Factors controlling photo-degradation and photo-flocculation of DOM across acidic and basic watersheds in Southeast USA

Terrestrial dissolved organic matter (DOM) is efficiently removed by photochemical processing in the middle and low latitude watersheds before reaching the coastal ocean. DOM can be photo-degraded and re-mineralized to small molecules (e.g., CO₂, NH₃), as well as photo-flocculated to form suspended particles and further sequestered to sediments. To better understand the factors controlling the rates of photochemical processes and the associated reaction mechanisms, DOM-rich waters from a variety of watersheds were 0.1 um filtered and UV-irradiated in a solar simulator for up to 6 days. Study sites include Florida karst basic watersheds, South Carolina rivers, and a Virginia/North Carolina acidic swamp. Physico-chemical factors such as pH, metals (e.g., Ca, Mg, Fe, Zn), nutrients (e.g., SO₄^2-, NO₃^-, NH₄⁺, PO₄^3-), organic matter bulk C and N concentrations, and molecular markers for black carbon (BPCAs) were investigated across samples. DOM before and after photo-irradiation was characterized using a series of analytical techniques, such as UV-Vis and fluorescence spectroscopies, liquid-state ¹H NMR, and Ultrahigh Resolution Mass Spectrometry (FT-ICR-MS). There are dramatically different photo-degradation rates observed among samples, with DOM from low latitude watersheds being less degradable relative to DOM from the middle latitude watersheds, likely due to the fact that the DOM from lower latitude had been intensively photo-irradiated in the field prior to sample collection. Preliminary evidence suggests that the interplay of metals with DOM results in different degrees of photo-flocculation. The diversity of selected samples in this study provides a unique opportunity to examine the influences of multiple inorganic and organic factors on photochemical process, towards improving our knowledge of the role and photochemical stability of DOM within the biogeochemical cycles.