EFFECT OF PHOTOCHEMICAL TRANSFORMATION ON DISSOLVED ORGANIC CARBON CONCENTRATION AND BIOAVAILABILITY FROM WATERSHEDS WITH VARYING LANDCOVER

The transformation of freshwater dissolved organic carbon (DOC) has important implications for the health of aquatic ecosystems, status of climate, and water quality. Aquatic ecosystems rely on the bioavailability of dissolved nutrients through biogeochemical cycles to remain productive. Evolution of Vermont’s land cover and land use (LCLU) as well as our changing climate can have important consequences on the biogeochemistry of local waterways. Specifically, these landscape alterations can affect the concentration (quantity) and bioavailability (quality) of carbon within the watershed. In this study water samples were obtained from the three major LCLU watersheds (forested, agriculture, urban) in the Lake Champlain basin of Vermont under different environmental (flow) conditions and irradiated with sunlight in order to understand how changing LCLU and significant weather events affect the availability of DOC and its photochemical processing. Since heterotrophic organisms rely on the more protein-like, fraction of DOM for metabolic processes, this study analyzed how sunlight degraded DOC to alter the quality and increase the accessibility for aquatic heterotrophs. Absorbance spectra were used to determine SUVA254 and spectral slope values that could infer changes in DOC aromaticity and molecular weight. Additionally, fluorescence was measured to produce excitation-emission matrices (EEMs) in order to determine the relative abundance of humic-like and protein-like fluorescing molecules. The EEMs obtained from the samples for all three landscapes demonstrated a positive correlation between quantity of bioavailable DOC and length of exposure to sunlight, which was consistent with the observed decrease in aromaticity (decrease in SUVA254) and molecular weight (increase in spectral slope ratio). Heterotrophic microbes were inoculated into identical samples to measure the biodegradation and to calculate the DOC that was consumed overall. The inoculation and spectral data concluded that photodegradation increased the bioavailable fraction of DOC in all three landscapes analyzed. Photochemical processing of DOC appears to transform an originally recalcitrant nutrient into a usable form, ultimately increasing nutrient resources for heterotrophic organisms near the base of the food web.