Sustained fecundity when phytoplankton resources are in short supply: Omnivory by Calanus finmarchicus in the Gulf of St. Lawrence

Abstract

Despite low ambient concentrations of phytoplankton, Calanus finmarchicus sustained relatively high rates of egg production (11–45 eggs female−1 d−1) in the open waters of the Gulf of St. Lawrence in late June–early July. These rates were comparable to egg production rates observed in high‐chlorophyll waters of the lower St. Lawrence estuary. Contrary to classical, linear food‐chain models, variations in egg production of C. finmarchicus were therefore not predictable from variations in standing stocks of autotrophic cells. Egg production rates were independent of the concentration of chlorophyll in situ, the abundance of autotrophic microplankton cells, in situ gut fluorescence, and the rate of ingestion of autotrophic particles. Heterotrophic microplankton—including ciliates and dinoflagellates presumed to be heterotrophic—constituted a disproportionate fraction of the ration of C. finmarchicus in the open Gulf of St. Lawrence, despite their relatively low cell densities. Evidence suggests that heterotrophic microplankton frequently provided a prey resource sufficient for net lipid synthesis as well as for egg production.