MEGABENTHIC COMMUNITY ECOLOGY - LIFE UNDER ICE- ICE-SHELVES

Julian Gutt¹, Harald Bohlmann², Werner Dimmler³, Gesche Kohlberg¹, Stephanie Langner¹, Jan Seiler¹

1 Alfred Wegener Institute, Bremerhaven, Germany

2 ISITEC, Bremerhaven, Germany

3 FIELAX, Bremerhaven, Germany

Objectives

The main objective of this subproject is exactly in line with the two major subprojects of the CAML related topics: benthic ecological processes under the former Larsen A/B and cold seep biology under the former Larsen B Ice Shelf. The specific aspects of this sub-project were to focus on the mega-epibenthos and apply non-invasive methods mainly by a video-equipped remotely operated vehicle (ROV). The reasons for this decision are three-fold, firstly, only this way the mega-epibenthos can be quantitatively surveyed on both types of sediments, soft and hard, in a comparative way. Secondly, in Antarctica the megabenthos plays the dominant role in most regions in comparison with the infauna. Thirdly, climate related ecological long-term processes such as iceberg scouring can be best detected using a quantitative method with a high spatial resolution, which means that the video footage provides discrete information for each single metre on a transect. The same holds true for the search for seep spots. The results can be compared with those from the "normal" high latitude shelf in the eastern Weddell Sea, e.g.

from expeditions ANT-XIII/3, ANT-XV/3, ANT-XXI/4. In order to evaluate the uniqueness or normality of the findings from the Larsen area and to test general ecological hypotheses, e.g. on the ecological background of very rare species, two core stations north of this area at Dundee Island and Snow Hill Island were investigated.

Work at sea

The ROV (“Cherokee” owned by MARUM/RCOM, University of Bremen) was deployed 18 times in total: One test-station at the South Shetland Islands without scientific results, 10 times in the Larsen B area ("B_South", "B_West",

"B_Seep", B_North") and 3 times at Larsen A ("A_North" and "A_South"), and 4 times in the reference area near Dundee Island and Snow Hill Island (“DI”

and SHI”). Each cast lasted approx. 100 min representing an averaged length of almost 1 km. As a consequence, a total length of approx. 15 km sea-floor was videographed. Assuming an average strip width of 0.8m and an average

ANT-XXIII/8, Scientific reports

speed of the ship and ROV, respectively, the total area covered during all casts was 12,000 m². In addition a total of approx. 3500 photographs have been made primarily for a better identification of organisms depicted in the videos.

Preliminary results

Unexpectedly, the observations at station "B_South" showed at water depths

>100m bedrock colonized by a very specific sessile fauna with hydrocorals, being the most obvious animal group (Fig. 2.1). A few patches of scraped rock, due to iceberg scouring were found. At the same spot but at a greater water depth the sediment was composed of smaller grain sizes and stones.

The megafauna was very poor.

Fig. 2.1 True hard bottom community at “B_South”. © AWI/MARUM, Univ. Bremen.

At "B_West" soft sediments with few icebergs scour marks, few stalked crinoids, and very few intermediate sized hexactinellid sponges were most conspicuous. At "B_North" and "A_South" numerous ascidians especially in shallow water depths, may represent first pioneer organisms, with massive growth (Fig 2.2).

At "B_Seep" core-station the seep was found with patches of shells of cf Calyptogena. The identification was based on material obtained with the Agassiz trawl and multibox corer. Some of these approx. 30cm large patches

CAML and related topics

are distributed only over a relatively small area. At station "A_North" very close to the steep rocky coast but close to the Drygalski Glacier the sediment was obviously very fine and colonized by many infaunal polychaetes. Among the observed mobile animals mysids resting on the sediment and euphausids in the water column only few decimetres above the bottom were quite abundant. The station with maybe the highest megabenthic diversity in the Larsen area was "A_South" surveyed during two casts. This station is not far from "B_North", the sediment was very diverse. Consequently, different species of solitary ascidians were also abundant at this station. In addition, numbers of hexactinellid sponges including juveniles were higher compared to other stations in the area. Various cnidarians and various echinoderms contributed considerably to the relatively high species richness.

Fig. 2.2 Ascidians were frequent at “B_North” and “A_South”. © AWI/MARUM, Univ. Bremen.

The reference stations at Dundee Island and Snow Hill Island were both surveyed twice. At the shallower replicate at approx. 100m the benthos showed, based on a comparison with the eastern Weddell Sea, various stages of recolonization after iceberg scouring covering the entire area. This corresponds very well with the high number of grounded and floating icebergs observed in the area. At the deeper replicate (approx. 200m) the local diversity was higher, especially various cnidarians, echinoderms, bryozoans, demosponges and hemichordates contributed to this variety. In the eastern Weddell Sea hexactinellid sponges were good indicators for areas without

ANT-XXIII/8, Scientific reports

iceberg disturbance but were extremely rare in the vicinity of both reference stations.

ROV footage and underwater photographs contributed considerably to the efficient outreach program on board and will contribute to similar public relation activities in the future.

2.1.2 MACROBENTHIC COMMUNITIES - ASTEROID AND POLYCHAETE