Biologically- induced Mixing of Stratified Waters by Swimming Zooplankton
Final Report Abstract
Motile aquatic organisms, ranging in size from bacteria to whales, impart momentum and kinetic energy on the surrounding water and thus leave a hydrodynamic footprint along their swimming trajectories. It has been suggested that marine organisms generate as much mechanical energy in water motion, such as the wind and tides. If this was true, marine life potentially affects the global climate by controlling the vertical transport of heat and salt and therewith the global oceanic circulation. Moreover, the biologically-generated flows are important for the swimming efficiency of the organisms and are affecting their nutrient uptake, interactions with predators and prey, and perception of hydrodynamic and chemical signals. Because the actual impact of swimming organisms on larger-scale mixing and heat or mass transport in aquatic systems is still unknown, current figures are rather speculative. Using the water flea Daphnia as a model organism with a widespread abundance in inland waters, we studied the flow fields around freely swimming zooplankton as a function of organism size, swimming mode and abundance. For the first time, we have measured the rate of vertical mixing in density-stratified waters caused by migrating zooplankton and have investigated its swimming performance under turbulent flow conditions. As the most important results of this project, we found that despite of surprisingly high rates of kinetic energy dissipation in the trails of swimming Daphnia, their contribution to vertical heat and mass transport in stratified waters is rather small. Within a volume about 500 times the size of the organisms, energy dissipation rates were comparable to dissipation rates of turbulent kinetic energy typically found in lakes. Although the footprint size of the swimming organism in terms of density and solute concentration fluctuations is exceeding the size of the hydrodynamic footprint by another factor of 100, the effective diffusivity within this volume is only slightly exceeding the diffusivity of most solutes. Therewith we provided first experimental evidence for the very low mixing efficiency of the small swimmers. This work has been referred to in prominent news reports. However, we have further demonstrated that the hydrodynamic footprints of the small swimmers are hotspots of kinetic energy dissipation also under turbulent flow conditions and that dense aggregations of Daphnia, which follow a localized attractor such as light, can generate much larger flow structures with potentially higher mixing efficiency. Observations in lakes, which have been obtained as part of another ongoing research project revealed that biogeochemical interfaces in lakes are often characterized by only weak vertical transport, with vertical diffusivities comparable to those observed in the Daphnia trails, as well as by high abundances of zooplankton. More detailed field observations of the small-scale patchiness of zooplankton are required to assess the relevance of swimming-induced mixing at such localized biogeochemical hotspots. http://news.nationalgeographic.com/news/2014/09/140930-sea-monkeys-laser-ocean-animals-science/ http://news.sciencemag.org/biology/2014/09/can-sea-monkeys-stir-sea
Publications
- (2012), Zooplankton induced currents and fluxes in stratified waters, Water Qual. Res. J. Can., 47(3-4), 276-285
Noss, C., and A. Lorke
(See online at https://doi.org/10.2166/wqrjc.2012.135) - (2013), Three- Dimensional Analysis of the Swimming Behavior of Daphnia magna Exposed to Nanosized Titanium Dioxide, PLoS One, 8(11), e80960
Noss, C., A. Dabrunz, R. R. Rosenfeldt, A. Lorke, and R. Schulz
(See online at https://doi.org/10.1371/journal.pone.0080960) - (2013), Three-dimensional tracking of multiple aquatic organisms with a two camera system, Limnology and Oceanography: Methods, 11, 139-150
Noss, C., A. Lorke, and E. Niehaus
- (2014), Direct observation of biomixing by vertically migrating zooplankton, Limnol. Oceanogr., 59(3), 724–732
Noss, C., and A. Lorke
(See online at https://doi.org/10.4319/lo.2014.59.3.0724) - (2014), Hydrodynamic Trails Produced by Daphnia: Size and Energetics, PLoS One, 9(3), e92383
Wickramarathna, L. N., C. Noss, and A. Lorke
(See online at https://doi.org/10.1371/journal.pone.0092383)
