Recent Changes and Patterns in the Water Chemistry of Lake Simcoe

Abstract

After the Great Lakes, Lake Simcoe is the largest lake in southern Ontario. Located within a 1 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 μg/L/year) and Cook's Bay (0.31–0.41 μg/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°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.