Beschreibung:
<p>Acidification effects on diatom communities in lakes have been studied by analyses of sediment samples. In the surface sediment (0-1 cm) of 31 lakes in SW Sweden, the frequency ratios of valves of planktonic species to valves of periphytic species decreased with decreasing pH of the lakes. Net annual accumulation values of diatom valves to the bottom of Lake Gårdsjön were estimated using quantitative diatom analyses of a sediment core from the lake. These results, which are assumed to reflect past productivity changes, show that an extensive impoverishment of the plankton diatom community has taken place concomitant to the acidification during recent decades. The annual accumulation rate of euplanktonic species, mainly Cyclotella spp., has decreased to 10-40% of the values recorded from the pre-acidification period. In fact, this remaining part apparently represents re-deposited and upwards biotransported older diatom valves. The annual accumulation rate of valves of periphytic species, however, has increased since the lake became acidified. These results of the quantitative diatom analyses indicate that the principal effect of the acidification on the diatom communities is a shift in productivity from the pelagic zone to the littoral and profundal zones and not necessarily decreased total diatom production.</p>