Browsing by Author "Johnston, Tyler B."

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Biogeochemical and microbial seasonal dynamics between water column and sediment processes in a productive mountain lake; Georgetown Lake, MT, USA
(2016-08) Parker, Stephen R.; West, Robert F; Boyd, Eric S.; Feyhl-Buska, Jayme; Gammons, Christopher H; Johnston, Tyler B.; Williams, George P; Poulson, Simon R
This manuscript details investigations of a productive, mountain freshwater lake and examines the dynamic relationship between the chemical and stable isotopes and microbial composition of lake bed sediments with the geochemistry of the lake water column. A multidisciplinary approach was used in order to better understand the lake water-sediment interactions including quantification and sequencing of microbial 16 S rRNA genes in a sediment core as well as stable isotope analysis of C, S, and N. One visit included the use of a pore water sampler to gain insight into the composition of dissolved solutes within the sediment matrix. Sediment cores showed a general decrease in total C with depth which included a decrease in the fraction of organic C combined with an increase in the fraction of inorganic C. One sediment core showed a maximum concentration of dissolved organic C, dissolved inorganic C, and dissolved methane in pore water at 4 cm depth which corresponded with a sharp increase in the abundance of 16 S rRNA templates as a proxy for the microbial population size as well as the peak abundance of a sequence affiliated with a putative methanotroph. The isotopic separation between dissolved inorganic and dissolved organic carbon is consistent with largely aerobic microbial processes dominating the upper water column, while anaerobic microbial activity dominates the sediment bed. Using sediment core carbon concentrations, predictions were made regarding the breakdown and return of stored carbon per year from this temperate climate lake with as much as 1.3 Gg C yr (super -1) being released in the form of CO (sub 2) and CH (sub 4) . Abstract Copyright (2016), . American Geophysical Union. All Rights Reserved.
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.