COPEPOD GRAZING DURING A MESOCOSM STUDY OF AN EMILIANIA-HUXLEYI (PRYMNESIOPHYCEAE) BLOOM

Five bottle incubation experiments were done to investigate the grazin g and food selection by the copepod Calanus finmarchicus (Gunnerus) du ring a phytoplankton bloom in a N and P fertilized mesocosm. The phyto plankton biomass was dominated by diatoms and lager dinoflagellates (> 35 mum), while the haptophyte Emiliania huxleyi (Lohmann) Hay et Mohl er dominated numerically (up to 10 . 10(3) ml-1). The concentration of dinoflagellates (< 35 mum) was always low (less-than-or-equal-to 39 m l-1). The ciliate abundance in the mesocosm was within the lower end o f previous recordings at the start of the period (3.8 ml-1), and decre ased throughout the bloom. The copepod C. finmarchicus preferentially fed upon ciliates and dinoflagellates (< 35 mum), and selective copepo d predation may have controled the development of these protists in th e mesocosm. However, diatoms made up the largest part of the copepod c arbon ingestion due to their dominance in the plankton. The haptoiphyt es E. huxleyi and Phaeocystis cf. pouchetii were not preferred food ty pes, and never made up a significant part of the copepod diet. However , there was a significant grazing on small single cells, presumably sw arming Phaeocystis, at the end of the bloom.