Physico-chemical changes in two northern headwater lakes in the Northwest Territories, Canada, during winter to spring seasonal transitions

This note describes extended monitoring of two small headwater lakes in the Lockhart River in the Northwest Territories (Canada) during consecutive late winter and spring transitions. During these transitions, the lakes changed from isolated ice-bound water bodies to interconnected lakes, throughout which they exhibited similar modifications to their physico-chemical characteristics. Lake temperature changes were linked to ambient conditions, with warming after May 1 initiating snow- and ice melt inflows. Lake temperatures of 3 °C to 4 °C were measured in both lakes at the start of monitoring, which were maintained until a steady increase commenced in early June 1 as ice cover started to recede. This period was also associated with peak water levels and lake discharges. Conductivity decreases (~ 26 to ~ 12 μS/cm in the larger, shallower lake, Area 8, to 16 to 11 μS/cm in the smaller, deeper lake, Lake I1) commenced around mid-May, in advance of lake temperatures increases, and stabilized after approximately 3 weeks once lake ice had melted. The extent of lake dilution based on conductivity decreases (54-56% in Area 8, and 31% in Lake I1) was attributed primarily to the ice cover melt volume on each lake at the start of monitoring. DO concentrations were mostly undersaturated at the start of the monitoring period, with values below aquatic life guidelines; Area 8 reached saturation once lake conductivity stabilized, but Lake I1 was saturated once conductivity started to decrease. pH transitioned from slightly acidic (~ 5.8; below aquatic life guidelines) to circumneutral, with the transition slightly different between lakes depending on the lake conductivity transition.