Recent changes and patterns in the water chemistry of Lake Simcoe

After the Great Lakes, Lake Simcoe is the largest lake in southern Ontario. Located within a I hour drive of half the population of Ontario, there is currently major concern over the impact of rapid urbanization on the nutrient status of the lake. However, despite a more than doubling of the human population in the Lake Simcoe watershed over the past two decades, average total phosphorus (TP) levels were lower in 2000-2003 compared with 1980-1983 at six of eight lake stations, and declines were significant at the most nutrient-enriched near-shore sites located in Kempenfelt Bay (0.29 mu g/L/year) and Cook ' s Bay (0.31-0.41 mu g/L/year). Total P concentrations varied two-fold across the lake, and phytoplankton bio-volume followed a similar pattern, with greatest phytoplankton abundance occurring at high TP sites, particularly in Cook's Bay. Nevertheless, steep declines in bio-volume occurred at all sites beginning in the mid-1990s, and water clarity (Secchi disk depth) improved concurrently such that Secchi depths were 40-80% greater in 2000-2003 compared with 1980-1983. Zebra mussels, which became established in Lake Simcoe around 1995, likely contributed to decreased phytoplankton bio-volume and related improvements in water clarity, which began during the mid-1990s. Despite major reductions in phytoplankton bio-volume, average rates of dissolved oxygen (DO) depletion (18 m-bottom zone; normalized to 4 degrees C) did not change substantially over time, and were similar in 2000-2003 (average 1.25 +/- 0.21 g O-2/m(3)/month) compared with 1980-1983 (average 1.26 +/- 0.19 g O-2/m(3)/month). In contrast, minimum, end-of-summer DO levels (18 m-bottom) increased slightly over the period of record. Although there has been some improvement in end-of-summer DO availability, DO concentrations continue to decline to levels that are limiting to many fish species (e.g., 3.8 mg O-2/L in 2001) by the end of the summer stratified period.