Oxygen dynamics in a boreal lake responds to long‐term changes in climate, ice phenology, and DOC inputs

Abstract Boreal lakes are impacted by climate change, reduced acid deposition, and changing loads of dissolved organic carbon (DOC) from catchments. We explored, using the process‐based lake model MyLake, how changes in these pressures modulate ice phenology and the dissolved oxygen concentrations (DO) of a small boreal humic lake. The model was parametrized against year‐round time series of water temperature and DO from a lake buoy. Observed trends in air temperature (+0.045°C yr −1 ) and DOC concentration (0.11 mg C L −1  yr −1 , +1% annually) over the past 40 years were used as model forcings. A backcast of ice freezing and breakup dates revealed that ice breakup occurred on average 8 days earlier in 2014 than in 1974. The earlier ice breakup enhanced water column ventilation resulting in higher DO in the spring. Warmer water in late summer led to longer anoxic periods, as microbial DOC turnover increased. A long‐term increase in DOC concentrations caused a decline in lake DO, leading to 15% more hypoxic days (<3 mg L −1 ) and 10% more anoxic days (<15 µg L −1 ) in 2014 than in 1974. We conclude that climate warming and increasing DOC loads are antagonistic with respect to their effect on DO availability. The model suggests that DOC is a stronger driver of DO consumption than temperature. The browning of lakes may thus cause reductions in the oxythermal habitat of fish and aquatic biota in boreal lakes.