Total phosphorus budgets and nitrogen loads: Lake Simcoe, Ontario (1990 to 1998)

Prompted by concerns over eutrophication, the total phosphorus (TP) budgets of Lake Simcoe were measured from 1990 to 1998. Total nitrogen (TN) loads from a portion of the catchment and from atmospheric deposition were also measured. Mean concentrations of TP and TN over the study period were highest in rivers draining subcatchments with the highest proportions of vegetable polders and urban development, and lowest in those draining a higher proportion of forest and scrubland. The mean annual exports of TP and TN measured were highest from vegetable polders (109 and 2,540 kg/km(2)/yr respectively). High TP export (65 kg/km(2)/yr) was also measured from the subcatchment with the highest proportion of urban land. Export from mixed agricultural subcatchments ranged from I I to 27 kg/km(2)/yr for TP and 220 to 790 kg/km(2)/yr for TN, while export from catchments with a high proportion of forest and scrubland ranged from 6 to 7 kg/km(2)/yr for TP and 170 to 270 kg/km(2)/yr for TN. Mean (1995 to 1998) atmospheric deposition directly to the lake was 56 kg/km(2)/yr for TP and 920 kg/km(2)/yr for TN. The largest sources in annual TP budgets were atmospheric deposition (23 to 56%), the tributaries (17 to 49%) and urban non-point sources (9 to 22%). Overall the annual load of TP to the lake ranged from 85 to 157 t over the study period, remaining well above a 75 t/yr target, and concentrations of TN and TP in tributaries draining agricultural and urban areas were above recommended levels. Continued efforts are thus required to reduce nutrient inputs to Lake Simcoe from non-point sources.