Major changes in fish thermal habitat diversity in Canada's Arctic lakes due to climate change

Climate warming is a major disruptor of fish community structure globally. We use large-scale geospatial analyses of 447,077 Canadian Arctic lakes to predict how climate change would impact lake thermal habitat diversity across the Arctic landscape. Increases in maximum surface temperature (+2.4–6.7 °C), ice-free period (+14–38 days), and thermal stratification presence (+4.2–18.9%) occur under all climate scenarios. Lakes, currently fishless due to deep winter ice, open up; many thermally uniform lakes become thermally diverse. Resilient coldwater habitat supply is predicted; however, thermally diverse lakes shift from providing almost exclusively coldwater habitat to providing substantial coolwater habitat and previously absent warmwater habitat. Across terrestrial ecozones, most lakes exhibit major shifts in thermal habitat. The prevalence of thermally diverse lakes more than doubles, providing refuge for coldwater taxa. Ecozone-specific differences in the distribution of thermally diverse and thermally uniform lakes require different management strategies for adapting fish resource use to climate change.

 

The following is an overview of the methods that we used in this paper. Each paragraph has an accompanying sub-section within the Methods section that provides more details. To develop the approach used in this paper, we applied both empirical and semi-mechanistic methods to build the set of predictive models needed to fulfill our primary objective: (i) predicting the impacts of climate change on the seasonal progression of thermal structure in Canadian Arctic lakes: and (ii) assessing how those impacts would change the character and diversity of the fish communities resident in those lakes^24,35. A summary of issues addressed, and methods used follows:

 

(i) Ground-Truthing Lake Morphometry: Lake shape is a primary determinant of lake thermal structure. We used the GIS-based estimates of Canadian Arctic lake morphometry as the basis for our study, hereafter the Arctic GIS lake database¹³. Our Arctic GIS lake database provides the basic information (lake area, mean depth, maximum depth) needed to characterize lake shape¹³. We confirmed the accuracy of those estimates by comparing them to an empirical database of 167 Arctic lakes obtained from a variety of sources (see Ground-Truthing Lake Morphometry section below) with morphometrics directly measured from field surveys.

 

 (ii) Lake-Specific Predictions of Ice Cover Phenology and Maximum Surface Water Temperature: These are two of the primary elements determining the seasonal pattern of a lake's thermal structure. We mobilized published empirical models to predict the impacts of climate change on these characteristics of Arctic lakes.

 

(iii) Lake-Specific Predictions of Thermal Stratification Patterns: Gorham and Boyce (1989) developed a semi-mechanistic model linking the character of summer thermal stratification in lakes to the following lake-specific characteristics: the density difference between surface and bottom waters in mid-summer, summer wind strength, and lake fetch²⁵. Gillis et al. (2021) successfully used this model to predict the presence or absence of seasonal lake thermal stratification²⁷. We mobilized various sets of empirical data to ground-truth this model for our set of Arctic lakes. We then used it, along with our other models, to forecast the impacts of climate change on:

(iv) Regional Predictions of the Impact of Climate Change on Stratification Patterns in Canadian Arctic Lakes: We accessed spatially explicit datasets for historical (1986–2005) climate and projected changes in climate for 2050 and 2100 under RCP4.5 and RCP8.5 emission scenarios using historical and future climate data for each lake from the Government of Canada's climate data extraction tool29 to generate historical and future climate conditions for each lake in our Arctic GIS data base. Using these climate conditions and lake-specific morphometric data as input to the models described above, we estimated historical and future seasonal patterns of thermal structuring for each lake. We then summarized these projected changes across all lakes in each of the eight terrestrial ecozones comprising the Canadian Arctic region³⁰.

 

(v) Regional Predictions of the Impact of Climate Change on Fish Habitat Diversity in Canadian Arctic Lakes: North American limnetic fish species can be classified into three thermal guilds (cold, cool, warm) based on their thermal preferences. We used the historical and projected future patterns of thermal structuring for each lake in our Arctic data base to generate annual, lake-specific estimates of the supply (the volume-days: m³ days) of habitat suitable for each thermal guild. We then summarized projected changes in suitable thermal habitat supply across all the lakes found in each of the eight Canadian Arctic terrestrial ecozones.

 

 

13. Campana, S. E. et al. Arctic freshwater fish productivity and colonization increase with climate warming. Nat. Clim. Chang. 10, 428–433 (2020).

24. McMeans, B. C. et al. Winter in water: differential responses and the maintenance of biodiversity. Ecol. Lett. 23, 922–938 (2020).

25. Gorham, E. & Boyce, F. M. Influence of lake surface area and depth upon thermal stratification and the depth of the summer thermocline. J. Great Lakes Res. 15, 233–245 (1989).

27. Gillis, D. P., Minns, C. K. & Shuter, B. J. Predicting open-water thermal regimes of temperate North American lakes. Can. J. Fish. Aquat. Sci. 1–78 (2021) doi:10.1139/cjfas-2020-0140.

29. Government of Canada. Climate data extraction tool. (2021).

30. Ricketts, T. A. et al. Terrestrial Ecoregions of North America - A conservation assessment. (Island Press Books, 1999).

35. Van Zuiden, T. M., Chen, M. M., Stefanoff, S., Lopez, L. & Sharma, S. Projected impacts of climate change on three freshwater fishes and potential novel competitive interactions. Divers. Distrib. 22, 603–614 (2016).