Pathways of Snowmelt Water into an Ice-Covered Lake

Abstract:

Discharge of water into ice-covered arctic lakes during snowmelt can be high, but no general framework exists to quantify the pathway of the flow into the lakes and the associated distribution of incoming resources including dissolved organic carbon (DOC) or greenhouse gases. In this study, we characterize the fate of the snowmelt water flowing into 1.5 km² Toolik Lake, Alaska, in 2014 and 2015. We deployed arrays with temperature, conductivity, and oxygen sensors in the water column over the winter, performed high temporal and spatial resolution CTD surveys on four 500 m to 1 km long transect lines during spring, and obtained correlative meteorological and discharge data. During both study spring periods, we observed different snowmelt inflow regimes based on the discharge rate (low and high) which led to differences in the extent of vertical and horizontal dilution of the lake water. Our first estimates of horizontal dispersion of snowmelt water in Toolik Lake under a high discharge regime are in the upper range of values found for ice-covered lakes (O ~ (10²) cm² s^-1). In both years, the incoming water spread over ~75% of the basin near the surface with associated loading of DOC and methane. Spring 2014 was typical of other years with a gradual snowmelt and restricted depth of penetration of the incoming water. In fact, the increased density gradient in the upper few meters created conditions which retarded subsequent mixing at ice off. In contrast, persistent high pressures over the Alaskan region caused an exceptionally warm spring and rapid snowmelt in 2015. The subsequent warming of stream waters meant that the within lake vertical density gradient was weakened and facilitated later mixing. The differences in magnitude of discharge and temperature of incoming water during the more average and the warm springs enable interpretations and predictions of the fate of solutes flowing into lakes during snowmelt under variable weather regimes.