Long‐term pattern of alternative stable states in two shallow eutrophic lakes

Abstract

SUMMARY: Lake Tåkern and Lake Krankesjön, two moderately eutrophic, shallow lakes in southern Sweden, have during the past few decades shifted several times between a clear‐water state with abundant submerged vegetation and a turbid state with high phytoplankton densities. Between 1985 and 1991, Lake Takern was in a clear state, whereas Lake Krankesjon shifted from a turbid to a clear state. During this shift, the area covered by submerged macrophytes expanded, followed by an increase in water transparency, plant‐associated macroinvertebrates, and piscivorous fish. Nutrient concentrations, phytoplankton biomass and abundance of planktonic cladocerans decreased. In both lakes, water level fluctuations were the most common factor causing shifts, affecting submerged macrophytes either through changes in light availability or through catastrophic events such as dry‐out or mechanical damage by ice movement. Our data give further support for the existence of two alternative stable states in shallow lakes maintained by self‐stabilizing feedback mechanisms.