Photochemical bleaching of oceanic dissolved organic matter and its effect on absorption spectral slope and fluorescence

Photobleaching of open-ocean dissolved organic matter (DOM) is typically treated as a removal mechanism; however, photobleaching also encompasses a poorly characterized suite of transformative processes. To examine the qualitative changes to DOM optical properties during photobleaching, 674 m N. Pacific DOM, concentrated and desalted by reverse osmosis with electrodialysis (RO/ED), was subjected to 68 days of continuous irradiation in a UV solar simulator. Approximately 84% of chromophoric and fluorescent DOM (CDOM and FDOM respectively) and 38% of dissolved organic carbon (DOC) were lost during the irradiation. Based on these results the concentration of photochemically refractory DOC in the surface pacific is estimated to be 27 μmol of carbon per liter. In addition, the spectra of the remaining CDOM and FDOM were shifted towards shorter wavelengths, a result which has important implications for the interpretation of fluorescence excitation emission matrix (EEM) spectra because the relative positions of fluorescence maxima are often attributed to differences in FDOM source. Qualitative indices derived from CDOM and FDOM spectra for the irradiated deep DOM sample resembled those for surface waters of the North Pacific Ocean indicating that photobleaching has a significant influence upon the optical properties of DOM in the open ocean.