Past anthropogenic activities offset dissolved inorganic phosphorus retention in the Mississippi River basin

The rapid acceleration of anthropogenic phosphorus (P) loadings to watersheds has fuelled massive freshwater and coastal eutrophication and completely changed the global P cycle. Within watersheds, emitted P is transported downstream towards estuaries. Reservoirs can retain a significant proportion of this P. In the long term, this accumulated P can however be re-mobilized, a process lacking in current global P budgets. Here, we include P cycling in a coupled integrated assessment-hydrology-biogeochemistry framework with 0.5 by 0.5-degree spatial resolution and an annual time resolution, and apply it to the Mississippi River basin (MRB). We show that, while reservoirs have aided in the net retention of P, they serve as dissolved inorganic P (DIP) sources due to the transformation of legacy P in sediments. The increasing DIP sourcing in the MRB has been offsetting P retention in streams, especially towards the end of the twentieth century. Due to its bioavailability, DIP is the most likely form to trigger eutrophication. Although P inputs into the MRB have decreased since the 1970s, legacy effects are delaying positive outcomes of remediation measures.