Grazing by Calanus helgolandicus and Para-Pseudocalanus spp. on phytoplankton and protozooplankton during the spring bloom in the Celtic Sea

Feeding rates and selectivity of the calanoid copepods Calanus helgolandicus and Para-Pseudocalanus spp. on natural assemblages of microplankton were evaluated in the English Channel and western Celtic Sea during non-bloom and bloom conditions in April 2002. Ingestion rates of total chlorophyll-a were low at non-bloom stations where the phytoplankton community was dominated by cells < 5 mu m in length and higher during the bloom when the > 5 pm size fraction was dominant. Protozooplankton contributed to the copepod diet in all experiments, C. helgolandicus clearance and ingestion rates were highest for the ciliate Myrionecta rubra (626-1347 ml cop(-1) d(-1); 0.3-27 mu g C cop(-1) d(-1)). C. helgolandicus ingested between 1 and 18 mu g C cop(-1) d(-1) (1-12% body C) from phytoplankton+protozooplankton food sources. The total carbon ingested by Para-Pseudocalanus spp. was lower (0.5-6 mu g cop(-1) d(-1)) but this was equivalent to between 6 and 78% of body carbon being ingested daily. Our data suggest that C. helgolandicus selected prey according to size; this was not the case for Para-Pseudocalanus spp. which became more selective as chlorophyll-a concentration increased. Grazing impact of the entire copepod community on protozooplankton was assessed. We found that at non-bloom stations between 12 and 17% of the protozooplankton community was being removed daily by the copepod community, whereas during the peak of the bloom the proportion being removed daily was only 2%. We conclude that during the spring bloom period copepods gained the majority of their carbon from phytoplankton ingestion but during non-bloom periods, protozooplankton and the ciliate M. rubra made a significant contribution to copepod diet. (c) 2007 Elsevier B.V. All rights reserved.