Spatio-temporal structure of the epipelic diatom assemblage from an intertidal mudflat in Marennes-Oléron Bay, France

Spatio-temporal changes in taxonomic composition and structure of an epipelic diatom assemblage from an intertidal mudflat on the French Atlantic coast was studied over an annual cycle along a cross-shore transect. The assemblage structure was described by estimating both relative abundance and contribution to biovolume of each species. Results showed that the assemblage was numerically dominated by small-sized species (mean relative abundance of 91%). Large species, however, significantly contributed to the total biovolume (mean contribution to biovolume of 49%). A factorial correspondence analysis indicated that the epipelic assemblage was relatively homogeneous along the cross-shore transect but emphasized the seasonal succession of diatom species. In relative abundance, the assemblage structure was characterized by the dominance of the small species Navicula phyllepta throughout the year with a seasonal succession of secondary species, including only one large diatom (Gyrosigma peisonis). In biovolume, small (N. phyllepta and Navicula gregaria) and large species (Pleurosigma angulatum and G. peisonis) alternatively dominated the assemblage. Describing the epipelic assemblage using cell volume emphasized the contribution of large species and revealed that the assemblage contained two diatom fractions, characterized by different biological and physiological behaviours, which may alternatively represent a large proportion of the biomass.