Abstract
Abstract. The aquatic eddy covariance technique stands out as a powerful method for benthic O2 flux measurements in shelf environments because it integrates effects of naturally varying drivers of the flux such as current flow and light. In conventional eddy covariance instruments, the time shift caused by spatial separation of the measuring locations of flow and O2 concentration can produce substantial flux errors that are difficult to correct. We here introduce a triple O2 sensor eddy covariance instrument (3OEC) that by instrument design eliminates these errors. This is achieved by positioning three O2 sensors around the flow measuring volume, which allows the O2 concentration to be calculated at the point of the current flow measurements. The new instrument was tested in an energetic coastal environment with highly permeable coral reef sands colonised by microphytobenthos. Parallel deployments of the 3OEC and a conventional eddy covariance system (2OEC) demonstrate that the new instrument produces more consistent fluxes with lower error margin. 3OEC fluxes in general were lower than 2OEC fluxes, and the nighttime fluxes recorded by the two instruments were statistically different. We attribute this to the elimination of uncertainties associated with the time shift correction. The deployments at ∼ 10 m water depth revealed high day- and nighttime O2 fluxes despite the relatively low organic content of the coarse sediment and overlying water. High light utilisation efficiency of the microphytobenthos and bottom currents increasing pore water exchange facilitated the high benthic production and coupled respiration. 3OEC measurements after sunset documented a gradual transfer of negative flux signals from the small turbulence generated at the sediment–water interface to the larger wave-dominated eddies of the overlying water column that still carried a positive flux signal, suggesting concurrent fluxes in opposite directions depending on eddy size and a memory effect of large eddies. The results demonstrate that the 3OEC can improve the precision of benthic flux measurements, including measurements in environments considered challenging for the eddy covariance technique, and thereby produce novel insights into the mechanisms that control flux. We consider the fluxes produced by this instrument for the permeable reef sands the most realistic achievable with present-day technology.
Highlights
This study introduces a new eddy covariance instrument and demonstrates its functionality through a measuring series addressing benthic oxygen flux in a dynamic backreef area covered by highly permeable carbonate sands
The aquatic eddy covariance technique is a powerful technique for quantifying fluxes at the seafloor as it measures over any type of substrate and integrates over a relatively large area (Berg et al, 2003; Lorrai et al, 2010; McGinnis et al, 2008)
The trajectories of the cumulative fluxes were similar in both instruments, but in the 3OEC, the additional sensor and elimination of errors associated with time shift corrections reduced fluctuations of the averaged signal trajectories (Fig. 2c)
Summary
This study introduces a new eddy covariance instrument and demonstrates its functionality through a measuring series addressing benthic oxygen flux in a dynamic backreef area covered by highly permeable carbonate sands. The aquatic eddy covariance technique is a powerful technique for quantifying fluxes at the seafloor as it measures over any type of substrate and integrates over a relatively large area (Berg et al, 2003; Lorrai et al, 2010; McGinnis et al, 2008). The technology so far has been adapted to measure temperature, salinity, oxygen, hydrogen, sulfide, and nitrate fluxes (McGinnis et al, 2011; Johnson et al, 2011; Long et al, 2015; Crusius et al, 2008; Weck and Lorke, 2017)
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