Biogeochemical Microsensors For Boundary Layer Studies

Abstract

Abstract Interfacial processes in sediments, biofilms, and other benthic aquatic systems can be strongly affected by the presence of hydrodynamic boundary layers. The boundary layer (BL) can limit mass transfer between the overlying water and the solid matrix, thus affecting biological processes. The BL region adjacent to reactive surfaces that contains significant chemical gradients is known as the diffusive boundary layer (DBL). The DBL thickness is dependent on the flow velocity, surface topography and porosity of the solid matrix, and it ranges from <0.1 mm to several millimeters in thickness (see also chapters 5, 9, 14 and 15). As the interface is approached, mass transfer of solutes in the DBL becomes less dependent on advective motions (turbulence) and increasingly dependent on diffusion. Any net solute consumption or production within the matrix relative to the surrounding water, therefore, leads to the formation of concentration gradients over the DBL as well as within the matrix. The sediment-water interface thus can be the site of steep gradients of physicochemical variables over distances of <0.1 mm, and methods for studying these variables near the interface must resolve variations over very small distances without significant disturbance to these gradients. Noninvasive optical analysis, by direct microscopic observation, analysis of water current with laser-Doppler anemometry, nuclear magnetic resonance imaging (Wieland, 2000) or positron emission tomography (Khalili et al., 1998) fulfills these requirements, but most methods with high spatial resolution are based on microsensors.