Modelling of water circulation and the dynamics of phytoplankton for the analysis of eutrophication

The eutrophication and algal blooms are problems in many Finnish lakes and sea areas. One possible way to approach the problem is mathematical modelling. If the most important processes have been successfully modelled, it is easy to calculate the effects of nutrient load changes. This thesis presents experiences on developing and applying eutrophication models. The pre-calculated flow fields are the most important input to the water quality model, so the applications of the flow model are examined in two papers. The results indicate that the flow model can produce typical flows caused by wind. The simulations of the timing of upwelling were also successful. The excessive vertical diffusion can cause problems in eutrophication model applications. The results of simulations indicate that the greenhouse effect will advance and enlarge the spring bloom of phytoplankton. However, the average phytoplankton biomass in the Gulf of Finland will be near current level, defined in these scenarios by the available amount of nutrients almost unchanged due to the insignificant changes in the external nutrient inputs. The more significant nutrient load reductions from the major loading points have a clearly decreasing effect on the total algal biomasses even in one year long simulations. When the nitrogen fixing cyanobacteria was taken into account in the simulation, the results indicated that the phosphorus removal from St. Petersburg would decrease both average cyanobacteria and other algae biomasses. The biomass of other algae might increase in the zone where additional nitrogen is transported from the east. The Finnish national agenda, which is nitrogen dominated, does not seem to be an effective way to fight against accumulations of cyanobacteria. However, the total phytoplankton biomass will decrease near the Finnish coast.