Point sources and agricultural practices control spatial-temporal patterns of orthophosphate in tributaries to Chesapeake Bay

Orthophosphate (PO₄) is the most bioavailable form of phosphorus (P). Excess PO₄ may cause harmful algal blooms in aquatic ecosystems. A major restoration effort is underway for Chesapeake Bay (CB) to reduce P, nitrogen, and sediment loading to CB. Although PO₄ cycling and delivery to streams has been characterized in small-scale studies, regional drivers of PO₄ patterns remain poorly understood because most water quality trend assessment focus on total P. Moreover, these trend assessments are usually at an annual timestep. To address this research gap, we analyzed PO₄ patterns over a 9-year period at 53 monitoring stations across the CB watershed to: 1) characterize the role of PO₄ in total P fluxes and trends; 2) describe spatial and temporal patterns of PO₄ concentrations across seasons and streamflow; and 3) explore factors explaining these patterns. Agricultural watersheds exported the most total P compared with watersheds under different land uses (e.g., urban or forest), with PO₄ comprising up to 50% of those exports. Although PO₄ exports are declining at many sites, some agricultural regions are experiencing increasing trends at a rate sufficient to drive total P trends. Regression modeling results suggest that point source load reductions are likely responsible for decreasing PO₄ concentrations observed at many sites. Watersheds with more Conservation Reserve Program enrollment had lower summer PO₄ concentrations, highlighting the effectiveness of this practice. Manure inputs strongly predicted PO₄ concentrations at high flows across all seasons. Both manure applications and conservation tillage were correlated with changes in PO₄ concentrations at high flow, suggesting these activities could contribute to increasing PO₄ concentrations. This study highlights the effectiveness of point source control for reducing PO₄ exports and underscores the need for management strategies to target sources, practices, and landscape factors determining PO₄ loss from soils where manure inputs remain high.