way at Virginia Institute of Marine Science. The 

 relative magnitudes of inputs and losses are not yet 

 known, but some nutrient losses are probable. 

 The zooplankton compartments of the concep- 

 tual Bay model also contain icthyoplankton and 

 larvae of benthic organisms. Any animal feeding on 

 zooplankton can also consume larval fishes and 

 benthos, thus playing a role in regulation of those 

 populations. All larval fishes are zooplankton 

 feeders, and may be a very significant factor in the 

 trophic dynamics of the Bay system. 



BENTHOS 



Benthic organisms are important in the flow 

 dynamics of C, N, and 2 in the Bay (D. Boesch, 

 pers. coram.). 



Eelgrass (Zostera) communities cover much ol 

 the shallow bottom from mean low water to about 

 2 m depth in the upper mesohaline and polyhaline 

 areas of the Bay (Orth 1975). Other seagrass spe- 

 cies, generally more abundant on the eastern shore 

 where there are wide shallow areas, include 

 Potamogeton and Vallisneria in fresh and brackish 

 water, and Ruppia in middle and higher salinities 

 (Lippson 1973). 



Seagrasses (fig. 5) provide structure and habitat 

 for epiphytic plants and a diverse epifauna of 

 amphipods, isopods, barnacles, tunicates, poly- 

 chaetes, and gastropods (Marsh 1973, 1976). 

 Macrofauna consume about 55 percent of the net 

 production of eelgrass, phytoplankton, and benthic 

 algae of a Zostera community, with the rest avail- 

 able to bacteria, microfauna, and meiofauna 

 (Thayer et al. 1975). There is little grazing pressure 

 on the leaf blades (Zieman 1975, Marsh 1970), 

 which enter the food chain as detritus. Benthic in- 

 fauna densities are higher in Zostera communities 

 than any other benthic habitat in Chesapeake Bay 

 since the grass stabilizes the sediments (Orth 1973). 

 Seagrass communities also provide protection for 

 larval and juvenile fishes and blue crabs in soft and 

 peeler stages, as well as food for fishes, crabs, 

 shrimps, and water birds. Fishes associated with 

 eelgrass [e.g., anchovies (Anchoa spp.) in summer, 

 spot (Leiostomus zant hunts) and silver sides 

 (Antherinidac) in winter] feed on detritus, plank- 

 tonic copepods, and epifaunal crustaceans, deriving 

 about half of their nutrition from the seagrass com- 

 munity (Adams 1976a, b, c). 



The cownosc ray {Rhinoptera bonasus) tends 



to uproot seagrasses as it grubs for benthic or- 

 ganisms (Orth 1975). 



Oyster reefs constitute another type of benthic 

 community (Larsen 1974). Oysters feed on dino- 

 flagellates and detritus, and possibly bacteria and 

 lipids; clams feed on coarser particles (D. Haven, 

 pers. comm.). Oysters, clams, and other filter 

 feeders remove sediments and detritus from the 

 water column much faster than possible through 

 sinking alone. Fecal pellets make the material avail- 

 able to benthic grazers. Biodeposits on the sedi- 

 ment surface are enriched by bacteria, and turned 

 over by sediment mixers, such as shrimp and 

 worms (D. Haven, pers. comm.). 



Benthic organisms also are present in sandy and 

 muddy bottoms without seagrasses or oyster-reef 

 structure (see fig. 6). Standing stocks are lower, 

 but turnover may be very rapid. Exclosure studies 

 indicate that blue crabs and some demersal fishes 

 are voracious predators, and may control benthic 

 standing stocks on unprotected bottoms (Boesch et 

 al. 1976, Virnstein 1976). Benthic populations 

 may also be partly controlled by predation on 

 planktonic reproductive stages during the summer. 

 A bimodal spring and fall setting pattern is com- 

 mon, with setting reduced during the summer 

 when predation by ctenophores and fishes on zoo- 

 plankton is highest (D. Boesch, pers. comm.). 



The distinction between shallow and deep parts 

 of the bay is more obvious for benthos. Benthic 

 communities are characterized by surface and sus- 

 pension feeders in shallower waters, and deposit 

 feeders in deeper waters. Predation on infauna may- 

 be the most important controlling factor for shal- 

 low populations, and competitive interference is a 

 controlling factor in deeper waters. Species also tend 

 to be associated with sediment type; shallow 

 sediments are usually sand, and deeper sediments 

 are usually mud. In the deep channels experiencing 

 anoxic conditions due to stratification of the water 

 column, benthos are depauperate (D. Boesch, pers. 

 comm.). Presumably, low oxygen conditions also 

 exclude fish and other mobile organisms lrom 

 some of the deeper waters of the Bay during the 

 summer (Haefncr 1971). 



Biological and chemical processes in the sedi- 

 ments are thought to be important in regulating 

 the abundance and chemical form of N and P in 

 the water column. More research is necessary to 

 elucidate the mechanisms as well as the magnitude 

 of these processes (K. Webb, pers. comm.). 



8 



