Diving under the ice: A first attempt at quantitative krill assessments in the Antarctic winter

Krill shape the structure of the Antarctic marine ecosystem due to their central position within the Southern Ocean food web as prey of a wide range of predators such as birds, penguins, seals and whales (Figure 1), and as effective grazers on large phytoplankton blooms.

Figure 1: Krill occupy a central role in the Antarctic food-web

The region around the Antarctic Peninsula is the main population centre of krill in the Southern Ocean. A decline in sea ice cover has been observed during the last 20 years and linked to a concomitant decline in krill abundance (Figure 2, left). However, despite the importance of winter sea ice duration for krill recruitment and spawning success (Figure 2, right), investigations of krill associated with sea ice habitat, especially during the Antarctic winter, are rare due to obvious logistical and environmental challenges.

In the Austral winter of 2013, a nine-week expedition was led by the Alfred Wegener Institute (AWI) and several collaborating international institutes, including SAEON, to investigate the relationship between biological and physical sea ice conditions and the condition of krill, especially the larval stages. It was anticipated that the resulting information would aid the prediction of likely effects of environmental change (e.g. decrease of sea ice due to climate change) on the organisms under study and the response of the pelagic ecosystem to a shift in species composition.

From 16 August until 14 October 2013, the icebreaker RV Polarstern was home to several research groups from various countries including Germany, Australia, England, South Africa, France, Turkey, Sweden and Spain. Research foci included ice physics, ice chemistry, polar atmospheric science, large predators, macro and micro plankton communities, under-ice remotely operated vehicle (ROV) and diving surveys.

Figure 2: Krill abundance has decreased substantially in the past 20 years in the region around the Antarctic Peninsula (left). Antarctic winter sea ice duration as a determinant of krill abundance and spawning success (right) (after Atkinson et al., 2004).

Albrecht Götz from SAEON’s Elwandle Node and Sven Kerwath and Lutz Auerswald from the Department of Forestry and Fisheries (DAFF) participated as collaborating members of the krill research group led by Professor Bettina Meyer (AWI) and formed part of the Scientific Diving Group led by Ulrich Freier (AWI). The diving research was one of the main aims of the expedition, as diving surveys under the sea ice away from the continent in the Antarctic winter were only attempted once before and with limited success.

The aim of the group was to quantitatively assess the abundance and distribution of krill larvae in relation to environmental factors. Moreover, sampling of larvae from their habitat directly under the sea ice was important for physiological examinations, since this habitat is otherwise not accessible to conventional sampling methods such as nets.

Boat diving operations

The marginal ice zone is considered ecologically important for the larval krill as it functions as a transitional zone connecting the highly productive open water zone with the pack ice zone which offers shelter, habitat complexity and additional food sources such as ice algae.

At two sites dives were carried out from a five-metre rubber duck. A support vessel was on standby in the vicinity. This type of operation was carried out in the marginal ice zone where floes were smaller, more mobile and less stable and therefore not suited to the installation of an ice camp. The main diving boat was attached to a suitable floe with ice screws and a diver entered the open water to survey the under-ice habitat under the floe. Before all diving operations from boats, the area was scanned for leopard seals during helicopter flights and a fully kitted safety diver remained on standby.

Ice camp diving operations

Deeper in the pack ice, away from the marginal ice zone, multi-day ice camps were established for diving and remotely operated vehicle (ROV) operations. Reconnaissance helicopter flights and remote-sensing data were used to identify large, stable floes suitable for ice camp operations, including the use of skidoos to transport equipment and a small caterpillar to drill the main dive holes and several security holes.

The main diving boat was attached to a suitable floe with ice screws     The ice camp comprised a heated igloo (red tomato), a modified Scott tent (orange tipi) for the generators, a hospital sled (not shown here) to provide a rapid rescue chain to RV Polarstern in case of emergencies and the white dome tent, in which the main dive hole was situated.

The ice camp was then set up. It comprised a heated igloo (red structure), a modified Scott tent (orange tipi) for the generators, a hospital sled to provide a rapid rescue chain to RV Polarstern in case of emergencies and a white dome tent, in which the main dive hole was situated. Outside the dive tent and a short distance away, an under-ice security camera was installed to observe the divers, but also to record possible visits of dangerous wildlife such as leopard seals during the night.

Krill sampling and preliminary results

During the cruise across the South Atlantic from Punta Arenas (Chile) to Cape Town, two ice camps were established and two sites were visited for diving operations from small boats. The first ice camp was situated in the pack ice zone east of the South Orkney Islands. Ten degrees further east of the first ice camp, boat dives were conducted in the marginal ice zone. The next ice camp was established in the pack ice zone south of the South Sandwich Islands and the last boat dive site was situated in the marginal ice zone to the east of this island chain. Overall, 54 dives were conducted during daytime and 16 at night, totalling almost 26 hours of under-ice observation time.

Ice camp on a floe with the white dome tent (diving operations), the orange Scott tent (generators), the red igloo (day room) and the red-and-white ROV sled tent (left). Also visible are a skidoo, the blue rescue sled and the RV Polarstern in the background.         View larger version Inside the dome tent with a diver getting ready to enter the ice hole in the centre

Krill larvae density was assessed through diver-operated photo quadrats and fixed video transects during day and night. In addition, krill larvae were sampled from under the ice with a diver-operated pump for physiological examinations. Sea-ice chlorophyll data and thickness were assessed by means of sensors attached to a ROV and sampling with ice corers from top-side. Comprehensive video documentations (roving dives) of the environment and the ice habitat provided in-depth and small-scale information on habitat features.

A diver takes standardised photo quadrates to determine the density of krill larvae under the ice A swarm consisting of thousands of juvenile krill associated with protective structures under the ice during the day

Video and photo material was analysed quantitatively using the image processing software and the krill sampled with the pump were differentiated into larval and juvenile stages. Subsequent preliminary analysis was carried out, from which a number of partly novel or for the first time quantitatively supported patterns could be inferred.

A diver works with an underwater pump to sample krill larvae for size structure analysis and physiological experiments     Every winter an area of about 13 million square kilometres (= combined area of USA and India) is added to the Antarctic sea ice. The expedition was timed to coincide with the maximum annual sea ice extent during the winter of 2013, which was an above average year for total sea ice area (red line).

Conclusions

Through a pioneering effort of under-ice diving from moving floes floating on top of ~5,000 metres of water in the Antarctic winter, we could obtain quantitative data on krill larvae in one of the most extreme environments on earth. Due to the logistically challenging survey work, this kind of information has not been collected before, although krill, in terms of overall biomass, is the most abundant animal on earth.

Krill is the single most important species in the Southern Ocean ecosystem as it - directly or indirectly - provides food for all top predators, including the largest mammals (whales) and largest mammal populations in terms of biomass globally (seals, > 30 million individuals).

With this research we will show how krill larvae and juveniles use and depend on the shelter, bio-film and ice algae provided by the winter sea ice in the Southern Ocean. With over 15 million square kilometres (approximately the size of Russia), the surface area of frozen sea in the Antarctic winter supports a vast number of krill larvae and juveniles. This support will be compromised if climate change leads to a further shrinking and/or thinning of the sea ice in the Southern Ocean.

With the cunning use of flags, scientists from SAEON and DAFF claimed this area for South Africa, making it temporarily the second biggest country in the world. From left: Sven Kerwath, Lutz Auerswald and Albrecht Götz