Evaluating the form and mobility of phosphorus in urban streambed sediment

Abstract

Phosphorus (P) enrichment remains a primary driver of eutrophication in freshwater systems, yet the processes governing its storage, transformation, and mobility in urban watersheds are not well documented. Sediment is the dominant vector for P transport in aquatic systems, and its fate is governed by interacting physical and biogeochemical processes. Once deposited, streambed sediments act as both sinks and sources of P, regulating downstream P transport during environmentally sensitive summer baseflow conditions through sediment-water dissolved P exchange. This study investigates the distribution, speciation, and mobility of sediment-associated P in a small (76 km²), predominantly urban watershed (Laurel Creek, Ontario, Canada), with an emphasis on fine-grained streambed sediments and longitudinal variability along the river continuum. Sequential extraction was used to quantify PP fractions (NAIP, AP, OP), while the major element composition of streambed sediment (Fe, Al, Mn, Mg, Ca, Na) was measured to assess geochemical controls on PP partitioning. Phosphorus mobility was evaluated using the equilibrium phosphate concentration (EPC0) and phosphate exchange potential (PEP) to determine the potential of stream sediments to function as sources or sinks of soluble reactive P (SRP). Results demonstrate that both PP form and mobility are strongly influenced by land use and impoundments. Total particulate phosphorus (TPP) ranged from 424 to 1220 µg g⁻¹, indicating substantial P storage within streambed sediments. Bioavailable PP (NAIP) was significantly enriched in agricultural headwaters (U = 0, p < 0.01) and was strongly correlated with Fe, Al, Mn, and organic matter. Downstream trends in sediment geochemistry, characterized by decreasing Fe, Al, and Mn and increasing Ca and Mg, were associated with reduced bioavailable P fractions. Similarly, higher EPC0 values and positive PEP were observed at agricultural sites, indicating a greater potential for SRP release during baseflow. In contrast, urban sites displayed lower and more variable EPC0 and PEP values, showing both potential source and sink behavior. Impoundments appear to play a key role in attenuating P transport in Laurel Creek, with reduced PP concentrations and lower EPC0 observed downstream, suggesting retention of P-rich sediments within upstream reaches. Overall, sediment P dynamics in Laurel Creek watershed are controlled by interactions among land use, geochemistry, and urban impoundments. Fine sediments function as both legacy P reservoirs and regulators of SRP, varying along the river continuum. These findings highlight the importance of integrating P speciation and mobility assessments to improve understanding of in-stream P cycling and to inform management strategies aimed at reducing downstream eutrophication in urbanizing watersheds.