Limited ecosystem response to alternative stable-state manipulation: Results from a whole-lake deepening experiment to completely remove macrophytes

Background/Question/Methods  Previous work has demonstrated that over a wide range of nutrient concentrations shallow lakes can exhibit two alternative equilibria: a clear-water state dominated by submerged aquatic vegetation and a turbid state dominated by algae. Macrophytes help to preserve a clear-water state by sequestering nutrients, providing a refuge for herbivorous zooplankton from predators, reducing sediment re-suspension, and impeding nutrient influx from internal sources. Consequently, elimination of macrophytes should increase algal biomass and water turbidity. We evaluated this hypothesis using an $18M whole- lake experiment in which macrophyte-rich, clear-water Wascana Lake, Regina, Canada, was deepened from ~1.5 m to ~6 m during a single winter and macrophyte habitat and populations were largely eliminated. Nearby Buffalo Pound Lake was monitored as a reference.

Results/Conclusions  Contrary to theoretical expectations, no significant change was observed for algal abundance (as Chlorophyll a), zooplankton density or community composition, nitrogen content, or water transparency for at least 4 years following experimental deepening, although concentrations of phosphorus declined substantially during 3 of 4 post-manipulation summers. These findings suggest that experimental deepening can improve water quality for recreational use (e.g., boating) by eliminating macrophytes without initiating a regime shift to a turbid state with noxious algal blooms.