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Collaborative Research:

Robust optode-based eddy correlation systems for oxygen flux measurements in aquatic environments

Markus Huettel, Peter Berg

Funded by NSF Award OCE-1334117

The aquatic eddy correlation technique can produce high-quality oxygen flux records due to its non-invasive nature, high temporal resolution, and ability to integrate over a large sediment surface area. No other existing flux method integrates these advantages. The main shortcoming of the technique is associated with the fragile oxygen microelectrode that often breaks or malfunctions and limits effective deployment times. This project is designed to solve this problem by producing robust eddy correlation technology and interpretation software for oxygen flux measurements. The requirements for these sensors are that they 1) can reliably measure oxygen concentrations over extended time periods in water with suspended particles and other matter, 2) can capture oxygen fluctuations caused by the eddies carrying the oxygen flux, and 3) do not distort turbulent eddies carrying the flux signal.

 

Eddy instrument components

 Eddy correlation instrument for benthic aquatic oxygen flux measurements. The tripod carries an oxygen microelectrode with signal amplifier (2), and an acoustic Doppler Velocimeter (ADV) (3) that measures the three dimensional flow field within a small measuring volume (~1.5 cm3). The experimental instrument shown carries a second oxygen sensor, which is a robust oxygen optode (1). Different optode systems will be tested in this project. The red dot (4) below the ADV represents the location, where flow and oxygen are measured simultaneously (typical 5 to  30 cm above the sea floor). The sea floor area contributing to the flux is highlighted by the reddish color (5). The dark irregular shapes (6) are patchy areas with increased oxygen consumption, for example due to buried seagrass debris or microalgae. Because vertical oxygen transport towards or away from the sea floor is facilitated by turbulent motions, the vertical oxygen flux can be calculated by integrating over time the product of vertical flow fluctuation (Insert 7, blue line) and associated oxygen fluctuation (Insert 7, red line), both measured at high temporal resolution (16 to 64 Hz). (Insert 8) Breaking of the fra-gile glass microelectrode typically ends the deployment. No robust oxygen sensor is available today.

 

Durable optode eddy correlation systems can significantly improve the reliability of benthic oxygen flux estimates, a key parameter in local and global carbon budgets, and climate change studies. Existing flux estimates for shelf sediments rely mostly on short-term point measurements and typically exclude natural flow and light, thereby influencing transport processes, biogeochemical reactions, benthic photosynthesis and organism behavior. A robust eddy correlation instrument will be a powerful tool for measuring oxygen fluxes in such environments and will combine simple deployment and efficient collection of unbiased flux data. This will benefit both eddy correlation studies that typically include 24 h deployments, and future long-term monitoring of benthic systems, including underwater observatory platforms. It will also allow reliable collection of eddy correlation data in environments with high particle loads or in environments with strong currents and wave action as are frequently found on the continental shelf. An effective solution of the problems of microelectrode breakage and malfunctioning that appear to be overwhelming to many interested users will stimulate a substantially wider use of the approach. As a reference, in the atmospheric boundary layer, where similar sensor problems have been eliminated, the eddy correlation technique is the standard flux method today.

 

Measuring principle of the oxygen optode

AOptode

BFibertip

         

Ruthenium dye


CQuenching

I0/I= 1 – KSV * [O2]           Equation 1


D Lifetime

Eq 2 Equation 2

Eq 3Equation 3

    

 (A) Planar optode with bluegreen LED emitting excitation light and red LED emitting a reference light. The photodiode receives the oxygen-dependent luminescence emitted from the planar optode. (B) Fiber optode tip coated with an oxygen-sensitive dye (fluorophore) (C) Oxygen acts as a dynamic quencher, decreasing the fluorescence1 quantum yield of the sensing dye fluorophore (Kautsky, 1939) according to the Stern-Volmer relationship (Equation 1, I and I0are the fluorescence intensities in the presence and absence of oxygen, KSV is a constant expressing the quenching efficiency (Stern and Volmer 1919)). (D) State of the art optode systems measure decay of the luminescence2 after a light pulse, allowing background noise suppression, e.g. due to chlorophyll fluorescence (Tengberg et al., 2006). After the light pulse excited the sensor dye (L), the decay of the resulting luminescence is quantified in two bins (A1 and A2). The ratio A1/A2 is a function of the luminescence decay and calibrated against oxygen concentration. The decay time τ is calculated according to Equation 2 for a constant Δt of the two gates (Liebsch et al., 2000). The relationship between oxygen concentration and τ is described by Equation 3, where τ0 is decay time in the absence of oxygen.

1Fluorescence - Light emitted during absorption of radiation of another wavelength
2Luminescence - Light emission at normal and lower temperatures, here after the light pulse

 

 

 

Huettel, M., Berg, P. and Kostka, J.E. (2014) Benthic Exchange and Biogeochemical Cycling in Permeable Sediments. Annual Review of Marine Science, 6:23–51

Berg, P., Long, M. H., Huettel, M., Rheuban, J.E., JcGlathery, K.J., Howarth, R.W., Foreman, K.H., Giblin, A.E., Marino, R. (2013). Eddy correlation measurements of oxygen fluxes in permeable sediments exposed to varying current flow and light. Limnology and  Oceanography, 58(4), 2013, 1329–1343

Chipman, L., Huettel, M., Berg, P., Meyer, V., Klimant, I., Glud, R.N., Wenzhoefer, F. (2012), Oxygen optodes as fast sensors for eddy correlation measurements in aquatic systems, Limnology and Oceanography Methods, 10, 2012, 304–316

Berg, P., and Huettel, M. (2008) Monitoring the Seafloor Using the Noninvasive Eddy Correlation Technique: Integrated Benthic Exchange Dynamics, Oceanography 21, 164-167.

Berg, P. and others 2003. Oxygen uptake by aquatic sediments measured with a novel non-invasive eddy-correlation technique. Marine Ecology-Progress Series 261: 75-83.

Berg, P., H. Roy, and P. L. Wiberg. 2007. Eddy correlation flux measurements: The sediment surface area that contributes to the flux. Limnology and Oceanography 52: 1672-1684.

Canfield, D. E. and others 1993. Pathways of organic carbon oxidation in three continental margin sediments. Mar. Geol. 113: 27-40.

Nelson, J. R., J. E. Eckman, C. Y. Robertson, R. L. Marinelli, and R. A. Jahnke. 1999. Benthic microalgal biomass and irradiance at the sea floor on the continental shelf of the South Atlantic Bight: Spatial and temporal variability and storm effects. Continental Shelf Research 19: 477-505.