Breathing in sleep: oxygen dynamics during the polar night in an Arctic lake

Seasonally ice-covered Arctic lakes lack solar radiation during polar nights, and snow cover can extend the dark period up to half a year of permanent darkness. We used high-resolution temperature and dissolved oxygen data from three winters in Arctic Lake Kilpisjärvi to quantify oxygen conditions. We applied vertically resolved single-column oxygen budget and lake-averaged oxygen budget approaches to estimate drivers of deep-water oxygen depletion. Deep-water hypoxia occurred in all 3 years, with timing and magnitude strongly influenced by under-ice temperature and mixing. Basin-scale internal waves with periods exceeding diurnal lengths were persistent, generating turbulence indicated by a vertical diffusion coefficient exceeding conductive values by an order of magnitude as the only large-scale motions under ice. The major physical factor on oxygen dynamics was the ice formation date. A 1–2 weeks earlier ice-on resulted in temperatures close to the maximum density, reduced vertical mixing, and intensification of bottom gravity currents. Downslope lateral transport by gravity currents produced accumulation of low oxygenated waters in deep layers, responsible for 55–85% of the near-bottom oxygen decrease, the rest contributed by local oxygen uptake by sediment. A delayed ice-on led to stronger vertical mixing and weaker oxygen depletion. Oxygen consumption was mainly controlled by bottom oxygen flux, while water column respiration contributed about 20% of total depletion. The results highlight sensitivity of under-ice oxygen dynamics to physical transport processes. The effect of ice formation date on under-ice oxygen content suggests potentially strong response of Arctic lakes to shortening of the ice cover.