The role of charophytes in increasing water transparency:: a case study of two shallow lakes in Estonia

The hydrochemical regime and the biota in two lakes of Vooremaa landscape protection area, Central Estonia, were studied in 2000-2001 within the frames of the EC project ECOFRAME aimed to work out water quality criteria for shallow lakes in Europe. Lake Prossa is a macrophyte-dominated lake with an area of 33 ha and a mean depth of 2.2 m. Most of the bottom in this lake is covered by a thick mat of charophytes all the year round. Lake Kaiavere located at a distance of 10 kin from the former is a much larger (250 ha, mean depth 2.8 m) plankton-dominated lake. Despite the different area and the Chara meadows in Lake Prossa, the nutrient dynamics were very similar in both lakes. The first vernal phytoplankton peak was expressed in reduced Secchi depth in both lakes. After that peak, the water became clear in L. Prossa but remained turbid in Lake Kaiavere. In the Chara-lake the mean individual weight of zooplankton reached its maximum in both years in June. In the year 2000 it coincided with the peak of the zooplankton/phytoplankton ratio and with the minimum of soluble reactive phosphorus both contributing to the initiation of the clear-water period. Towards autumn, the individual mean weight of zooplankton decreased in the Chara-lake and remained smaller than in the plankton-dominated lake. Hence, zooplankton grazing could initiate the clear-water phase in the Chara-lake but other factors were needed for its maintenance. Another parameter that showed a clear difference between these two takes was the carbonate alkalinity. It was rather stable or even increasing during spring in the plankton-dominated lake, while it decreased by nearly 50% between April and July in the Chara-lake. The calculated saturation level of calcite remained still lower in the Chara-lake showing that the co-precipitation of algae with autochthonous calcite could not explain the observed increase in water transparency above Chara meadows. The reduced sediment resuspension and the possible allelopathic influence of charophytes on phytoplankton remain the main explanations for the maitenance of the extensive clear-water period in the Chara-lake.