Phytoplankton and the physicochemical background in an assessment of the ecological and trophic conditions in vendace-type lakes

This study focused on the phytoplankton and environment relationships as well as on the ecological and trophic conditions of lakes inhabited by coregonid fish. Studies were carried out in deep and stratified vendace-type lakes called Lake Pluszne and Lake Łańskie (the Olsztyn Lake District, north-eastern Poland) in 2007-2008. Ecological and trophic conditions were determined on the basis of the phytoplankton multi-metric PMPL and Trophic State Index. Both lakes were characterized by a similar thermal and mictic regime, with the occurrence of hypolimnetic oxygen deficits. They were classified as hydrocarbonate-calcium type and medium-sized eutrophicated water bodies. According to the integrated trophy assessment, proposed in this research, the final Trophic State Index TSIAV indicated meso-eutrophy of both lakes, although the phytoplankton-based PMPL indicated that the ecological potential in Lake Łańskie was good and less than good - due to large cyanobacteria biomasses - in Lake Pluszne. The actual loads of phosphorus and nitrogen significantly exceeded permissible levels, especially in Lake Łańskie (5-fold at the most of the P content), at similar morphometric and sedimentation conditions in both lakes. However, more intensive water-exchange rate and more favorable conditions for phosphorus runoff in Lake Łańskie could limit the phytoplankton growth, especially cyanobacteria. Summing up, better oxygen conditions and less abundant phytoplankton suggested that Lake Łańskie had more suitable conditions for coregonids during the surveyed period than Lake Pluszne, where the vendace biomass has recently decreased drastically.