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What do warmer temperatures mean for the health of our lakes?

Author: Arunemathi Shanmugam, University of Waterloo

This July was the warmest on record and we expect to continue to see record-breaking weather in the near future. Warmer air temperatures will cause a variety of changes, including changing ice-covered periods, areas that experience ice cover, reducing snow cover, decreasing snowfall, and increasing water temperatures1. These changes are obviously bad news for our friends who enjoy snowmobiling and ice fishing, but what do they mean for the health of our lakes?

Climate change is expected to decrease dissolved oxygen (DO) concentrations in Ontario lakes. DO comes mainly from the atmosphere and supports aquatic animals2. In particular, increased water temperatures caused by climate change will likely have a significant impact on DO concentrations in the deepest parts of our lakes. Differences in shallow and deep water DO are influenced by water temperature. Cold water holds more DO than warm water because colder water temperatures increase the solubility of oxygen in water3. DO will decrease in shallow waters as water temperatures rise, due to rising air temperatures and reduced wind speeds4. DO concentrations throughout a lake are affected by the distinct layers of water that form due to temperature differences5. Water near the lake’s surface is warmed by the sun and becomes less dense than the colder water underneath it6. The differences in temperature between surface waters and deeper waters produce layers of water of different densities that do not easily mix7. So, this layering of water prevents water from circulating throughout the lake, so deep water DO is not readily replenished by atmospheric oxygen like the shallower layers.

Changes in DO concentrations can make it more difficult for lakes to support aquatic life. Most aquatic animals have a preferred range of water temperature and DO concentration. For instance, Lake Trout (Salvelinus namaycush) prefer cold water habitats that are well oxygenated8. Decreases in DO in deep water can force deep-water fish up into warmer waters, where their survival is jeopardized9. Overtime these fish could die as they struggle to compete with fish that are more tolerant to low oxygen habitats10.

Loss of deep water DO also degrades water quality11. Really low levels of DO can cause chemical reactions that release nutrients from lake sediments. The release of nutrients, like phosphorous, can stimulate the growth of algae, which can produce harmful algal blooms12. These algal blooms can produce dangerous toxins that are harmful to humans and animals13.

Due to the importance of DO for healthy lake ecosystems, it is often used to measure the health of lakes and streams. The RLCA has purchased a DO data logger to monitor DO levels in our lakes. DO monitoring will help give us an early warning sign if climate change is harming our lake ecosystems. The RCLA is collaborating with the University of Waterloo this fall in a study looking at comparing DO concentrations across lakes to better understand if the deep locations currently monitored by the Lake Partner Program is sufficient as our only monitoring location.

1 United States Environmental Protection Agency, “Climate Change Indicators: Snow and Ice”, July 26, 2023,

2 Water Science School, “Dissolved Oxygen and Water,” last modified June 5, 2018,

3 United States Environmental Protection Agency, “Dissolved Oxygen”, May 18, 2023,

4 Stephen F. Jane et al. “Widespread Deoxygenation of Temperate Lakes,” Nature 594 (2021): 66-70, doi: 10.1038/s41586-021-03550-y.

5 Paul Fafard, “How and Why Lakes Stratify and Turn Over: We explain the science behind the phenomena,” International Institute for Sustainable Development, May 16, 2018,

6 Ibid.

7 Ibid.

8 We Fish Canada, “Fish Species,” accessed August 11, 2023,

9 Emily Chung et al. “Bad news for fishing: Climate change is sucking the oxygen out of lakes, study suggests,” CBC News, June 10, 2021.

10 Ibid.

11 Stephen F. Jane et al. “Widespread Deoxygenation of Temperate Lakes,” Nature 594 (2021): 66-70, doi: 10.1038/s41586-021-03550-y.

12 Ibid.

13 Ibid.

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