Tanentzap, Andrew

Increasing wildfire frequency, driven by climate change, can change the concentration andcomposition of dissolved organic matter (DOM) exported from land into receiving waters by removing terrestrial vegetation, changing soil hydrology, and interrupting microbial degradation. In this thesis, I tested how wildfire impacts the molecular composition and reactivity of DOM. I compared DOM from boreal headwater streams in northwestern Ontario, Canada between 10 catchments recently affected and 10 comparable catchments that were undisturbed for at least 20 years. Using optical spectroscopy, ultra-high-resolution mass spectrometry, and incubation experiments, I found that burned streams had 29% higher average DOM concentrations and contained less bioavailable DOM, which resulted in microbes respiring more CO2 in burned streams rather than using carbon to build biomass. These results indicate that the impacts of wildfire on carbon sequestration have been underestimated and highlight the need to consider wildfire in forest carbon budgets.

Author Keywords: boreal forest, carbon flux, dissolved organic matter, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), headwater streams, Wildfire

Climate-driven permafrost thaw releases microorganisms and dissolved organic matter (DOM) into northern lakes, where their interactions with microbial communities and seasonal processes shape greenhouse gas emissions. In a factorial experiment mixing DOM and microbes from thermokarst ponds and lakes, we found that both DOM and microbial identity strongly influenced degradation. Lake microbes preferentially consumed thermokarst DOM, producing 3× more CO₂ due to low growth efficiency, while thermokarst microbes altered DOM with little CO₂ release. A survey of 40 lakes across a climate gradient showed CO₂ fluxes peaking in spring from under-ice buildup and CH₄ fluxes peaking in fall after summer accumulation. Dissolved gas concentrations served as early indicators of these events, with CH₄ linked to reduced DOM and CO₂ to multiple pathways. Overall, DOM quality, microbial traits, and seasonal dynamics interact to control lake carbon cycling, emphasizing the need for year-round monitoring under climate change.

Author Keywords: Dissolved Organic Matter (DOM), Fluxes, Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), Greenhouse Gases (GHGs), Lakes, Thermokarst