Phosphorus legacy and equilibrium dynamics in up and downstream sediments of Lake Urmia

The potential for sediments in the Lake Urmia basin to sorb and retain dissolved inorganic phosphorus (P) is unknown. Land use has impacted Sediment P sorption capacity along several tributaries to Lake Urmia and its wetlands. The following hypotheses were tested: (1) upstream agricultural P sources are more significant than downstream locations, (2) the storage of P and its subsequent release is controlled by potential differences in physicochemical properties of upstream versus downstream sediments, and (3) the differences in algae growth will correlate with its native adsorbed P (NAP) and equilibrium values (EPC0). We employed several strategies to link sediment physicochemical properties, sediment P adsorption characteristics, and potential algal response to understand upstream and downstream P cycling characteristics better. The results suggest that P sorption capacity was generally higher in the downstream sediments than in upstream locations. There was a significant correlation between sediment properties, mainly clay and organic matter, and sorption parameters. The equilibrium phosphorus concentration (EPC0) values were higher than the soluble reactive phosphorus (SRP) concentration in the water column, suggesting that sediment is a source of P in the water column. River sediments have a high potential for P sorption and, depending on the landscape and biogeochemical processes can be considered an internal source of pollution in the river system.