Cruising and jumping: the effect of microplastics on the swimming behavior of copepods measured by 3D Lagrangian particle tracking velocimetry.

Calanoid copepods are key components of marine and estuarine food webs. Exposure to various classes of pollutants induces changes in their swimming behavior. This raises concerns about potential effects on critical processes such as feeding, mating, predator avoidance and vertical migration. The effect of pollution by microplastics is not well known. We investigated in a large experimental tank the effects of the smallest size fraction of microplastics on the swimming behavior of the estuarine copepod Eurytemora affinis. Because the motion of zooplankton is intrinsically linked to that of the ambient fluid, we recorded copepods moving freely in calm water and in grid-generated turbulence to recreate some of the hydrodynamic conditions they experience in their natural environment. Using an advanced implementation of 3D Lagrangian particle tracking velocimetry, we simultaneously measured copepod trajectories and the surrounding flow field at high temporal resolution. In calm water, copepods alternated between periods of cruising and sudden relocation jumps. In turbulence, copepod motion was dominated by transport by the flow, yet jumps allowed copepods to deviate from the flow streamlines. The measurement of the relative velocity of copepods with respect to the underlying flow enabled us to characterize the statistics of these jumps. Turbulence significantly increased jump frequency without modifying their amplitude or duration. Following a 12-hour exposure to polyethylene fragments at 300