Zooplankton Aggregations Induced by a 2D Laboratory Model of Langmuir Circulation

Langmuir Circulation is a widespread form of wind shear-driven turbulence at the ocean surface made up of a series of counterrotating vortex pairs. The upwellings and downwellings within Langmuir circulation may concentrate vertically moving particles or plankton. In particular, upwards swimming zooplankton (e.g., copepods, daphniids, and mysids) matching the downwelling flow speed may be trapped and form aggregations within regions known as Stommel Retention Zones (SRZs). Here we investigate SRZ formation by mysids (Americamysis bahia) and daphniids (Daphnia magna) within a laboratory facility which creates a two-dimensional model of Langmuir circulation. Using light to induce upwards swimming, we measured zooplankton spatial distribution and swimming characteristics in simulated Langmuir Circulation of various flow strengths. At flow speeds which do not exceed their swimming capabilities, the mysids and daphniids aggregated in the downwelling, forming SRZs. At higher flow speeds, the zooplankton were more uniformly distributed as they were primarily advected by the flow. In order to investigate how Langmuir Circulation may affect trophic interactions, we also describe how the spatial distribution of the mysid is altered by the presence of prey.