Multi-Scale Drivers of Lake Methane Dynamics

Lakes vary widely in limiting nutrients (nitrogen and/or phosphorous) and trophic status as a result of natural variation in geology and land cover and due to intensification of agriculture in watersheds of many freshwater lakes. Since eutrophic lakes have the potential to absorb greater quantities of atmospheric carbon dioxide (CO₂; a greenhouse gas), eutrophic lakes might be a benefit in terms of carbon burial and mitigating climate warming (Finlay et al., 2009; Lazzarino et al., 2009; Balmer and Downing 2011). However, upon sinking and decay, phytoplankton biomass creates an environment suitable for fermentation and methanogenesis (Kiene 1991). In fact, lake eutrophication may yield greater generation of warming potential as CO₂ emissions are traded for release of methane (CH₄), a greenhouse gas with 25x greater radiative forcing than CO₂ (Huttunen et al., 2003). However, influence of phytoplankton carbon on methanogenesis and subsequent CH₄ emissions from lakes has hardly been studied, and the only evidence of the influence of eutrophication on CH₄ emissions is derived from weak correlations between lake trophic status and CH₄ emission (Huttunen et al., 2003; Juutinen et al., 2003; Bastviken et al., 2004; Sepulveda-Jauregui et al., 2014). The implications of enhanced primary productivity for microbial-mediated CH₄ production and ecosystem scale greenhouse gas emissions must be investigated further to fully evaluate whether eutrophication enhances CH₄ emissions from freshwater lakes.

While my research does not assess the direct effects of agricultural eutrophication on CH₄ emissions, and most of my research is conducted on natural lakes, the research in this dissertation does highlight the potential for enhanced eutrophication (natural or anthropogenic) to increase contributions of CH₄ to the atmosphere from freshwater lakes. To address key questions regarding the influence of eutrophication on CH₄ emissions, my dissertation research utilizes experimentation and observation to test hypotheses at multiple temporal and spatial scales, and employs concepts from microbial, community, and ecosystem ecology.