Physical and Biogeochemical Impacts of Migrating Zooplankton Aggregations

Biologically generated turbulence has been proposed as an important contributor to nutrient transport and ocean mixing. However, for swimming animals to produce non-negligible transport and mixing, they must produce eddies at scales comparable to the length scales of stratification in the ocean. It has previously been argued that biologically generated turbulence is limited to the scale of the individual animals involved, which would make turbulence created by highly abundant centimeter-scale zooplankton such as krill irrelevant to ocean mixing. Their small size notwithstanding, zooplankton form dense aggregations tens of meters in a vertical extent as they undergo diurnal vertical migration over hundreds of meters. In this work, we investigate the potential for this behavior to introduce additional length scales — such as the scale of the aggregation — that are of relevance to animal interactions with the surrounding water column. Utilizing laboratory experiments, we show that the collective vertical migration of centimeter-scale swimmers generates aggregation-scale eddies that mix a stable density stratification, resulting in a significantly enhanced effective turbulent diffusivity. The large-scale fluid transport similarly enhances mixing of other relevant scalars, such as dissolved oxygen, leading to cascading biogeochemical effects upon the water column. Altogether, the results illustrate the potential for marine zooplankton to considerably alter the physical and biogeochemical structure of the water column, with potentially widespread effects owing to their frequent vertical migrations and high abundance in climatically important regions of the ocean.