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Monitoring the Seafloor Using the Noninvasive Eddy Correlation Technique: Integrated Benthic Exchange Dynamics

BackgrouNd The eddy correlation technique has been used for 60 years in atmospheric boundary layer research to measure land-air exchanges of different constituents (e.g., Swinbank, 1951). Only recently, the technique has also been applied to the benthic boundary layer by Berg et al. (2003), who determined sediment-water fluxes of dissolved O 2 and validated their findings against in situ chamber measurements. It is technically challenging to measure the key eddy correlation variables at a point near the sediment-water interface and at the high frequency required to fully resolve turbulent eddies. However, if feasible, the reward is high in that major limitations inherently linked to other flux methods, such as lab measurements in sediment cores and deployments of in situ chambers, can be bypassed. Specifically, eddy correlation measurements integrate over a larger area (Berg et al., 2007) and are done under true in situ conditions with no disturbances of sediment, light, and bottom boundary layer flow. The latter is particularly important for permeable sediments, where current and wave-driven porewater flushing can significantly alter biogeochemical cycling (Huettel et al., 1998; Jahnke et al., 2000; Reimers et al., 2004). As methodological bias is minimized, this technique can significantly improve monitoring at the seafloor. Here, we present our latest progress on the eddy correlation technique within our integrated Benthic Exchange Dynamics (iBED) project. Although eddy correlation is a general flux method, we focus only on O 2 , which has been the theme of this project. The eddy correlation technique relies on measuring vertical water column velocity and O 2 concentration in a ~ 1 cm 3 volume located 5 to 30 cm above the sediment surface and at a rate of 16 to 64 Hz. With the underlying assumption that all O 2 transported vertically toward or away from the sediment surface is facilitated by turbulent motions, the vertical flux can be derived from these two variables. Specifically, the eddy flux is defined as u z 'C ' where u z 'C ' is the fluctuating vertical velocity away from the mean, u z 'C ' is the fluctuating concentration away from the mean, and the bar symbolizes an averaging over time (Berg et al., 2003). Typically, 10 to 15 minutes of continuous data are required to obtain one statistically sound flux estimate. The flux is valid for an oval-shaped sediment surface area, the so-called footprint, located upstream of the measuring point. A …