Nutritive value and selection of food particles by copepods during a spring bloom of Phaeocystis sp in the English Channel, as determined by pigment and fatty acid analyses

In this study phytoplankton pigments and fatty acids were used as biomarker s to study trophic relationships between phytoplankton and zooplankton. The se markers permit the characterization of both suspended matter and copepod s, allowing examination of the transfers from food to zooplankton. A drogue study was carried out to follow a water mass in the coastal waters off the eastern English Channel over a 3-day period, with samples collected every 3 h. The study focused on the dominant calanoid copepod species: Temora lon gicornis, Acartia clausi and Pseudocalanus elongatus. Our study was perform ed during the spring phytoplankton bloom when solitary cells of Phaeocystis SP. formed 90% of the total phytoplankton. Fatty acid analyses provided an indication of the low nutritive value of these algal cells; in contrast to other algal species which had higher nutritional value (e.g. colony-formin g diatoms, Cryptophytes and dinoflagellates). Our results suggest that all species selectively grazed on Phaeocystis sp. and non-selective v on diatom s. Dinoflagellates were avoided by all species. Temora longicornis selectiv ely grazed on Cryptophytes, which may be related to the nutritional value o f this algae. The fatty acid composition of the three copepod species indic ated an 'herbivorous' diet for Pelongatus and an omnivorous one for A. clau si and T longicornis, which is less opportunist.