Beach ecosystems extend from dune to offshore areas along most coasts, and provide essential services that are not provided by any other ecosystem. Indeed, sandy systems contain unique biodiversity and supply nursery and foraging areas for numerous commercially important marine species, such as flatfish. However, these systems are threatened by increasing anthropogenic pressure. Green tides (GT, i.e. accumulations of green opportunistic macroalgae) are a major human-induced threat to marine ecosystems, from inshore to nearshore. This eutrophication process greatly affects both benthic invertebrate communities and flatfish communities, within sheltered and non- or microtidal systems. However, the responses of dynamic open macrotidal sandy systems to eutrophication in the form of macroalgal mats are not yet fully understood. In particular, understanding the effects of GT on two connected biological compartments (infauna and flatfish) within two connected habitats (intertidal and subtidal) is crucial. Here, we set out to assess the influence of several environmental variables, including Ulva biomass, on the variability in infauna and flatfish communities in both the intertidal and the subtidal at four sites impacted or not by GT. In total, 110 biodiversity samples were analysed with classic and novel analytical approaches. Our results demonstrate that the presence of GT specifically impacts intertidal macroinvertebrate communities. However, small effects of GT on subtidal infauna communities, as well as on species-specific flatfish at both intertidal and subtidal, were still detectable. Our findings underline the vulnerability of highly dynamic ecosystems exposed to anthropogenic stress, in particular intertidal sandy shores.

The composition (% relative abundance) of the benthic diatom community was studied in the littoral zone (0–5 m) of the NW and E coasts of the brackish, non-tidal Gulf of Riga, in the Baltic Sea. The samples were collected concomitantly with measurements of sediment primary productivity and chlorophyll a during three cruises, representing three seasons (autumn, spring, summer). At all depths, the living diatom community was dominated by epipsammic species, the most abundant taxa being Martyana atomus, a species of Fragilariaceae and Achnanthes delicatula. The community was characterized by a stable composition and a high species diversity throughout the study period (Shannon–Weaver, mean H′ = 4·4). Principal Component Analyses (PCA), together with Redundancy Analysis (RDA) based on 129 taxa, revealed both spatial (site and depth) and seasonal differences in the diatom communities. However, these differences were mainly caused by the varying proportions of taxa common in both sampling areas in all three seasons, rather than by actual major taxonomic changes in the species present in communities. The distribution of the typical epipsammic species along the depth gradient was not uni-directional. The dominance of attached life forms, and the absence of a well-developed epipelic community, reflects the physically dynamic substratum, particularly in the eastern part of the Gulf. As a result, primary productivity is kept rather low, decreasing rapidly with depth due to frequent turbidity of the water column.