Water level changes in a large shallow lake as reflected by the plankton:periphyton-ratio of sedimentary diatoms

Biostratigraphic diatom analyses were carried out on a short sediment core from the large shallow-water Lake Vortsjarv, Estonia, in order to relate the diatom composition to the instrumental water level record. We dated the sediment core by radiometric methods (210Pb, 137Cs, 241Am) and spheroidal fly-ash particle abundance chronology and evaluated the statistical significance of the relationships between the percentage of planktonic diatoms and the water level continuously monitored since 1871. Before the 1960s, the percentage of planktonic diatoms in the sediment showed quite strong positive relationship to water level. The impact of eutrophication after the 1960s presumably masked the influence of water level changes on the diatom community. In addition, statistical analysis of the upper part of the sediment core (1970—present day) together with measured limnological parameters of the lake showed that water transparency had the strongest influence on diatoms, while temperature, pH and alkalinity had lesser impacts. Our study shows that the planktonic:periphytic diatom ratio in the sediment can be used to track overall trends of the lake-level changes in Lake Vortsjarv before the onset of cultural eutrophication; however, the results have to be interpreted carefully, taking into consideration other possible limnological factors such as water transparency, nutrients and wind.