Depth-Resolved Photochemical Lability of Dissolved Organic Matter in the Western Tropical Pacific Ocean

Water samples collected from various depths of the offshore South China and Philippine Seas were exposed to solar-simulated radiation. Photomineralization of dissolved organic carbon (DOC) and photobleaching of chromophoric dissolved organic matter (CDOM) and its humic-like fluorescent constituent (FDOM) were observed in all samples. Protein-like FDOM was, however, either photo-decomposed or photo-produced, depending on the sample's depth. The photobleaching of CDOM and humic-like FDOM was much faster in deep than in shallow water samples while photomineralization displayed a weaker vertical zonation. Prior-irradiated deep water inoculated with surface-water bacteria showed enhanced microbial DOC removal but CDOM production. Results from this study suggest that deep-ocean CDOM and FDOM can barely survive photobleaching during one ocean mixing cycle, but photochemical turnover of the bio-refractory deep DOC is considerably longer than its average radiocarbon age. Coupled photochemical-microbial processes can not only remove part of the bio-refractory deep DOM but also regenerate part of it during ocean overturning circulation.