Spatio-temporal variation of microphytobenthos biomass, diversity and assemblage structure in the Loire Estuary, France

Diatom-dominated microphytobenthos (MPB) communities of 4 intertidal mudflats along the meso- and polyhaline reaches of the Loire Estuary, France, were investigated during a year cycle. They were analysed in terms of biomass, diversity, species composition and growth form distribution. The assemblages of the 2 upstream sites were characterised by high biomass and lower diversities and were mostly dominated by epipelon. The 2 downstream, most haline sites had lower biomass and higher diversities and were dominated by both epipelon and tychoplankton. Diversity did not exhibit a clear seasonal signal in the upstream mudflats, but it was higher during the first half of the study in the downstream sites. The coexistence of 2 growth forms seems to increase diversity of the mudflat assemblages. Species distribution was mainly linked to changes in sediment texture and salinity, both with a marked seasonal variability. MPB biomass was inversely related to MPB diversity and positively related to both mud content and the epipelon. MPB diversity was not, however, significantly correlated with mud content. Slight changes in sediment texture, even if causing variations in assemblage composition, did not change overall diversity. The existence of an important and even dominant tychoplanktonic fraction could be considered a distinctive feature of these benthic environments in the Loire, as well as in other macrotidal estuaries.