Restoring shallow lakes impaired by eutrophication: Approaches, outcomes, and challenges

Shallow lake ecosystems are widely impaired by eutrophication. Lake restoration projects frequently fail or are only partially successful, reflecting that environmental managers face difficult challenges in addressing this complex problem. Key uncertainties relate to selecting appropriate methods and predicting the resulting response trajectory, that is, "what should be done, how much improvement should be expected, and how long will it take?" This review critically evaluates approaches applied to address eutrophication in shallow lakes. A set of shallow lake restoration case studies from across the globe, involving moderate to high effort monitoring, was evaluated to determine common features and to identify advantages/disadvantages and drivers of successful restoration projects. Approaches are broadly grouped into four categories: 1) external nutrient load reductions, 2) internal nutrient load reductions, 3) biomanipulation, and 4) hydrological manipulations. Case studies highlight that external load reduction is an essential component of successful restoration, although supplementary measures can support restoration goals. To inform planning and help set realistic expectations, the following key challenges to the restoration of shallow lakes were identified: 1) insufficient reductions in external nutrient loads, 2) biogeochemical lags, 3) resistance of a stable, algal-dominated state due to hysteresis in the turbidity-nutrient load relationship, 4) delayed macrophyte reestablishment, 5) climate change, 6) need for lake-specific approaches, and 7) social and political shortcomings. Managing these challenges is crucial to the success of shallow lake restoration projects.