Decomposition of plankton‐derived dissolved organic matter in permeable coastal sediments

We tested the hypothesis that dissolved organic carbon (DOC) is degraded when filtered through permeable shelf sediments. In a laboratory flume experiment the concave shape of DOC profiles observed in the upper 10 cm of Gulf of Mexico sublittoral sands was reproduced by the combination of DOC filtration and production in the sediment surface layer and mineralization and adsorption in the subsurface layer. Six percent to 14% of 13Clabeled, highly degradable DOC was mineralized during filtration through 5.8-cm–long sand-filled column reactors, up to nine times more than in dark, water-filled column reactors. Filtration through 50-cm–long sand columns removed all highly degradable DOC pumped through the sediment, translating to fluxes of up to 379 mmol DOC m22 d21, corresponding to DOC removal from an approximately 1-m–deep water layer each day. Bacterial incorporation of 13C identified a diverse group of sedimentary aerobic and anaerobic microbes processing DO13C filtered through sand columns, dominated by Gammaproteobacteria, Deltaproteobacteria, Firmicutes, Bacteroidetes, and Planctomycetes. Excitation-emission matrix spectroscopy analysis of chromophoric dissolved organic matter contained in diatom-derived DOC indicated that microbes preferentially removed components around the fluorescence peak of tryptophan or protein-like substances, which may have the highest nutrient value. Mass spectra analysis revealed that filtration through sand also removes a broad spectrum of substances from less degradable humic and fulvic acid–like DOC and produces new DOC components. The flushed sand layer between the water column and deeper anoxic sediment layers acts as an effective DOC filter, with subsurface horizontal pore-water flows promoting decomposition of DOC.