| Summary: | Until the early 1960s, Lake Ladoga was oligotrophic and characterized by good water quality, but within the past 20–30 years, as a result of human impact, the ecological state seems to have deteriorated. Especially since the 1970s, its trophic state has changed to mesotrophic, with elevated nutrient concentrations and decreased transparency. Conditions at some of the worst polluted sites have actually improved in recent years, due to closing down of some sources of industrial pollution, but there are alarming signs of general eutrophication of the main body of water. The total concentration of phosphorus has increased 2–5-fold since the 1960s. During the last decades, several planktonic, benthic and fish species sensitive for eutrophication have disappeared. The present species composition and the biomass of the plankton and zoobenthos typify different trophic conditions in different parts of the lake. Primary production, phytoplankton and zooplankton biomass and species composition display mesotrophic and eutrophic conditions in the coastal regions, whereas the pelagial areas are mainly oligo-mesotrophic. The species composition of the macrobenthos and its small biomass characterize the deep central part of Lake Ladoga as oligotrophic.
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