PS106/2 - Weekly Report No. 8 | 09 - 16 July 2017
Week 8: Returning to Svalbard
[17. July 2017]
After concluding our 4th ice station at the northernmost location of this expedition, Polarstern set a south-westerly course, heading for the position of our well-known PASCAL ice floe of PS 106/1. This time, many open leads allowed a mostly gentle passage through the ice. Our journey was inter-spaced with stations where we set out Polarstern’s rubber boats Laura and Luisa to sample the surface microlayer, conducted CTD casts and performed hauls with our full range of zooplankton and under-ice fauna sampling gear: LOKI with AquaScat, Multinet, RMT and SUIT. In parallel, the helicopters went out for missions aiming at light measurements over melt ponds, bird and mammal surveys, and ice thickness measurements with the EM-Bird.
In spite of the more open sea ice, chlorophyll values in the water were low. The catches of the nets, too, contained extremely low biomass, and seabirds or mammals were rarely seen, letting the northern part of our research area appear like a biological desert.
Only very shortly before we reached the PASCAL ice floe, this impression abruptly changed. While in the snow-covered north the ice looked solid and heavy, still very much like winter sea ice, the sea ice in the south showed obvious signs of decay and was densely covered with melt ponds. Abundances of seabirds and seals suddenly increased, and the water column showed signs of a beginning phytoplankton bloom. The sea ice workers were caught by impatient excitement, as we approached ‘our’ old floe. Hours before, a helicopter reconnaissance flight had already shown that all installations on the floe had survived, but that the appearance of the floe had totally changed (Picture 1). Nonetheless, Polarstern could be moored to the exact same place where it was placed a month ago. This time, apart from the usual work with melt pond sampling, ice coring and ROV, various autonomous instruments needed to be collected. On board, a first inspection indicated that most instruments had worked fine, and had been collecting a wealth of valuable data during the past five weeks.
The sea ice biologists were particularly keen to observe the changes the ecosystem had made since we had left the floe on PS 106/1. At that time, we had already observed that ice algae were losing their habitat as the ice underside started to melt. Now, algae melted out of the ice formed floating aggregates several millimetres to centimetres in diameter. Possibly, these floating aggregates help the ice algae to re-colonize the ice as soon as circumstances are better. Microscopic analyses showed that some algae in the aggregates were in good condition, whereas others were dying. After five weeks of melting, the ‘gardening area’ of PS 106/1 had profoundly changed. In the area with initially thin snow cover, a melt pond had formed. Even in our “high-snow” area with a thick snow layer when we first worked on this ice floe, the snow was completely gone, and the ice was strongly decayed. Brine channels had widened to arm-wide pores, in which aggregates of the under-ice alga Melosira arctica rose to the surface. In general, also this alga had suffered from the strong melting. Only poor remains were left of the beautiful under-ice gardens of Melosira strands we had observed during PS 106/1. Big Melosira aggregates floated in melt ponds which had opened to the sea underneath. With on-going melting, these aggregates will ultimately sink, becoming a food pulse for organisms living on the sea floor. Since Melosira arctica was widely distributed this year, these animals can expect a resourceful summer…
The physical oceanography team of the University of Gothenburg performed a second hydrographic transect just a few hours after the end of the ice station. Starting from the deep ocean, they conducted CTD casts at densely spaced stations, measuring vertical profiles of temperature, salinity, oxygen and chlorophyll concentrations in the water, until the ship reached the shelf. During this second transect, the physical oceanographers also deployed four autonomous bottom temperature sensors. Those sensors record the temperature at the bottom of the ocean every 30 minutes for two years, before floating back to the surface and sending the data by satellite back to Gothenburg. Thanks to these instruments, we will know how often and by how much the temperature changes over the transect. It helps us to assess how representative our point measurements on-board Polarstern these last days were when compared to the rest of the year.
