Direct observation of biomixing by vertically migrating zooplankton
Limnol. Oceanogr., 59(3), 2014, 724-732 | DOI: 10.4319/lo.2014.59.3.0724
ABSTRACT: The potential contribution of swimming zooplankton to the vertical mixing of stratified waters has been the topic of an ongoing scientific debate. Current estimates, which are primarily based on scale analyses and numerical simulations, range from negligible effects to significant contributions that are comparable in magnitude to physical driving forces, such as wind and tides. Here, we analyzed laboratory observations of fluid mixing that are caused by vertically migrating zooplankton (Daphnia magna) in a density-stratified water column. Mixing rates were quantified at the scale of individual organisms in terms of the dissipation rates of small-scale spatial variance of tracer concentration measured by laser-induced fluorescence. At the bulk scale, we analyzed temporal changes in the mean density stratification. Organism and bulk scale observations were used to estimate apparent diffusion coefficients in trails of individuals and organism groups. Mean diffusivities of 0.8–5.1 × 10−9 m2 s−1, which were averaged over trail volumes of 1.5–13 × 10−5 m3, are on the same order of magnitude as the molecular diffusivity of salt. A comparable diffusivity (1.1 × 10−9 m2 s−1) was estimated on the bulk scale, and the initial density stratification, although frequently passed by migrating Daphnia, was preserved over the 5 d experimental period. The present results agree with scaling arguments and suggest the negligible enhancement of vertical transport in comparison with the turbulent mixing that is typically observed in oceans and lakes.