Turbulent mixing in experimental ecosystem studies

Abstract

ABSTRACT. Turbulent mixing is an integral aspect of aquatic ecosystems Turbulence affects ecosystem features ranglng from phytoplankton blooms at large scales through microscale interactions in the plankton. Enclosed experimental ecosystems, if they are to mimic the function of natural ecosystems, also must mimic natural turbulence and its effects. Large-scale velocity gradients and unstable buoyancy fluxes generate turbulent mixing in nature, most often at the surface and bottom boundaries and in the pycnocline. Large eddy sizes are controlled by the mixed layer depth, boundary layer thickness, or overturning length in the pycnocl~ne. Turbulent energy cascades through smaller and smaller eddy scales until it can be dissipated by molecular viscosity at the smallest scales. In contrast, artificial apparatuses frequently are used to generate turbulent m~x i n g in the interior of experimental ecosystem enclosures Large eddy sizes are controlled by the size of the generation apparatus, and they usually are much smaller than in nature Mismatched large eddy length scales and differences in turbulence generation mechanisms are responsible for the difficulties in mimicking natural turbulent mixing in experimental enclosures. The 2 most important turbulence parameters to consider in experimental ecosystem research are overall mixing time, T,,,, and turbulence dissipation rate, E. If the levels, spatial dlstribut~ons, and temporal variability of T,,, and E can be matched between an enclosure and the natural system it IS to model, then potential mixing artifacts can be minimized. An important additional consideration is that benthic ecosystems depend on the time-averaged boundary layer flow a s much as the turbulence. Existing designs for m i x~n g experimental ecosystems are capable of reasonably representing some aspects of natural turbulent mixing Paddle and grid stirring are the best available techniques for water column mixing, and flumes are best for benthic turbulence. There is no design at present that represents both environments adequately. More work also is needed on mixing of flexiblewall in situ enclosures. A more serious problem, however, is that turbulent mixing in experimental ecos)~stem studies too often is ignored, inadequately characterized, or unreported. Several methods are available for reasonable characterlzation of mixing in enclosures without sophisticated technology, and the technology for direct velocity measurements is becoming more accessible. Experimental ecosystem researchers should make a concerted effort to implement, characterize, and report on turbulent mixing In t h e~r enclosures.

Evaluation of oscillating grids and orbital shakers as means to generate isotropic and homogeneous small‐scale turbulence in laboratory enclosures commonly used in plankton studies

“…This implies, for example, that all relevant scales influencing the investigated process should be contained in the fully developed cascade of turbulent eddies (i.e., within the inertial subrange of the turbulent energy spectrum). This is difficult since the scales of generation of turbulent Evaluation of oscillating grids and orbital shakers as means to generate isotropic and homogeneous small-scale turbulence in laboratory enclosures commonly used in plankton studies motion in the field are much larger than in the containers used for experiments (Sanford 1997). One needs to be aware of the dimensions of the organisms and the process under study with respect to the dimensions of the container.…”

Effect of oysters Crassostrea virginica and bottom shear velocity on benthic-pelagic coupling and estuarine water quality

“…Benthic boundary-layer processes can be realistically represented in flumes but, with few exceptions (Asmus et al 1992), water-column processes generally are not. In isolated tanks, watercolumn processes can be mimicked well (Peters & Redondo 1997, Sanford 1997, but processes at the sediment -water interface are distorted (Sullivan et al 1991, Crawford & Sanford 2001, Porter et al 2004. In linked flume/tank mesocosms (Porter et al 2004), both water-column processes and benthic boundary-layer flow can be represented realistically, allowing for ecosystem experiments with enhanced bottom shear velocity, realistic water-column turbulence, and a realistic ratio of water-column turbulence levels to bottom shear velocity.…”

Gypsum dissolution is not a universal integrator of ‘water motion’

“…Alternatively, fluctuations in the flow can decrease the diffusive sublayer's thickness and increase mass transfer (Jaehne et al 1987;Sanford 1997) with or without an accompanying steady flow. Fluctuations in the flow can be generated by waves, oscillating flow, and small-scale turbulence such as from eddies or wakes around roughness elements or in aggregations.…”