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 C, 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.