Climatic sensitivity of seasonal ice-cover, water temperature and biogeochemical cycling in Lake 239 of the Experimental Lakes Area (ELA), Ontario, Canada

Regional climate plays an important role on in-lake physical and biogeochemical processes and determines the state of water quality and ecology of the lakes. This paper presents the result of a one-dimensional numerical modelling of lake ice cover, water temperature, as well as nutrient and phytoplankton dynamics in Lake 239 (Rawson Lake) located in the Experimental Lakes Area (ELA), Ontario, Canada. Using observed meteorological and inflow/outflow data, the Multi-year Lake simulation model (MyLake) was setup to replicate measured lake water temperature, ice-cover, dissolved organic carbon, particulate and dissolved phosphorus, and chlorophyll-a concentrations over the entire depth of the lake. The sensitivity of the lake to changes in climatic drivers, specifically to changes in surface air temperature and wind speed, was examined by incrementally changing the model input forcing for temperature and wind speed and mapping the resulting changes in the simulated outputs. The results show that lake processes are generally as sensitive to changes in wind speed as they are to changes in air temperature. Simulated changes in lake variables corresponding to projected changes in Global Climate Models (GCMs) under RCP4.5 and RCP8.5 emission scenarios reveal a general increase in lake water temperature and the corresponding decrease in lake ice thickness and ice cover duration under the changing climate. While lake water temperature increases at all depths, the shallow epilimnion layer is more sensitive to the changing climate than the deeper hypolimnion layer, reaching mean annual change value of +3.7 °C for RCP 8.5 and the 2090s as compared to the 1990s baseline period. While the simulated change in lake ice thickness reaches −20 cm by 2090s for the RCP 8.5 scenario, the corresponding change in ice-cover duration reaches -47 days. These changes are shown to result in seasonal changes in the biogeochemical cycling over the depth of the lake with projected increases in chlorophyll-a concentration during ice covered season of December to May reaching up to +0.15 mg/m3 for RCP 8.5 and the 2090s followed by a decrease during open water season of June to November reaching −0.15 mg/m3 compared to the 1990s baseline period.