Long-term water quality responses to sediment removal in a small, shallow, urban lake

Sediment removal is a promising strategy for lake restoration that potentially provides long-lasting water quality improvement by directly tackling the source of internal phosphorus (P) loading. However, more studies evaluating the longevity of intervention results are needed to viably assess the method's restoration performance. We used long-term data to evaluate the water quality impacts of sediment removal in small, shallow Lake Gallträsk in southern Finland, where sediment removal by suction dredging was conducted in 2009–2011. The top 50–100 cm of surface sediment was removed from 18 % of the lake's total area to increase the lake depth for recreational purposes. Data before and after sediment removal were analyzed using a combination of structural break and interrupted time series analyses. Our results revealed a significant reduction in the water column P concentration after sediment removal, suggesting that lake recovery from past loading was substantially promoted by the intervention. Additionally, suspended solid concentration reduced after sediment removal, enabling favorable conditions for submerged macrophyte recolonization. Water color and chemical oxygen demand increased after the intervention, possibly due to both internal and external drivers. Nitrogen (N), in turn, was not unaffected by the sediment removal, and the N/P ratio subsequently increased. The long-term changes in water quality and submerged macrophyte abundance after suction dredging have resulted in the absence of harmful algal blooms. Our results demonstrate that sediment removal can enable the long-term recovery of lakes from legacy P loading and act as a powerful measure to mitigate eutrophication.