LAUSR

Abstract Understanding riverine phosphorus (P) dynamics, its transport, and transformation mechanisms from sources in the watersheds to receiving water bodies are essential for the development of effective strategies to decrease excess P loading and reduce eutrophication. Karst watersheds are thought to be especially vulnerable to P pollution due to their geomorphological features. In this study, we investigated the dynamics of particulate P (PP) chemical phases in two karst watersheds from a Great Lakes Area of Concern in southern Ontario (Bay of Quinte, Canada). Suspended solids were collected during different hydrological regimes representing storm events with low and high discharge, and particulate P binding forms were measured using extraction techniques. Our results show that in these karst watersheds, particulate P is dominated by chemical species that are likely to be highly bioavailable and contribute 62–75% to total PP. Concentrations of these bioavailable P binding forms in suspended solids increase two- to three-fold on dry mass basis during periods of high river flow. Electron microprobe and energy dispersive X-ray spectroscopy suggest that the primary carriers of particulate P are aggregates of Fe oxyhydroxides and Al-silicates. Our results underscore the influence of particulate P sources on the ongoing eutrophication in the Bay of Quinte Area of Concern.