The Fate of Terrestrial Dissolved Organic Matter in Ocean Margins Investigated through Coupled Microbial-Photochemical Incubations of Vascular Plant Leachates

Biological productivity, air-sea CO₂ exchange and nutrient cycling in ocean margins is strongly affected by mineralization of terrigenous dissolved organic carbon (tDOC) delivered by rivers. The decomposition of tDOC was investigated with coupled photochemical-microbial incubations to assess the combined effects of microbial and photochemical processes on the structure and extent on removal of tDOM. For these incubations, vascular plant material leachates were prepared with five different materials from the Sacramento River Valley and estuarine wetlands: foothill pine, blue oak, mixed annual grasses, mixed Tule, and cattails. Incubations were done with controlled light exposure and known spectral irradiation. Samples collected along a continuum of degradation stages were analyzed for dissolved organic carbon (DOC), total hydrolysable enantiomeric amino acids (DLAA), total hydrolysable neutral sugars (THNS), total hydrolysable amino sugars (THAS), lignin phenols, and optical properties. The loss of vascular plant material was calculated at different stages of decomposition by comparison of measured C-normalized concentrations to C-normalized values in fresh leachates. This was matched with calculation of microbial contributions based on D-amino acids. As a result, calibrated biomarkers describing vascular plant decomposition and input of microbial DOC were developed for different stages of tDOC decomposition. Application of these calibrated biomarkers will be used to study riverine DOM in river plumes using transect samples from the San Francisco Bay Estuary during summer of 2014, and as well as a transect from the Brazos River mouth into the Gulf of Mexico collected during the 2015 summer flood events.