Lakes may have alternative states due to excessive phosphorus (P) input: a clear‐water state and turbid one with high chlorophyll concentrations. Because shifts between these states have large ecosystem effects, and restoration after the shifts is costly or sometimes impossible, precise evaluation of the possibility of alternative states is needed for lake management. Yet the shifts are quite variable and seem to depend on many factors, including lake morphometry, temperature, and dominance of macrophytes. Here we evaluated the role of these factors using an empirically based model that included more mechanistic detail than earlier models of regime shifts in trophic state. Mean depth and temperature strongly influenced the susceptibility of lakes to regime shifts and lake restoration. The macrophyte effect of preventing P recycling from sediments was critical to the susceptibility of shallow lakes to regime shift. With warmer temperatures, eutrophication was more likely and restoration was less successful due to increased internal P recycling from the sediment. Lakes with intermediate depths were most susceptible to regime shifts and were least restorable. These lakes were too deep to be protected by macrophytes in their littoral zones and were too shallow to mitigate P recycling through hypolimnetic dilution. Our results illustrated the interplay of multiple physical, chemical, and biotic mechanisms in regime shifts, a complex type of causality that may arise in regime shifts of other types of ecosystems.

Enrichment is an increasingly serious trend in natural ecosystems. A theoretical model of a predatorp rey system with a natural assumption of satiation in predation predicts that enrichment causes the populations to £uctuate to stochastic extinction. However, this`paradox of enrichment' does not always occur in experimental and natural communities. Here we present a theoretical model that describes a novel mechanism for resolving the paradox in the case of a predator with optimal selective feeding. Speci¢cally, a less pro¢table but edible (thus`unpalatable') prey species sharply reduces the amplitude of population oscillations and ¢rmly prevents the minimum abundances of species from falling below certain values. The presence of such an unpalatable prey thus guarantees the robustness of the system against enrichment.

No abstract

Alternative stable states in shallow lakes are typically characterized by submerged macrophyte (clear-water state) or phytoplankton (turbid state) dominance. However, a clear-water state may occur in eutrophic lakes even when macrophytes are absent. To test whether sediment algae could cause a regime shift in the absence of macrophytes, we developed a model of benthic (periphyton) and planktonic (phytoplankton) primary production using parameters derived from a shallow macrophyte-free lake that shifted from a turbid to a clear-water state following fish removal (biomanipulation). The model includes a negative feedback effect of periphyton on phosphorus (P) release from sediments. This in turn induces a positive feedback between phytoplankton production and P release. Scenarios incorporating a gradient of external P loading rates revealed that (1) periphyton and phytoplankton both contributed substantially to whole-lake production over a broad range of external P loading in a clear-water state; (2) during the clear-water state, the loss of benthic production was gradually replaced by phytoplankton production, leaving whole-lake production largely unchanged; (3) the responses of lakes to biomanipulation and increased external P loading were both dependent on lake morphometry; and (4) the capacity of periphyton to buffer the effects of increased external P loading and maintain a clear-water state was highly sensitive to relationships between light availability at the sediment surface and the of P release. Our model suggests a mechanism for the persistence of alternative states in shallow macrophyte-free lakes and demonstrates that regime shifts may trigger profound changes in ecosystem structure and function.

Macrophytes may enhance grazing on phytoplankton by providing a refuge for zooplankton against fish predation. Loss of macrophytes can trigger sudden degradation of water clarity (regime shift) in lakes. However, the presence of piscivores may drive planktivorous fish to take refuge amongst littoral macrophytes. To address the possibility of regime shifts, I here constructed an empirically based model that combined population dynamics of organisms with game theory for optimal habitat selection, taking into consideration the trophic structure, lake size and eutrophication. The model showed that macrophytes generally acted as a refuge for zooplankton, rather than for fish. The model predicted that regime shifts were more likely in small, shallow lakes and that the presence of macrophytes raised the possibility of regime shifts. The present study demonstrated that the fast dynamics of animal behaviour could lead to regime shifts, in connection with slower variables such as nutrient loading.