The recovery of Vesijärvi, a lake in southern Finland: water quality and phytoplankton interpretations

The recovery of formerly heavily eutrophicated Vesijärvi from noxious cyanobacterial blooms is described in terms of changes in phytoplankton and water quality. Substantial water protection and restoration measures were introduced since 1975. Two main turning points in the phytoplankton community structure occurred: a shift from Planktothrix agardhii- to Aphanizomenon flos-aquae-dominant blooms in the early 1980s and the total collapse of the cyanobacterial blooms in 1990. Both shifts were closely connected to the decrease in the nutrient concentrations (total N decreased from 800 mg m–3 to 600 mg m–3 and total P from 60 mg m–3 to 30 mg m–3), to the increase in water transparency, and to the elimination of extremely high pH values (> 9). In the mid-1990s the water quality as well as the species composition and the biomass of phytoplankton in Vesijärvi resembled the situation in the late 1950s.