Increasing nitrate concentrations in streams draining into Lake Ontario

Nitrate (NO3-N) concentrations in offshore waters of Lake Ontario increased by approximately 60% between the 1970s and 2000s, although the drivers are unclear. Here, we show that NO3-N concentrations also increased significantly in at least one season at 13 of 15 large southern Ontario tributaries (8.4 to 2779 km2) that drain into Lake Ontario and have no known upstream wastewater treatment plants. Average NO3-N concentrations more than doubled between the 1970s and 2000s at some streams. Only the two most urbanized streams did not show any increase in NO3-N and NO3-N declined at the most urbanized catchment in this study (Sheridan Creek; 92% urban). Agriculture is the predominant form of human activity at the 13 watersheds where NO3-N increased, accounting for 51-71% of total land cover. Both the total area of agricultural land and the type of agriculture have changed dramatically in southern Ontario; and these shifts could alter nutrient transfer to waterways. Specifically, shifts in agriculture towards more N-demanding annual row crops like corn could result in higher NO3-N leakage to streams, and the impact of this form of land use change on nutrient export requires further investigation. Overall, these results suggest that changes in tributary loading may have contributed to recent observations of increasing NO3-N levels in offshore waters of Lake Ontario.