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There has been a lot of ice in the waters north of Svalbard this autumn. It does not mean that the sea is cold.
Photo: Thomas Wenneck
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Ice-covered seas do not need to be cold

Much ice on the sea surface does not mean that the sea is cold. North of Svalbard we saw ice and melt water on the sea surface, while we measured seven degrees at 30 meters depth. During the first cruise in the project SI_Arctic, cod was the fish species caught the most places. Between the ice-free areas in the Fram Strait and the ice covered areas north of Svalbard there were clear differences in species and quantity of plankton, fish and marine mammals.

The first SI_Arctic (Strategic Initiativ Arctic) survey has come to an end. The goal of the SI_Arctic project is to explore and understand the marine ecosystem of the Arctic Ocean north of Svalbard, and this was also the goal with the first survey that was conducted with RV Helmer Hanssen during 19 August – 7 September 2014.

The survey covered the region west of Svalbard (Fram Strait) and north of Svalbard (see Figure 1). Part of the survey was in open water and parts of it in ice covered regions. Underway we measured oceanography, phyto- and zooplankton, fish, benthic organisms, marine mammals and sea birds. We collected large amounts of data and numerous samples that are still to be analysed.

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Figure 1. Survey coverage and stations taken during the SI_ARCTIC survey.

Ice cover

The ice cover during the summer made the survey somewhat challenging. The ice concentration in the Arctic in August–September was below average (1981–2010) everywhere, except north and east of Svalbard where the survey should go. In this region there were much more ice than usual. The reason is persistent northerly winds during summer that has caused both lower air temperatures as well as brought ice into the region north and east of Svalbard.

 

 

Ocean temperature

Despite large amounts of ice, the Atlantic Water showed high temperatures. Both in Fram Strait (near 79o40’N) and on the shelf-break north of Svalbard (80o45’N and 14oE) temperatures close to 7 oC was observed. In Fram Strait the highest temperatures were observed close to the surface. On the shelf-break north of Svalbard the Atlantic current was covered by ice and a thin layer of melt water. The surface water was close to 0 oC, but already at 30 m depth there were water with temperatures close to 7 oC. The ice melts rapidly if getting in contact with the warm Atlantic Water, and this is why the region has been free-of-ice in the last years. However, this year the persistent northerly winds bringing new ice into the region more or less continuously gave much ice despite high temperatures in the Atlantic Water.

Fish

The highest catch rates of fish were found at the shelf and shelf break at 200–500 m depth. Going into deeper and colder water down the shelf, the catch rated decreased and the species composition changed. The most dominating species in terms of number of stations they were caught was cod. It was found both at bottom and pelagic, both at the shelf/shelf break and over deeper waters off the shelf break (Figure 2), and both in ice free waters and under the ice.   

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Figure 2. Catch rates of cod in pelagic (blue) and bottom (red) trawl hauls.

Mesopelagic layer

Both in Fram Strait and north of Svalbard the acoustics showed a pronounced mesopelagic layer between 300 and 450 m depth (shown for Fram Strait in Figure 3). Trawling showed that the mesopelagic layer consisted of fish (cod, capelin, redfish, Greenland halibut and others) and planktonic organisms, small mid-water fish and micronekton. An interesting feature was that the layer in Fram Strait changed from being dominated by fish from the slope to about two-thirds of the distance to the west, to being dominated by plankton for the western part of the section. The abrupt change in the contributions of these two fractions occurred about where there was a cross-over from warm Atlantic Seawater to cold polar and deep seawater.

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Figure 3. Acoustic total backscatter (Sa values) in the upper 1000 m in northern Fram Strait showing a mesoplagic layer in 300-450 m depth. The section crosses the shelf break at about 79o40’N and runs from the shelf (to the right in the figure) and into the deeper Fram Strait (to the left in the figure).

Fram Strait vs north of Svalbard

The sections showed some interesting differences between the mostly ice free Fram Strait and the partly ice covered arctic regions north of Svalbard. In eastern Fram Strait there was a significant amount fish present both at bottom and in the mesopelagic layer and small amounts of whales was observed. Along the slope north of Svalbard on the other hand, the mesopelagic layer seemed to be mostly dominated by plankton, and also species that most often are found further south was observed here. In this region there were also more frequent observations of whales, probably due to better feeding conditions associated with the zooplankton concentrations.