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ANT-XXIX/3 started 22/01/13 in Punta Arenas after a delay of almost two days due to crane damage. In the beginning it was already known that it would be difficult to reach the three planned study areas east of the Antarctic Peninsula. Discussions about alternative plans showed that the linkage between oceanography, bathymetry and biology would not be possible anymore as planned. The alternative areas of oceanography and krill research were merged. The question about a source of the krill-population in the SW-Atlantic was maintained and in terms of oceanography the fade of the deep water formed in the Larsen areas could still be studied instead of its origin. For the general biological question a nested approach was developed which allowed a large-scaled comparison between different environmental regimes (ice, temperature, primary production) and also considered different mesoscale habitats at core stations providing representative results for all three benthic fractions (meio-, macro-, megabenthos). This concept should contribute to answering questions about drivers of the marine ecosystem and allow predictions of future scenarios for climate-induced environmental changes. It also contributes significantly to the new SCAR-biology program AnT-ERA. After crossing the Drake Passage 26/01 benthic sampling (multi-corer, dredges, imaging method OFOS, box corer) supported by bathymetric surveys took place at two core stations off Joinville Island, it was finally decided on 18/02 to give up reaching the Larsen areas. Based on satellite images it appeared impossible to work in the inner embayments. There was a narrow strip of open water in front of Larsen A, but it was surrounded by solid sea-ice. Since there was no clear way through the ice to reach this open water and no guarantee for a return out of the ice there was no convincing argument to reach and efficiently work in this small opening. During an oceanographic transect Polarstern crossed an unusually shallow area, which became the location for all sampling activities for 2.5 days. Benthic sampling was continued in the Erebus & Terror Gulf at the eastern entrance of the Antarctic Sound. Polarstern passed through the Antarctic Sound and into the Bransfield Straight 22/02 where the biologists sampled at three core stations each containing four stations. A short oceanographic transect was also completed in the straight. Deception Island was visited 06/03 and water samples were collected in the caldera. The concept for general marine biology was also applied to an area northwest of the South Shetland Islands in the Drake Passage but with reduced effort. Station work ended 13/03 with a final CTD-transect northwest of the South Shetland Islands. The alternative programs of oceanography, krill research, and general marine biology supported by bathymetry were almost completely finished despite the difficult sea-ice conditions. The first whale helicopter surveys east of the peninsula focused on the relationship between the Southern minke whale and sea ice. Helicopter surveys were not carried out in the Bransfield Strait due to difficult weather conditions, but were continued in the Drake Passage due to unexpectedly good conditions. Polarstern arrives in Punta Arenas 18/03, one day earlier than scheduled due to the crane damage mentioned above.

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Expedition ANT-XXIX/3 (PS81) started in Punta Arenas, Chile, on 22 January after a delay of almost two days due to crane damage and consequential loading problems. Due to the difficult ice conditions it was already known from the beginning that it would be difficult to reach the three planned study areas, Larsen A/B (marine ecosystem research), Larsen C (physical oceanography) and east of the tip of the Antarctic Peninsula in the Weddell Sea (krill research), as well as adjacent areas. It became obvious that the integration of physical oceanography, bathymetry and general marine biology could not be realized anymore, as it had been planned for the Larsen-specific scientific questions. Therefore, once on board the larger working groups substantiated the alternative plans as described below.

The project to investigate the formation of deep water in the western Weddell Sea should have been carried out off the Larsen C ice shelf close to its source area, based on results from ANT-XVIII, 2000 (Schröder et al., 2002) and ANT-XXII/2 (ISPOL, Absy et al., 2008, Hellmer et al., 2008, Huhn et al., 2008) and also within the Larsen A and B embayments. The region of investigation was shifted to an area between 60°S and 65°S where the northward expansion of the newly formed water mass along the continental slope into the Powell Basin could be followed. Stations were planned along five quasi-zonal transects until 50°W, of which the results can also provide a basis to reconstruct the fate of the cold deep water originating in the southern Weddell Sea. Other meridional transects at 55°W and approximately 58°W in the Bransfield Strait and north of Livingston Island complemented the physical oceanography program, which aims to estimate the proportion of Weddell Sea water in the eastern Bransfield Strait and in the deep counter current of the Drake Passage.

The original planning of the krill and plankton program had the aim to reach an area in the Weddell Sea between 63°S and 66°S to carry out the first standardised survey on potential krill stocks and plankton communities. It was an open question of whether an independent krill population exists in the Weddell Sea, which either releases krill larvae into the Scotia Sea or contributes to the high density of krill in the Southwest Atlantic. The difficult ice situation demanded a modification of the original plan, leading to a joint station design with the physical oceanographers. The study area was shifted northward between 60°S and 65°S with stations in partly ice-free areas in the eastern Bransfield Strait on a north-south transect at 55°W. This shift of the study area to the north should allow, nevertheless, a survey in an area of the outflow from the Weddell Sea in less difficult ice conditions. These stations can also provide information about the composition of krill populations and plankton communities in the area west of the Antarctic Peninsula. Based on this concept the original aim could be maintained, even though stations were cancelled in the southern margin of the study area.

