This paper presents experimental results for the fluid transport induced by tethered as well as freely swimming zooplankton organisms (Daphnia) in a density-stratified water column. Particle image velocimetry and planar laser induced fluorescence were used to measure the instantaneous velocity and density distributions and to estimate kinetic energy, dissipation rates, vertical mass fluxes, and apparent diffusion coefficients around individual organisms. For freely swimming organism we found dissipation rates of kinetic energy of 10−6W kg−1 and an apparent diffusion coefficient of 10−5m2 s−1. Both values are valid for a volume of influence of approximately 1 cm3, which is about two orders of magnitude larger than the size of the organism. In the case of tethered Daphnia all measures are significantly larger but – associated with the artificial situation – of less environmental significance. The results indicate that swimming-induced fluid and mass transport in the vicinity of zooplankton organisms is characterized by apparent diffusion coefficients comparable in magnitude to turbulent diffusivities typically found in stratified aquatic environments. Upscaling of these results using abundances of Daphnia in lakes further suggests that biologically-induced fluid transport by zooplankton might potentially be important for vertical mixing in stratified waters if background mixing is weak.

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