Effects of salinity on epiphytic diatom communities on Pilayella littoralis (Phaeophyceae) in the Baltic Sea

The distribution of brackish water diatoms epiphytic on the brown filamentous macroalga Pilayella littoralis was studied in the Öresund, the Baltic Sea proper, and the Bothnian Sea. The main environmental variables considered were salinity (4-12‰) and wave action. Biological data collected were species composition, cell density, biomass, diversity, cell size and life form. Salinity was the overriding environmental factor regulating the distribution of epiphytic diatoms in the Baltic Sea as a whole. Degree of exposure of sites to wave action was less important but significant. Different species in the natural communities described here proved to have different salinity ranges and optima. Between 5 and 6‰, a marked transition from an epiphytic flora dominated by diatom taxa with marine affinities to one dominated by taxa with freshwater affinities was observed. At this critical salinity, marine epiphytes attached by mucilage pads (Tabularia fasciculata, T. waernii) or long stalks (Licmophora hyalina) were replaced by freshwater species in zig-zag colonies (Diatoma moniliformis, D. vulgaris). Another sharp transition in species composition was found at the entrance of the Baltic Sea (between 8 and 10‰), where some species suddenly decreased in abundance (Tabularia investiens, T. ktenoeides, Licmophora oedipus, L. communis) and others increased (Tabularia tabulata, T. cf. laevis, Licmophora cf. gracilis, Rhoicosphenia curvata). Cell density and biomass of epiphytic diatoms showed weak increasing trends as salinity decreased. Species richness was not correlated with salinity, but Shannon-Weaver diversity (H') decreased with lower salinities through increased dominance within the diatom communities. Wave action had a larger effect on H' than salinity; H' decreased with higher exposure to wave action.