Global climate change, particularly amplified in the Arctic, is drastically altering ecosystems. Arctic lakes, integrating atmospheric and catchment inputs, serve as robust indicators of climate change impacts, relatively free from other anthropogenic stressors. Lake warming is size-dependent: larger, shallower lakes become thermally uniform, while smaller, deeper lakes stratify, creating thermally diverse systems. Climate warming is expected to increase stratification, especially in deeper lakes. Earlier ice break-up and later freeze-up, lengthening ice-free seasons, are also projected. These changes will significantly impact water quality, aquatic biota, and ecosystem productivity. Arctic freshwater fish communities are dominated by coldwater species, which are important ecological, subsistence, and economic resources. Climate warming may reduce suitability for coldwater fish in some waterbodies but create new resources in previously unsuitable lakes. Simultaneously, it promotes the expansion of coolwater and warmwater habitats, potentially negatively impacting coldwater fish. Competition from resident and invading coolwater/warmwater species will also intensify. This study assesses the impacts of climate change on the thermal dynamics of Canadian Arctic lakes and the resulting changes in habitat diversity for different fish thermal guilds.

Prior research has documented the profound effects of climate change on global ecosystems, particularly in the polar regions where warming is amplified due to positive cryospheric feedback mechanisms. Lakes, acting as integrators of atmospheric and catchment-level processes, are recognized as valuable indicators of climate change impacts. Studies have demonstrated size-dependent lake warming patterns, with larger, shallower lakes warming more uniformly and smaller, deeper lakes exhibiting thermal stratification. The timing of lake ice break-up and freeze-up are highly sensitive to climate, with projections indicating earlier break-up and later freeze-up, leading to extended ice-free seasons. These shifts influence lake water quality, habitat availability for aquatic organisms, and overall ecosystem productivity. The literature also points to the vulnerability of coldwater fish communities, which are predominant in Arctic regions, to climate warming. Potential negative impacts include reduced habitat suitability in some areas, increased competition with coolwater and warmwater species, and the potential invasion by species from warmer regions. Previous research has highlighted the complex interplay of climate change and other factors influencing fish communities and the need for comprehensive assessments to understand these dynamic interactions.

This study estimated the thermal structure of all Canadian Arctic lakes (>10 ha; N = 447,077). Lake morphometry (maximum and mean depth) was estimated using GIS-based methods and validated using empirical data from 167 Arctic lakes. Historical (1986-2005) and future climate scenarios (RCP4.5 and RCP8.5; 2050 and 2100) were used with validated models to predict seasonal changes in lake thermal structure, considering how lake morphometry influences climate change impacts on thermal habitat diversity for different fish thermal guilds. Specific models included empirical models for ice break-up, freeze-up, and thickness, and models for maximum surface water temperature (combining the TMAX and SWT models), and a semi-mechanistic model (Gorham-Boyce) linking lake characteristics to thermal stratification patterns. Lakes were classified as barren (winter ice thickness > mean lake depth), thermally diverse (stratified in summer), or thermally uniform (mixed in summer). Annual habitat supply (volume-days) for coldwater, coolwater, and warmwater fish guilds were estimated, and changes summarized across ecozones. Establishment and persistence thresholds for each fish guild were established based on existing research on temperature preferences and population viability.

The study projects significant lake warming, longer ice-free periods, and increased thermal stratification under all climate scenarios. Maximum surface water temperature increases ranged from +2.4–6.7 °C, ice-free periods from +14–38 days, and thermal stratification presence from +4.2–18.9%. Lakes previously too cold to support fish may become habitable. The prevalence of thermally diverse lakes is projected to more than double, offering refuge for coldwater species. Coldwater habitat shows resilience, but thermally diverse lakes will shift to include coolwater and warmwater habitats, opening opportunities for southerly species. Ecozone-specific differences exist, with the greatest changes in the far north (Arctic Cordillera and Northern Arctic ecozones). The number of barren lakes decreases, with those that transition more likely to become thermally uniform than thermally diverse. Thermally uniform lakes decrease in prevalence but will also become more favorable to coolwater and warmwater species. Ice-free coldwater habitat decreases, while coolwater and warmwater habitat increases, with variations across ecozones. The greatest relative decreases in coldwater habitat are projected for the Taiga Shield, while the largest increase in coolwater habitat is predicted for the Southern Arctic. The Taiga Plains, Taiga Shield, Boreal Plains, and Southern Arctic show the largest relative increases in coolwater and warmwater habitat. Coldwater habitat remains at similar levels for thermally diverse lakes, while thermally uniform lakes shift from coldwater-dominant to coolwater-dominant (except in the far north). Under RCP8.5, warmwater habitat increases substantially. Overall, open-water volume-days increase, with coldwater habitat decreasing under RCP8.5. The study shows a much-needed estimate of historical thermal habitat supply in Arctic lakes and how it is projected to change.

The findings demonstrate that climate change will substantially alter the thermal habitat diversity of Canadian Arctic lakes, with significant implications for fish communities. The increased prevalence of thermally diverse lakes provides refuge for coldwater species, but the emergence of coolwater and warmwater habitats will alter species composition and interspecific competition. The projected changes are not uniform across the Arctic landscape, emphasizing the need for ecozone-specific management strategies. The resilience of coldwater habitat in some areas, particularly in the far north, may offer opportunities for increased sustainable harvests. However, in other regions, particularly the south, the invasion of coolwater and warmwater species poses a significant threat to coldwater fish production. The limited connectivity of the Arctic hydrological network may help maintain fish diversity, but the potential for species invasions through connected waterways necessitates vigilant monitoring and management. The study's projections align with global lake projections, highlighting the accelerated warming of ice-covered lakes. The results highlight the vulnerability of Arctic freshwater fish communities and the importance of incorporating climate change projections into conservation and management planning.

This study provides a comprehensive assessment of the impacts of climate change on thermal habitat diversity in Canadian Arctic lakes. The projected increases in thermal diversity and the resilience of coldwater habitat in some regions offer a nuanced understanding of the effects of climate change. However, the study also highlights the significant threats posed by the expansion of coolwater and warmwater habitats and the potential for species invasions. Future research should focus on refining models to incorporate greater detail on under-ice winter conditions, changes in freshwater distribution, impacts on thermal diversity within the coldwater guild, and the application of new technologies for monitoring.

The study employs simplified models of lake stratification dynamics, which may not fully capture the complexity of real-world processes. The use of a fixed percentile value for characterizing summer storm wind strength could influence the accuracy of stratification projections. The study's focus on ecozones, rather than individual lakes, limits the resolution of the predictions. The defined thermal guild boundaries are somewhat arbitrary and could affect the interpretation of the results. Furthermore, the lack of daily temperature sampling in some of the ground-truthing datasets may have introduced uncertainties into the model validation.