The sediment-water interface is one of the most important boundary layers of the earth system with crucial filter functions for the mass exchange between the seafloor and the water column. One of the most important exchange processes across this layer is the oxygen flux, which is a predominant control factor for the biogeochemical activity in the sediment. The eddy correlation system (ECS) allows measuring exchange rates across the water-sediment interface in a non-invasive manner providing direct measurements with temporal resolution of minutes and a spatial resolution of several meters.
The instrument is composed of an Acoustic Doppler Velocimeter (ADV), a fast responding O2 microelectrode (< 0.2 to 0.3 s) connected to an Auto-Zero-Amplifier and a battery, all fixed on a frame. Eddy correlation relies on measuring vertical velocity and oxygen concentration in the same measuring volume fast enough to calculate the momentary advective flux due to turbulent motions. Integrated over time for a period long enough to get a statistically sound average, it gives a net O2 transport toward or away from the sediment. The determined flux is integrated over a several m2 large footprint (typically 10 – 100 m2), located up-stream of the measuring point and depending primarily on water depth, sediment surface roughness, and current velocity.
The eddy correlation system (ECS) has the advantage of being applicable to many kind of surfaces (i.e. coral reefs, sea grass meadows) not feasible for other instrument deployments.