Weekly report 4, 06.05.2001
The last week of our cruise started out well: On Sunday afternoon we recovered the mooring that we had deployed at the beginning of our cruise. Shortly after the release signal was given, the glass spheres covered in yellow and orange plastic popped up 200 m from the ship. Sediment traps and current meters were also recovered; the latter had recorded northeasterly currents superimposed on tidal oscillations. These currents are part of a larger ocean circulation, the Antarctic Circumpolar Current, which circles the Antarctic continent permanently and thus insulates warmer subpolar waters from the cold Antarctic water masses. It also forms a barrier for many plankton organisms. Krill may make use of its strong drift to transport swarms from areas west of the Antarctic Peninsula to the islands in the north.
We were also lucky in catching krill! Krill larvae were highly abundant in the upper water layers all over the continental shelf. During nighttime we also caught adults in the upper 5m. Sub-adults were found between 180 and 250 m in the open ocean (although these were different species, Euphausia triacanta, Thysanoessa spp.) occurring in patchy but dense swarms, and being hunted by small myctophid fish. Close to the coast we also found swarms of sub-adult krill in the deeper layers, this time Euphausia superba. The remaining question is what will these swarms do in the winter to come and this will be addressed by our American colleagues working with two ships in the following weeks.
Our results on krill distribution west of Adelaide and Alexander Islands supplement current knowledge. In April 2001 the ice cover west of the Antarctic Peninsula was very thin and patchy. Krill are known to feed under sea-ice and ice algae are an important food source to support high krill biomass during winter. In our experiments on board POLARSTERN copepods were also tested as an alternative food by krill. The experimentally determined biochemical turnover rates indicate that krill larvae were in excellent condition. Food supply seemed to be sufficient to support krill biomass for the long winter.
Information on total krill standing stock in Antarctica is still scarce. Krill stocks of 200 million tonnes were initially determined from whale feeding rates, but these are much higher than estimates in recent years. Either there is much less krill around nowadays or we simply did not find the spots of high biomass. The area hosting krill in Antarctica is around 200 million km2 (twice the size of the USA) and coincides with the area of seasonal sea-ice coverage. Given an average krill biomass of 30g m-2 this would translate into 20 individuals m-2. Imagine that as 20 locusts per square meter of your lawn at home! Regardless of the actual number, it is very interesting that so much of the overall plankton biomass is concentrated in this single species. Before sea-ice forms, krill seem to school in deeper ocean layers and along the coastline, where we found amazingly high numbers of top predators. Thus, Euphausia superba is rather flexible and highly adapted to live in the sea-ice zone. But to accurately determine krill biomass and distribution under the sea-ice is a very difficult task.
We spent the remaining research time doing station work along one transect perpendicular to Adelaide Island in the open sea. A depression system hit us on Monday, with moderate winds, but because of the long fetch, the swell reached 6m. Again chairs started moving across our cabins and the tables had to be set all over again. The huge plankton bloom found during the first transect 2 weeks ago had vanished.
Samples from 600m water depth obtained by means of a multicorer showed thick algal fluff on the sediment surface. The multicorer looks like a spider with 8 long legs stretching out, which reach the sea floor first. Shortly after, 12 Plexiglas tubes are pressed into the mud by heavy lead, very carefully so that the sediment surface is not disturbed. As the instrument is pulled up again, cups cover the tubes and prevent sediment from falling out. Then they are carefully retrieved on board ship and transported to the laboratories for further analysis.
The dense algal fluff at the sediment surface was indeed our plankton bloom that had sunk out. At the end of the growing season phytoplankton cells and chains stick together and form aggregates, which experience higher sinking rates (more than 100 m per day) compared to single cells. This process of sedimentation is the main route of transport of organic material into the deep ocean and also the main support route for organisms living there. The fundamental difference between terrestrial and marine production is that the latter produces biomass of about the same biochemical composition in terms of proteins, carbohydrates and fat as the animals living on that biomass. Terrestrial plant biomass contains large amounts of cellulose and lignin, not accessible for most animals without the help of bacteria or fungi. Thus the transfer efficiency between plants and animals in the ocean is much higher compared to land. Planktonic krill and copepods do not only use the algal good but also by animals on the sea floor as the phytoplankton sinks out of the water column. Especially the larvae of such benthic organisms, which populate the water layers just above the sediments, rely on this kind of food, as they do not have sufficient storage products, a theme investigated by one of our groups.
At the end of our cruise we also had to fulfil some logistic requirements. Research stations in Antarctica are not frequently visited by supply vessels or planes. Therefore it is good practice to support these stations without bureaucratic hassle and beyond national responsibilities if time and weather permits. The British research station Rothera is located on Adelaide Island and we had parcels for them on board. A short window of good weather allowed helicopter transport of these boxes on 1 May, and also a short visit to the station. We also had equipment for the Argentine station Jubany and the German Dallmann laboratory, both located on King George Island. In addition, two German technicians had spent three months maintaining the station and wanted to return home before winter freezes other transport possibilities. The weather was perfect on 3 May, and we established a helicopter shuttle between the ship and Jubany station, which we used for a short visit ashore. The Argentine crew provided a warm welcome with home made cake and we invited them to visit our ship. Many of the ships party enjoyed walking along the black beach (volcanic ash!) and photographing fur seals and penguins from some distance. Glaciers calve into the cove close to the station - some of us had the luck to experience such a spectacle from nearby. The remainder of the ice shelf - blocks of ice tons in weight - is exposed on the beach with ice in white, blue or totally transparent, sometimes crafted to weird forms by nature. These were objects for our art photographer. Thus the day brought personal highlights to most of us; a most appreciated farewell from a short but intensive cruise.
The way home was not only busy because we had to pack all instruments and material but also entertaining. Two delicious, roasted pigs were the highlight of a barbecue on the deck followed by a party and dancing into the small hours.
We are most thankful to the captain, the officers and the crew of POLARSTERN for their prompt, competent and reliable help on any problems arising during the cruise - sometimes on very short notice.
We soon have to say goodbye to the ship but await walking on solid ground again.
Uli Bathmann (chief scientist)