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 log10-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 log10-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 log10-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.