Browsing by Author "Poulson, Stephen"

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Stable isotopes track biogeochemical processes under seasonal ice cover in a shallow, productive lake
(Springer, 2014) Gammons, Christopher H; Henne, William; Poulson, Stephen; Parker, Stephen R.; Johnston, Tyler B.; Dore, John E.; Boyd, Eric S.
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.