Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
  • Problem Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

A new aquatic eddy covariance instrument was deployed that reduces flux errors through parallel measurements with three oxygen sensors positioned around the flow measuring volume (Merikhi et al. 2021).

All data were recorded with the aquatic eddy covariance instrument and reference sensors (light, oxygen, temperature) carried by the instrument tripod. The instrument tripod was lowered from the Florida Institute of Oceanography (FIO) Florida Keys Marine laboratory research vessel "Diodon" into the water at the study site and then placed by SCUBA divers at the exact location indicated by the lat/long coordinates provided (24° 43.523' N, 80° 49.855' W). Each of the deployments lasted one day; start and end times are provided in the dataset. After completion of the measuring period, the instrument was recovered by divers and returned to the FIO Florida Keys Marine laboratory for data download and sensor replacement.

After instrument retrieval, the signals of the three optodes were averaged for each measuring time point. The potential influence of oxygen concentration changes in the water column over time on the flux (i.e. storage effect) were negligible. Oxygen fluxes were calculated using the procedures explained in Merikhi et al. 2021. Specifically, flow and oxygen data were reduced to 8 hertz (Hz) data by averaging adjacent data points. In cases where the instrument was tilted relative to the main horizontal flow, the software rotated the flow velocity field for each 15-minute measuring interval such that mean transverse and vertical velocities became zero. Within each 15-minute interval, the mean vertical velocity and mean oxygen concentration then were calculated as least-square linear fits to the 8 Hz data. Subtraction of these means from each data point within the measuring interval produced the fluctuating concentration O₂' and fluctuating vertical velocity Vz' values away from their respective means that were used in equation 1 (Merikhi et al. 2021). Mean fluxes of four 15-minute periods were averaged to produce hourly fluxes expressed in units of millimoles per square meter per hour (mmol m⁻² h⁻¹). Further details of the data processing are provided in Merikhi et al. 2021.

- Imported original file "250421 Data upload.xlsx" into the BCO-DMO system.
- Added columns for site latitude, longitude, and depth.
- Renamed fields to comply with BCO-DMO naming conventions.
- Converted Date field to YYYY-MM-DD format.
- Added column for date-time in ISO8601 format.
- Saved the final file as "965718_v1_fl_keys_benthic_o2_flux.csv".

NSF Award Abstract:

The PIs request funding to build and test robust eddy correlation instruments for unidirectional and oscillating flow environments based on sturdy fiber- and planar-optical sensors and novel signal-processing electronics. The new hardware will be supported by software development to correct potential flux underestimations caused by inadequate oxygen sensor response time and spatial offsets between oxygen and flow sensors. The fragility of the thin glass microelectrode used in aquatic eddy correlation instruments severely limits the use of this powerful technique for flux measurements in benthic environments. This problem represents the major bottleneck preventing the widespread use of this approach.

Broader Impacts:

The PIs have very strong records both in spreading the use of EC technology through the community and in graduate and undergraduate education. They outline clearly the ways in which they will continue their ongoing endeavors in both areas. In addition, the application of this technology to the geochemistry and ecology of shallow-water regions has broad implications for carbon cycling and ocean acidification studies, both of which have important societal ramifications. Better quantify oxygen fluxes in the aquatic environment is important for society. It can e.g. help predict when and if the health of an aquatic system is being weakened, and when e.g. hypoxia or anoxia is approaching. Anoxia leads to death of all higher life

NSF Award Abstract:
This project focuses on the role of large underwater sand formations, particularly at the mouths of estuaries, in the cycling of nutrients and organic materials. "Megaripples" are common features on marine sediments where bottom currents are rapid. In the northeastern Gulf of Mexico, these sand dune-like features are up to half a meter in height, with wavelengths reaching 20 meters. The investigators propose that megaripples act as large filter systems that rapidly convert dissolved and solid organic matter into inorganic carbon and nutrients, and thus influence the biological productivity of coastal waters. They propose to use a one-kilometer long megaripple field in the inlet of Chotawhatchee Bay, in the northeastern Gulf of Mexico, as a natural laboratory for studying these processes. In addition, they will conduct laboratory experiments to investigate the filter processes at a smaller scale. By producing data on the functioning of megaripples, the project addresses a knowledge gap that has implications on our understanding of the cycles of matter in coastal waters. The project offers opportunities for both graduate and undergraduate students in learning state-of-the-art techniques. The students will gain experience in working on high frequency data acquisition and analysis of 'big data'. To enhance outreach, the researchers will develop and teach two courses on Permeable Sediment Biogeochemistry and Aquatic Eddy Covariance Studies for the Saturday at the Sea program offered by Florida State University. Results will be disseminated via scientific journals, conference presentations and public lectures, and directly to the Apalachicola-National Estuarine Research Reserve (NERR), which will make the results available to the other 28 NERR sites.

The two main project objectives are: 1) demonstrate the general function of megaripples as biocatalytical filters, and 2) demonstrate that common inlet megaripples contribute to nutrient retention in coastal bays. A 1 km long megaripple field in the inlet of Chotawhatchee Bay (wavelengths: ~20 m, amplitudes: 20 to 40 cm) in the northeastern Gulf of Mexico will be used as an in-situ laboratory. Measurements will characterize megaripple topography and the water flowing over them. Salinity and suspended particles are utilized as natural tracers to quantify solute and particle entrainment into the megaripples. This project will deploy non-invasive aquatic eddy covariance instruments equipped with newly developed robust sensors to quantify sedimentary remineralization in the flushed megaripple bed. This technique integrates the benthic oxygen flux over a large section of the megaripple field, while including the natural dynamics of currents and light. Real-time water column measurements with a boat-mounted flow-through analyzer permit rapid quantification of large horizontal gradients of key water column parameters. The in-situ measurements will be combined with laboratory column reactor experiments that quantify nutrient re-mobilization through organic carbon mineralization in flushed megaripple sand. The megaripple field in the inlet of Chotawhatchee Bay is easily accessible from shore and by small boat, expediting instrument deployments and in-situ measurements while reducing project costs. Synthesis of all data will produce a conceptual and quantitative understanding of megaripples as natural biocatalytical filters. Because transport and reaction in megaripples are governed by basic physical and biogeochemical processes, these results will reveal information on the general biogeochemical functioning of megaripples that so far is not available.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.