The original approach of general biology, which mainly focused on the benthos, was centred around the response of the marine system to the disintegration of the Larsen A and B ice shelves. This would have continued surveys initiated during ANT-XXIII/3 in the framework of the Census of Antarctic Marine Life (CAML; Gutt et al., 2011) and also conducted during ANT-XXVII/3. The assumed turn-over from an oligotrophic to a “normal” Antarctic system had been the focus.

The alternative program was consistent with this general ecological idea and also contributes to SCAR’s new biology program “Antarctic Thresholds - Ecosystem Resilience and Adaptation” (AnT-ERA). An approach had been decided that considered benthic patterns and processes both at large and intermediate spatial scales. It was assumed that differences in the quality and quantity of food supply for different ecological guilds exist (mobile and sessile fauna, epi- and infauna, mega-, macro- and meiofauna) between an area east of the tip of the Antarctic Peninsula in the Weddell Sea (WS), in the Bransfield Strait (BS) and west of the South Shetland Islands in the Drake Passage (DP). This assumption is derived from long-term results on the seasonal summer primary production in the euphotic zone (Bracher et al., pers. comm.), averaged summer sea-ice cover, bottom water temperature (Clarke et al., 2009), and two macrobenthic community studies, one based on by-catches from scientific fishery surveys (Lockhart and Jones, 2008) and the other on a scientific benthos survey (Piepenburg et al., 2002). In addition, it was assumed that differences in food availability to the benthos within these areas are influenced by bottom topography and currents. These ideas resulted in a nested approach that covered large-scale investigations (between study areas) and intermediate-scales (within study areas, between core stations). This may provide representative results for single areas and will allow comparisons at larger spatial scales along ecological gradients. The large-scale comparison will contribute to the development of future scenarios for benthic communities in case of ongoing climate-induced environmental changes, e.g., in terms of in food availability, temperature and ice cover.

The heterogeneous bottom topography served as a proxi for food availability because topography considerably affects the near bottom current. Four different habitats were selected within each core station, a location for a suite of benthic work: shallow bank (approximately 200 m), upper exposed slope (approximately 270 m), deeper slope (approximately 450 m), and canyon (approximately 500 m). This concept comprised the deployment of all general sampling equipment including the RMT­net for krill catches, CTD­measurements and water samples at all stations. Additionally, regional bathymetric surveys in the study areas should provide valuable information for general bathymetric studies, for the detailed planning of the sampling design and later for the interpretation of the biological results. Although it could be expected that some gear could not be used at all stations mainly due to sediment characteristics, this concept provides results for major benthic components at these core stations. This concept and sampling design was adjusted for the area east of the Antarctic Peninsula and north of the South Shetland Islands in the Drake Passage by reducing the number of sampling sites per core station or modified definitions of the habitats since not all ecological scenarios were present at each station.

After leaving Punta Arenas to the west Polarstern crossed the Drake Passage and arrived at an ice­covered area north of Joinville Island and near the tip of the Antarctic Peninsula on 26 January. A first benthos station was sampled for two days using the multi-corer, imaging gear OFOS, box-corer, and dredges attributed later to the Bransfield Strait area. Depending on the ice conditions these activities were complimented by a local bathymetric survey. After that Polarstern carried out a number of transects (F, G, D, E, C, B), which formed a major part of the krill research (in the context of CCAMLR) and the physical oceanography study of the fate of newly formed deep bottom water (Fig. 1.1). The CTD-stations were successfully completed, as planned, quasi-synoptically until 7 February despite high ice cover. The RMT-net was deployed successfully at 70 % of these alternatively planned stations. The benthos work resumed in the Weddell Sea at the south-eastern entrance of the Antarctic Sound in the Erebus and Terror Gulf. Two more krill/CTD transects were conducted in the south (A, O) from 11 to 18 February. These were situated at the northern margin of the originally planned krill research box.

It was finally decided on 18 February to give up reaching the Larsen areas. Based on satellite and radar images it appeared impossible to work in the inner embayments. There was a narrow strip of open water in front of Larsen A, but it was surrounded by solid sea-ice. There was no clear way through the ice to reach this open water and no guarantee for Polarstern’s return. Because of the unpredictable trajectories of the ice, there was no argument to reach and work in this small area of open water. During one of the krill/CTD transects Polarstern crossed a very shallow area, which became the focus of the ecological sampling for the next 2.5 days. This shallow bank was an interesting object for research on pelagic-benthic coupling of the ecological drivers, and of evolutionary as well as ecological dispersal of marine organisms. This shallow bank is probably a capped volcano. During a phase of strong winds a chain of grounded icebergs kept the sea-ice from this site.

