Zooplankton of the bay are dominated by two species of Acartia 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 comsumed 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. 
362 
