Muskoka Lake is the largest of a group of scenic lakes in the rocky uplands of southern Ontario located 200 to 240 km north of Toronto and 20 km east of Georgian Bay. The term "Muskoka" may come from the Huron Indian names "musquash" and "ooka" meaning "the place where the large rocks are red" (17).

The swift flowing Muskoka River empties the combined discharges of the Muskoka District Lakes into Lake Muskoka which discharges into the Moon River which empties into Georgian Bay. Between Precambrian Shield outcrops of igneous metamorphic rocks are dense forests of balsam fir, pine and spruce, as well as numerous bogs, and some farm pasture fields. The many birch and maple trees add color to the annual autumn display which attracts thousands of visitors to the area each year. The Muskoka Lakes District is one of the most famous summer cottage and resort areas in Canada. Well known resort areas include Bala, Baysview, Bracebridge, Dorset, Gravenhurst, and Port Sydney.

In 1850, the government of what was then Upper Canada, opened the 1,620 km2 Muskoka Lakes District for settlement but very few people came. Eighteen years later the new government of Ontario offered 40 hectares of Muskoka land free to individuals and another 40 free to families on condition that they cleared 6 ha and built a substantial log house within 5 years (17). Americans and Canadians began visiting the Muskoka Lakes in increasing numbers in the 1870's when rail and paddlewheel ships provided access to many of the larger Muskoka Lakes. The lower Muskoka Lakes, including Muskoka, Rosseau and Joseph, were water connected by a small lock which allowed boat deliveries to resort hotels along the 480 km of combined shoreline.

Initially, logging of vast stands of white pine provided a mainstay for the economy of the area but by 1910 the vast majority of the white pine had been logged out. While there was in the past some industry in the towns, (e. g. textile mills in Bracebridge and boat builders in some of the lower lakes), limited farming and servicing the influx of tourists to the area each summer became the main economic activity and it remains so to this day. Today there are more than 20,000 cottages (average cost $ 250,000) and 130 hotels and motels crowded into the Muskoka Lakes District. The number of visitors to the district is now approaching 1.5 million per year (17).

Gravenhurst Bay on Lake Muskoka has been sampled every year from 1969 to 1985 by the Ontario Ministry of Environment. During the "70's" Gravenhurst Bay experienced periodic algal blooms associated with excessive nutrient inputs (2). In 1971 the Gravenhurst Bay Sewage Treatment Plant became one of the first sewage treatment plants in Ontario to be equipped with phosphorus removal technology. As a result, phosphorus loading and chlorophyll concentrations declined until 1975 when a second sewage treatment plant in Gravenhurst Bay began discharging nutrient rich effluent into the bay. This was corrected in April of 1976 and further improved by diverting sewage from the bay in 1989. While most of Lake Muskoka maintains oligotrophic conditions, restricted areas such as Gravenhurst Bay at the southern end of the lake near the town of Gravenhurst have shown deterioration of water quality. However, today the levels of enrichment are low (mean annual phosphorus = 5.3 micro g l-1) as are whole lake chlorophyll concentrations (1.52 ag- 1)(1).

The Muskoka lake drainage basin receives between 30 and 40 kg ha-1 yr-1 of sulfate from acid precipitation. The alkalinity of Muskoka Lake is low and therefore the lake has been placed in the moderate risk class with regard to its potential for lake acidification (1).

There is sport-fishing for lake trout, rock bass and small mouth bass but the presence of mercury in all three of these species has resulted in their limited consumption as recommended by the provincial government. These limits suggest that lake trout larger than 45 cm and small mouth bass larger than 35 cm should not be eaten. It is believed that the mercury enters the lake from the natural weathering of mercury-rich soils in the drainage basin and from long range transport. Both phenomena are currently under investigation by the Ontario Ministry of the Environment.