Polarstern passed through the Antarctic Sound on 22 February and the general marine biologists continued their benthos work on the shelf of the Antarctic Peninsula in the Bransfield Strait as described above. A short oceanographic transect with krill catches was also completed on this side of the Sound. At the four different habitats (bank, upper slope, slope, canyon) all sampling equipment was deployed with minor exceptions, e.g., no multicorer on gravely sediments. The strategy was applied to all three core stations, each comprising four habitats. Consequently, for each of these core stations a comprehensive image of their benthic composition can be achieved, ranging from meio- and macro- to megafauna with some benthic-pelagic coupling processes, which shape this structure. After station work on the shelf of the Antarctic Peninsula was finished, a station in the crater of Deception Island was sampled oceanographically, especially for trace elements. Polarsternreached the area north of the South Shetland Islands in the Drake Passage 7 March where general marine biological research was conducted at three core stations with reduced effort. The alternative benthologically-dominated sampling targeting three areas in the Weddell Sea, Bransfield Straight, and Drake Passage at different spatial scales and applying a nested approach ended 12 March after completing a total of nine core stations with up to four single stations each, including the bathymetric surveys. Finally, the oceanographic work finished with a short transect north of Livingston Island on 13 March.

The scientific observations and counting of whales were mainly carried out throughout the cruise depending on weather conditions. Especially east of the Antarctic Peninsula fog and low clouds often prevented survey flights. East of the peninsula the major scientific focus was the distribution of Antarctic minke whales in relation to sea-ice conditions. Unexpectedly, good weather conditions in the Drake Passage allowed several successful whale survey flights north of the South Shetland Islands, which showed higher numbers of species and an overall higher whale density than in the area east of the peninsula. During 40 surveys a total of 267 sightings of 669 individuals belonging to 7 species were recorded.

An observer from Argentina and one from Chile were on board in accordance with their respective country’s research permits of the Polarstern in territorial waters.

A two person broadcast team from the ZDF (Zweites Deutsches Fernsehen) reported about the expedition to the news station, news magazines, a blog, and prepared documentaries throughout the cruise. Younger scientists on board designed another blog for the Association of Polar Early Career Scientists (APECS).

The expedition ended in Punta Arenas, Chile, on 18 March, one day earlier than scheduled due to the logistic problems described above.

References

Absy JM, Schröder M, Muench R, Hellmer HH (2008) Early summer thermohaline characteristics and mixing in the western WEDDELL Sea. Deep-Sea Research II, 55, 1117-1131. doi: 10.1016/j.dsr2.2007.12.023.

Clarke A, Griffiths HJ, Barnes DKA, Meredith MP, Grant SM (2009) Spatial variation in seabed temperatures in the Southern ocean: Implications for benthic ecology and evolution. Journal of Geophysical Research, 114, G03003, doi:10.1029/2008JG000886.

Gutt J, Barratt I, Domack E, d’Udekem d’AcozC, Dimmler W, Grémare A, Heilmayer O, Isla E, Janussen D, Jorgensen E, Kock K-H, Lehnert LS, López-Gonzáles P, Langner S, Linse K, Manjón-Cabeza ME, Meißner M, Montiel A, Raes M, Robert H, Rose A, Sañé Schepisi E, Saucède T, Scheidat M,Schenke H-W, Seiler J, Smith C (2011) Biodiversity change after climate-induced ice-shelf collapse in the Antarctic. Deep-Sea Research II, 58, 74-83. doi:10.1016/j.dsr2.2010.05.024.

Hellmer HH, Schröder M, Haas C, Dieckmann GS, Spindler M (2008) The ISPOL drift experiment. Deep-Sea Research II, 55, 913-917. doi: 10.1016/j.dsr2.2008.01.001.

Huhn O, Hellmer HH, Rhein M, Roether W, Rodehacke C, Schodlok M, Schröder M (2008) Evidence of deep and bottom water formation in the western Weddell Sea. Deep-Sea Research II, 55, 1098-1116. doi: 10.1016/j.dsr2.2007.12.015.

Lockhart SJ, Jones CD (2008) Biogeographic patterns of benthic invertebrate megafauna on shelf areas within the Southern Ocean Atlantic sector. CCAMLR Science, 15, 167-192.

Piepenburg D, Schmid MK, Gerdes D (2002) The benthos off King George Island (South Shetland Islands, Antarctica): further evidence for a lack of a latitude biomass cline in the Southern Ocean. Polar Biology, 25, 146-158.

Schröder M, Hellmer HH, Absy JM (2002) On the near-bottom variability in the northwestern Weddell Sea. Deep-Sea Research II, 49, 4767-4790

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