Modelling phytoplankton dynamics of the eutrophic Lake Vortsjarv, 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 eu trophic and it has a high value for fisheries and recreation. A water quali ty model was developed in order to make predictions of the effects of the d ifferent possible restoration measures (reduction of nutrient loading, wate r level regulation) planned to be carried out. Two phytoplankton groups, di atoms and cyanobacteria, were included in the model. Their contribution to the total algal biomass is dominating. The other state variables of the mod el are concentration of soluble reactive phosphorus (SRP), concentration of inorganic nitrogen and concentration of dissolved oxygen. The ecological m odel was constructed using continuously stirred tank reactor (CSTR) hydraul ics. The zero-dimensional approach could be used because the lake is shallo w and has no thermal stratification. According to the model, external nutri ent loading has a very small effect on the biomass of cyanobacteria, wherea s the effect is clearer when diatoms are concerned. The contribution of wat er level to biomass is very significant for both diatoms and cyanobacteria.