Limnetica 34

Feedback mechanisms between macrophytes and water clarity resulting in the occurrence of alternative stable states have been described in a theoretical concept for shallow lakes. Here, I review recent studies applying the concept to other freshwater systems, unravelling stabilizing mechanisms and discussing consequences of regime shifts. Recent modelling studies predict that abrupt changes between clear and turbid water states can also occur in lowland rivers, both in time and in space. These findings were supported by long-term data from rivers in Spain and Germany. A deep lake model revealed that submerged macrophytes may also significantly reduce phytoplankton biomass by 50-15 % in 100-11 m deep and oligotrophic lakes. Some of the mechanisms stabilizing clear-water conditions are still far from fully understood. Available data suggest that the macrophyte community composition affects number and type of mechanisms stabilizing clear-water conditions. Allelopathic effects of macrophytes on phytoplankton are no longer doubted, however, bacterial colonization of macrophytes and phytoplankton, phytoplankton interactions, local adaptations and strain-specific sensitivities have been found to modulate these interactions. New aspects on stability properties of shallow lake ecosystems arose from experimental and modelling studies on terrestrial organic matter input, both in dissolved (tDOM) and particulate (tPOM) form. These suggest that the likelihood that shallow lakes will shift to or stay in the turbid state is enhanced with a predicted future increase in tDOM and tPOM input. Shallow lake restoration still suffers from knowledge gaps such as the role of propagule availability and dispersal for the re-establishment of a diverse submerged macrophyte vegetation. The importance of lake regime shifts, however, is increasingly supported by studies on quantitative consequences for processes such as primary production, carbon emissions and carbon burial.