Estimation of total phosphorus loads for a large, flashy river of a highly developed watershed—seasonal and hysteresis effects

Estimation of nutrient loads is important for the assessment of the links between phosphorus loading and biotic impairment. The challenge for many monitoring programs is to make the best use of available information (flow and concentration data) for load estimations. This study evaluates several total phosphorus loading estimation procedures using long-term (1990–2009) data from a large, highly developed watershed that flows into Lake Simcoe. The procedures involved the grouping of long-term data into 7-year moving windows, stratifying flow and concentration data, and calculating annual load for the central year of each window. A procedure involving seasonal stratification and then further stratification of the winter/spring strata by hydrograph limb performed better than stratification by flow. The former stratification distinguished more homogenous groups based on concentration versus flow relationships (significant slopes and higher r2) and had better sampling of flow ranges. Annual loads calculated using the Beale Ratio load estimation method ranged from 3920 to 20,642 kg (1993–2005) and were greater and more consistent than loads calculated using the method historically used to calculate loads to Lake Simcoe (the Midpoint method). It is recommended, for this river, and rivers of other regions with similar characteristics (urban/agricultural watersheds affected by seasonality and hysteresis) that sampling aims at capturing the full range of flows of the hydrograph in all seasons. Data stratified by season, hydrograph or flow should be investigated further to identify combinations of these factors; then more understanding of the concentration/flow dynamics and more accurate loads can be acquired for a system.