Distribution of benthic diatoms in the littoral zone of the Gulf of Riga, the Baltic Sea

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 ail 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.