Interest in validating the eddy covariance (EC) technique under wave-induced flows led to a series of experiments in a 104-m-long large wave flume (LWF) using an acoustic Doppler velocimeter (ADV) and two oxygen microelectrodes (tips ~2 mm apart) mounted on a sturdy tripod. Four additional ADVs positioned within the flume provided comparative near-bed velocity measurements during experiments with irregular waves over a sand bed. These measurements revealed that modifications of local turbulence by the tripod frame were insignificant. However, errors in velocity measurements were at times observed for setups where the microelectrode tips protruded into the ADV’s measurement volume. Disparate oxygen microelectrode velocity effects (stirring sensitivities) combined with response time offsets were also identified as problems, adding biases to EC flux derivations. Microelectrode velocity effects were further investigated through modeling designed to mimic the LWF data, and through examination of a 12-h dataset from the Oregon shelf. The modeling showed that under progressive waves, an artificial EC flux, or bias, arises most severely when the velocity sensitivity of the microelectrode is unequal in opposing flow directions or augmented by horizontal currents, and the velocity and oxygen data are not perfectly aligned in time. Sensitivities to wave motions were seen in the oxygen measurements from the Oregon shelf, contributing to an average flux of +2.7 ± 0.6 mmol m−2 day−1 (SE, n = 22) at wave frequencies. Since overall EC fluxes equaled only −4.1 ± 1.8 mmol m−2 day−1 (SE, n = 22), sources of EC biasing coupled to waves cannot be ruled out as potential problems for estimating exact benthic oxygen fluxes under common continental shelf field conditions.

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 chal-lenging 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 inte-grate 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.

We present and compare small sediment-water fluxes of O-2 determined with the eddy correlation technique, with in situ chambers, and from vertical sediment microprofiles at a 1450 m deep-ocean site in Sagami Bay, Japan. The average O-2 uptake for the three approaches, respectively, was 1.62 0.23 (SE, n = 7), 1.65 0.33 (n = 2), and 1.43 0.15 (n = 25) mmol m(-2) d(-1). The very good agreement between the eddy correlation flux and the chamber flux serves as a new, important validation of the eddy correlation technique. It demonstrates that the eddy correlation instrumentation available today is precise and can resolve accurately even very small benthic O-2 fluxes. The correlated fluctuations in vertical velocity and O-2 concentration that give the eddy flux had average values of 0.074 cm s(-1) and 0.049 mu M. The latter represents only 0.08% of the 59 mu M mean O-2 concentration of the bottom water. Note that these specific fluctuations are average values, and that even smaller variations were recorded and contributed to the eddy flux. Our findings demonstrate that the eddy correlation technique is a highly attractive alternative to traditional flux methods for measuring even very small benthic O-2 fluxes.