Marsh-Exported Dissolved Organic Matter Fate in Estuaries

Dissolved organic matter (DOM) is a significant driver of estuarine productivity and nutrient cycling. The colored component of DOM, chromophoric dissolved organic matter (CDOM), impacts coastal optical properties, ocean color, and light attenuation. While marshes are largely considered sinks for carbon due to their high productivity and low soil carbon degradation rates, laterally they are sources of carbon as optically and chemically distinct DOM to surrounding aquatic ecosystems; these inputs are often essential in sustaining a net heterotrophic system. However, the photoreactivity and bioavailability of marsh-exported DOM is largely uncategorized, thus making it difficult to quantify its impacts on estuarine dynamics. The photoreactivity and bioavailability of chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) from various sources within the Rhode River, a brackish, eutrophic, sub-estuary during the summer, and seasonally across four Chesapeake Bay marshes, were examined. Microbial (14 d in the dark) and photochemical incubations (7 d in light followed by 7 d in dark) were performed, and changes to DOM optical properties were measured using spectrophotometry (CDOM absorption coefficients and spectral slopes) and spectrofluorometry (FDOM excitation-emission matrices analyzed using parallel-factor analysis, PARAFAC). 0.2 μm filters were used for “photochemical-only” treatments, whereas glass-fiber filters (GF/Fs) were used for microbial treatments. Initial results indicated the presence of bacterial cells in 0.2 μm filtrates; however, heating the filtrates in a water bath at 60°C for 30 min successfully killed the bacteria, while still preserving the DOM optical properties. Re-filtering both 0.2 μm or GF/F filtrates with a 0.2 μm filter at the end of a dark incubation resulted in the loss of 5-10% of the CDOM absorption coefficient at 300 nm (proxy for CDOM amount), indicating the removal of aggregates that may have formed over the 7 d incubation; studies that have utilized this method for estimating microbial bioavailability could be greatly overestimating the microbial degradation of CDOM, since reported losses due to microbial degradation are often of similar magnitudes. Results showed that CDOM absorption coefficients, molecular weight, and total FDOM were greatest at the marsh site in the Rhode River, and that these parameters decreased down-estuary; CDOM absorption coefficients and total FDOM at marsh sites decreased in colder seasons, indicating lower export in colder months, while CDOM quality remained the same throughout. Photobleaching resulted in a net loss of CDOM and humic-like FDOM and a decrease in CDOM molecular weight, while microbial degradation resulted in a net loss, or no change, of CDOM absorption coefficients and a net increase in humic-like FDOM. Photoreactivity decreased down-estuary, with distance from terrestrial DOM sources, while microbial degradation increased. Seasonally, marsh-DOM photoreactivity increased in the colder seasons, most likely due to the lower previous exposure to UV-radiation in the winter compared to the summer; microbial degradation was much more variable seasonally. Photobleaching increased the bioavailability of marsh-derived CDOM and FDOM, resulting in higher rates of production of humic-like DOM, and a greater loss of CDOM. Jug Bay, a freshwater marsh down-stream of a major sewage treatment plant, had the greatest loss of CDOM by photobleaching and microbial degradation, and the smallest increase in humic-like FDOM due to microbial degradation compared to the other marshes. It also showed the greatest change in CDOM quality (e.g., molecular weight) with photobleaching, indicating that it is particularly photoreactive as well. This highlights the significance of human activity on marsh-exported DOM quality and export, and the potential impacts of these changes on estuarine carbon dynamics