Copepods are only a few mm by size and thus can’t move against oceanic currents that often are in the order of several centimetres per second. But the copepods can, and do, move over long distances (several 100 m) vertically in the water column. Vertical water velocities are usually small and in the order of the swimming speed of copepods. But just how fast are the copepods? We are going to try to find out in an ongoing experiment at UiT – The Arctic University of Norway.
We will put copepods of the species Calanus finmarchicus in a large chamber filled with sea water and measure how well they can move against down-welling, artificially induced currents. Thus we aim to find out if upward swimming against downward currents can allow the copepods to remain at the surface and thus aggregate into large surface patches. For this, the results will later also be coupled to a model of oceanic currents.
To start with, we were measuring how fast the downwelling velocities in the chamber are. We regulated the water velocity by adjusting the speed of a pump, which pumps the water into the chamber. Then, we were putting small green particles into the chamber and filmed them (video above). Using image analysing software we then traced the particles in the video and calculated their speed. Now we know how fast the downwelling current is at given pump velocities.
We run this experiment in cooperation with Dr. Claudio DiBacco from DFO Canada, who earlier measured the swimming speed of cyprid larvae and from whom we could borrow most of the experimental equipment.
Soon we will go out at sea to collect copepods for the experiments, stay tuned!