A comparison of nutrient export at two agricultural catchments: insight into the effect of increasing urban land cover in southern Ontario

Abstract

Abstract:This study compared concentrations and export of total phosphorus (TP; including dissolved and particulate forms) and nitrogen (N) between quickflow and delayed flow at two adjacent agricultural catchments (Jennings and McLarens Creeks), one of which is undergoing increased urbanization. Annual runoff, frequency of quickflow events in the growing season (GS), and chloride (Cl À ) concentrations and export were greater at the more urbanized Jennings Creek. There were only modest differences in TP and total dissolved P between the two catchments, with the largest difference occurring in the GS, when the urbanized Jennings Creek had higher TP concentrations and export. In contrast, the majority of N export was as dissolved inorganic nitrate-N (NO 3 À -N), and NO 3 À -N concentrations and export were consistently greater at the more agricultural McLarens Creek. Particulate P was sensitive to increases in quickflow, with particulate P accounting for approximately 75% of TP export during GS events. In contrast, NO 3 À -N export was less influenced by hydrologic conditions than P and appears to be delivered to these streams primarily by subsurface flow, which resulted in greater NO 3 À -N export during delayed flow compared with events at both catchments. Overall, these results suggest that urban cover at Jennings Creek has impacted streamflow with more frequent quickflow events during the GS and substantially higher stream Cl À , although the influence of urban increases at the expense of agricultural land on P and N export was less clear.