Photomineralization of organic carbon in a eutrophic, semiarid estuary

Scientific Significance Statement Photochemical remineralization of dissolved organic carbon (DOC) is considered an important DOC sink in the aquatic environments, and previous studies revealed that photochemical reactions enrich the heavy carbon isotope (C-13) in the residual DOC pool. To date, few studies have examined the significance of photomineralization in subtropical estuaries, particularly those subject to eutrophication and high DOC levels. We found that photomineralization plays an important role in not only removing DOC, but also shifting the stable isotopic composition of residual DOC to be more C-13-depleted, contrary to findings in other coastal and open ocean environments. The effect of photomineralization on the carbon cycle in a eutrophic, semiarid estuary (Baffin Bay, Texas) was investigated using closed-system incubations. Photochemical production rate of dissolved inorganic carbon ranged from 0.16 to 0.68 mu M hr(-1), with a daily removal of 0.3 similar to 1.5% of the standing stock of dissolved organic carbon (DOC). The photomineralization rate was negatively correlated with chlorophyll a concentration, suggesting that plankton-derived DOC was less photoreactive to solar radiation. The stable carbon isotope composition (delta C-13 similar to -18.6 parts per thousand) of degraded DOC, as calculated using the DIC "Keeling" plot, further indicated high photochemical lability of C-13-enriched DOC in this semiarid environment. Our finding showed that photomineralization of C-13-enriched DOC is an important component of carbon cycle in this system, and this process does not necessarily remove C-13-depleted organic carbon as observed in other coastal systems.