GRAZING PRESSURE OF THE CALANOID COPEPOD TEMORA-LONGICORNIS ON A PHAEOCYSTIS DOMINATED SPRING BLOOM IN A DUTCH TIDAL INLET

Between 30 March and 11 May 1990, total copepod abundance and the abundance, biomass and gut fluorescence of Temora longicornis were determined and related to the abundance and succession of phytoplankton development in a Dutch tidal inlet. Gut pigment values were highest in females and lowest in young copepodites, but weight-specific pigment concentrations were about similar. Pigment levels measured in the guts were relatively high at the beginning and end of the period of investigation when diatoms dominated the phytoplankton community, and low during the Phaeocystis dominated period, when ambient chlorophyll concentrations were highest. For the latter period, calculated ingestion rates in T. longicornis were low and estimated daily consumption amounted to less than 1% of the phytoplankton standing stock, suggesting a negligible grazing impact on the development of the Phaeocystis bloom. In spite of the low grazing on phytoplankton, T. longicornis biomass increased by one order of magnitude. The discrepancy between low grazing pressure and copepod development is explained by assuming that T. longicornis switched to heterotrophic food: a bloom of ciliates present during the Phaeocystis dominated period.