Copepod grazing in a subtropical bay: species-specific responses to a midsummer increase in nanoplankton standing stock

Ingestion rates of 4 small copepod species (Oithona simplex, O. nana, Acrocalanus inermis and Parvocalanus crassirostris) were investigated in Kaneohe Bay, Hawaii, during a midsummer increase of the pico- and nanoplankton communities. There was no evidence that adult female copepods fed significantly on picoplankton-sized cells. However, all the species responded behaviorally to variations in the concentration (10 to 110 μg C l–1) and size spectrum (relative increase of cells >5 μm) of nanoplankton prey. The copepods generally behaved as opportunistic particle feeders, demonstrating higher consumption rates on the most abundant cells (2–5 μm nanoplankton); however, autotrophs were usually selected over heterotrophs of similar size. Maximum ingestion rates were similar for the 2 calanoids and O. nana (around 120 000 cells copepod–1 d–1) and lower for O. simplex (around 40 000 cells copepod–1 d–1), but biomass-specific rates of O. simplex equaled those of the other species. At the highest nanoplankton concentrations, the ingestion rates of copepods appeared saturated, daily rations ranging from 100% body C d–1 for A. inermis to 260% body C d–1 for P. crassirostris. The differences between ingestion rates measured as cells per copepod per day and those converted to carbon suggested that ingestion might be held below potential by the cumulative handling times of individual prey rather than the physiological constraints of food consumption and digestive processing.