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 biomarkers to study trophic relationships between phytoplankton and zooplankton. These markers permit the characterization of both suspended matter and copepods, 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 longicornis, Acartia clausi and Pseudocalanus elongatus. Our study was performed 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-forming diatoms, Cryptophytes and dinoflagellates). Our results suggest that all species selectively grazed on Phaeocystis sp. and non-selective v on diatoms. Dinoflagellates were avoided by all species. Temora longicornis selectively grazed on Cryptophytes, which may be related to the nutritional value of this algae. The fatty acid composition of the three copepod species indicated an 'herbivorous' diet for Pelongatus and an omnivorous one for A. clausi and T longicornis, which is less opportunist.