Water and sediment as sources of phosphate in aquatic ecosystems: The Detroit River and its role in the Laurentian Great Lakes

Eutrophication of freshwater ecosystems and harmful algal blooms (HABs) are an ongoing concern affecting water quality in the Great Lakes watershed of North America. Despite binational management efforts, Lake Erie has been at the center of dissolved reactive phosphate driven eutrophication research due to its repeated cycles of algae blooms. We investigated the Detroit River, the largest source of water entering Lake Erie, with the objectives to (1) characterize Detroit River phosphate levels within water and sediment, and (2) use multiple chemical and isotopic tracers to identify nutrient sources in the Detroit River. Riverine water and sediment samples were collected at 23 locations across 8 transects of the Detroit River. The bulk δ15N values from sediments were enriched compared the δ15N values of nitrate from water samples, consistent with biogeochemical cycling in the sediment. Principle component analysis of multiple chemical tracers from water samples found spatial variation consistent with multiple sources including synthetic and manure-derived fertilizers and wastewater effluent. The concentrations of phosphate dissolved in water were within regulatory guidelines; however, sediments had elevated concentrations of both water- and acid-extractable phosphate. Sediment-sequestered legacy phosphorus historically deposited in the Detroit River may be transported into Lake Erie and, if mobilized into the water column, be an unrecognized internal-load that contributes to algal bloom events. Globally, freshwater ecosystems are impacted by numerous non-point source phosphorus inputs contributing to eutrophication and the use of multiple tracer approaches will increase our ability to effectively manage aquatic ecosystems.