Analysis of dissolved organic matter photodegradation under ultra-violet and visible light conditions

Abstract:

Sunlight has been shown to be an important contributor to degrading dissolved organic carbon (DOC) in aquatic ecosystems. Researchers have explored this photodegradation using a variety of artificial and natural light sources in the literature. We created three experiments using different sources of light (natural sunlight, artificial UV-B plus UV-A repair, and artificial UV-B) to assess how each light source impacts the photodegradation of DOC. Samples for these experiments were collected in June 2021 from the surface (DOC ~5 mg/L) of Lake Lacawac (Lake Ariel, PA, USA) and from a shallow groundwater well (lysimeter; DOC ~60 mg/L) in the wetlands surrounding Lake Lacawac. Triplicate treatments were prepared for initial, dark control (foil wrapped), and photo (light exposure). In each experiment, samples were exposed to one of the three light sources for a 7 day period.  Light intensity data was collected for normalization of data to exposure (dose rates).  We measured changes in DOC concentration, dissolved inorganic carbon (i.e., CO2) and DOC quality. DOC quality was assessed utilizing absorbance scans (800-200 nm) and excitation emission matrices. The two water sources (surface vs. lysimeter) demonstrated differences in photodegradation, which was expected based on prior experimentation. The initial analysis supports DOC being transformed differently under the three types of light exposure. This information is critical to our understanding of food web responses that DOC does not change predictably across sources; particularly given the changes in UV-R we have observed over the last decade in natural systems.