The article presents the results of many years' research (1991-2007) on the chlorophyll and nutrient concentrations, biomass and primary production of phytoplankton in the Curonian Lagoon. The main factors affecting the level of biological production and the trophic status are determined by the comparison of hydrological and chemical indicators. Water temperature is a key environmental factor that regulates the phytoplankton production and abundance in the Curonian Lagoon. The greater seasonal rise in water temperatures in 1990-2000, presumably, a consequence of the climate change, alongside with other factors (freshness, slow water flow) creates conditions for cyanobacterial «hyperblooms». Probably, the 1990-2000 climate warming is the cause of the eutrophication of the Curonian Lagoon, which continues despite significant reduction in external nutrient load.