GEOCHEMISTRY AND STABLE ISOTOPES OF A EUTROPHIC LAKE IN WESTERN MONTANA, USA

Georgetown Lake is a shallow (5 to 10 m), man-made lake and prodigious trout fishery in southwestern Montana. Previous studies have shown that severe dissolved oxygen (DO) depletion occurs in the lake each winter under ice cover. The very high productivity in summer coupled with acute winter anoxia is what makes this lake “extreme”. Here we report the results of monthly water chemistry and stable isotope sampling during the winter of 2010-2011.

By mid-winter, two sampling sites had established a robust vertical profile with the following characteristics: a) a rapid lowering of DO concentration to values near zero about 2 to 3 m above the bottom, with a 3 to 7 ‰ increase in δ¹⁸O-DO; b) an increase in alkalinity, CO₂ partial pressure, and dissolved inorganic carbon (DIC) with depth, with a 2 ‰ decrease in δ¹³C-DIC; and c) an increase in ammonium, H₂S, silica, phosphate, Ca²⁺, Mn²⁺ and Fe²⁺ towards the lake bottom. These trends are explained by a combination of aerobic and anaerobic respiration, coupled with dissolution of calcite in lake sediment. The shallower of the two sampling sites had higher DO concentrations and a less steep slope of δ¹⁸O-DO vs. DO concentration, suggesting that below-ice photosynthesis was partly offsetting consumption of DO by respiration. Photosynthetic, purple, H₂S-oxidizing bacteria were observed at the shallow site (6 m), but not at the deeper site (10 m). Interestingly, the δ¹³C-DIC of the deeper samples at the shallow site shifted several ‰ to more positive values late in the winter, with continued increase in DIC concentration. This observation is tentatively attributed to fermentative breakdown of organic matter to isotopically-heavy DIC and isotopically-light dissolved organic carbon (DOC). The water column of the entire lake became completely mixed shortly after ice-off in late May of 2011, and is normally mixed and well-oxygenated throughout the unfrozen months due to high winds.

The ability of photosynthesis to continue in certain portions of the lake during the 6+ months of ice cover could well be a critical reason why fish survive the winter season in Georgetown Lake. However, the lake is near an ecological tipping-point, and is vulnerable to increased residential development and associated nutrient loads.