Grazing by the copepods Calanus helgolandicus and Acartia clausi on the protozooplankton community at station L4 in the Western English Channel

Seasonal variation in community composition, abundance and biomass of protozooplankton (PZP) was investigated at L4 in the English Channel from 2002 to 2004 and the importance of feeding of Acartia clausi and Calanus helgolandicus on natural populations of phytoplankton and PZP was investigated between May and August, 2002. Calanus helgolandicus ingested between 1 and 12 µg phytoplankton carbon ind.−1 day−1 (2–13% total body carbon); A. clausi ingested between 1 and 6 µg C ind.−1 day−1 (20–166% total body carbon). Clearance rates on PZP prey increased linearly with increase in food concentration and ranged from 120 and 304 mL ind.−1 day−1 and between 26 and 150 mL ind.−1 day−1 for C. helgolandicus and A. clausi, respectively. Calanus helgolandicus obtained 2–27% of its daily ration as a percentage of body carbon from phytoplankton and 2–17% of its daily ration from PZP. Acartia clausi obtained 22–161% of its daily ration from phytoplankton and 3–44% daily ration from PZP. Calanus helgolandicus and A. clausi populations together grazed up to 21% of the total PZP standing stock daily. Both C. helgolandicus and A. clausi demonstrated selective feeding on large choreotrich ciliates and heterotrophc dinoflagellates. Our results support the hypothesis that PZP comprise a significant proportion of the copepod diet after the spring bloom when the phytoplankton community is dominated by small cells.