The Lake Simcoe Region Conservation Authority

Riverine Flood Status
Lake Simcoe Shoreline Status
Low Water Status

The Lake Ecosystem

The Lake Ecosystem

Lake Simcoe provides watershed residents with many valuable socioeconomic and ecosystem services, including drinking water and wastewater assimilation, tourism, recreation, and irrigation, contributing an estimated $922.7 million annually to Ontario’s economy. Unfortunately, climate change is impacting significant aspects of the lake ecosystem, and these important services will be impacted as a result.


Lake Ice

​Lake ice cover is not only critical to ecosystem functions in the lake but also to the winter recreational economy, cultural heritage, and winter transportation. Ice record data indicates that the period of ice cover on Lake Simcoe has been decreasing, and in recent years, open water has even remained throughout the winter. Lake ice is highly vulnerable to climate change. Climate modelling predicts that this will only continue and that the duration of open water may extend by up to 47 days (from 257 days in the baseline period [1971-2000] to 304 days by 2100). This can impact lake processes as thin ice or open water allows sunlight to enter the water column in the winter.


Lake stratification is the separation of the water in a lake into distinct layers, based on water temperature and density, with the warmest (less dense) layer near the top and the coldest (most dense) layer at the bottom of the lake. Stratification is a natural process and occurs on an annual basis, with intervening periods of mixing as seasonal air temperatures change. Measurements of the timing and duration of water column stratification in Lake Simcoe since the 1980s show that stratification is occurring earlier in spring and ending later in fall. As this period of stratification extends, deep waters in the lake may become more depleted in oxygen, impacting the coldwater fishery.

Dissolved Oxygen

During summer, dissolved oxygen in deep water decreases primarily because of the decomposition of organic matter. This has the potential for negative impacts to fish and other organisms in the lake, particularly those coldwater loving species that return to the colder deeper waters of the lake during the summer. Since stratification is expected to extend under climate change, the end-of-summer dissolved oxygen level in Lake Simcoe may decrease by up to 1 mg/L by 2100. However, the amount of dissolved oxygen may actually increase in winter, as decreasing lake ice allows light into the water and photosynthesis can occur, producing oxygen.


Phosphorus is an important element in the lake ecosystem, but in high concentrations can lead to eutrophication, where excessive nutrients can stimulate the growth of aquatic plant life, resulting in decreased oxygen levels. Excess nutrients can also cause harmful algal blooms. Although phosphorus loading from human activities is the main source in Lake Simcoe, low oxygen conditions can also result from the release of stored phosphorus from lake-bottom sediment. With climate change contributing to an extended period of stratification, this re-release of phosphorous from lake sediments will likely become an ever-growing challenge.


Algae is valuable to the lake. It forms the basis of the aquatic food chain, supporting organisms like invertebrates and fish. Climate change is projected to alter the size structure of algal communities as warmer, more stable water bodies favour smaller, lighter species over heavier forms that need frequent water column mixing to remain in the sunlit areas. Warming temperature and earlier ice-off timing also results in earlier spring algae blooms, which is expected to negatively impact young fish growth and survival, and reduce recreational enjoyment of the lake. A trend toward warmer surface waters, and a longer period of water column stratification, may also give a competitive advantage to more harmful cyanobacteria (i.e. blue-green algal blooms).

Aquatic Plants

Aquatic plants are an important part of the lake ecosystem. They provide habitat for fish and invertebrates, reduce shoreline erosion, and contribute to nutrient cycling and primary production. As water temperatures warm and the growing season lengthens, increased plant growth in the lake is expected. Unfortunately, non-native species from warmer climates may be the ones that benefit the most.


Climate change projections for the watershed forecast warmer temperatures and more frequent and intense precipitation events, combined with extended dry periods, which indicate that pathogen outbreaks (e.g. E. coli) and beach closures may become more common in the future.

Fish Communities

Projected increases in water temperature and decreased dissolved oxygen levels due to climate change are likely to impair coldwater fish species (e.g. lake trout) in the lake, impacting their spawning, growth and survival rates. On the other hand, warmer water conditions will allow warmwater species such as smallmouth bass to thrive. Additionally, warmer waters will allow more southern species to move into the lake if migration routes exist.​

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Integrated Watershed Management 
1-800-465-0437 Toll free

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