Algal grazing by the planktonic copepodsCentropages hamatusandPseudocalanussp.: Diurnal and seasonal variation during the spring phytoplankton bloom in the Øresund

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Abstract
Seasonal and diel variation in rate of algal grazing were estimated from measurements of gut content (plant pigments) and gut turnover in the copepods Centropages hamatus and Pseudocalanus sp. during spring (Januar–May) in the Øresund. Both species exhibited significant diel variation in gut content and ingestion rate at the three depths studied (5, 10 and 22 m), with the highest ingestion rates and gut contents during night. The variation was most pronounced in March, but almost insignificant in April. Since the copepods did not migrate vertically, the observed pattern is due to a variable feeding activity rather than caused by continuous feeding at varying food concentrations. We found a positive correlation between ambient algal concentration and algal ingestion rate and gut content for both species in weekly morning samples. The results indicated satiation of the ingestion rates at high algal concentrations. Maximum algal ingestion rates measured in the field were similar to maximum ingestion rates measured in the laboratory in C. hamatus at the same temperatures. However, due to the circadian feeding rhythm, the daily rations estimated in the field were considerable less than corresponding maximum daily laboratory rations, and were in addition relatively constant (∼6–12 ng chlorophyll-a · ind−1 · d−-1) and independent of depth and ambient algal concentration. An assessment of the total mesozooplankton algal grazing pressure, based on measured zooplankton densities and estimated algal rations, showed that less than 1–5% of the phytoplankton primary production was channeled through mesozooplankters in the Øresund during the period studied.