State of Lake Sysmäjärvi, Eastern Finland, after loading with mine water and municipal waste water for several decades

The deep mining of copper and nickel at Outokumpu, Eastern Finland, lasted from 1910 to the late 1980s, during which period metalliferous waste water of high conductivity and containing high concentrations of iron (Fe), sulphate, copper (Cu) and nickel (Ni) was discharged from the mine into Lake Sysmäjärvi. The same lake also receives municipal waste water, and a new soapstone dressing works began operation nearby in 1977. The waste water from the mine was alkaline during the period 1928–1938, and the municipal waste water load caused eutrophication in the lake. The phytoplankton community was rich in species and dominated by the blue-green Oscillatoria agardhii in 1930. Sphaerid clams (Pisidium) were common in the macrozoobenthos. During the period 1938–1964 the mine water was acidic, so that the lake water became clear, its pH in 1951 being < 4.5 and its Fe content varying from 6 to 24 mg l^-1. The phytoplankton community consisted of diatoms, the fish population almost disappeared and the number of macrozoobenthos species declined. Pisidium was found only at one littoral site and only Sialis spp. were found in the deep water. From 1964 onwards the waste water from the mine was limed, and the lake water became almost neutral within a few years. The pH changes were also evident in the outflowing River Sysmänjoki. Phytoplankton biomass was low and dominated by diatoms and Cryptophyceae flagellates in the 1970s and 1980s. The chlorophyll a content during the years 1987–2001 showed the lake to be eutrophic, and the phytoplankton biomass increased. Blue-green algae have been scarce since the first sampling in 1930. The Fe content of the water varies from 1000 μg l^-1 to 2000 μg l^-1 and the Ni content from 30 μg l^-1 to 80 μg l^-1, with a maximum of 200 μg l^-1. The high concentrations of metals both in the water column and in the sediment could be harmful to biota even today.