Spatial heterogeneity in groundwater and evaporative fluxes and their influence on lakes with varying methane dynamics within Mackenzie River Delta (Canada)

Hydrologic processes affecting Arctic lakes are sensitive to permafrost thaw depths and climate change. Lake water balances reflect changes in air temperature, precipitation, permafrost connectivity, and groundwater transport. The Mackenzie River Delta (MRD), one of the great river deltas of the Arctic coast, contains over 45,000 freshwater lakes. It is an essential place to explore lake hydrologic and biogeochemical processes. The MRD has discontinuous permafrost and previous studies indicate that the delta lakes have negative summer-time water balances, with evaporation greater than annual precipitation. Lake levels are replenished in the spring by the Mackenzie River flood.

Three lakes in the middle MRD near Inuvik, Northwest Territories, Canada were selected for study. The lakes are near one another and the Mackenzie River; they are small (0.2 ha - 3.1 ha), but have variable depths (1.5 m- 3.1 m). We measured lake level using pressure transducers deployed continuously over a two-year time period (August 2015 to August 2017). Deep lake water (25 cm from sediment surface) was collected from two lakes using autonomous, continuous samplers (OsmoSamplers) and integrated samples of 1-2 weeks of lake water were analyzed for their methane (CH₄), Cl, Ca, Mg, Sr, Ba, and Li concentrations.

All three lakes exhibited overall lake level decline by the end of the open-water period, but they were influenced by different hydrologic processes. Summer evaporation was the major hydrologic process affecting lake level for one of the three lakes. The shallowest lake exhibited lake level decline consistent with evaporation and low open-water dissolved CH₄, but the two deeper lakes had groundwater influx during the same time-period and variable dissolved CH­₄. Ion concentration due to summer lake evaporation was minimal compared to the much greater concentration in winter due to solute exclusion during ice formation. Surprisingly, in all three lakes, despite the expected groundwater connection to permafrost in two lakes, microbial activity produced modern methane. Taken together, these results indicate that lakes in delta systems should not be assumed to have similar hydrologic or biogeochemical behavior, especially if there are differences in lake geomorphology and permafrost site characteristics.