ASSESSING MONTHLY CHANGES AND THE DEGRADATION OF TERRESTRIALLY DERIVED DISSOLVED ORGANIC CARBON IN A TEMPERATE LAKE

Aquatic ecosystems are known to release significant quantities of carbon dioxide (CO₂) to the atmosphere. In the past, it has been thought that heterotrophic microbial communities were responsible for processing dissolved organic carbon (DOC) into carbon dioxide through a process known as biodegradation. Recent research in Arctic ecosystems showed that sunlight is also capable of processing DOC to CO₂ through photo-degradation. This study focuses on how terrestrially sourced DOC from three temperate lakes was processed on a seasonal basis. Changes in DOC concentration were measured and absorbance scans (800-200 nm) were generated to assess qualitative changes to the DOC. Samples were collected for the months of June, July, and August from shallow groundwater lysimeters installed near the lakes. After collection, samples were deployed at the surface of Lake Lacawac. The goal of this project was to understand if the terrestrial DOC source changed seasonally and whether photo-degradation or biodegradation was more important to the processing of DOC. The amount of DOC in groundwater as it was obtained from lysimeters increased throughout the summer for two of the study lakes (Giles and Waynewood), and decreased for the lake (Lacawac) with the highest DOC concentrations. Our data showed that the treatments that allowed both photo-degradation and biodegradation together processed the largest concentration of DOC. This was followed by the photo-degradation only samples and then by the biodegradation samples. This trend was observed across all three lakes that were sampled during the summer of 2016. DOC specific absorbance at 320 nm (SUVA₃₂₀) decreased in samples exposed to sunlight, while the spectral slope (S_r) increased in those same samples. These results have important implications for DOC in temperate lakes as they show how important sunlight is to the processing of DOC and that variable quality DOC can be produced from photo-degradation.