Incomplete mixing in a small, shallow Arctic delta lake: Implications for water column biogeochemistry

The extent to which lakes mix modulates dissolved oxygen and biogeochemical conditions within the water column. The mixing dynamics of Arctic delta lakes have not been investigated in detail and have been treated as cool, shallow systems influenced by routine flooding. Past biogeochemical studies have presumed these lakes mix and release winter-derived gases at ice-off. Here we assess stratification and extent of mixing over a full year in a 5-m deep, floodplain lake located in the Mackenzie Delta (NT, Canada). Moored arrays and a meteorology station enabled calculation of physical parameters such as density, Lake number, buoyancy frequency, and heat budgets. The lake received only minor river flooding and never completely mixed during the year. A chemocline prevented turnover at ice-off. This was followed by strong thermal and chemical stratification when ice-free, resisting wind-induced mixing across the thermocline even through fall. A 3-layer structure developed with an upper mixed layer, metalimnion, and monimolimnion. Each layer had distinct dissolved oxygen levels, with the highest concentrations in the metalimnion and hypoxia in the monimolimnion, where CH4 accumulated. These results, in combination with prior studies’ measurements of CH4 in bottom-waters, implies this lake is in a state of continuous multi-year stratification.