Mysid and copepod aggregation and trophic interaction in a 2D laboratory model of Langmuir circulation

Langmuir circulation (LC) comprises rows of counterrotating vortices running parallel to the wind. Upwelling and downwelling regions between Langmuir cells may aggregate plankton, with floating or upward-swimming organisms aggregating in downwellings and vice versa. In particular, upward-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 copepods (Acartia sp.) within a laboratory facility which creates a two-dimensional model of Langmuir circulation. Using light to induce upwards swimming, we measured zooplankton spatial distributions and swimming characteristics in simulated Langmuir circulation of various flow strengths. At all flow speeds, mysids form an SRZ in the downwelling, though the aggregation becomes more diffuse at higher flow speeds. Copepods are uniformly distributed at the surface in low flow and seem to avoid the turbulent downwelling region at higher flow speeds. As SRZs may affect trophic transfer by aggregating predators and prey, we also investigated how copepod presence affects mysid distributions and swimming behavior at various flow speeds and found that the mysid SRZ is somewhat diffused by the presence of copepods at high flow speeds, possibly because the mysids are seeking prey.