Photochemical production of dissolved organic carbon from resuspended sediments

A series of controlled photolysis experiments using simulated sunlight was conducted in the presence and absence of estuarine bottom sediments to address the role of sediment resuspension on dissolved organic carbon (DOC) cycling in estuarine and coastal waters. In 0.2-mm-filtered estuarine water samples with no added sediment, DOC concentrations decreased from 0.4 to 3 mmol L21 h21 as a result of photooxidation. When 0.2-mm-filtered water samples with the addition of 1–2 grams of estuarine sediment per liter were irradiated, DOC was produced at rates of 3 to 150 mmol L21 DOC per gram dry sediment. Photoproduction of DOC from resuspended sediments increased in direct proportion to the percent organic carbon content of the added sediment. Highenergy ultraviolet light was the most effective for photodegrading DOC in filtered samples and for photoproducing DOC from resuspended sediments. Photosynthetically active radiation (PAR; 400–700 nm) did not significantly degrade DOC in filtered water. However, PAR did produce half as much DOC from resuspended sediments in organic-rich regions of the estuary relative to full spectrum sunlight irradiations. The photoproduction of DOC from resuspended sediments, calculated for the top 1 m of coastal waters, resulted in fluxes that were significantly larger than benthic and riverine fluxes of DOC. Photoproduction from resuspended sediments therefore represents an episodically significant but previously unrecognized source of DOC to estuarine and coastal ecosystems receiving large sediment plumes. Estuarine sediments are naturally resuspended and mobilized by wind-driven, wave-induced water movement as well as tidal and riverine currents. Anthropogenic sources of resuspended sediments in estuaries include dredging, bow waves, propeller wash, and bottom-trawling fishing devices. In the typically shallow waters of estuaries, sediment-water interactions are a fundamental, yet commonly neglected, set of processes that can significantly impact a host of important biogeochemical processes, such as trace-metal mobilization, nutrient and organic carbon (OC) cycling, and release of anthropogenic contaminants (Komada and Reimers 2001; Komada et al. 2002; Koelmans and Prevo 2003). Sediment resuspension may affect OC concentrations in natural waters by dispersion of pore waters enriched in dissolved organic carbon (DOC) into overlying waters or by desorption or displacement of organics bound to disturbed sediments (Komada et al. 2002; Koelmans and Prevo 2003). Numerous studies have documented that organic matter is closely, and sometimes reversibly, associated with particles in river and estuarine systems (Hedges and Keil 1999). Resuspension and physical reworking of these sediments may significantly impact their remineralization rates (Aller 1998; Sun et al. 2002; Arzayus and Canuel 2004) and could alter the partitioning of organics between the dissolved and particulate phases. For example, Komada and Reimers (2001) found DOC increased during resuspension of estuarine and inner-shelf sediments in bottom waters, primarily due to desorption of mineral-bound OC. The fraction of easily releasable OC was only a small fraction (#0.3%) of sedimentary particulate organic carbon (POC), but was correlated to POC content (Komada and Reimers 2001). Koelmans and Prevo (2003) found that DOC was produced during laboratory simulations of resuspension events of lake and river sediments containing varying levels of organic matter. It was suggested that a significant fraction of particulate carbon could be mobilized from the sediments within a few weeks of incubation. Although these studies demonstrate the importance of dark release of DOC from resuspended sediments, they do not address the potentially significant role that photochemical transformations could play in the mobilization of DOC from sediments newly exposed to sunlit surface waters. Recently, Mayer et al. (2006) demonstrated the loss of POC in suspended sediments from the Mississippi River system after relatively long-term (.24 h total) exposure to simulated sunlight and inferred the progressive production of DOC from light-exposed sediments. Given the frequency of resuspension episodes and shallow depths in estuaries, the introduction of resuspended bottom sediments into the photic zone is a ubiquitous occurrence, although the magnitude and significance of DOC produced by light exposure remain poorly understood. The potential photoreactivity of particle-bound organics may produce dissolved photoproducts, analogous to the production of low-molecular-weight photoproducts during sunlight irradiation of dissolved organic matter (Kieber et al. 1990; Wetzel et al. 1995; Opsahl and Benner 1998). 1 Corresponding author ([email protected]). Acknowledgments Michelle Smith, Suzanne Zvalaren, and members of the Marine and Atmospheric Chemistry Research Laboratory group at University of North Carolina–Wilmington assisted with sampling and analyses. We also thank the captain and crew of the R/V Cape Fear for their efforts during sampling. This work was supported by the National Science Foundation (grants ATM 0342420 and OCE 0326685) and the Strategic Environmental Research and Development Program (contract DACA72-00-C-0015). Limnol. Oceanogr., 51(5), 2006, 2187–2195 E 2006, by the American Society of Limnology and Oceanography, Inc.