UC San Diego

Drier and hotter conditions linked with anthropogenic climate change increase wildfire activity which can influence terrestrial and aquatic carbon cycles at broad spatial and temporal scales. Other aspects of environmental change such as climate warming enhance soil decomposition leading to accelerated rates of sedimentation and leaching from soils into aquatic systems, a phenomenon known as “browning.” Large-scale ecological disturbances like wildfire can increase the occurrence of browning from post-fire erosion. I tested the effects of fire-treatment (burned vs. unburned plant material) and its interaction with terrestrial loading on dissolved organic carbon (DOC) composition, concentration, and degradation (biological vs. photochemical) in freshwater mesocosms. DOC concentration increased nonlinearly with added plant material in both burning treatments and degraded most rapidly at intermediate concentrations, indicating that decomposition at intermediate concentrations removes DOC at a faster rate than at higher concentrations. Excitation-Emission Matrix (EEM) fluorescence spectroscopy showed that the effect of fire and browning changed chemical signatures apparent in the EEMs, and that both mainly affected the humification index and the specific ultraviolet absorbance at 254 nm over time. Incubations showed that biodegradation contributed more to DOC decomposition than photodegradation, and that DOC decomposition was most rapid at intermediate loading levels likely because anoxia at high terrestrial input inhibited microbial activity. My thesis shows that fire and browning elicit non-linear responses in the dynamics and composition of DOC in aquatic systems, and that fire alters the chemistry of organic detritus in ways that impact its processing and role in aquatic environments.