Finally we collected some copepods in the fjords around Tromso. We took the small boat from UiT, R/V “Hyas”, on a calm and beautiful summer day and traveled to Ramfjord, 12 km to the south of Tromso. There we towed our plankton net 11 times to get enough copepods for the experiments.
We took the copepods (Calanus finmarchicus) to the laboratory and placed them inside our water circulation chamber, after an acclimation period. We then adjusted the water flow to speeds between 0 abd 6 cm/s, and observed how the animals reacted. Although we are still analysing all the videos, we have already noticed some clear swimming behaviours: At high downwelling speeds the animals seem to be washed away, but at 1 cm/s or less they are able to swim upwards against the currents. In the following video you can see how one copepod is swimming for 1 minute against a downward flow of approximately 1 cm/s. It managed to keep the position, and even move a little bit upwards.
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!
While spring has arrived many places in the northern hemisphere, you can read about the overwintering of Calanus finmarchicus in our second scientific publication. This small copepod migrates down to 800 m or more, and spends many months at this depth before coming to surface waters again!
These findings will help us to learn much more about zooplankton patches. How often do large surface patches occur? Do they follow the phytoplankton bloom? Are they purely shaped by oceanic currents? All these questions, which have been very difficult to answer based on traditional sampling methods, will now be much more easy to understand.
During this years research cruise from 16-27 June we performed sampling along 6 transects and at 18 stations, using a variety of sampling instruments. See who participated at the intense sampling here.
Our first study area was off the coast of Vesterålen islands, where we found high numbers of our key study species, the copepod Calanus finmarchicus.
After completing sampling there, we moved to study patches of this species at Tromsøflaket, a known fishing hot spot.