Perennial N₂ supersaturation in an Antarctic lake

Abstract

The dry valleys of southern Victoria Land in Antarctica contain several closed basins in which perennially ice-covered lakes are found. One of the most unusual features of these lakes is the occurrence of high O₂ concentrations in the water column; values ranging from slightly more than saturation to more than four times saturation have been reported^1–3. Recently, we considered a bulk O₂ budget for Lake Hoare, Antarctica, which led us to suggest that biological processes alone were not sufficient to explain the observed elevated oxygen levels³. Consequently, there must be a non-biological source of O₂. We suggested that this source results from the exclusion of O₂ during the freezing of aerated meltstream water at the bottom of the ice cover, and predicted that this physical mechanism should also enhance the other atmospheric gases. Here we report the results of a study which, for the first time, documents the supersaturation of N₂ in a lake. Dissolved N₂ levels of 145% and 163% were determined from samples taken just below the ice cover and at a depth of 12 m, respectively. The relative importance of biological and abiological sources is reflected in the ratio of N₂ concentration to O₂ concentration. In Lake Hoare this ratio was 1.20 at the ice/water interface and 1.05 at 12 m; considerably different from the ratio in equilibrium with air (∼1.8). Based on these results, we have determined that about half of the net O₂production in the lake is the result of biological processes.