Estimating the biodiversity of the shelf and oceanic zone of the d’Urville Sea (East Antarctica) for ecoregionalisation using the CEAMARC (Collaborative East Antarctic Marine Census) CAML surveys

A proposal for a representative system of MPAs (RSMPA) for eastern Antarctica (CCAMLR Divisions 58.4.1 and 58.4.2) has been developed to satisfy overall the principles of comprehensiveness, adequacy and representativeness (CAR).  The Dumont d’Urville Sea area, including the Mertz Polynya, is one of the areas identified in the proposal (140-150°E from the coast to 60°S). This area has been studied by a number of countries in the CEAMARC surveys (Collaborative East Antarctic Marine Census) as part of the Census of Antarctic Marine Life. This report details some of the outcomes of a workshop hosted by France in Brest in late May 2011.  It collates and synthesises available data from the CEAMARC and other studies to provide a detailed ecological picture of this area and to further evaluate this area relative to the CAR principles and relative to its use as a reference area for understanding ecosystem change in the region.

The offshore zone is separated latitudinally by the presence of three fronts separating species assemblages and by the presence of an eddy (62°- 64°S, 138-140°E). Two of the fronts, the Southern Boundary (SB) and the Antarctic Slope Front (ASF) were identified to have a major influence on the biogeography of the pelagic species. The SB is the northern limit of Euphausia superba and the ASF clearly separates most of the neritic species from the oceanic. Vertical segregation of assemblages of pelagic cnidarians and ctenophores, fish and plankton were observed. However, this segregation does not apply on the continental slope as deep pelagic fauna can be upwelled with the Modified Circumpolar Deep Water. These characteristics combined with complex canyon systems indicate the continental slope is a transitional area between the neritic and oceanic zone. It is also important to note that it is on the slope area that two Vulnerable Marine Ecosystems have been identified by CCAMLR. Two large basins are found on the shelf in this area, differing in their physical characteristics and one, the George V Basin, is where the Adélie Land Bottom Water (ALBW) is formed and flows offshore.

Revised physical pelagic and benthic regionalisations of the East Antarctic sector showed 5 predominant cluster types over the shelf and two offshore in the pelagic realm and 9 distinct cluster types over the shelf in the benthic realm. Ecoregions were then defined according to both benthic and pelagic characteristics. They are based primarily on the benthic regionalisation with some of the benthic regions being divided further to take account of the differentiation in the pelagic environment.

The shelf pelagic regionalisation is explained by sea-ice and water masses. Spatial differences in pelagic species assemblages are observed but it varies from year to year. The variability in spatial structure of these assemblages is because of the dominance of Pleuragramma antarcticum and Euphausia crystallorophias. These two species are prey of top predators and control the planktonic production. The Adélie Basin is an important area for seals and penguins of the Pétrels Archipelago when they forage at a short distance from their colonies.

The benthic ecoregions over the shelf clearly separate coastal zones from the deep basins, their upper edge and the banks. Close association between demersal fish and benthic communities was identified. Grounded icebergs and long-term sedimentation patterns play a major role in structuring the bottom seascape. Suspension feeders are abundant on rocky and sandy substrates, while detritus and deposit feeders dominate the muddy basin sediments. The coastal zone is also the place of specific assemblages at least for fish for which some species are mainly coastal.

The workshop held in Brest at the end of May 2011 agreed that the Mertz area will be more likely to satisfy the CAR principles and its use as a reference area by changing the western boundary from 140°E to 136°E and the eastern boundary from 150°E to 148°E.