Vertical distribution and abundance of pelagic cnidarians in the eastern Weddell Sea, Antarctica

The species composition, abundance and vertical distribution of micronektonic cnidarians has been investigated in the upper 2800 m at Discovery Station 9969, located in the cold regime in the eastern Weddell Sea, Antarctica. In total 22 siphonophore and 20 medusan species were identified. Overall siphonophores were more abundant than medusae, but the latter usually contributed much more to the biovolume. On average, cnidarians contributed more than 50% to the total biovolume of each catch; and they contributed >70% to the combined total of all the samples. Despite the high water content of these animals, these large biovolumes meant that the cnidarians formed similar to 25% of the total carbon in each catch. The possible impact of these gelatinous cnidarian populations on the ecosystem is discussed. The vertical distribution of some individual species showed that they occurred over relatively discrete depth ranges, implying that the usage of such general terms as epi-, meso- or bathypelagic could be misleading. Thus the three most common siphonophore species, Diphyes antarctica, Dimophyes arctica and Muggiaea bargmannae, each reached a peak abundance in a different depth range within the 0-500 m depth range, while a fourth, Crystallophyes amygdalina, was found almost exclusively between 400 and 600 m. Even deeper living species were found to occur within restricted depth ranges. Thus, with one exception in all cases, the medusae Atolla wyvillei and Halicreas minimum, and the siphonophore Chuniphyes moserae were found only in the 700-2200, 500-1800 and 1200-2800 m depth ranges respectively.