Boundary shear velocities and fluxes in the MEERC experimental ecosystems

Diffusion to solid-water interfaces is directly related to shear velocity. Shear velocities at the walls and bottoms of experimental ecosystem enclosu res of different sizes and shapes (the Multiscale Experiment Ecosystem Rese arch Center pelagic/benthic [MEERC P/B] tanks at the University of Maryland Center for Environmental Science, Horn Point Laboratory) were measured usi ng hot-film sensors. Spatially averaged bottom and wall shear velocities we re related to internal mixing, which was produced by rotating internal padd les and measured using a combination of gypsum dissolution and direct turbu lence measurements. Shear velocities always increased with increasing mixin g, but relationships between mixing and shear velocity changed with tank vo lume and shape. Spatially averaged bottom shear velocities decreased with i ncreasing tank volume at an internal mixing level of 2 cm s(-1), but averag e wall shear velocities were similar for most tanks. In contrast, the rate of increase in bottom shear velocity with increasing mixing was similar for most tanks, but the rate of increase in wall shear velocity with increasin g mixing was lower for the larger tanks. A bulk impeller Reynolds number ca ptured some, but not all, of the scale dependence of ratios of boundary she ar velocity to internal mixing intensity; mixing design and tank geometry w ere also important. Levels of all shear velocities in the MEERC P/B tanks w ere lower than levels in natural coastal environments for equivalent intern al mixing. Realistic levels of internal mixing in the tanks resulted in unr ealistically low boundary shear velocities. As a result, wall diffusive sub layer thicknesses were similar to those found in deep-sea environments, and benthic diffusive sublayer thicknesses were even larger. Most current meso cosm designs are likely to be affected similarly. The artificially low-ener gy benthic environment may have particularly important consequences for eco system processes affected by pelagic-benthic coupling.