Spatial distribution and temporal variability in the forms of phosphorus in the Beaver River subwatershed of Lake Simcoe, Ontario, Canada

Agricultural runoff is an important source of phosphorus (P) to surface waters. This paper investigated the relationship between agricultural land use and the forms of P (i.e., total phosphorus [TP], total dissolved phosphorus [TDP], and soluble reactive phosphorus [SRP]) in streams draining 8 headwater subcatchments in the Beaver River subwatershed, a major inflow to Lake Simcoe, which is a large hard-water lake in south-central Canada. The time period of analysis had a strong influence on the results. There was no relationship between percent total agriculture and average TP when concentrations were averaged over the entire 12-month (Jun 2010–May 2011) monitoring period, whereas there were significant positive correlations between agricultural land use and average TP during the summer season (Jun–Aug). Significant correlations between average stream TDP and SRP and percent pastureland were observed, although relationships were again dependent on the time period of analysis. Concentrations of TP, TDP, and SRP were highly variable over time, with maximum concentrations occurring during the winter months. This was illustrated by a single rain-on-snow event (11 mm) on 5 March 2011, when samples taken 3–4 h apart varied by as much as 100%. These results indicate that winter storm-targeted sampling is likely necessary to capture the full range of annual variability in P forms and concentrations