Wave tank study of particulate organic matter degradation in permeable sediments

We used a wave tank to study the influence of pore-water flow and diffusive transport on the degradation of labile particular organic matter (POM: Ulva lactuca pieces) embedded in permeable sediment. Pore-water advection, induced by the interaction of the wave-driven oscillatory boundary flow and stationary sediment ripples, reproducibly exposed POM buried in the top 2 cm of the sediment to oxic or anoxic conditions lasting between days and weeks. Planar oxygen optodes together with carbon and nitrogen analyses were used to visualize and quantify the degradation rates. Oxygen consumption rates (OCR) were up to 18-times higher at locations of the buried POM compared to the surrounding sediment. Elevated OCR were also detected downstream the POM locations. Despite high permeability of the sediment and exposure to oxygenated pore-water flows, suboxic and anoxic sites and suboxic pore-water ‘‘plumes’’ developed at and downstream of the locations of POM in otherwise oxygenated sediment regions. The carbon loss of the buried U. lactuca discs derived from the OCR measurements was only 4–15% of that measured by the carbon analysis of the recovered pieces, suggesting that the bacterial degradation of POM and the final degradation of dissolved organic matter (DOM) were spatially decoupled by the pore-water flow. Advection can thus enhance the rate of organic matter degradation by efficiently distributing DOM from the ‘‘hotspots’’ of organic matter mineralization to larger volumes of permeable sediments and associated microbial communities. Permeable sediments are abundant in the global continental shelf regions (Emery 1968). These nonaccumulating sands are generally poor in organic carbon and have been considered long time as relatively inactive habitats that do not contribute substantially to the cycling of organic matter (Boudreau et al. 2001). Recent studies, however, indicate that the low concentration of organics is more likely the result of rapid turnover and high exchange rates (Huettel et al. 1998; Rusch et al. 2003). When the interaction of the boundary-layer flow (e.g., unidirectional or oscillatory flow induced by currents and gravity surface waves, respectively) and sediment topography enables dynamic advective porewater flow, the solute exchange at the sediment-water interface can exceed transport by molecular diffusion by several 1 Present address: FAB Anlagenbau GmbH, Ostarastr. 5, 51107 Cologne, Germany ([email protected]). 2 Present address: DHI Wasser & Umwelt, Wiesenstrasse 10A, D28857 Syke, Germany 3 Present address: Florida State University, Department of Oceanography, 0517 OSB, West Call Street, Tallahassee, Florida 323064320. Acknowledgments We thank Björn Grunwald and Gerhard Holst for assistance with the MOLLI system. Ingo Klimant (Institute for Analytical Chemistry, Microand Radiochemistry, Technical University of Graz, Austria) and Claudia Schröder (Institute for Analytical Chemistry, Bioand Chemo-Sensors, University of Regensburg, Germany) are thanked for the recipe, the technical advice, and help with the preparation of planar O2 optodes. Bo Barker Jørgensen is acknowledged for constant interest and support of this work, Dirk de Beer for valuable comments and discussion. Georg Herz, Alfred Kutsche, Volker Meyer, and Paul Färber are acknowledged for their help with the wave tank setup and electronics. Valuable comments of Ronnie Glud and two anonymous reviewers are much appreciated. The study was funded by the German Federal Ministry of Education and Research (BMBF, project 03F0284A) and the Max Planck Society (MPG). orders of magnitude (Rutgers van der Loeff 1981; Precht and Huettel 2004; Reimers et al. 2004). Advection can significantly increase the oxygen penetration depths in sediments (Forster et al. 1996; Lohse et al. 1996; Ziebis et al. 1996a) as well as the transport of organic particles (Huettel et al. 1996; Rusch and Huettel 2000), resulting in enhanced sedimentary oxygen consumption and organic matter degradation (Forster et al. 1996; Ziebis et al. 1996b; Reimers et