Alternative stable states
Regime shifts, theoretical modelling as well as empirical studies has shown that natural systems may occur in alternative stable states and that shifts between states are abrupt, not gradual. One of the archetypal examples of abrupt regime shifts between alternative stable states comes from the study of shallow lakes subject to increase in nutrient loading. In pristine conditions with low nutrient loadings, shallow lakes exist in an alternative stable state that is characterized by clear water, lush stands of submerged macrophytes, high abundance of waterfowl and a high proportion of piscivorous fish. The resilience of the clear-water stage is maintained by a number of ecological feed-back processes in which macrophytes play a central role: they reduce sediment resuspension, compete for nutrients with algae and provide a refuge for zooplankton. However, as a result of increasing nutrient loads and internal ecosystem processes the lake may eventually and abruptly undergo a regime shift. The alternative stable state is characterised by having high turbidity caused by algae, no submerged macrophytes, few birds, and a fish community dominated by planktivores.
In order to understand the dynamics of shallow lake ecosystems it is thus necessary to understand the mechanisms creating resilience against changes and, further, the processes that eventually promote a transition between states. For this purpose we have for a number of years studied Lake Krankesjön, a lake known to shift between the turbid and the clear-water phase. Since the late 1980’s it has been in a clear-water state with lucid macrophyte beds. However, during the last two years it has turned turbid and stayed so throughout the summer and, thus, we may be in a unique and extremely intriguing situation in that we have the opportunity to document a state shift and, more importantly, have the potential to unravel the mechanisms behind the transition.
In our research group we have for many years run continuous monitoring programe for Lake Krankesjön and we have now accumulated a long-term data set on a range of abiotic and biotic variables, including fish, macrophytes,waterfowl, water chemistry, turbidity, phytoplankton and zooplankton. In a collaborative project with paleolimnologist we study the prevalence of regime shifts during historical times. Further, in field and laboratory experiments we study the effects of partial fish migration, snail/periphyton and zooplankton/phytoplankton interactions on the establishment and growth of submerged macrophytes.