Oxygen Supersaturation in Ice-Covered Antarctic Lakes: Biological Versus Physical Contributions

Freezing in ice-covered lakes causes dissolved gases to become supersaturated while at the same time removing gases trapped in the ablating ice cover. Analysis of N$_2$, O$_2$, and Ar in bubbles from Lake Hoare ice shows that, while O$_2$ is ∼2.4 times supersaturated in the water below the ice, only 11% of the O$_2$ input to this lake is due to biological activity: 89% of the O$_2$ is derived from meltwater inflow. Trapped bubbles in a subliming ice cover provide a natural "fluxmeter" for gas exchange: in Lake Hoare as much as 70% of the total gas loss may occur by advection through the ice cover, including ∼75% of the N$_2$, ∼59% of the O$_2$, and ∼57% of the Ar losses. The remaining gas fractions are removed by respiration at the lower boundary (O$_2$) and by molecular exchange with the atmosphere in the peripheral summer moat around the ice.