ANT-XXVII/3, Weekly Report No. 7
The iceberg resting place, Austasen, from the air. (photo: D. Gerdes)
21 March - 27 March 2011 (CAMBIO)
During this week we have worked in the sea area southwest of Neumayer Station between 70° and 72° southern latitude under high Antarctic autumn conditions. The program BENDEX got its name from benthic disturbance experiment, in the vicinity of an iceberg resting place. During the season 2003/4 the benthic surface fauna was removed using a modified bottom trawl in an area of 100 x 1000 m to simulate the effect of an iceberg running aground. This impact was carried out 7 years ago and the bottom fauna may have used this time to re-colonise the area. On one hand we are interested in how fast or slowly the area is re-colonised, and on the other hand, which pioneer species take part in this process and what succession of assemblages form new communities. There are many re-colonised areas in this part of the Weddell Sea, which is characterised by many scouring icebergs. However, as we do not know the exact time of the icebergs’ stranding, we can only partially determine the velocity of the processes from these natural events. The matter is interesting against the background of potentially increased iceberg calving during continued global warming, and taking into account the vulnerability of the Antarctic ecosystem to disturbance.
During this week the wind has little effect on us regarding the ship’s movement because the ice prevents swell, but after temporary decrease of the frost between Monday and Wednesday the temperatures plummet towards the end of the week. From Thursday the combination of strong wind and cold makes work on deck rather uncomfortable, the equipment and the catches freeze, and the formation of new sea ice increases strongly. We can observe all stages of ice formation at strong wind from grease ice, via pancakes of different size, to larger ice-flows, and if there is no wind the ice cover forms over night and rapidly increases to more than 10 cm thick. We loose Timo's fish trap, which probably became trapped under an ice flow as it approached the surface, and once again due to ice cover close to the coast, we were unable to search for the colonisation substrates that were deployed 13 years ago. Despite all this we have been able to do quite a lot of work during the week.
High Antarctic sponge community at about 200 m depth: glass, cabbage and other sponges; bottle brush gorgonians, holothurians (on sponges), hemichordates (or pterobranchs) (dark) and much more. (photo: T. Lundälv)
Against the background of the BENDEX concept, the most important thing is that the visual gear (cameras on the ROV and MG) recognise the experimental field clearly, and that the corers are able to take good samples from the disturbed area and the area outside to document differences in the quality of the sediment and the colonisation. The area which was swept free of fauna in the original experiment is even today clearly separated from the area around this field. The tracks of the otter boards of the ground trawl are still clearly recognisable. The sponges which during the experiment were transported towards the margins of the area are still lying there in heaps and partly covered by sediment. The underwater video cameras of the ROV and the MG also document that up until today there has not been a massive colonisation of pioneer species which we expected from our knowledge of processes of iceberg scours. It looks as though many species do not reproduce regularly (e.g. once a year), but perhaps over longer periods of time. For the ecosystem of the Antarctic sea floor this suggests a slowed down dynamics as has been assumed by many polar biologists for a long time. However, reasons for the retarded colonisation may also be found in the quality of the sediment, but this we will know only after Enrique has analysed his sediment samples in the lab.
Unfortunately we cannot describe in detail the fantastic pictures, which the three deployments of the ROV have yielded during the week. The two photographs we have attached can only give an impression. The three dimensional high Antarctic benthic communities here at the iceberg resting place are a classic example of species richness, colour and structure that every marine zoologist should have seen, and they are always full of surprises. The large glass sponges, which by themselves are beautiful, serve as a hiding place and breeding cave for fish and as a substrate for colourful crinoids, ophiuroids and holothurians. The cabbage sponges colonise the bottom in dense assemblages which reminds one of a cabbage field. Tile red sea urchins are sitting in dozens on dark green stones and graze on the soft white polyps of soft corals which grow from root-like threads. An intensely red mound turns out to be a gathering of sea stars feeding on a holothurian. Filtering brown holothurians are lying in such density that the sediment is no longer visible. Close to the Bordeaux red branched colonies of the hemichordates – our distant relatives as they are forming a kind of spinal cord in the larval stage – there appears a sister species that is semi-spheric. A ctenophore and a medusa (or sea jelly), both of which are normally animals of the plankton, here appear to be bottom dwelling forms. One can look at the ROV images for hours without being bored.
Benthic community at 60 m depth from the only known shallow-water site in the southeastern Weddell Sea. Orange, yellow and beige soft corals (alcyonarians) can be seen on and between the stones, which are washed out by the currents. Also visible are white hydrozoans, red sea stars and sea urchins, and brown holothurians. (photo: T. Lundälv)
Contrary to the ROV pictures, the AGT gave a miserable picture of benthic community outside the experimental area. It must not be deployed inside as it would be a new disturbance. The first of three catches is a heap of mud with little animal content which may be ascribed to an iceberg scour. However, the stones of the second haul and the huge pile of sponge spicules from the third haul give only a sad impression of the species richness in the area.
The fishery with the bentho-pelagic trawl, which is to catch live fish material for the physiologists, is not a great success either. Katja makes great efforts, but in a total of 12 hauls the yield is only a few kilograms of fish, particularly Pleuragramma, a few ice fish, and a couple of larvae or juveniles, among them a Bathydraconid. We see that if there is strong frost it is practically impossible to get the fish on deck alive because heaving the incredibly long net requires a lot of time, and picking up ice pieces is inevitable.
The physiologists on board the “Polarstern” are investigating the temperature tolerance of related species of fish and invertebrates between the Subantarctic and high Antarctic to study to what extent the environmental temperature influences the distribution of these species, which is of interest to the ecologists and the geneticists. For this purpose they are measuring different levels of the organisms; from single proteins, to the mitochondria (the power plant of the cells), to liver cells, right up to the whole animals. For example, high Antarctic icefish and octopus are very sensitive to small temperature oscillations when compared to their Subantarctic relatives, and certain groups of decapods are found only in Subantarctic waters. This cruise is particularly favourable for such comparative studies because it covers a large gradient of latitudes.
During the night on Friday we steam to Atka Bay, which in this season is only covered by a thin layer of new ice. This has the advantage that we can reach the shelf ice edge without any problems, but the disadvantage is there are no seals or penguins to be seen around the ship. On Saturday morning at 9am we are along side the ice shelf edge. Neumayer Station, which now is above the ice, can be recognised clearly from the ship thanks to the good visibility. After a safety lecture by the chief scientist the shuttle to visit the station begins and lasts until late afternoon. It is sunny but cutting cold at -21°C at the vessel and -24°C at the station. Scientists and crew members are flown the 13 km to the station in groups of three or four. At the station they are led around by the over-winterers. For some there is sufficient time for a coffee, others enjoy themselves with billiards, others play table football. All are impressed by the engineering of the huge hydraulic stilts. Still, only few can imagine living in this isolation. Most of the winterers pay us a counter visit on the ship, and say farewell on the afterdeck with mulled wine and warm soup. The “Polarstern” leaves at 20:30 to the tune of “time to say goodbye”.
On Sunday morning we wake up at -24°C and sunshine in the BENDEX area. There is again new ice around the ship and on the port side the shelf ice edge stretches for kilometres, which here, contrary to Larsen, is not yet affected by global climate change, which is also demonstrated by our measurements.
From a cosy warm ship we greet you from the cold in the name of all people on board.
Rainer Knust, Chief Scientist Wolf Arntz, Rapporteur
RV Polarstern at the ice shelf edge of Atka Bay. (photo: D. Gerdes)
