Biogeochemical Processes Surface Water to Sediments

Aquatic ecosystems are synchronized with the diurnal photocycle through photosynthesis by algae and macrophytes and through photochemical re- 

Fluorophores are another type of trace moiety that is lost through photolysis (Pullin and Cabaniss, 1997 Langford and Bruccoleri, 2000). The decrease in fluorescence by photolysis is confirmed by the observation that the fluorescence intensity per mg DOC is a good indicator of the production of hydrogen peroxide from DOM photolysis in a wide range of freshwaters (Scully and Lean, 1994). As discussed above, the relative fluorescence intensity of the fulvic acid fraction is fairly simple to measure. Furthermore, measurement of FI and/or 813C may allow for differentiation between a decrease in fluorescence intensity per mg C due to mixing with fulvic acid from microbial material and a decrease due to photolysis. 

Tracers of Incorporation of Organic Sulfur in Fulvic Acid under Anoxic Conditions 

Similar to the N content of fulvic acid, the S content can reflect differences in the S content of precursor organic material. However, the S content can also be influenced strongly by the chemical conditions of the environment in which the fulvic acid is generated. Fulvic acids from reducing 

FIGURE 8 Change in phenolic hydroxyl content of fulvic acids from Spirit Lake during the several years after the eruption of Mt. St. Helens (from McKnight et al., 1988). The measurement of phenolic hydroxyl content is by derivitization with 13C-enriched diazomethane that is quantitatively measured by 13C-NMR spectroscopy. 

---