Low grazing impact of mesozooplankton on the microbial communities of the Alboran Sea:: a possible case of inhibitory effects by the toxic dinoflagellate Gymnodinium catenatum

The grazing impact of the mesozooplankton community on algae and protozoans was assessed during September 1999 in the Alboran Sea (SW Mediterranean). Ingestion rates were obtained by incubating aliquots of net-collected mesozooplankton with natural seawater, The results showed a weak pressure of mesozooplankton on algae and protozoans, with grazing impacts on community stocks averaging 1.3, 1.4 and 1.5% for phytoplankton, ciliates and dinoflagellates, respectively. Chlorophyll a (chi a) > 5 mum was grazed at higher rates than chi a < 5 mum, and in some stations dinoflagellates were preferred over ciliates. In general, ingestion rates were low, yielding to daily rations ranging from 2 to 20% of body carbon, which were too low to cover the metabolic demand of mesozooplankton in most of the stations. We hypothesize that this unexpected result could be in relation to the presence of the toxic dinoflagellate Gymnodinium catenatum in the study area.