Solar Radiation and Dissolved Organic Matter in Aquatic Environments

Abstract

At 662 ± 32 Pg C, dissolved organic matter (DOM) is the second largest bioreactive reservoir of C in the ocean, and comparable in size to the atmospheric CO₂ pool. Photodegradation can be an important DOM sink but remains a poorly understood mechanism. Dissolved inorganic carbon (DIC) is a major product of DOM photodegradation in riverine and coastal waters. However, the in situ rates and magnitude of DIC photoproduction in the global oceans are poorly constrained due to analytical difficulties. The goal of my thesis was to fill some of the knowledge gaps in DOM photodegradation and DIC photoproduction. To assess the effect of increasing terrestrial DOM input to the coastal Baltic Sea, we conducted photoirradiation experiments during a mesocosm amendment experiment. We determined that humic substances added to the ecosystem can be photodegraded rapidly. Added humic substances reduced the CDOM fading rates but did not affect DIC photoproduction significantly. Photodegradation was likely not the dominant process for CDOM degradation in situ, and the importance of photodegradation needs to be further assessed. We also conducted experiments extracting DOM and enriching seawater with the extracted DOM. We determined that there was no difference in CDOM fading but difference in DIC photoproduction among the amended and unamended samples. It is likely that both solid phase extraction and electrodialysis reverse osmosis extracts had lower CDOM:DOC ratios than the original seawater. When extrapolating to rates in the original water sample from the amended samples, either method is likely to provide useful answers if one is interested in CDOM fading in coastal waters, but color extrapolation likely provides a better approximate to DIC photoproduction rate of the original sample, especially when using solid phase extraction-amended samples. In addition, we determined that photodegradation of microplastic fibers (microfibers) may be a source of DOM and CDOM, although the produced DOM may be highly photolabile. My research suggested that color change, or CDOM fading, may not be a good predictor of DOM photodegradation, particularly for DIC photoproduction. Photodegradation can be a sink of marine DOM, however, teasing out its relative importance remains difficult.