Reader, Heather Erin
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Marine dissolved organic carbon (DOC) is one of the largest and most dynamic pools of reduced carbon on earth. Photochemical processes have the potential to significantly affect the content of this DOC pool. Photochemical oxidation to carbon monoxide and carbon dioxide are two direct pathways for the removal of DOC from the marine system. Indirectly, photochemical processes can lead to the removal of DOC through the alteration of chemical structures rendering them more biologically labile. In order to assess the influence of photochemistry on the coastal carbon cycle the variability of these processes must be well constrained. To calculate photochemical production in marine waters, it is crucial to know how light is absorbed by chromophoric dissolved organic matter (CDOM), as well as the spectral efficiency of the resulting photochemical reactions (i.e. the apparent quantum yield (AQY) spectra). The challenges of using visible wavelength CDOM absorption data to model ultraviolet absorption data are investigated. Direct measurements of ultraviolet absorption data model photochemical processes best. When this is not possible, visible data can be used with an accuracy of +/- 10% in coastal waters. Either a hyperbolic absorption model or one using correction factors applied to a traditional exponential model will allow for similar accuracy in the ultraviolet portion of the absorption spectrum. The variability of photochemical oxidation of dissolved organic carbon in a coastal system was studied in order to constrain remote sensing calculations. The variability of CO and CO2 AQY spectra were relatively well constrained in three estuaries of Georgia, USA. The AQY for CO varied within +/- 12.7% year-round while CO2 varied within +/- 33.6% year-round. Hyperspectral remote sensing reveals finescale hydrodynamic structure in estuarine systems and is readily adaptable to photochemical modeling applications. Defining the variability of photochemistry’s influence on the biological lability of DOC is a complex undertaking. Unlike direct photochemical oxidation of DOC to CO and CO2, the biologically labile products (BLPs) of incomplete oxidation can themselves be photochemically reactive. Competition between production and destruction of BLPs during irradiation is a significant consideration when determining the quantitative influence of photochemistry on marine systems.