ANT XVIII/5b: Krill biology
Southern Ocean GLOBEC

Weekly report 3, 01.05.2001

At the beginning of last week POLARSTERN headed south into sea-ice southwest of Alexander Island. Satellite pictures had indicated large ice floes in the areas that our sea-ice specialists were keen to investigate. Our first iceberg appeared on Sunday at 5 pm with a heavy sea splashing its rims. On our way into the ice we first passed fields of newly formed pancake ice, 1m in diameter, that slowly aggregated and merged to bigger ice floes covered with snow; decoration by nature at its best. During the previous weeks, strong and cold (minus 15°C) southerly winds caused freezing of the water between the floes and consequently, the 50 cm thick floes were separated by stripes of 10 cm thin ice, dangerous to work on. We had expected good weather, but a depression system had sent its snow-showers onto the freezing ocean.

Finally at 72°S the ice field became more stable and safe to work on. The ice-team, of 4 women and 2 men, penetrated the floes and retrieved one ice core after the other. From these cores, measurements of temperature and salinity were taken and then the core were sawed into slices. These were incubated to measure the biological activity of the specialised ice biota. One single ice station took about 4 to 6 hrs depending on the weather conditions. On Tuesday for example we had minus 15°C with Beautfort 7 winds, creating a wind chill effect below minus 40°C. Under such conditions work slows down drastically as everything takes much longer and does not always work out as planned. Contrary to fresh water ice, sea-ice is characterised by a sponge like structure with brine channels being the habitat for a rich in-ice fauna and flora. Diatoms, silica shelled monocellular algae, dominate these ice communities and are its main primary producers. In diatoms, characteristic additional orange pigments cover the uniform green plant pigment chlorophyll. Assemblages of diatoms therefore colour ice floes in typical red-brown. Pieces of brown ice spotted amongst white floes in the ships track indicate just how patchy the distributions of ice algae are in this area. The diatoms are the basis for food webs in sea ice. To investigate biological processes linked to this production was one of the main goals of the sea ice research team, who was working in bitterly cold winds on the white plain around the ship protected by orange survival suits.

The diving team even dared to have a look under the ice in these conditions. Their objectives were to document the under-ice structure and the associated biology by photography and net sampling. Zooplankton collected by such nets contained a typical species only found in close association to sea ice, making the zooplanktologists on board very happy. Against our expectations, we did not see any adult krill between the ice floes. But a Minke whale circled the zodiac and had a closer look at the diver, who was also enthusiastic about this special meeting event. Interestingly enough, sea water around freezing temperature (minus 1.85°C) is a warm habitat to work in, compared to the platform exposed to the strong winds above. And diving from the zodiac or using holes cut into the ice floes is rather convenient. Nonetheless, diving in Antarctica is heavy work and needs much more logistic support compared to the diving operations in moderate climate usually performed by the AWI diving team.

Diving continued during night by means of a remote operated vehicle (ROV) equipped with videos and a camera. The ROV can go 200 m from the ship under the closed sea-ice cover and is controlled via monitors located in a lab on board ship. The first dives did not show any krill between or under the ice, but during the last dive, 4 cm long juveniles rushed through the video monitors spotted by the camera lights. We saw schools of krill in a 5 to 20 m water layer under the ice not knowing if adults were also among the jumping and flipping animals.

Our net trawls were not so successful so far, especially the rectangular midwater trawl (RMT) which caused some headaches. This instrument is equipped with two nets, a 1 sqm net with 1 mm gauze and an 8 sqm net with 4 mm gauze. Such nets caught copepods and juvenile krill and also small fish in high numbers but adult krill have been scarce in the samples. On Wednesday night the net became blocked by heavy sea-ice floes closing behind the ship. With the help of the able crew, we managed to get the instrument back in, but we had to sacrifice the 4 mm gauze to Antarctica. The hunt will continue for adult krill!

For only two days during the last week, and in fact during the entire expedition so far, we have seen sunshine and blue skies. Sunrise illuminates microstructures of the sea-ice in sequences of red and purple, while the walls of nearby icebergs reflect the golden sun. Caverns and gates of these icebergs appear in light green or dark blue depending on the structure of the ice. The whiteout during snow storms has vanished and is being substituted by the clear sight to the horizon which makes objects 20 km distant appear close. At last, the good weather allows our bird watchers and whale counters to fly in helicopters and enhance their ship-derived data. In the area of 10/10 rafted pancake ice and nilas, 30 Minke whales were spotted during one day. Also our ice physicist has used the good flying conditions to deploy three small buoys on sea-ice floes over the deep ocean area. These buoys will drift with the floes north during winter, with their tracks being recorded in the home institute in Cambridge, UK.

During Thursday we had to leave the MIZ. As we headed north, dense sea-ice fields became rare and pancake ice dominated. The nearby open ocean sends its swell into the pancake fields and made them undulate gently up and down. Icy sculptures on the ship´s bow started to melt and fog covered POLARSTERN, heralding the arrival of the next depression system.

As a reward, the Brazilian team on board gave a special night with a potent national drink and music from their home. Everybody had an enjoyable time.

Only a few days and nights are left to complete our work. We will repeat our first transect between Adelaide Island and retrieve the mooring at the continental slope. Hopefully weather will permit to the successful recovery of this mooring before the end of our cruise. American colleagues will continue this strategy so that we should have a good sequence of data for the same transect throughout the entire winter.

Many greetings from a ship grieving after leaving the amazing ice fields, but awaiting a safe return home.

Uli Bathmann (chief scientist)