The aquatic eddy-correlation technique can be used to noninvasively determine the oxygen exchange across the sediment-water interface by analyzing the covariance of vertical flow velocity and oxygen concentration in a small measuring volume above the sea bed. The method requires fast sensors that can follow the rapid changes in flow and the oxygen transported by this flow to calculate the momentary advective flux driven by turbulent motions. In this article, we demonstrate that fast optical oxygen sensors, known as optodes, represent a good alternative to the traditional Clark-type electrochemical microelectrodes for such measurements. Optodes have the advantage over microelectrodes of being insensitive to flow, less susceptible to signal drift, more durable under typical field conditions, less expensive, and repairable. Comparisons of the response times of optodes and microelectrodes to rapid oxygen changes showed that optimized optodes had the same response time (162 ± 66 ms) as the microelectrodes (160 ± 57 ms) and were fast enough to capture the oxygen fluctuations that are relevant for the eddy correlation flux calculations. Side by side comparisons of benthic O 2 flux data collected with microelectrode-based eddy correlation instruments and optode-based eddy correlation instruments in freshwater and marine environments showed no significant differences between the measured fluxes. The optodes allow the development of more user-friendly eddy correlation instruments that combine the advantages of noninvasive measurements and integration of fluxes over a large footprint area, using a relatively rugged and less expensive sensor.

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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 13C‐labeled, 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 m−2 d−1, 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.

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