Nitrogen versus phosphorus export in the Bay of Quinte, Ontario, Canada: Is the role of nitrogen fate and transport being overlooked in agricultural watersheds?

Study region: Two small agricultural watersheds — the Napanee River and Wilton Creek watersheds — are located in one of the most degraded sites in the Great Lakes basin: Bay of Quinte in northeastern Lake Ontario. Study focus: The present study uses the Soil and Water Assessment Tool (SWAT) model to delineate hotspots and moments of excessive Nitrogen (N) export, as well as to identify the underlying drivers. The efficiency of common Best Management Practices (BMPs), frequently advocated to control phosphorus (P), in also reducing N loading is tested with SWAT. We also offer a novel post-hoc probabilistic analysis that links the TN:TP ratios of the exported material from the watershed with the corresponding ratios in the adjacent watercourses that ultimately shape the nature of nutrient limitation of phytoplankton growth in the Bay of Quinte. New hydrological insights for the region: Our modelling exercise reveals a close association of cropland areas, steeper slopes, and the prevalence of certain soil types with excessive TN and TP export rates. Our TN:TP ratio analysis predicts a higher likelihood of P being exported in short supply in the northern and N in the southern part of the Napanee watershed, whereas a higher likelihood of excessive N export was registered near the outlet of the Wilton Creek watershed. Although the complete stacking of all BMPs led to a distinct nutrient loading reduction, our analysis suggests more effective P control compared to N using the implemented BMPs, especially in the Napanee River watershed. The present analysis signifies the importance of an integrated N and P management to address nutrient pollution and to improve ecosystem resilience in the Great Lakes region.