EUTROPHICATION DUE TO PHOSPHORUS RECYCLING IN RELATION TO LAKE MORPHOMETRY, TEMPERATURE, AND MACROPHYTES

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.