Technical Note: Novel triple O<sub>2</sub>-sensor aquatic eddy covariance instrument with improved time-shift correction reveals central role of microphytobenthos for carbon cycling in coral reef sands

Abstract

Abstract. The aquatic eddy covariance technique stands out as a method for benthic O2-flux measurements because it measures non-invasively, but in the conventional instruments, the spatial separation of the measuring locations of the velocity and O2 sensors causes a time-shift that can be substantial and difficult to correct. Here we introduce a triple O2-sensor-eddy covariance instrument (3OEC) that by positioning of the O2-sensors around the flow measuring volume allows eliminating these time-shifts through signal averaging. The new instrument was used to determine O2-production and consumption in an energetic coastal environment with highly permeable coral reef sands colonized by microphytobenthos. The measurement at ~10 m water depth revealed O2-fluxes that range among the highest reported for marine sediments despite relatively low organic content of the water and coarse sediment, indicating a central role of microphytobenthos for the carbon and nutrient cycling in the coral sand. High light utilization efficiency of the microphytobenthos and bottom currents increasing pore water exchange facilitated the high benthic production and respiration. The measurements documented a gradual transfer of the flux signal from the small turbulence generated at the sediment water interface to the larger wave-dominated eddies of the overlying water column with a delay influenced by the memory effect of eddies. These results demonstrate that the 3OEC can improve the precision of the flux measurements, including measurements in environments considered challenging for this 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.