Date of Award

Winter 2018

Project Type

Thesis

Program or Major

Earth Sciences

Degree Name

Master of Science

First Advisor

Ruth K Varner

Second Advisor

Joel E Johnson

Third Advisor

Michael W Palace

Abstract

Across the Arctic, postglacial lakes contribute a substantial amount of the total atmospheric methane (CH4), and their emissions are predicted to increase. However, there is still much uncertainty as to the contribution of northern water bodies to atmospheric CH4 emissions. This is mainly due to the spatiotemporal variability of the predominant pathway of emission from high latitude lakes: ebullition (bubbling). There are a myriad of factors that affect ebullition fluxes, including solar radiation input and atmospheric pressure, which make it difficult to model the impact on regional emissions. Very few studies have correlated sediment characteristics and submerged vegetation density with ebullition, to see what drives the variation across space and time. This study investigated the effect of submerged aquatic macrophyte (SAM) species distribution and abundance on CH4 dynamics in three postglacial lakes in Stordalen Mire, near Abisko, Sweden (68°21'N, 18°49'E). Submerged vegetation density maps developed from vegetation transects and sediment geochemistry derived from sediment cores were compared to ebullitive flux measured with bubble traps,. The source contribution of terrestrial and aquatic vegetation to the lake sediment carbon (C), the substrate for methanogens, was investigated using δ13C stable isotope analysis and organic carbon-to-nitrogen (C:N) elemental analyses. These data suggest that the organic C in postglacial subarctic lakes are a mixture of allochthonous and autochthonous inputs, with significant C being added by the in-situ decay of submerged vegetation, providing annual organic matter to the sediment. It was found that submerged vegetation density does not influence sediment CH4 concentrations, but rather, among shallow zone cores, the physical structure of the sediments drives most of the variation in ebullitive flux. Among shallow zones, the best predictor of overlying CH4 ebullitive efflux is the sediment porosity. It was also found that total sediment CH4 concentration has a strong negative relationship with ebullitive efflux, meaning that high sediment CH4 concentration is not an indication of high ebullition potential. Increased macrophyte density was not observed to ‘fertilize’ the sediment with organic C, nor did submerged vegetation density have any observed effect on sediment CH4 concentrations, downcore geochemistry, or ebullitive flux. Findings suggest that in a system that is not C-limited, it is perhaps the C quality and not the C quantity that drives the variability in methanogenesis. An investigation into which microbial communities exist in these sediments and in what abundance is required. These data also suggest that anaerobic oxidation of methane (AOM) might also be occurring even in freshwater lake environments, a finding that is implicative in terms of our understanding and modeling of CH4 ebullition and emissions across the Arctic, perhaps yielding new insights into how net emissions might change in the future.

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