Gradient responses of diatom communities in the Bothnian Sea (northern Baltic Sea), with emphasis on responses to water movement

This study analyses diatom communities on submerged stones in three areas of the Bothnian Sea, a subbasin of the brackish Baltic Sea. The communities were composed of mixtures of epilithic, epiphytic, epipelic, epipsammic and occasionally some pelagic diatoms. Altogether, 120 quantitative epilithic samples, 40 per area, were collected in spring. Exposure to wave action was the major environmental factor varying among sampling sites. Relative ignition loss, used in this study as an indirect measure of macroalgal cover, was positively correlated with exposure to wave action and negatively with dry weight and diatom species diversity. This suggests that macroalgae were more abundant on stones with higher exposure to wave action. The weight of sand grains on the sampled stones was negatively correlated with exposure to wave action. Relative abundance of diatom species was recorded in two separate biovolume classes. For both classes, canonical correspondence analysis revealed that the major environmental factor Structuring the diatom communities was exposure to wave action. Of secondary importance were differences in community composition explained by nutrient variables. Epipelic species, e.g. Navicula gregaria, N. rhynchotella and Surirella brebissonii, as well as small epipsammic species, e.g. Martyana atomus, Opephora mirtabilis and Staurosira punctiformis, were typical of no or low water movement. By contrast, the relative abundance of epiphytes, e.g. Diatoma moniliformis and Tabularia fasciculata, increased with stronger water movement. This was presumably related to the higher availability of macroalgal substrata at sites exposed to wave action. Dominance of epipsammic species separated stagnant water from all other levels of water movement. Large diatom species ( 1000 mum(3)) showed a continuous gradient response to water movement; small species (< 1000 mum(3)) gave a 'threshold' response.