Tidal marshes as a source of optically and chemically distinctive colored dissolved organic matter in the Chesapeake Bay

The role of tidal marshes as a source of dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) for adjacent estuarine waters was studied in the Rhode River subestuary of the Chesapeake Bay. Water in a tidal creek draining brackish, high-elevation marshes was sampled every hour during several semidiurnal tidal cycles in order to examine the tidal exchange of dissolved organic matter (DOM). Water leaving the marsh during ebbing tide was consistently enriched in DOC compared to water entering the marsh during flooding tide. There was a net DOC export from the marsh to the estuary during seasons of both low and high marsh plant biomass. Optical analysis demonstrated that, in addition to contributing to the carbon budgets, the marsh had a strong influence on the estuary’s CDOM dynamics. Marsh-exported CDOM had optical properties that were consistently and markedly different from those of CDOM in the adjacent estuary. Specifically, marsh CDOM had: (1) considerably stronger absorption, (2) larger DOC-specific absorption, (3) lower exponential spectral slope, (4) larger fluorescence signal, (5) lower fluorescence per unit absorbance, and (6) higher fluorescence at wavelengths .400 nm. These optical characteristics are indicative of relatively complex, high-molecular-weight, aromatic-rich DOM, and this was confirmed by results of molecular-weight-distribution analysis. Our findings illustrate the importance of tidal marshes as sources of optically and chemically distinctive dissolved organic compounds, and their influence on CDOM dynamics, DOC budgets, and, thus, photochemical and biogeochemical processes, in adjacent estuarine ecosystems. Dissolved organic matter (DOM) plays a key role in a broad range of processes and climate-related biogeochemical cycles in aquatic ecosystems, affecting carbon dynamics, nutrient availability, phytoplankton activity, microbial growth, and ecosystem productivity. The lightabsorbing component of the DOM pool, known as colored dissolved organic matter (CDOM), is a major determinant of the amount and quality of the underwater light field. This, in turn, controls exposure of aquatic organisms to biologically damaging ultraviolet (UV) radiation and affects aquatic photochemistry and ocean color (e.g., Bricaud et al. 1981; Vodacek et al. 1997; Pienitz and Vincent 2000). Estuarine and coastal-margin ecosystems are hot spots of DOM cycling because of intense physical and biological activity. In these systems, DOM composition and dissolved organic carbon (DOC) quality are controlled by the relative 1 Corresponding author ([email protected]; tel: 301-614-6048; fax: 301-614-5903). Acknowledgments We thank Jesse Phillips-Kress, James Duls, Sam Benson, and Sharyn Hedrick for assistance in the field, and Donald Weller for assistance in preparing the map of station locations. We also thank Associate Editor Elizabeth Canuel and two anonymous reviewers for their constructive comments. Support for this work was provided by the Smithsonian Institution Fellowship program and National Aeronautics and Space Administration–Goddard Space Flight Center. Field work on Chesapeake Bay was funded in part by United States Environmental Protection Agency, Coastal Intensive Site Network (CISNet) Program through grant R826943, and Science to Achieve Results (STAR) Program through grant RD83087801, and by the Smithsonian Environmental Sciences Program. This is Southeast Environmental Research Center contribution 359. Limnol. Oceanogr., 53(1), 2008, 148–159 E 2008, by the American Society of Limnology and Oceanography, Inc.