Photobleaching of Dissolved Organic Material from a Tidal Marsh-Estuarine System of the Chesapeake Bay † Maria Tzortziou* 1 , Christopher L. Osburn 2 and Patrick J. Neale 3 1 Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 2 US Naval Research Laboratory, Washington, DC 3 Smithsonian Environmental Research Center, Edgewater, MD Received 4 October 2006; accepted 1 March 2007; DOI: 10.1111 ⁄ j.1751-1097.2007.00142.x ABSTRACT Wetlands and tidal marshes in the Rhode River estuary of the Chesapeake Bay act as important sources of dissolved organic carbon and strongly absorbing dissolved organic matter (DOM) for adjacent estuarine waters. The effects of solar exposure on the photochemical degradation of colored DOM (CDOM) were examined for material derived from different sources (estuarine and freshwater parts of the Rhode River, sub-watershed stream, marshes) in this estuarine ecosystem. Consistent with changes in fluorescence emission, absorption loss upon exposure to different portions of the solar spectrum (i.e. different long-pass cut-off filters) occurred across the entire spectrum but the wavelength of maximum photobleaching decreased as the cut-off wavelength of the filter decreased. Our results illustrate that solar exposure can cause either an increase or a decrease in the CDOM absorption spectral slope, S CDOM , depending on the spectral quality of irradiation and, thus, on the parameters (e.g. atmospheric composition, concentration of UV-absorbing water constituents) that affect the spectral characteristics of the light to which CDOM is exposed. We derived a simple spectral model for describing the effects of solar exposure on CDOM optical quality. The model accurately, and consistently, predicted the observed dependence of CDOM photobleaching on the spectral quality of solar exposure. INTRODUCTION Dissolved organic matter (DOM), a major reservoir of organic carbon in the ocean, plays a central role in many biological and chemical processes in aquatic ecosystems, affecting carbon budgets, nutrient availability and ecosystem productivity (1–3). The colored fraction of DOM, CDOM, is one of the key water constituents determining the underwa- ter light field, affecting ocean color and aquatic photochem- istry (4–6). Estuarine and coastal margin ecosystems, such as the Chesapeake Bay and its surrounding wetlands, are complex and dynamic environments of intense DOM cycling in which DOM amount, quality and distribution reflect a balance between inputs and decomposition. In these systems, inputs include seasonal in situ production and organic matter of terrestrial and intertidal origin introduced through river run-off, wetland discharges and tidal exchanges (e.g. 1,7–9). Despite large inputs, DOM of terrestrial and intertidal origin contributes only a small part of the total DOM pool in the global ocean, suggesting rapid cycling and high mineraliza- tion rates of these components within estuaries and coastal marine environments (10,11). Photochemical and biological degradation processes play critical roles in regulating the residence time, cycling and fate of CDOM and dissolved organic carbon (DOC) in near shore waters (e.g. 12,13). Biological degradation is mediated primarily by bacterioplankton that use the labile DOM fraction for growth and respiration. Photochemical alter- ation of CDOM during solar exposure generates a variety of photoproducts, including reactive oxygen species (14), atmo- spherically important trace gases, such as CO, CO 2 and COS (e.g. 5), and low molecular weight labile carbonyl com- pounds that are readily available for consumption by microbial communities (3,15). This photochemical–biological coupling is not completely understood and previous studies have shown that photochemistry may form, as well as destroy, biological carbon substrates (e.g. 12,16,17). Accord- ing to Tranvik et al. (18) the net outcome of competing stimulatory and inhibitory effects of photochemical processes on the bioavailability of DOM varies across different environments depending on their water chemistry and on the origin and composition of DOM. Although marsh DOC inputs have been shown to contribute significantly to the carbon budget of the coastal zone (e.g. 7,10,19), the composition and optical properties of marsh-derived CDOM remain largely uncharacterized. Thus, we know little about the susceptibility of this material to photodegradation, or on the effects of solar exposure on its bacterial availability. One of the main objectives of this study was to provide some information on the optical quality and photoreactivity of the dissolved organic compounds exported from a tidal marsh system of the Chesapeake Bay. Brackish and fresh- water tidal marshes cover a large area (about 70 000 ha) along the western and eastern Bay shores, potentially playing an important role in the complex photochemical and biogeochemical processes taking place in the Bay system. The change in CDOM chemical structure during solar exposure is reflected in changes in its optical properties (e.g. 5,20) that provide a first-order measure of the photoreactivity †This invited paper is part of the Symposium-in-Print: UV Effects in Aquatic and Terrestrial Environments. *Corresponding author e-mail: martz@snarktoo.gsfc.nasa.gov, mtzortziou@ hotmail.com (Maria Tzortziou) Ó 2007 The Authors. Journal Compilation. The American Society of Photobiology 0031-8655/07 Photochemistry and Photobiology, 2007, 83: 782–792 782