Integrated 3D modelling of water circulation and the dynamics of phytoplankton; the effects of a planned reservoir

The FINNALGA model has been used to assess the effects of a planned reservo ir, Vuotos, on a downstream lake, Kemijarvi, in northern Finland. The model is based on 3D calculations of water currents and water quality. The simul ation results are compared with the measurements and the effects of Vuotos on the present situation are assessed. The assessments on increased nutrien t inputs are based on the earlier experiences of an existing similar reserv oir at Porttipahta. Many scenarios were made to obtain as versatile assessm ent as possible. According to the model simulations, the average concentrat ions of algal biomass will increase by 20 - 60 %, while the maximum concent rations will double during the first years of the reservoir Vuotos. The eff ects will be most appeared in the northern part of the; lake, near the estu ary of Kemijoki. In the eastern parts of the lake the effects will be small er, because it is not on the through flow route of Kemijoki.