EGG-PRODUCTION BY THE CALANOID COPEPOD ACARTIA-TONSA IN THE MESOHALINE CHESAPEAKE BAY - THE IMPORTANCE OF FOOD RESOURCES AND TEMPERATURE

Egg production of Acartia tonsa females was measured simultaneously with grazing and other biological and physical parameters in the mesohaline portion of Chesapeake Bay, USA, during periods of 1987 and 1988. We hypothesized that egg production would be related to food abundance in situ, depending principally on phytoplankton in spring and microzooplankton in summer. Step-wise multiple regression analysis indicated that egg production (female C d-1) was significantly correlated with temperature (parabolic transform), protozoan microplankton biomass, and the C:N ratio of suspended particulate matter. Egg production was not correlated with any measures of phytoplankton biomass, production or ingestion. Egg production was significantly higher at night while there was no observed diel trend in ingestion of phytoplankton by A. tonsa. The amount of carbon ingested as phytoplankton was adequate to support calculated growth and metabolic requirements of A. tonsa females roughly half the time. Carboy experiments were used to compare the relative amounts of phytoplankton and microzooplankton carbon ingested by A. tonsa in the bay on 2 occasions in May and August of 1988. Copepod carbon-specific ingestion rates on phytoplankton were low [5.5 mug C (mg C)-1 h-1] in May, but relatively high on microzooplankton [22.7 mug C (mg C)-1 h-1]. In August, microzooplankton were ingested at lower rates compared to May [7.2 mug C (mg C)-1 h-1] but phytoplankton were ingested at much higher rates [48.2 mug C (mg C)-1 h-1]. For both experiments, the total C ingested as phytoplankton and microzooplankton was sufficient to balance the calculated respiration and measured egg production requirements of A. tonsa females. From these results we conclude that microzooplankton can provide an important food source for estuarine copepods, and that temperature and microzooplankton rather than phytoplankton abundance may be the best indicators of A. tonsa reproductive potential in Chesapeake Bay.