Seasonal nutrient export dynamics in a mixed land use subwatershed of the Grand River, Ontario, Canada

Abstract

Algal blooms in the Great Lakes are a concern due to excess nutrient loading from non-point sources; however, there is uncertainty over the relative contributions of various non-point sources under different types of land use in rural watersheds, particularly over annual time scales. Four nested subwatersheds in Southern Ontario, Canada (one natural woodlot, two agricultural and one mixed agricultural and urban) were monitored over one year to identify peak periods (‘hot moments’) and areas (‘hot spots’) of nutrient (dissolved reactive phosphorus, DRP; total phosphorus, TP; and nitrate, NO3−) export and discharge. Annual nutrient export was small at the natural site (0.001 kg DRP ha−1; 0.004 kg TP ha−1; 0.04 kg NO3—N ha−1) compared to the agricultural and mixed-use sites (0.10–0.15 kg DRP ha−1; 0.70–0.94 kg TP ha−1; 9.15–11.55 kg NO3—N ha−1). Temporal patterns in P concentrations were similar throughout the sites, where spring was the dominant season for P export, irrespective of land use. Within the Hopewell Creek watershed, P and N hot spots existed that were consistently hot spots across all events with the location of these hot spots driven by local land use patterns, where there was elevated P export from a dairy-dominated sub-watershed and elevated N export from both of the two agricultural sub-watersheds. These estimates of seasonal- and event-based nutrient loads and discharge across nested sub-watersheds contribute to the growing body of evidence demonstrating the importance of identifying critical areas and periods in which to emphasize management efforts.