Grazing activity of mesoplanktonic copepods in a shallow bay during an algal spring bloom (Marennes-Oleron Bay, France)

In situ grazing activity of mesoplanktonic copepods was investigated by the fluorometric method during an algal spring bloom in a zone of oyster farmi ng in Marennes-Oleron Bay The grazing activity of copepods was overall high er during the night than during the day for three species (Temora longicorn is, Paracalanus parvus and Acartia clausi), but peaks also appeared during the day for all of them. Individual ingestion rates and daily rations were higher during neap tide :low suspended particulate matter (SPM) concentrati ons) than during spring tide thigh SPM concentrations). During ebb tide (wh en SPM concentrations were the lowest) the feeding activity of T. longicorn is, P. parvus and A. clausi was significantly negatively correlated with al gal concentration. Our work suggests that during the alg al spring bloom in the farming area of Marennes-Oleron Bay the pressure exerted by mesoplankt onic copepods on the algal stock was very low, as a consequence of: (i) hig h algal concentrations in the field (resuspension and high phytoplanktonic production); (ii) low ingestion rates when high algal concentrations were o bserved. The likely ingestion of non-fluorescent particles by copepods is d iscussed.