Aggregation of zooplankton in a Stommel Retention Zone in a laboratory model of Langmuir circulation

Langmuir circulation is a common form of surface turbulence comprising a series of counterrotating horizontal vortex pairs. Upwellings and downwellings in Langmuir circulation may suspend and trap passive particles or active swimmers like zooplankton in regions known as Stommel Retention Zones. For zooplankton, Stommel Retention Zone formation depends on flow speed and animal swimming speed and direction. Here we explore this biophysical interaction using Daphnia magna in a laboratory model of Langmuir Circulation. The phototactic daphniids were induced to perform different levels of upwards swimming (mimicking diel vertical migration) via constant or intermittent illumination. Some daphniids were additionally exposed to chemically dispersed crude oil, which impaired swimming. We characterized the swimming speed and direction, trajectories, and spatial distribution of daphniids in still water and in response to Langmuir circulation-like flows of various strengths. In still water, constantly illuminated and oil-exposed daphniids swam upwards more often than intermittently illuminated animals. Greater levels of upwards swimming in still water corresponded to stronger daphniid aggregations in the downwelling when a Langmuir Circulation-like flow was present. However, at flow speeds exceeding their swimming abilities, daphniids were generally advected with the flow and uniformly distributed, an effect particularly evident for the weakly-swimming oil-exposed daphniids. An individual-based model was also used to investigate the effects of active swimming vs. passive behavior and of swimming direction on Langmuir circulation-associated aggregations. Our study of zooplankton Stommel Retention Zones generated in a laboratory facility offers insight into how and when Langmuir circulation-associated plankton aggregations may occur in the field.