Original paper

Shifts between clear and turbid states in a shallow lake: multi-causal stress from climate, nutrients and biotic interactions

Hargeby, Anders; Blindow, Irmgard; Hansson, Lars-Anders

Abstract

We used long-term monitoring data to assess causes behind a recent shift from a clear to a turbid water state in Lake Tåkern, Sweden. The lake has a previous record of shifts between clear-water and turbid states, but the causes behind these shifts are not well known. During the recent shift, which occurred in 1995 –1997, submerged vegetation subsequently declined after a 30-year period of clear-water and abundant vegetation. Among the possible explanations we identify several processes unlikely to have contributed to the recent shift from clear to turbid conditions, including long-term changes in external input of phosphorus, fluctuations in water level, and changes in zooplankton grazing pressure. Instead, likely scenarios to have contributed to the macrophyte decline, and thereby to the shift were: (1) a series of mild winters with short ice cover and absence of winter-kills of fish, leading to high biomasses of benthivorous and planktivorous fish before the shift, and thereby increased bioturbation and internal nutrient recycling, (2) unusually cool and windy springs the years before and during the shift, leading to unfavourable conditions during the establishing phase of submerged macrophytes. Both shorter periods of ice cover and high wind velocity in winter and spring were associated with climate, approximated by the North Atlantic Oscillation (NAO). We argue that none of these processes alone can force the lake from the clear to the turbid state, but that several stress factors in concert are necessary to initiate a shift. Hence, we conclude that climate variability is likely to have contributed to a multi-causal stress, reducing the resilience of the clear-water state and finally triggering the shift through inter-year dependent changes in biomass of submerged macrophytes and fish, organism groups known to have key roles in the dynamics of shallow lakes.

Keywords

alternative statesecosystem resiliencenaosubmerged vegetation