Subice layering and origin of acidic waters in a small boreal lake during the spring runoff

Abstract

In a small boreal forest lake, the subice circulation of waters from five distinct origins (lake premelt water, lakeshore snowpack meltwater, ice cover meltwater, groundwater and hillslope through‐flow water) was studied during the spring melt period. A network of collared access holes was used to monitor the temperature, conductivity and pH profiles of the water column beneath the ice cover. The study also included the tracing of snowbank meltwaters and through‐flow waters with rhodamine. Conductivity and pH appeared to be efficient parameters for discriminating most of the water types considered. During the spring melt three distinct low‐pH episodes were recorded at the lake outflow. The first episode (pH 5) occurred in the early melt period; snow and ice meltwaters layered beneath the ice forming a stratum ≈20 cm thick. It moved toward the outlet under laminar flow conditions (observed average velocity, 1m/h). Two subsequent pH episodes of pH 5.3 and 5.1 were observed during the massive melt period. These episodes were due to peak flows of hillslope through flow which entered the lake and formed a layer 20–40 cm thick in the water column (observed average velocity, greater than 20 m/h). The ice cover thus not only modified the nearshore water quality by inducing thermal layering but also strongly influenced the quality of the outflow waters due to the segregation and rapid evacuation of the through flow and surface meltwaters.