The Polar Encyclopedia

Scientists shave little information about the distribution, evolution and impact of plankton on the overall balance of matter (and CO2 in particular) during the Arctic winter. The hypothesis most often put forward is that the absence of light inhibits the mechanisms that govern the production of organic matter. In that case the biological absorption of CO2 by photosynthesis – on the part of autotrophic planktonic algae – is zero. On the other hand, the production of CO2 via the breathing of heterotrophic organisms (zooplankton) continues.Furthermore, the conditions in winter certainly have a decisive impact on the biological cycle of numerous planktonic species, and consequently on the build-up of a stock of nourishment for the larvae of several species of fish. Certain copepods in particular go through hibernation stages at great depth during which they are passively carried along by the currents. These animals would not be able to produce a new generation during the spring unless the individuals were transported towards zones where they is high production of spring phytoplankton – zones where they could continue to grow or, if already adult, reproduce.
Other species survive and grow despite the extremely low temperatures.
The results published after an expedition around the Svalbard Archipelago show that during the warm season most of the zoological groups that play a part in the production or destruction of sedimentable particles are present there: crustaceans (copepods, amphipods, ostracods); molluscs (pteropods, chaetognaths, appendicularians, coelentrates, siphonophora and jellyfish).

THE GREENLAND SEA

To what extent is zooplankton “inoculated” by the currents in the Greenland Sea?
The Arctic waters bring with them only small quantities of living organisms, but the branch of the North Atlantic Current that flows up the eastern side of the strait that provides an exit for polar waters brings zooplanktonic species. But which are the dominant groups that subsist there in winter? And what role does the zooplankton play in the overall balance of matter (particularly CO2) in the region?
Firstly phytoplankton. Seawater samples taken in February and May in a fjord on the western coast contain only flagellates (between 0.1 and 37 million cells per litre). It is only in April that scientists will observe diatoms and a marked surge in production of vegetal biomass. Strong growth of a flagellate species can trigger a sudden influx of vegetal matter towards the zooplankton. The concentrations of chlorophyll that have been measured correspond to annual primary production of 150 mg of carbon per m2 (between April and September), which is comparable to the Norwegian fjords.
However, it should be noted that there is a relatively low concentration of nutritious salts in the upper layer of the sea, roughly equivalent to the concentration in the deeper waters of the Mediterranean and 2 or 3 times less than the concentrations recorded across upwelling zones in inter-tropical coastal waters.
Zooplankton, on the other hand, subsists in winter at depths between 300 and 400 metres.
The most abundant species are probably copepods. Calanus hyperboreus is a “resident” species and the dominant one in the Greenland Sea, while Calanus finmarchus (finmarchicus plus haut) is imported by deep currents and its numbers diminish in autumn. The species reappears in April at depths of 200-300 metres and by June it occupies the surface water.
Pteropods such as Limacia sp., which subsists at the end of the autumn, as well as the chaetognath Eukrohnia sp. also add to the overall biomass. Ostracods such as Boroecia borealis have also been observed during this period.
The microbial network (bacteria, etc.) must also remain quite active, even during the winter, because scientists have observed one of the ten most abundant species of appendicularians, which indicates the existence of small-sized prey.

LIFE IN AMONG THE SEA ICE

On occasions, the sea ice can turn brown and give off a marked odour due to the presence of a multitude of diatoms growing on the underside of the sea ice.
But within the mass of ice itself, life forms subsist among the crystals of fresh-water ice and in the thin seawater channels. A complete food chain can be found here, including planktonic algae and their decomposers (bacteria). Some of these organisms spend their whole life cycle inside the sea ice and others only part of their cycle, but all of them have adapted to a wide range of salinity and sunlight strength. This means that even during the Arctic winter, some algae can continue to photosynthesise using the very weak light of the polar night.Some of these organisms originate in the sea and others are washed down to the sea by rivers. In autumn, when the sea starts to freeze over once more, they remain there, clinging to the ice. At first, both their diversity and the quantities present are quite low, but as soon as spring arrives numbers and diversity skyrocket, along with those of their predators. In sections of ice that have built up over several years, and thus several life cycles, the ice displays several strips or layers of microscopic communities that allow scientists to date successive spring “flowerings”.