Oxygen optodes as fast sensors for eddy correlation measurements in aquatic systems

Abstract

The aquatic eddy-correlation technique can be used to noninvasively determine the oxygen exchange across the sediment-water interface by analyzing the covariance of vertical flow velocity and oxygen concentration in a small measuring volume above the sea bed. The method requires fast sensors that can follow the rapid changes in flow and the oxygen transported by this flow to calculate the momentary advective flux driven by turbulent motions. In this article, we demonstrate that fast optical oxygen sensors, known as optodes, represent a good alternative to the traditional Clark-type electrochemical microelectrodes for such measurements. Optodes have the advantage over microelectrodes of being insensitive to flow, less susceptible to signal drift, more durable under typical field conditions, less expensive, and repairable. Comparisons of the response times of optodes and microelectrodes to rapid oxygen changes showed that optimized optodes had the same response time (162 ± 66 ms) as the microelectrodes (160 ± 57 ms) and were fast enough to capture the oxygen fluctuations that are relevant for the eddy correlation flux calculations. Side by side comparisons of benthic O 2 flux data collected with microelectrode-based eddy correlation instruments and optode-based eddy correlation instruments in freshwater and marine environments showed no significant differences between the measured fluxes. The optodes allow the development of more user-friendly eddy correlation instruments that combine the advantages of noninvasive measurements and integration of fluxes over a large footprint area, using a relatively rugged and less expensive sensor.

Technical note: Time lag correction of aquatic eddy covariance data measured in the presence of waves

“…Clark-type oxygen microelectrodes used for eddy covariance typically have response times (t 90% ) of 0.2 to 0.5 s Attard et al, 2015;Donis et al, 2015). Newer optical sensors that have been developed in recent years have comparable or somewhat longer response times of 0.2 to 0.8 s (Chipman et al, 2012;Murniati et al, 2015;Berg et al, 2015). This means that the time series of the two key variables from which eddy fluxes are derived, the vertical velocity and oxygen concentration, are never perfectly aligned in time.…”

Using a High-Frequency Fluorescent Oxygen Probe in Atmospheric Eddy Covariance Applications

“…This is most likely due to the combination of the limited resolution and the oversampled system (sampling at 20 Hz but the sensor outputs only at about 2 Hz). For eddy covariance measurement in water, the stepwise behavior was shown to have a minor effect on the calculated fluxes (Chipman et al 2012). However, this does not necessary have to be the case for atmospheric eddy covariance measurements of oxygen where the concentration fluctuations are small relative to the absolute concentration.…”

“…This method is widely used within the meteorological community to determine, for example, fluxes of CO 2 , and sensible and latent heat. Oxygen eddy covariance measurements in water have been performed successfully over the last decade in studying water-sediment exchange (e.g., Berg et al 2003Berg et al , 2009Kuwae et al 2006;Brand et al 2008;McGinnis et al 2008;Reimers et al 2012;Chipman et al 2012). Atmospheric eddy covariance measurements of oxygen have not previously been performed, since instrumentation with sufficient response time and resolution was not available.…”

Acoustic monitoring of O2 production of a seagrass meadow

“…For such measurements, O 2 optodes provide for optical detection, and are much more stable with time than the older Clark-type O 2 electrodes (Tengberg et al, 2006). The input-output mass balance approach also requires a large number of O 2 profiles from ships or moorings and data on water transport, while eddy covariance methods are not at present possible on long deployments due to limitations of sensors (Chipman et al, 2012).…”

“…Open water methods include O 2 input-output mass balance (Gazeau et al, 2005), diel cycle mass balance, i.e. the Odum (1956) method (Champenois and Borges, 2012), and eddy covariance (Chipman et al, 2012). Incubation chambers provide a direct and precise local measurement of O 2 exchange between the organisms and the surrounding water either via electrodes or chemically by Winkler titration, but they have the caveat of being discrete temporal and spatial measurements (Silva et al, 2009); (Staehr et al, 2011).…”