Modelling phytoplankton dynamics of the eutrophic Lake Võrtsjärv, Estonia

The aim of the study was to assess the effect of nutrient loading and water level regulation of water quality in Lake Vortsjarv. The lake is highly eutrophic and it has a high value for fisheries and recreation. A water quality model was developed in order to make predictions of the effects of the different possible restoration measures (reduction of nutrient loading, water level regulation) planned to be carried out. Two phytoplankton groups, diatoms and cyanobacteria, were included in the model. Their contribution to the total algal biomass is dominating. The other state variables of the model are concentration of soluble reactive phosphorus (SRP), concentration of inorganic nitrogen and concentration of dissolved oxygen. The ecological model was constructed using continuously stirred tank reactor (CSTR) hydraulics. The zero-dimensional approach could be used because the lake is shallow and has no thermal stratification. According to the model, external nutrient loading has a very small effect on the biomass of cyanobacteria, whereas the effect is clearer when diatoms are concerned. The contribution of water level to biomass is very significant for both diatoms and cyanobacteria.