SUSTAINED FECUNDITY WHEN PHYTOPLANKTON RESOURCES ARE IN SHORT SUPPLY - OMNIVORY BY CALANUS-FINMARCHICUS IN THE GULF OF ST-LAWRENCE

Despite low ambient concentrations of phytoplankton, Calanus finmarchi cus sustained relatively high rates of egg production (11-45 eggs fema le-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 observ ed in high-chlorophyll waters of the lower St. Lawrence estuary. Contr ary to classical, linear food-chain models, variations in egg producti on of C. finmarchicus were therefore not predictable from variations i n standing stocks of autotrophic cells. Egg production rates were inde pendent of the concentration of chlorophyll in situ, the abundance of autotrophic microplankton cells, in situ gut fluorescence, and the rat e of ingestion of autotrophic particles. Heterotrophic microplankton-i ncluding ciliates and dinoflagellates presumed to be heterotrophic-con stituted a disproportionate fraction of the ration of C. finmarchicus in the open Gulf of St. Lawrence, despite their relatively low cell de nsities. Evidence suggests that heterotrophic microplankton frequently provided a prey resource sufficient for net lipid synthesis as well a s for egg production.