis that animals of one trophic level 

 modify the environment and affect the dynamics 

 of members of another trophic level, and called it 

 trophic group amensalism. They found suspension 

 feeders in the subtidal to be generally restricted to 

 sandy or firm mud bottoms, and deposit feeders to 

 be more numerous in soft silty substrates. The 

 polychaete results generally support this 

 hypothesis. An exception noted by Young and 

 Rhoads (1971) was the case in which it was 

 hypothesized that tube-building polychaetes (both 

 suspension and deposit feeders) make it possible 

 for higher densities of bivalve and polychaete 

 suspension feeders to coexist with deposit feeders 

 in silty sediments because of their ability to bind 

 particles together and thereby stabilize 

 sediments. This hypothesis may be useful in 

 explaining why suspension feeders, e.g., the tube 

 builder Oiceniafusiformis and the members of the 

 Veneridae, are so numerous in subarea IB, as well 

 as why the tube building terebellid Thelepus 

 cripus, a surface level deposit feeder, reaches its 

 maximum density in subarea IB. The combination 

 of tube building coupled with the feeding behavior 

 of suspension feeders may provide these organ- 

 isms a survival advantage in this otherwise soft 



946 



