EGG-PRODUCTION OF THE COPEPOD ACARTIA-TONSA IN FLORIDA BAY DURING SUMMER - 1 - THE ROLES OF FOOD ENVIRONMENT AND DIET

The diet and egg production rate of Acartia tonsa were measured during the thermally stable period between June and October 1995 at four loc ations in inner and outer Florida Bay. We sought to characterize the r ole of A. tonsa in the bay's pelagic food web, which has been changing since 1987, when the dominant submerged vegetation began shifting fro m benthic seagrasses to planktonic algae. At Rankin Lake, a shallow ba sin on the north side of the inner bay, where extensive seagrass morta lity and persistent cyanobacteria blooms have occurred, microplankton biomass was relatively high and dominated by heterotrophic protists an d dinoflagellates. Nanoplankton at Rankin Lake, while numerically abun dant, usually contributed only a small portion of the biomass. The ing estion rate of A. tonsa in Florida Bay varied independently of food co ncentration.(i.e., total microplankton biomass), but rates were higher (mean +/- SD = 3.88 +/- 0.73 mu g C copepod(-1) d(-1)) on the north s ide of the bay than on the south side (0.78 +/- 0.11 mu g C copepod(-1 ) d(-1)). Microzooplankton and dinoflagellates were important dietary constituents, especially in the vicinity of Rankin Lake. Egg productio n in this region (mean I SD = 14.2 +/- 7.7 eggs female(-1) d(-1)) was considerably higher than the baywide mean (5.8 +/- 0.81 eggs female(-1 ) d(-1)), and principal components analysis revealed associations betw een egg production and both dietary microzooplankton and dinoflagellat e biomass. However, although grazing rates were relatively high in the inner bay, A. tonsa removed only 1-6% of the primary production from the water column during the summer and its egg production rates were l ow relative to typical rates for the species.