Effects of transient bottom water currents and oxygen concentrations on benthic exchange rates as assessed by eddy correlation measurements

Eddy correlation (EC) measurements in the benthic boundary layer (BBL) allow estimating benthic O-2 uptake from a point distant to the sediment surface. This noninvasive approach has clear advantages as it does not disturb natural hydrodynamic conditions, integrates the flux over a large foot-print area and allows many repetitive flux measurements. A drawback is, however, that the measured flux in the bottom water is not necessarily equal to the flux across the sediment-water interface. A fundamental assumption of the EC technique is that mean current velocities and mean O-2 concentrations in the bottom water are in steady state, which is seldom the case in highly dynamic environments like coastal waters. Therefore, it is of great importance to estimate the error introduced by nonsteady state conditions. We investigated two cases of transient conditions. First, the case of transient O-2 concentrations was examined using the theory of shear flow dispersion. A theoretical relationship between the change of O-2 concentrations and the induced vertical O-2 flux is introduced and applied to field measurements showing that changes of 5-10 mu M O-2 h(-1) result in transient EC-fluxes of 6-12 mmol O-2 m(-2) d(-1), which is comparable to the O-2 uptake of shelf sediments. Second, the case of transient velocities was examined with a 2D k-epsilon turbulence model demonstrating that the vertical flux can be biased by 30-100% for several hours during changing current velocities from 2 to 10 cm s(-1). Results are compared to field measurements and possible ways to analyze and correct EC-flux estimates are discussed.