Deliverable 3.1

Abstract: Effects of temperature induced changes in mixing depth on nutrient concentrations in lakes.

This deliverable presents results from work undertaken on three shallow, eutrophic and frequently mixing European lakes, Säkylän Pyhäjärvi (Finland), Võrtsjärvi (Estonia) and Müggelsee (Germany). These were analysed to determine the importance of stratification and mixing episodes (thermal stability) in shaping the nutrient and oxygen concentrations in the water column, and the abundance and diversity of phyto- and zooplankton during summer. The results were obtained by analyzing high-frequency monitoring data for water temperature, dissolved oxygen, chlorophyll a and biomass of cyanobacteria, weekly to monthly data on inorganic and total nutrients, and the phytoplankton and zooplankton composition in June-August 2008-2011.

The thermal regimes of the study lakes were characterized by low water column stability and short stratification periods of 1-34 days in duration followed by a partial or complete mixing of the water column. During stratification, the partitioning of the concentrations of oxygen, inorganic nutrients (phosphorus and nitrogen) and iron (Fe) increased between the epi- and hypolimnion. Chlorophyll a and phytoplankton biomass were higher during mixing episodes/weak stratification in Pyhäjärvi and Võrtsjärvi. Increased mixing lead to a higher biomass of centric diatoms, but in Pyhäjärvi cyanobacteria were also favoured by the breakdown of stratification. Phyto- and zooplankton diversity (taxa richness, H', Evenness) were not clearly affected by different thermal regimes of the study period. The results show that shallow lakes can react rapidly to changing thermal regime and therefore their nutrient dynamics and functional traits of plankton are sensitive to temperature-induced changes.

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