Determining Near-Bottom Fluxes of Passive Tracers in Aquatic Environments

In aquatic systems, the eddy correlation method (ECM) provides vertical flux measurements near the sediment-water interface. The ECM independently measures the turbulent vertical velocities w' and the turbulent tracer concentration c' at a high sampling rate (> 1 Hz) to obtain the vertical flux (w'c') over bar from their time-averaged covariance. This method requires identifying and resolving all the flow-dependent time (and length) scales contributing to (w'c') over bar. With increasingly energetic flows, we demonstrate that the ECM's current technology precludes resolving the smallest flux-contributing scales. To avoid these difficulties, we show that for passive tracers such as dissolved oxygen, (w'c') over bar can be measured from estimates of two scalar quantities: the rate of turbulent kinetic energy dissipation.. and the rate of tracer variance dissipation chi(c). Applying this approach to both laboratory and field observations demonstrates that (w'c') over bar is well resolved by the new method and can provide flux estimates in more energetic flows where the ECM cannot be used.