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.