THE EFFECT OF GROUNDWATER DISCHARGE ON MEROMIXIS AND GEOCHEMISTRY IN UPPER SUBALPINE O’BRIEN LAKE, BANFF NATIONAL PARK, CANADA

Located in the Rocky Mountains of Alberta Canada, we posit that 21-meter-deep O’Brien Lake is meromictic. The mixolimnion of O’Brien shows consistent dissolved oxygen, specific conductance, and temperature indicating that the water is well mixed. The chemocline features a localized temperature inversion of warmer water from about 8-11 m deep surrounded by colder temperatures both above and below in the water column. A putative spring is hypothesized to feed warmer, higher conductivity water into the middle depths of O’Brien forming the chemocline and causing meromixis of this lake. We show that there is a significant chemical contribution to density determining lake stability and stratification despite inconsistent temperature variations with depth. Below the chemocline, cold, anoxic (sulfidic) waters with relatively higher chemical and nutrient concentrations, keeps this deepest layer in the water column dense and resistant to mixing. The chemocline includes a persistent sharp peak in turbidity at 11 m as well as the oversaturation of calcite, dolomite, and rhodochrosite, indicating possible precipitation of these minerals, driven by an increase in pH. The deep chlorophyll maximum migrates from about 8 m at ice off in early July to just above the anoxic monimolimnion (~11 m) in late-July through October where increased concentrations of both dissolved phosphorus and ammonium likely drive primary productivity. O’Brien offers us insight into how springs can contribute to meromixis in subalpine lakes. The preservation and research of this lake are critical to maintaining the unique conditions that meromictic lakes provide.