Microsensor for in situ flow measurements in benthic boundary layers at submillimeter resolution with extremely slow flow

The currents in the lowest few millimeters of the bottom boundary layer of lakes are highly important for the dissipation of kinetic energy and for chemical processes such as oxygen transfer into the sediment. So far, no high-resolution flow velocity profiles close to the sediment - water interface have been reported for such systems because a suitable flow meter was lacking. This article introduces a novel sensor for the measurement of extremely lowflow velocities. The sensor is based on a gas transducer surrounded by a gas reservoir. It measures the change in the partial pressure of a tracer gas outside the reservoir tip due to advective transport. The sensor is suitable for measurements of velocities smaller than 1 mm s(-1), with a spatial resolution of 100 to 250 mu m. The flow measurements prove to be insensitive to temperature changes between 5 and 15 C. The sensor is robust against relative pressure changes, and angular differences in the sensitivity can be calibrated. We present high- resolution in situ measurements at the bottom of a pre- alpine lake with shear velocities as low as 0.13 +/- 0.02 cm s(-1). The velocity profile nicely resolves the transition zone between the viscous and the logarithmic boundary layer.