Feeding rates and productivity of the copepod Acartia bifilosa in a highly turbid estuary; the Gironde (SW France)

Acartia spp. are the dominant copepod species in the Gironde estuary, seaward of the turbidity maximum area. Acartia bifilosa develop a large population in spring and early summer whereas Acartia tonsa appear in late summer. High values and high variability of chlorophyll a/suspended particulate matter ratio are found seaward of the turbidity maximum area. Feeding rates of A. bifilosa were measured by fluorometry. Phytoplankton ingestion was found to be highly variable, between 8 to 80% of copepod carbon body weight. Except for adult females, copepods were heavier in summer than in winter. PB ratios, estimated by the instantaneous growth rate method, varied from 0.03 d^-1 to 0.14 d^-1. The gut contents and P/B ratios of Acartia bifilosa were related to chl a/SPM ratio. From those data, and a few obtained for A. tonsa, it is concluded that only in summer months phytoplankton ingestion is enough to maintain secondary production.