Meiofauna towards the South Sandwich Trench (750-6300m), focus on nematodes

Meiobenthos (excluding foraminifers) from the South Sandwich Trench was studied in sediment samples collected by multiple core (MUC) or multiple grab (MG). Sampling stations were chosen along a depth transect that covered the continental slope and abyssal plain towards the trench (water depths from 747 to 6319 m). Total abundance and biomass of meiobenthos ranged between 354 and 1675ind. 10 cm(-2), and between 16.3 and 80.8 mugC 10 cm(-2), respectively. Standing stock decreased, though not linearly, with increasing water depth. Biomasses and densities were situated above the world ocean's regression line of meiobenthic stocks against water depth, and higher on the trench floor (354-930ind 10 cm(-2), 18.7 mugC 10 cm(-2) at 6300 m) compared to most other oceans' trench regions. This observation confirms earlier results from the Antarctic continental margins that Antarctic meiofauna is characterised by dense populations in which nematodes predominate over the other taxonomic groups (87-98% of total abundance). Taxonomic investigations at genus level revealed that Chromadoridae, Monhysteridae, Desmodoridae, Desmoscolecidae, Xyalidae and Cyatholaimidae were dominant. Most of the nematode genera (in total 94) occurred in all depths, with only a slight distinction between the shallow (Daptonema, Dichromadora and Molgolaimus preferentially at 750-2300 m) and deep (with Tricoma preferentially from 3000 m on). The shallowcommunities were comparable to those from the Weddell Sea continental margin; the sediments at 2300-6300 m harboured in big lines similar compositions as abyssal and trench communities worldwide. Two main morph clusters were distinguished among the South Sandwich Trench nematodes: plump nematodes, with average L: W = 9, belonging to the taxonomic similar selective deposit-feeding genera of the Desmoscolecida, and comparable slender nematodes (L : W = 26; biomass size class -4 to -2) such as Monhystera, Acantholaimus, Daptonema, Dichromadora, Molgolaimus, Microlaimus and some species from Paracanthonchus, mixed among mainly selective and epistratum feeding modes. They formed the bulk of the trench community. The results did not fit well with the general decrease in standing stock with increasing water depth and distance from the continental shelf. Similarly, nematode size (length from 124 to 2991 mum, biomass from 0.023 to 0.042 mug dwt) did not show clear bathymetric trends. The lack of the usual deep-sea trends of structural characteristics of meiofauna in general (density, total biomass, taxon composition) and functional attributes of nematodes (morphometrics, biomass, trophic guild structure, and maturity index) is explained by the particularly complexity of the sedimentary environment of the South Sandwich Trench, which was the result of physical processes (turbidites), bioturbation activity by invertebrate taxa and food supply along the depth transect. (C) 2004 Elsevier Ltd. All rights reserved.