Chemical and Microbial Drivers of Pond Ecosystem Function in the Copper River Delta, Alaska

My dissertation assessed the chemical and microbial drivers of ecosystem function in high-latitude wetland ponds and how they might be influenced by climate change. Using a landscape ecology approach, I investigated how landscape position influenced physical, chemical, and biological characteristics of multiple ponds along a 20-km trajectory from glacial headwaters to ocean in southcentral Alaska, USA. Ponds more strongly influenced by glaciers tended to be more heterotrophic, exhibiting higher rates of decomposition and ecosystem respiration. In contrast, ponds near the ocean tended to be more autotrophic, exhibiting greater gross primary production and net ecosystem production. Despite this heterogeneity, nutrient diffusing substrate experiments demonstrated that pond biofilms responded similarly to nutrients. Nitrate amendments reduced biofilm biomass by about 60% with corresponding changes in microbial communities. The community patterns suggested that nitrate can strongly affect microbial interactions during biofilm formation by altering redox conditions. Because redox conditions structure functional processes in these systems and ponds play a disproportionately large role in the methane (CH₄) budget of global inland waters, I also examined physicochemical and biological factors that influence the CH₄ cycle. Ebullition, or bubbling, often dominated total CH₄ emissions from these ponds and appeared to integrate alternate sources of CH₄ other than current production. The predictability of processes in the CH₄ cycle diminished from CH₄ production to diffusive emissions to total emissions. Although specific processes in the CH₄ cycle were challenging to predict, I demonstrated that carbon stable isotopes of detritus are a potential tool for tracing CH₄ production. Tracing this process is important because global change is likely to affect the CH₄ cycle. My experiments suggested that CH₄ production in coastal wetlands will be sensitive to increased organic matter availability and potentially seawater intrusion. Overall, my dissertation shows that environmental heterogeneity contributes to differences in microbial processing and ecosystem function. Global environmental change is likely to disproportionately affect coastal wetlands at northern latitudes because they are subject to increased glacial melt, sea-level rise, higher temperatures, and longer growing seasons, all of which have the potential to influence the physicochemical properties that shape microbial communities that are largely responsible for ecosystem function.