INSITU COPEPOD FEEDING AND GRAZING RATES DURING A SPRING BLOOM DOMINATED BY PHAEOCYSTIS SP IN THE ENGLISH-CHANNEL

Gut pigment contents, ingestion rates and abundances of dominant copepod species were analysed from March to May 1990 before and during a spring bloom dominated by Phaeocystis sp. in coastal waters off Plymouth (south-western coast of England). Three different fractions of phytoplankton were considered (0.2-2, 2-10 and > 10-mu-m) as well as three size classes of copepods (200-350, 350-710 and > 710-mu-m). The gut fluorescence method was applied to estimate feeding rates in the different copepod size fractions. The dominant species in each one were Calanus helgolandicus, Pseudocalanus elongatus and Oithona sp. respectively. The results showed lower gut pigment contents, ingestion rates and copepod abundances during the Phaeocystis bloom when compared with the previous period of diatom dominance. Community daily grazing rates calculated by using the average individual ingestion rates and the estimated copepod abundances in each size fraction. decreased dramatically during the Phaeocystis bloom mostly due to the decrease in copepod abundances. Grazing impact on phytoplankton, estimated by comparing community grazing rates with phytoplankton standing stock (total chlorophyll a and the fraction > 10-mu-m), accounted only for 3 and 0.3% of total chlorophyll a and 7 and < 1% of the fraction > 10-mu-m, respectively, before and during the Phaeocystis bloom; this grazing being mainly due to the contribution of large and medium size copepods, These results suggest that Phaeocystis has a negative effect on copepods which might indirectly contribute to the development of Phaeocystis blooms because of the reduced grazing pressure and decrease in copepod abundances.