Flowpath and retention of snowmelt in an ice-covered arctic lake

The extent to which snowmelt flowing into ice-covered lakes spreads horizontally and mixes vertically influences retention of solutes derived from the landscape. To quantify these transport processes and retention, we combine time series temperature and specific conductance measurements in Toolik Lake (Alaska) and its major inflow, with measurements of discharge and meteorology, and profiles of specific conductance, temperature, fluorescence, chlorophyll a, and dissolved organic carbon (DOC) in spring of 3 yr. During early snowmelt, the concentration of DOC in the stream was 750 μM, twice that in the lake. During slow melt (discharge (Q) < 4 m³ s-1), the incoming solute-rich intrusion spread lakewide below the ice. During melt with Q > 6 m³ s-1, the incoming water partially flushed the inlet basin and the more dilute water flowed over the original intrusion with a preferential flowpath to the outlet. Penetrative convection was restricted by the increased density gradients from the incoming plume and initially constrained to shallow mixing zones associated with the step changes in density. As ice thickness decreased to less than 1 m, heating caused density instabilities at the base of the intrusions that mixed solutes ~ 10 m vertically, contributing to retention. Near-surface layers enriched with DOC persisted for ~ 10 d during a rapid melt and for over 3 weeks when the melt was slow. Retention, of order 10–20%, also depended on the rapidity of melt and magnitude of discharge.