Hydrodynamics of swarms at intermediate Reynolds number*

The significance of biologically generated turbulence is dependent upon the ability of swimming organisms to generate mixing eddies at scales comparable to the length scales of stratification in the ocean. Despite their small size, marine zooplankton undergo diurnal vertical migration over hundreds of meters and aggregate in dense swarms ranging from 10-50 m in vertical extent, which introduces additional length scales of relevance to their interaction with the surrounding water column. In recent work, we show that representative centimeter-scale swimmers (Artemia salina) migrating collectively perturb a stable density stratification at scales corresponding to the vertical extent of the laboratory controlled swarm, approaching 50 cm. This observed formation of aggregation-scale mixing eddies is the result of coalescence of the flows in the wakes of the individual organisms and leads to mixing of a stratified water column at a rate three orders of magnitude larger than molecular action alone. These results illustrate the potential for zooplankton to significantly alter the physical and biogeochemical structure of the water column with consequences for local or regional dynamics and ecology.