Zooplankton of the bay are dominated by two species of A cart ia which 

 switch dominance depending on season. They are generally present in greatest 

 biomass in late spring. During summer they are heavily grazed by larval fish, 

 menhaden and ctenophores. The benthos of the bay consists mostly of 

 heterotrophic soft bottom communities with Mediomastis sp. and Nucula sp. 

 dominating numerically. Several areas of the bay have communities dominated 

 by amphipods; where coarser sediments occur, large bivalves such as Mercenaria 

 mercenaria and Pitar morrhuana may provide the most biomass. 



A eutrophication gradient exists in Narragansett Bay due to sewage inputs 

 from the Providence River (about 380,000 m-^/day). However, the lower bay is 

 relatively clean and the water quality excellent. Average primary productivity 

 at one station in the Bay, mostly due to phytoplankton, has been estimated to 

 be 308 g C/yr (4) of which 45 percent may be consumed by the benthos (12). 



Narragansett Bay, as well as much of the marine coastal waters of the 

 northeast coast of the United States, is characterized by ecosystems in which 

 most of the photosynthesis is carried out by phytoplankton, but in which the 

 benthos plays an important part in the total cycling of energy and nutrients. 

 The microcosm tanks were designed to maintain ecosystems functioning in a 

 similar manner. The stirrers were designed to direct turbulent energy onto the 

 sediments, thus effecting a resuspension of flocculent material. The tanks are 

 exposed to natural sunlight, and their temperature regime follows that of the 

 bay within a few degrees. 



Description of microcosms 



A brief description of the facility was presented by Pilson et al. (1977). 

 Twelve fiberglass tanks are set up outdoors on land adjacent to a laboratory 

 building. Figure 24-2 and Table 24-1 provide information on the tanks and 

 some physical characteristics of the systems. All piping to the tanks is PVC or 

 fiberglass, and water is pumped from a pier 30 m offshore by a diaphragm 

 pump that appears to be non -destructive to plankton. 



Sediment in the microcosms is held in fiberglass containers in the bottom 

 of each tank. The containers were filled with sediment collected north of 

 Conanicut Island (near "benthic station," Figure 24-1). An attempt was made 

 to place the sediment in the right orientation in the containers, but inevitably 

 considerable mixing occurred. Nevertheless, the major features of the benthic 

 community in the tanks were similar to those in the bay during the period of 

 the experiment (F. Grassle, personal communication). 



Nine of the tanks were first filled during August, 1976, and maintained on 

 a flow-through regime (330 ml/min) giving a turnover time of about 27 days. 



364 



