MEPS 210:1-12 (2001)  -  doi:10.3354/meps210001

ABSTRACT: Diffusion to solid-water interfaces is directly related to shear velocity. Shear velocities at the walls and bottoms of experimental ecosystem enclosures of different sizes and shapes (the Multiscale Experiment Ecosystem Research Center pelagic/benthic [MEERC P/B] tanks at the University of Maryland Center for Environmental Science, Horn Point Laboratory) were measured using hot-film sensors. Spatially averaged bottom and wall shear velocities were related to internal mixing, which was produced by rotating internal paddles and measured using a combination of gypsum dissolution and direct turbulence measurements. Shear velocities always increased with increasing mixing, but relationships between mixing and shear velocity changed with tank volume and shape. Spatially averaged bottom shear velocities decreased with increasing tank volume at an internal mixing level of 2 cm s^-1, but average 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 increasing mixing was lower for the larger tanks. A bulk impeller Reynolds number captured some, but not all, of the scale dependence of ratios of boundary shear velocity to internal mixing intensity; mixing design and tank geometry were also important. Levels of all shear velocities in the MEERC P/B tanks were lower than levels in natural coastal environments for equivalent internal mixing. Realistic levels of internal mixing in the tanks resulted in unrealistically low boundary shear velocities. As a result, wall diffusive sublayer thicknesses were similar to those found in deep-sea environments, and benthic diffusive sublayer thicknesses were even larger. Most current mesocosm designs are likely to be affected similarly. The artificially low-energy benthic environment may have particularly important consequences for ecosystem processes affected by pelagic-benthic coupling.

KEY WORDS: Diffusion · Shear velocity · Interface · Experimental ecosystem · Mesocosm