Modelling the impact of increasing dissolved organic carbon load on seasonal anoxia in a boreal humic lake.

Summary

Boreal lakes are impacted by climate change, reduced acid deposition and changing loads of dissolved organic carbon (DOC) from the catchment. We set to explore how these changes, in particular the increasing DOC load, modulate ice phenology and dissolved oxygen (DO) of a boreal humic lake located in southeastern Norway. Observed trends in daily air temperature (+0.045 oC yr-1) and weekly DOC concentration (0.1 mg C yr-1, +1% annually) measured over the past 40 years at the study site were used as forcings for the lake model MyLake. The model was parametrized against year-round time-series of water temperature and DO from a high-frequency lake buoy. A backcast of ice freezing and break-up dates to 1974 reveals that ice break-up occurs on average 8 days earlier in 2014 than in 1974. An earlier ice break-up enhances water column ventilation, resulting in higher DO in the spring. Later in the season, warmer water in late summer lead to longer anoxic periods, as microbial DOC turnover increases. Long-term increase in DOC concentrations causes decline in lake DO, leading to 15% more hypoxic days (< 3 mg L-1) and 10% more anoxic days (< 15 ug 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. DOC increase thus has the potential to reduce the oxythermal habitat of fish and aquatic biota in boreal lakes.