Lakes vary in response to minor disturbances and, where stabilising feedbacks exist, tend to return to a stable state. If the disturbance is strong enough the stabilising forces may be overcome and the lake passes a threshold whereby it shifts into a state controlled by a new suite of negative feedbacks. A typical switch in state is thought to occur in shallow lake systems whereby a pulse of sediment or nutrients may drive an increase in phytoplankton impacting the light regime. This impacts negatively on submerged plants which results in greater entrainment of benthic sediments and release of nutrients. These strengthen the competitive advantage of phytoplankton and entrench the lake in a new state.
Multiple diatom-based sedimentary records of change in wetlands in the lower River Murray, Australia, have revealed assemblage turnover following river regulation and impoundment. Typically, benthic and epiphytic flora have yielded to planktonic and disturbance taxa indicative the loss of aquatic plants and a decline in the light regime consistent with regime shift theory. This evidence is supported by changes in the remains of macrophytes, cladocerans and stable isotopes which reveal the loss of plants and the shift to a pelagic system. However, rather than exhibiting flickering before changing abruptly, these systems have changed gradually over several decades. It could be argued that this represents a slow response to a threshold change. Alternatively, it is merely an ongoing response to the persistent pressure exerted by increased fluxes of sediments and nutrients.
Grundell, R. et al. (2012) Interaction between a river and its wetland: evidence from spatial variability in diatom and radioisotope records. J. Paleolimnol. 47: 205-219.
Kattel, G. et al. (2014). Tracking a century of change in trophic structure and dynamics in a floodplain wetland: integrating palaeo-ecological, and palaeo-isotopic, evidence. Freshw Biol. 60(4): 711-723.