Stable isotopes track biogeochemical processes under seasonal ice cover in a shallow, productive lake
Gammons, Christopher H
Parker, Stephen R.
Johnston, Tyler B.
Dore, John E.
Boyd, Eric S.
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Biogeochemical dynamics under seasonal ice cover were investigated in the shallow (<10 m) water column of highly productive Georgetown Lake, western Montana, USA. This high altitude (1,800 m) reservoir is well-mixed in summer, but becomes strongly stratiﬁed under ice cover (mid-November–mid-May). A rapid drop in dissolved oxygen (DO) concentration and rise in dissolved inorganic carbon (DIC) concentration was observed after the onset of ice, with a corresponding increase in δ18O-DO and decrease in δ13C-DIC, likely caused by respiration (R) of organic carbon. Photosynthesis/respiration ratios (P/R) estimated from simultaneous measurement of DO and δ18O-DO were near unity prior to ice formation but then systematically decreased with time and depth in the lake under ice cover. P/R in the water column was higher at a shallower monitoring site compared to a deeper site near the dam outlet, which may have been important for over-winter survival of salmonids. By March, the bottom 3 m of the water column at both sites was anoxic, with the bottom 1 m being euxinic. Elevated concentrations of dissolved sulﬁde, ammonium, phosphate, Fe2+, and Mn2+ in deep water suggest coupling of organic carbon degradation with reduction of a number of electron acceptors (e.g., Fe3+,NO3-, SO24-). The concentrations and δ34S values of H2S in the deep water and SO2i in the shallow water were similar, indicating near-complete reduction of sulfate in the euxinic zone. Late in the winter, an inﬂux of isotopically heavy DIC was noted in the deep water coincident with a buildup of dissolved CH4 to concentrations >1 mM. These trends are attributed to acetoclastic methanogenesis in the benthic sediments. This pool of dissolved CH4 was likely released from the lake to the atmosphere during spring ice-off and lake turnover.
Gammons, Christopher H., William Henne, Simon R. Poulson, Stephen R. Parker, Tyler B. Johnston, John E. Dore, and Eric S. Boyd. "Stable isotopes track biogeochemical processes under seasonal ice cover in a shallow, productive lake." Biogeochemistry 120, no. 1-3 (2014): 359-379.