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Climate change impacts on a eutrophying lake : Cultus Lake, British Columbia, Canada Sumka, Mark Gregory


This study characterizes the thermal dynamics of Cultus Lake, British Columbia, Canada, and assesses the impacts of climate change on critical habitat for species-at-risk Sockeye Salmon (Onchorynchus nerka) and Cultus Lake Pygmy Sculpin (Cottus aleuticus). Historical field data spanning 1920s–1930s, 2001–2003, and 2009–2016 were analyzed and a one-dimensional hydrodynamic model (General Lake Model) was calibrated using field data collected in 2016, and validated using field data from 2001–2003 and 2009–2016. The thermal structure of the lake was simulated to 2100 using outputs from the downscaled Canadian Regional Climate Model (CanRCM4) for two climate change scenarios (RCP4.5, moderate emissions scenario; and RCP8.5, extreme emissions scenario). Historically (1923–2016), the total lake heat content increased at a rate of 0.80 MJ m⁻² a⁻¹ and is projected to warm by 2.0 MJ m⁻² a⁻¹ (4.2 MJ m⁻² a⁻¹) for RCP4.5 (RCP8.5). Historically, the Schmidt stability increased by 2.2 J m⁻² a⁻¹ and is projected to increase by 2.6 J m⁻² a⁻¹ (6.5 J m⁻² a⁻¹) for RCP 4.5 (RCP8.5). The duration of stratification has historically been increasing at a rate of 0.18 d a⁻¹ and is projected to increase by 0.18 d a⁻¹ (0.50 d a⁻¹) for RCP4.5 (RCP8.5). The onset of stratification is now two weeks earlier than the 1920s–1930s and currently occurs around 23 March while the breakup date has not changed and occurs around 15 December. However, it is predicted that there will be no change in the date of onset of stratification while breakup will be delayed to 12 January (25 January) for RCP4.5 (RCP8.5). Lake surface temperature and outflow temperature is most important for salmon survival in August through November corresponding with the salmon run. Historical change in mean monthly temperature ranged from 0 °C a⁻¹ in November to a maximum of 0.016 °C a⁻¹ in August. This is predicted to increase to 0.016 °C a⁻¹ (0.046 °C a⁻¹) in November and 0.031 °C a⁻¹ (0.069 °C a⁻¹) in August for RCP4.5 (RCP8.5). Projections indicate fundamental changes to the thermal characteristics of Cultus Lake, which may further degrade water quality, particularly in conjunction with ongoing eutrophication, eliciting fundamental changes in the structural and functional attributes of critical habitat for species-at-risk.

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