Benthic contribution to pelagic microalgal communities in two semi-enclosed, European-type littoral ecosystems (Marennes-Oléron Bay and Aiguillon Bay, France)

Long-term changes of the taxonomic composition of the phytoplankton communities of two French littoral ecosystems (Marennes-Oléron Bay and Aiguillon Bay) were investigated in order to estimate the benthic contribution in terms of biomass. A long-term phytoplankton survey (REPHY - IFREMER) provided phytoplankton densities at different taxonomic levels of identification from 1995 to 1999. The database was homogenised at the taxonomic level of the genus and the log 10-density was used as an indicator of the biomass. At all stations investigated, a strong seasonality governed the changes in the number of genera and the log 10-density. No significant differences were found between the stations. Factorial Correspondence Analyses were performed to provide a typology of sampling dates and corresponding functional groups of genera. Pennate diatoms were dominant in winter periods, while centric diatoms and dinoflagellates characterised summer and fall periods. Moreover, the log 10-density of pennate diatoms did not follow a seasonal pattern. The relative contribution of this group, which appeared to be the most representative of the benthic microalgal communities, had a maximum variation range of 1 to 84% and was well correlated with the turbidity recorded at the same time. This study was a first step to provide a frame for further studies about the coupling between benthic and pelagic microalgal biomasses in semi-enclosed European-type littoral ecosystems.