Selective feeding on natural phytoplankton by Calanus finmarchicus before, during, and after the 1997 spring bloom in the Norwegian Sea

Selective feeding by the calanoid copepod Calanus finmarchicus was investigated during a 3-month study in spring 1997 at a permanent station in the Norwegian Sea (Sta. M, 66°N, 02°E). Phytoplankton biomass increased from 317 ng chlorophyll a (Chl a) liter^-1 in the prebloom phase to 2,095 ng liter^-1 during the bloom and declined after the bloom to 1,260 ng Chl a liter^-1. In the prebloom phase, clearance rates of C. finmarchicus females were between 22 and 100 ml copepod^-1 d^-1, while during the bloom, they ranged from 75 to 92 ml copepod^-1 d^-1, with a decline in the postbloom phase (41 ml copepod^-1 d^-1). After the phytoplankton bloom, the C. finmarchicus population was dominated by copepodid stages CIV and CV, with clearance rates ranging from 7 to 23 ml copepod^-1 d^-1. Grazing rates of adult females on the phytoplankton standing stock were low in the prebloom phases (5-23 Chl a copepod^-1 d^-1, respectively), increased during the bloom from 82 to 219 ng Chl a copepod^-1 d^-1, and declined after the bloom (64 ng Chl a copepod^-1 d^-1 for adult females and 7-27 ng Chl a copepod^-1 d^-1 for copepodid stages CIV and CV). C. finmarchicus showed a selection for diatoms throughout the study period and for dinoflagellates, before and after the spring bloom, despite the low concentration of both groups in the pre- and postbloom phases. During the postbloom period, no differences were observed in the selective feeding behavior of the copepodid stages compared to the adults. The contribution of diatoms to the overall phytoplankton biomass was 8 and 14% in the pre- and postbloom periods, respectively, while dinoflagellates were ≤3%. Haptophytes (dominated by Phaeocystis pouchetii) and cryptophytes were ingested according to their abundance. Avoidance of cyanobacteria (Synechococcus spp.), pelagophytes, and 'green algae' was observed throughout the study period.