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 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.

A new procedure for autonomous acquisition of photosynthesis-irradiance curves on a microphytobenthic biofilm

“…Using the autonomous system we developed, a P-I curve was constructed on a scatterplot of 122 points, with 8 replicates for each light intensity. Contrary to the method of measuring fluorescence (Morelle et al, 2018) or eddy covariance (Merikhi et al, 2021) which are techniques that do not take depth into account, to our knowledge, no study on microphytobenthos using microsensor technique has allowed the construction of a P-I curve with such a high number of values. This was made possible by the synchronization and automation of the different elements composing the autonomous system.…”

Benthic Fluxes of Fluorescent Dissolved Organic Material, Salt, and Heat Measured by Multiple-Sensor Aquatic Eddy Covariance

“…The FACT sensor operates with an analyte sensing volume for conductivity and fluorescence that is displaced from the physical structure of the probe tip, and thus can be made to overlap with the velocity sensing volume without interfering with velocity measurements. This closer overlap between sensing volumes avoids the need for time shift corrections due to sensing volume displacement, as well as uncertainties that increase as the horizontal velocity direction departs from a downstream sensor orientation [20,21].…”

A high‐resolution submersible oxygen optode system for aquatic eddy covariance

“…Over the last two decades, the aquatic eddy covariance (AEC) technique (Berg et al 2003) has improved the quality of benthic O 2 flux measurements for many ecosystems, including seagrass meadows (Lee et al 2017; Berger et al 2020; Koopmans et al 2020), oyster and mussel reefs (Attard et al 2019 b ; Volaric et al 2020), macroalgal and Maerl beds (Attard et al 2019 a ; Polsenaere et al 2021), permeable sands (Chipman et al 2016; Merikhi et al 2021), freshwater systems (McGinnis et al 2008; Koopmans and Berg 2015), and coral reefs (Long et al 2013; de Froe et al 2019). It has also recently been used for upside‐down measurements of gas exchange at the air–water interface (Berg and Pace 2017; Long and Nicholson 2018; Berg et al 2020).…”

“…Biofouling of both fiberoptodes and microelectrodes can cause erroneous signals (Huettel et al 2020), and collisions with debris can lead to signal spikes, sensor defects and data loss (Koopmans et al 2020(Koopmans et al , 2021. To increase robustness and thus, deployment success, some studies use multiple microelectrodes or fiber-optic optodes (Attard et al 2014(Attard et al , 2019bMerikhi et al 2021). An alternative to these sensing systems is the microplanar optode sensor, which is more robust because of its larger tip (8 mm diameter) that does not break and is rarely affected by floating debris (Berg et al 2016;Amo-Seco et al 2021).…”