Recovery of the fish community and changes in the lower trophic levels in a eutrophic lake after a winter kill of fish

A severe oxygen deficit induced a fish kill in the eutrophicated two-basin Lake Aimajarvi in southern Finland during winter 2002-2003, resulting in cascading effects on the lower trophic levels of the lake. Pikeperch disappeared from the lake and bleak and white bream decreased to low numbers. The recovery of the populations of other species started immediately when strong year-classes of roach and perch appeared in summer 2003 and onwards. A sharp increase in the growth of perch and roach was recorded, and perch became the dominant fish species during 2004-2006. Consequent responses after the fish kill included increased Secchi depth, expansion of submerged macrophytes, decreased nutrient concentrations and reduction of Cyanophyta from the more eutrophic northern basin of the lake, and a temporary increase in the size of Daphnia in the early summer 2003. However, the ecosystem of the lake returned gradually to the earlier structure and level of eutrophication. Bluegreen algae have returned since 2005, the water has become more turbid, macrophytes declined in 2007 and the fish community was again dominated by small cyprinid fishes in 2008.