Article - Plankton motion in turbulent flow

Calanoid copepods can perform active and directed motion and control the distribution of their populations even in turbulent environments such as estuaries or the mixed layer of the ocean. This is surprising, because the strong turbulence present in these ecosystems disperses small particles very efficiently. How can entire populations of copepods migrate and regroup in a turbulent water column, if the strong transport due to flow motion overwhelms their limited swimming abilities? What are the individual behavioral mechanisms that permit these population-scale features? To address these questions, we used an original flow measurement technique based on a high-speed multi-camera system to track simultaneously the motion of freely-swimming copepods and flow tracers. We reconstructed in three dimensions the trajectories of the organisms and the instantaneous flow field around them. We subtracted the local flow velocity from the velocity of the organisms to retrieve the behavioral component of their motion, and accurately quantified the local instantaneous flow field along their trajectories. Our results show that copepods modulate their swimming effort depending on the intensity of background turbulence: we measured a much higher jump frequency in turbulent flow compared to still conditions. Jumps were not individually triggered by sudden hydrodynamic signals in the immediate vicinity of the organisms and they differ markedly from the extreme startle response to unexpected stimuli that usually occurs when a copepod detects an approaching predator. These results indicate that copepods integrate hydrodynamic information from the background flow and regulate their swimming effort accordingly, and also that they can recognize flow signals generated by turbulence and do not wrongly interpret them as if generated by an approaching predator. Because during jumps copepods reach velocities that are typically much higher than those of the underlying flow, this strong increase in jump frequency allows for the possibility of active, directed displacements in a motion that would otherwise be entirely determined by flow transport. Using direct measurements and modeling, we also found that jumping more often enables these small organisms to reduce dispersion by vigorous turbulence through active swimming. This is relevant to limit excessive dilution of a patch and it may help them to perform interactions between neighbor organisms.