Phosphorus loading in impaired wetlands: The role of temperature and simulated dredging under controlled experimental conditions

Restoring wetlands can increase biodiversity and water quality in both wetlands and connecting ecosystems; however, failing to consider past land uses can result in negative water quality effects. Prior agricultural practices at a coastal wetland restoration site in West Michigan (USA) have caused high legacy phosphorus (P) concentrations in sediments, as well as high P levels in the water column. Sediment dredging is being considered as a restoration tool to control internal phosphorus loading (IPL). Additionally, climate warming is increasing both mean temperature and temperature extremes, which can affect IPL. The influence of both sediment dredging and climate warming on P dynamics was examined in this wetland complex by incubating sediment cores in a controlled laboratory experiment. Both ponds exhibited hypereutrophic conditions, although mean water column and sediment total phosphorus (TP) concentrations were ∼ 2.5 and ∼ 8 times higher, respectively, in the north pond compared to the south pond. Unexpectedly, sediment cores with the top 10 cm removed (simulated dredging) had slightly higher mean soluble reactive phosphorus (SRP) release rates in both ponds (north ∼22 mg m^-2 d^-1, south ∼1.9 mg m^-2 d^-1) compared to undredged cores (north ∼18.6 mg m^-2 d^-1, south ∼1.5 mg m^-2 d^-1). Equilibrium P concentrations suggest sediments are a source of SRP to the overlying water column regardless of dredging treatment. Increases to both average temperature and simulated heatwaves increased P release and P accumulation in core tubes in the less enriched south pond but not in the north pond. Our results emphasize the need for precursory research before implementing management actions and show that dredging alone might not be the appropriate management technique for this restoration site. Successful management of internal and external P loading is crucial to reducing eutrophication worldwide, leading to healthier freshwater ecosystems and enhancing the beneficial ecosystem services they provide.