Last updated: December 3, 2020
Article
Lake Temperature Trends
NPS/Paul Gabriel
We know from past studies that summertime surface water temperatures are warming in lakes, worldwide. Less well studied are changes in lake temperatures throughout the year and water column. Given that many fish–including juvenile salmon–experience more than just summertime surface water temperatures, a deeper understanding of lake temperature trends is warranted. In a recently published report, we used year-round, multi-depth data from moored installations known as temperature arrays to assess trends from 2006 through 2018 in four lakes with relatively easy access, heavy use, and focused concern for management. Although the temperature array dataset is relatively limited in both space and time, it encompasses more than 4 million records in a region where continuous water temperature data are generally lacking. Our findings indicate that:
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monthly mean surface water temperatures have warmed at all arrays since monitoring began;
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at some arrays, strong warming trends extend to deeper waters; and
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the rate of warming varied from 0.02 to 0.24 °C per year, depending on the array and depth.
At the low end of this range, the rates are small, when considered on an annual timestep. However, if the rates persist, they may amount to ecologically significant changes over time, particularly in the surface waters where juvenile salmon feed. The optimal temperature range for juvenile salmon is 12-15 °C, with the upper end of this range marking the state’s water quality threshold for freshwater habitats that support fish rearing. It is noteworthy, then, that daily mean surface water temperatures have already exceeded the state’s threshold at some point during the period of record at all arrays, while deeper waters (below 20 m) have not. Collectively, this suggests that deeper waters may offer refuge to juvenile salmon–now and in the future–even when temperatures higher in the water column are suboptimal.