Since the middle of the 20th century, the average temperature of the atmosphere near Earthsurface has increased. The global warming causes many effects in hydrological systems, suchas changes in thermal structure, water quality, aquatic ecosystems, etc. This thesis studies theimpact of climate change on Akkajaure reservoir, the second largest regulated reservoir inSweden, by simulating a predicted temperature rise based on the climate and hydrologicalconditions of Akkajaure in 1998-2002. The congeal duration, ice thickness and the turbulentkinetic energy (TKE) in the lake were calculated by the catchment hydrological model and thelake model. The movement of phytoplankton and their mean net production (MeanNP) rateare simulated by the dispersion model and the photosynthesis model. By comparing thesimulation results of past situation and three predicted scenarios, it is obtained that theincreases of temperature shorten the congeal duration, which is a lead factor for shortening thetrough period and amplification of peak value of TKE. The comparison of plankton particlesposition illustrates that the particles stay in a deeper position for a longer time because of thechanges of TKE. Though the plankton stays in euphotic zone longer as the temperatureincreases, the comparison of the mean production rate between the real scenario and thepredicted scenarios concludes that the mean production rate grows as the temperatureincreases because the shortened ice cover period makes the duration of absorbed sunlightincreases in lake. The effects of global warming may influence the distribution of microalgaein on high latitude lakes and reservoirs. The phytoplankton will stay in deeper water layers fora longer time.