Rehabilitation of shallow lakes: time to adjust expectations?

Shallow lake managers seek strategies to improve water quality and ecological features of these habitats, but lake responses are unpredictable and factors responsible for changes are often unclear. We summarized results of eight whole-lake rehabilitation projects in Minnesota, USA, an area with many shallow waters highly impacted by anthropogenic activities. To assess lake responses, we compared characteristics of managed sites to those of other regional shallow lakes manifesting clear- or turbid-state conditions. Managed lakes showed modest similarity to clear-water reference lakes in terms of phytoplankton (as chlorophyll a ), nutrients, and submerged aquatic plants. Responses of aquatic invertebrate communities were more equivocal, with relatively little similarity to clear-water sites following management. These patterns indicate that these lakes either failed to undergo transitions to clear-water states, or that clear-water conditions did not persist throughout the 2–3 year period following treatment and prior to our evaluation. We believe these results show responses of shallow lakes that have been pushed beyond boundaries where they maintain sufficient natural resilience to resist local stressors. This means that shallow lake rehabilitation efforts will not always succeed and that, when improvements occur, management may need to be repeated to maintain favorable ecological conditions in highly modified landscapes.