Zoobenthic biodiversity, biomass and abundance at Adelaide Island, Antarctica

The waters surrounding Antarctica are amongst the most isolated large areas of continental shelf, cut off for about 15 to 30 million yr by both deep water and the oceanographic barrier of the Polar Frontal Zone (PFZ). Although certain taxa are notably absent, shelf seabed richness (of the mostly endemic species) can be very high. Most of Antarctica's shallow shelf lies between 67 and 72degreesS, although virtually all the growing southern polar marine biology literature has been carried out to the north or south of this. Here we report one of the first, and the most detailed, quantitative studies of benthic faunal abundance, diversity and biomass from within this latitudinal belt (at Adelaide Island, Antarctic Peninsula). Representatives of 16 phyla, 25 classes, 34 orders and at least 75 species were found in the 40 samples of 0.25 m(2) area. This is rich, especially for polar localities described to date. Faunal abundance increased logarithmically from <100 to >10 000 individuals m(-2) from the intertidal to 35 m respectively. Annelids and bryozoans were the most numerous, and cryptofauna (such as these 2 phyla) exerted a major influence on both patterns and absolute values of diversity. Subtidal biomass increased from 500 to 10 000 g m(-2) at 3 to 35 m respectively and is, overall, the highest for any polar locality within the 0 to 40 m depth range. The echinoid Sterechinus neumayeri was the principal cause of these high values as it dominated biomass at all subtidal depths, although molluscs (particularly the limpet Nacella concinna) were important in the shallows. Striking subtidal zonation was apparent, demarked by both inter- and intra-specific characteristics. We suggest that faunistic bathymetric organisation essentially forms 3 zones comprising different suites of species and is also demarked by the population structure of the species S. neumayeri. Not only do the species S. neumayeri and N. concinna show strong zonation in occurrence but also their grazing activity is probably a strong agent producing similar patterns in cryptobenthos (through removal of recruits). We propose a general schematic (a diagram) of zonation as declining in the littoral southwards towards the PFZ and increasing in the subtidal southwards from the PFZ.