DIEL FEEDING-BEHAVIOR AND INGESTION RATE IN THE COPEPOD CALANUS-FINMARCHICUS IN THE SOUTHERN GULF OF MAINE DURING LATE SPRING

In situ feeding was measured on late stage Calanus finmarchicus in the southern Gulf of Maine during late spring 1988 and 1989. The region i s a spring feeding ground for the planktivorous right whale, Eubalaena glacialis. Measurements in 1988 were made during the declining spring bloom, when phytoplankton showed considerable spatial variability. 19 89 observations were made after summer stratification had become estab lished, when phytoplankton biomass was low and dominated by the <7 mu m size fraction. In situ ingestion rates of C. finmarchicus reflected these differences in food, and ranged from 30.2% body C day(-1) at a s pring bloom station to 0.6% body C day(-1) after stratification. These values compare with a maximum ingestion rate of 44.3% body C day(-1), measured in a shipboard grazing experiment using cultured algae as fo od, and indicate that C. finmarchicus became food limited in the south ern Gulf of Maine after stratification. Calanus finmarchicus diel feed ing behavior and diel vertical migration patterns changed during the s easonal progression from spring bloom to summer stratification. Diel f eeding rhythms and vertical migration were absent during the bloom, bu t developed ast eh bloom declined. During the post-bloom stratified co nditions, diel feeding rhythms continued but vertical migration ceased . Most of the C. finmarchicus population remained near the surface, wh ile a small population of nonmigratory individuals resided at depth an d appeared to be feeding upon sedimented spring bloom diatoms. At the spring bloom station C. finmarchicus ingested only a small proportion of the phytoplankton standing stock in the >7 mu m size fraction (2.9% day(-1)), reflecting the high biomass of phytoplankton and the stage composition of the C. finmarchicus population, dominated by early cope podite states with low biomass. In contrast, at the post-bloom station s in 1989, the dense surface aggregations of C. finmarchicus populatio ns were dominated by late copepodite stages whose total biomass was co mparable to that of the phytoplankton, and C. finmarchicus consumed a significantly larger fraction of the >7 mu m phytoplankton standing cr op (up to 62.5% day(-1)).