ARK-XXII/2, 4th Weekly Report
20 – 25 August 2007
Atlantic Water flow – two uneven partners and their passengers
At the beginning of the last week we worked along a section in the Voronin Trough to monitor the confluence of two branches of Atlantic water flow. The Norwegian Current brings warm and salty waters to northern Europe, and then into the Arctic Ocean in two branches. One branch goes through the deep Fram Strait and turns east to flow north of Svalbard. The second branch flows through the shallow Barents Sea. For a long time fishermen have appreciated the warmth of this Atlantic water that keeps the Barents Sea mostly ice-free during winter. Through direct contact with the atmosphere the water is cooled, and in some areas reaches its freezing point. When ice is eventually formed the salt either remains in thin brine channels in the ice or it is released to the surface water. The cooling and the salinity increase make the water more dense than the surrounding water, which causes it to sink. At the ocean floor (about 200 m deep) it flows towards the shelf edge and sinks into the 3000 m deep Nansen Basin.
There it rejoins its partner that took the pathway through Fram Strait. This branch of Atlantic Water is modified only a little during its passage. The region north of Svalbard is sometimes ice-free due to the oceanic heat provided by the Atlantic water and favourable winds (it has been called “Whaler’s Bay” because in the old days it was the northernmost region for whale hunting), but farther downstream this branch is covered by fresh water and ice which hampers further cooling.
The two branches reconvene north of the Kara Sea. Depending on the differences in temperature and salinity between these two branches, the Barents Sea branch either sinks below the Fram Strait branch because of its higher density, or both branches flow at a similar depth divided by a sharp front. The temperatures and salinities may vary with time. We know that the Fram Strait branch has become less dense over the last decade due to increased temperatures – although we found the inflow this year to be somewhat colder than last year. But no observations exist about the Barents Sea outflow since 1996. Our measurements at 86°E show that, since 1996, the Barents Sea water has become less saline and therefore less dense as well.
Like everywhere on our globe, the fauna found in the Arctic are very much adapted to their environment. Strong seasonality of light conditions, due to insulation and ice cover, as well as low temperatures dictate living conditions here. Polar bears, for some the most spectacular species of the Arctic, are only one end of a food chain that originates with very small species. In order to understand how the Arctic ecosystem adapts to the changing sea ice, one must consider more than just the shrinking of the sea ice extent as a decrease in the polar bears’ walking area. To this end, three biological groups study the exiting life in the ice, in the water and at the bottom.
An important link in the marine carbon cycle is the zooplankton. In the Arctic, the largest amount is made up of three species of the millimetre sized Copepods. To assess their role in the food chain it is important to know their life strategies and to understand which factors determine their composition and their distribution. The ecosystem in the ice is entirely restricted to the Arctic but the composition of the organisms in water is not. Due to the advection of waters from the Atlantic there is also input of Atlantic zooplankters, because their swimming abilities are not sufficient to resist the currents.
We sample the zooplankton with a multi-net from distinct depth intervals so that we obtain a picture of the three-dimensional distribution of species and total abundance. We observe a large diversity: there are vegetarians that prefer to live in the uppermost floors during summer when the phytoplankton blooms like a meadow in spring. Then there are the carnivores and opportunists that eat almost everything, including left-overs from others, anything that contains some calories. Their niche is farther in the deep where they sit and wait to catch what is sinking from above. For the first time we sample systematically the deep layers below 2000 m, which is thought to be the vertical depth limit of the Copepods. It turned out that the abyssal is of no interest for the Copepods. Instead, the catches bring mostly animals on board that seem hardly known so far. For example, we collected a worm (Nemertini) about 10 cm long on which, because we have no deep sea worm specialists onboard, we can not even tell the one end from the other. It has the bright orange color that is found often with abyssal animals; why is it necessary to have such a color in the dark?
However, the largest diversity is found at a few hundred meters depth in the Atlantic layer. Calanus finmarchicus is the small cousin of the Arctic inhabitant Calanus glacialis and is originally from the subarctic North Atlantic. Cousin C. glacialis is bigger than C. finmarchicus which is typical for polar species. Because of the slower metabolism, it takes longer for them to become mature, so they have to live longer and thus they become larger than their southern relatives. Little C. finmarchicus is carried to the north with or against its will by the warm Atlantic water flowing through Fram Strait. In the Arctic he seems to feel quite happy – at least he is found to survive everywhere along the Eurasian continental slope. The toughest guys are found even beyond the Lomonosov Ridge in the Canadian Basin. But they do not seem comfortable enough to reproduce. So far, no new generation has been found in the Arctic and the immigrating generation does not live longer than a year, as is known from many studies. Does that change with the warmer inflow? In experiments with specimen from a catch in the northern Kara Sea carried out onboard we found for the first time several female C. finmarchicus producing eggs!
The whole week we were surrounded by three low pressure systems: one at the North Pole, one over Novaya Zemlya and one at Severnaya Zemlya, which resulted in everlasting clouds, fog and sometimes snow showers. Not only does it look a bit depressing, it also prevents our helicopters from flying and does not allow us to carry out extended measurements of sea ice thickness and other parameters.
I am grateful to Kate Lepore who every week transfers my bumpy English into an elegant language.
Best regards in the name of all participants,
Ursula Schauer