Disentangling Drivers of Lake Methane Dynamics across Scales

Freshwater lakes are an important component of the global methane cycle and a contributor to atmospheric methane concentrations. From recent estimates, freshwater lakes account for 6-16% of total natural methane emissions and lake methane emissions are estimated to offset an equivalent of 25% of the land carbon sink each year. However, methane production and emission are incredibly variable across lakes, and a comprehensive understanding of the mechanisms that contribute to this variation has yet to be achieved. At the ecosystem scale, variation in lake methane dynamics is correlated with temperature, anoxia, and organic matter supply. However, methane is the product of microbial metabolism and variation in how microbes are influenced by environmental conditions and how they utilize organic matter sources may also provide meaningful differences in overall rates of methane production. Recent advances in sequencing technologies now allow us to determine differences in microbial community composition across space and time and to explicitly consider the influence of these differences on ecosystem-scale processes. In this dissertation, I use field and laboratory approaches to investigate sources of variation in lake methane dynamics across both microbial and ecosystem scales.

My dissertation draws mechanistic links between sediment microbial community composition and lake methane dynamics by investigating how both microbial communities and ecosystem processes are individually influenced by the environment, as well as how microbial communities may mediate ecosystem responses to environmental change. Using comparative field data, I first determine landscape level patterns in lake sediment microbial community composition (Chapter 2) and quantify the relationships between lake methane dynamics and primary productivity across both time and space (Chapter 3). I demonstrate that lake methane responses to changes in primary productivity are variable and I provide evidence for the role of the microbial community in influencing this variation as mediated by cross-lake variation in pH. Based on these results, I directly test the effects of pH on microbial community assembly and function using experimental incubations (Chapter 4). I find that pH is a driver of community assembly, and that microbial community composition influences sediment function independent of pH. Finally, I utilize the confirmed relationship between pH and microbial community composition to experimentally test whether variation in methane responses to organic matter supply is related to microbial community composition (Chapter 5). I find that microbial community composition, but not abundance, is significantly related to the response of sediment methane production to experimental additions of organic material. Together, these four chapters enhance our understanding of the regulators of lake methane production and provide critical evidence for the role of sediment microbial community composition in mediating ecosystem responses to environmental change.