Comparisons of Alive and Dead Copepod Velocity Statistics in Various Turbulence Intensities: Laboratory Study Using High Speed Cameras

Planktonic copepods are tiny crustaceans, with a typical size of the order of mm, living in suspension in marine or freshwaters during their entire life cycle. They have swimming and jumping abilities and are known to be well adapted to their turbulent environment. Turbulence is known to increase their contact rate and feeding flux, while a too intense turbulence is believed to have a negative effect so that a qualitative bell-shape is classically invoked to represent the contact rate of copepods versus turbulence intensity. However, such bell-shape has still needs quantitative understanding. The objective of this work is to quantify the influence of ambient turbulence on copepods behavior, using trajectory analysis. Copepods of the species Acartia tonsa have been placed in a laboratory generated turbulence system, characterized by Taylor-scale based Reynolds numbers from 140 to 380. Trajectories of alive and dead copepods, and of polystyrene particles of size 600 m, have been measured in the dark with infrared lights, using a high-speed camera at 1200 fps. Using adequate filtering, velocity and acceleration information have been extracted. The velocity probability density (pdf) functions of dead copepods are very close to the one of spherical particles of the same size. The pdfs of alive and dead copepods are different for low Reynolds numbers, and become superposed for Reynolds numbers larger than 270. This shows that copepods' swimming behavior is detectable under moderate turbulence, and not for high intensity turbulence. This gives information about the optimal turbulence levels for copepods and that when turbulence is of too high intensity, copepods have no specific behavior.