From the CTD casts of PS 106/2, large amounts of water were sampled for further physical and biogeochemical analysis. The majority of the water was filtered by the biogeochemistry (BGS) team of AWI. The BGS team was mostly interested in parameters linked to phytoplankton, such as chlorophyll concentration or organic carbon. In total, the BGS team filtered more than 900 L of seawater, which will be analyzed in the laboratories of the AWI. This series of measurements belongs to a monitoring study that started in 1993, aiming to investigate changes of ecosystem functioning in the central Arctic Ocean.
The last station of the oceanographic shelf slope transect was also the last station where we deployed the Surface and under Ice Trawl (SUIT, Picture 2) and the Rectangular Midwater Trawl (RMT, Picture 3). Both trawls are designed to sample animals of up to several centimetres size, from small crustaceans to young fish. The SUIT is currently the only sampling device able to collect these animals directly from the underside of sea ice over distances of several kilometres. SUIT is equipped with strong floaters that keep its heavy steel frame at the surface. Car wheels mounted on its upper front bar ensure both smooth contact with, and optimal gliding under, the ice. After being deployed from the stern of the ship, SUIT shears to starboard, until it slips under the ice sideways of the ship’s trajectory. A suite of sensors is mounted to the net, assembling continuous profiles of sea ice- and water column properties, e.g. ice thickness, temperature, salinity, light transmission and fluorescence. During PS 106/2, SUIT was deployed successfully at 20 sampling stations. Spread over two south-north transects from the shelf into the deep-sea and back, the change in the composition of under-ice fauna over these gradients could be studied for the first time in a systematic manner. A first qualitative inventory of the catch composition showed a transition from a copepod-dominated shelf community via a krill-dominated slope community, into the ‘desert-like’ deep-sea. Here, amphipods and jellyfish dominated, but abundances were extremely low. These low abundances in the deep-sea contrasted with earlier studies in the Eurasian Basin of the Arctic Ocean. RMT catches appeared to show a similar pattern, but were largely lacking ice-associated fauna.
Sea ice habitat properties such as thickness and roughness have been linked to the distribution and community structure of ice-associated animals. This means that as the sea ice environment continues to change, so will the habitat of many polar organisms with unknown consequences for the future Arctic ecosystems. Thus, one objective during PS 106/2 was to quantify physical-ecological properties of the sea ice environment at multiple spatial scales. The largest scale has been covered using a helicopter-borne electromagnetic sea ice thickness instrument, the so-called EM-Bird. During PS 106/2 we conducted five EM-Bird surveys, with an average survey length of ~200 km, well covering the entire study area. Modal sea ice thickness, which is a good indicator of the most dominant, level-ice type in the region, ranged between 1.0 and 1.5 m (Picture 4). Ice properties at smaller scales were sampled with the SUIT (kilometres), and the ROV (meters to hundreds of meters). Combining the information derived from sensor array observations of SUIT and the ROV, the physical-ecological relationships of different sea ice habitats can be quantified at multiple spatial scales. These relationships can then be combined with larger scale EM-bird and satellite sea ice thickness observations to classify and model regional and pan-Arctic sea ice habitats.
Yesterday, we made our way back into the Svalbard archipelago through the same passage we had used to head out into the Arctic Ocean. In this region, our focus will be on bottom trawling. Luckily, the first two hauls finally yielded significant amounts of the long-missed polar cod.
Best regards from scientists and crew,
Hauke Flores, Chief Scientist
With contributions from Ilka Peeken (AWI), Cèline Heuzé (University of Gothenburg, UGOT), Elin Andrée (UGOT), Sarah Salin (UGOT), Anique Stecher (AWI), Pim Sprong (AWI), Benjamin Staufenbiel (AWI) and Benjamin Lange (AWI) (translation by Ulrich Küster
Contact
Science
Hannes Eisermann
+49(471)4831-2122
hannes.eisermann@awi.de
Scientific Coordination
<link ueber-uns organisation mitarbeiter rainer-knust.html _self personal-page-link>Rainer Knust
+49(471)4831-1709
Rainer Knust
Assistant
<link ueber-uns organisation mitarbeiter sanne-bochert.html _self personal-page-link>Sanne Bochert
+49(471)4831-1859
Sanne Bochert