Gradient responses of diatom communities in the Bothnian Bay, northern Baltic Sea
Busse, Svenja; Snoeijs, Pauli
The Bothnian Bay in the northern Baltic Sea is characterized by a stable north-south salinity gradient (0.4-3.3 psu) as a result of river runoff and the absence of tides. Diatom communities occurring on submerged stones in the upper littoral along this gradient were studied. The communities were composed of mixtures of epilithic, epiphytic, epipelic, epipsammic and occasionally some pelagic diatoms. Altogether, 151 quantitative epilithic samples, taken between 27 May and 1 June 1991 from four study areas, were analyzed for dry weight (DW), ash-free dry weight (ADW) and diatom relative abundance in two biovolume classes. Relative ignition loss (ADW%), used in this study as an indirect measure of macroalgal cover, was positively correlated with salinity and negatively with DW. This suggests that diatoms were more abundant in lower salinity and macroalgae in higher salinity at the time of sampling. Altogether, 290 diatom taxa were recorded. Canonical correspondence analysis (CCA) revealed that the major environmental factors structuring the diatom communities were salinity and exposure to wave action. The availability of macroalgal hosts strongly co-varied with salinity in the data set and affected diatom community composition on the stones. However, when excluding the dominant epiphytes in CCA, salinity and exposure to wave action were still the major factors affecting the distribution of the remaining species (predominantly epilithic and epipsammic). Small diatoms (biovolume 1000 μm3) responded differently to environmental factors. Small diatom species were more affected by exposure to wave action and large species more by the salinity gradient.