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Abstract. The aquatic eddy covariance technique stands out as a powerful method for benthic O2 flux measurements in shelf environments because itintegrates effects of naturally varying drivers of the flux such as current flow and light. In conventional eddy covariance instruments, the timeshift caused by spatial separation of the measuring locations of flow and O2 concentration can produce substantial flux errors that aredifficult to correct. We here introduce a triple O2 sensor eddy covariance instrument (3OEC) that by instrument design eliminates theseerrors. This is achieved by positioning three O2 sensors around the flow measuring volume, which allows the O2concentration to be calculated at the point of the current flow measurements. The new instrument was tested in an energetic coastal environment with highly permeablecoral reef sands colonised by microphytobenthos. Parallel deployments of the 3OEC and a conventional eddy covariance system (2OEC) demonstrate thatthe new instrument produces more consistent fluxes with lower error margin. 3OEC fluxes in general were lower than 2OEC fluxes, and the nighttimefluxes recorded by the two instruments were statistically different. We attribute this to the elimination of uncertainties associated with the timeshift correction. The deployments at ∼ 10 m water depth revealed high day- and nighttime O2 fluxes despite the relatively loworganic content of the coarse sediment and overlying water. High light utilisation efficiency of the microphytobenthos and bottom currents increasingpore water exchange facilitated the high benthic production and coupled respiration. 3OEC measurements after sunset documented a gradual transfer ofnegative flux signals from the small turbulence generated at the sediment–water interface to the larger wave-dominated eddies of the overlying watercolumn that still carried a positive flux signal, suggesting concurrent fluxes in opposite directions depending on eddy size and a memory effect oflarge eddies. The results demonstrate that the 3OEC can improve the precision of benthic flux measurements, including measurements in environmentsconsidered challenging for the eddy covariance technique, and thereby produce novel insights into the mechanisms that control flux. We consider thefluxes produced by this instrument for the permeable reef sands the most realistic achievable with present-day technology.
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Technical note: Measurements and data analysis of sediment–water oxygen flux using a new dual-optode eddy covariance instrument
Abstract. Sediment–water oxygen fluxes are widely used as a proxy fororganic carbon production and mineralization at the seafloor. In situ fluxescan be measured non-invasively with the aquatic eddy covariance technique,but a critical requirement is that the sensors of the instrument are able tocorrectly capture the high-frequency variations in dissolved oxygenconcentration and vertical velocity. Even small changes in sensorcharacteristics during deployment as caused, e.g. by biofouling can result inerroneous flux data. Here we present a dual-optode eddy covarianceinstrument (2OEC) with two fast oxygen fibre sensors and document howerroneous flux interpretations and data loss can effectively be reduced bythis hardware and a new data analysis approach. With deployments over acarbonate sandy sediment in the Florida Keys and comparison with parallelbenthic advection chamber incubations, we demonstrate the improved dataquality and data reliability facilitated by the instrument and associateddata processing. Short-term changes in flux that are dubious in measurementswith single oxygen sensor instruments can be confirmed or rejected with the2OEC and in our deployments provided new insights into the temporal dynamicsof benthic oxygen flux in permeable carbonate sands. Under steadyconditions, representative benthic flux data can be generated with the 2OECwithin a couple of hours, making this technique suitable for mappingsediment–water, intra-water column, or atmosphere–water fluxes.
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- PAR ID:
- 10215169
- Date Published:
- Journal Name:
- Biogeosciences
- Volume:
- 17
- Issue:
- 17
- ISSN:
- 1726-4189
- Page Range / eLocation ID:
- 4459 to 4476
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Organic carbon mineralization and nutrient cycling in benthic environments are critically important for their biogeochemical functioning, but are poorly understood in coastal upwelling systems. The main objective of this study was to determine benthic oxygen fluxes in a muddy sediment in the Ria de Vigo (NW Iberian coastal upwelling), by applying the aquatic eddy covariance (AEC) technique during 2 campaigns in different seasons (June and October 2017). The main drivers of benthic fluxes were studied and compared among days in each season and between seasons. The 2 campaigns were characterized by an upwelling-relaxation period followed by a downwelling event, the last of which was due to the extratropical cyclone Ophelia in October. The mean (±SD) seasonal benthic oxygen fluxes were not significantly different for the 2 campaigns despite differences in hydrodynamic and biogeochemical conditions (June: -20.9 ± 7.1 mmol m -2 d -1 vs. October: -26.5 ± 3.1 mmol m -2 d -1 ). Benthic fluxes were controlled by different drivers depending on the season. June was characterized by sinking labile organic material, which enhanced benthic fluxes in the downwelling event, whereas October had a significantly higher bottom velocity that stimulated the benthic fluxes. Finally, a comparison with a large benthic chamber (0.50 m 2 ) was made during October. Despite methodological differences between AEC and chamber measurements, concurrent fluxes agreed within an acceptable margin (AEC:benthic chamber ratio = 0.78 ± 0.13; mean ± SD). Bottle incubations of water sampled from the chamber interior indicated that mineralization could explain this difference. These results show the importance of using non-invasive techniques such as AEC to resolve benthic flux dynamics.more » « less
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Aquatic eddy covariance (AEC) is increasingly being used to study benthic oxygen (O 2 ) flux dynamics, organic carbon cycling, and ecosystem health in marine and freshwater environments. Because it is a noninvasive technique, has a high temporal resolution (∼15 min), and integrates over a large area of the seafloor (typically 10–100 m 2 ), it has provided new insights on the functioning of aquatic ecosystems under naturally varying in situ conditions and has given us more accurate assessments of their metabolism. In this review, we summarize biogeochemical, ecological, and biological insightsgained from AEC studies of marine ecosystems. A general finding for all substrates is that benthic O 2 exchange is far more dynamic than earlier recognized, and thus accurate mean values can only be obtained from measurements that integrate over all timescales that affect the local O 2 exchange. Finally, we highlight new developments of the technique, including measurements of air–water gas exchange and long-term deployments.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/bg-17-4459-2020
