Quaternary benthic Ostracoda from the Tasman Sea: Distribution patterns within circumpolar deep-waters

Benthic Ostracoda were recovered from box-core samples collected during the cruise SO 136 of the German R.V. Sonne in the Tasman Sea and Southern Ocean (SW Pacific sector). The distribution of 33 ostracod genera (73 species) identified in 15 sediment samples is analysed in contact with the Circumpolar Deep Water (CPDW). Q-mode cluster and detrended correspondence analyses of the data partially support the observations made by previous authors that ostracod genera have specific thresholds controlled by water mass characteristics. In fact, the subdivision of the CPDW in three distinct units is well recorded by the ostracod assemblages. Concerning epi-faunal genera such as Bradleya and Poseidonamicus, temperature and dissolved oxygen play a main role in determining specific generic thresholds. Instead, the parameters controlling the distribution of the infaunal genus Krithe are not clear. Within the sediment, the distribution pattern could be driven mostly by the amount of organic matter, oxygen supply and sediment grain size. At the specific level, R-mode detrended correspondence analysis clearly shows that distribution patterns must be driven by other factors, covering the influence of the water-mass characteristics. Bottom topyraphy, deep ocean currents and flux of organic carbon interplay with the water-mass characteristics, thus creating a complex network of ecological gradients. Within each of the three main communities identified through the Q-mode analysis, the species-specific thresholds are not identical and often the range of the ecological gradients of the most common species does not include those of the rare species. Most likely, dynamics of co-occurring species take and there involve Ostracoda and other benthic species belonging to the same trophic groupings.