Only the latest decades the interest in the hyperbenthos, the fauna that lives. in close association with the bottom, has grown. Before, this fauna was relatively poorly examined. The diversity of the most important representatives of the hyperbenthos, the peracarid crustaceans, increases with increasing depth, while commonly the diversity of macrofauna organisms decreases with depth. In this study, the hyperbenthos composition was examined across the European continental slope. This continental slope is one of the most important topographical features of the earth. The sea-floor descends via the shelfbreak across the continental slope into the deep-sea. For this research, this continental slope was examined in Meriadzek Terras, southeast of Ireland. The sampled gradient was situated between 47° 53.24' N to 47° 41.01' N and 7° 50.57 W to 8° 12.54' W. Eight stations were sampled with a hyperbenthic sledge (Sorbe, 1983) from 200 to 1250 m in waterdepth, with a variation of approximately 150 m in depth between two successive stations. In this study the hyperbenthos compromised 65.5 % of the total sampled fauna with a number of 22381 organisms and with representation of 12 taxa and larval forms. The Peracarida, being the most important representatives of the hyperbenthos, were responsible for 59.36 % of the present hyperbenthic fauna. A total number of 13285 peracarid crustaceans were collected, with representation of 210 species (Amphipoda, Isopoda, Cumacea, Mysidacea and Tanaidacea). The amphipods are the most abundant with 38.46 %, followed by the mysids with 32.69 % and the cumaceans 18.46 %. The isopods and the tanaids completed the peracarid fauna with respectively 9.94 % and 0.45 %. The Arnphipoda, Isopoda, Cumacea and Tanaidacea showed an increase in diversity with increasing depth. The Mysidacea on the other hand rather showed a decrease in diversity with increasing depth. The hyperbenthic taxa and peracarid species were studied in terms of density, diversity and community analysis. Relying on the first two components, two bathymetric patterns were found. A first one occurred around 650 m of depth, where a shift in species composition, a decrease in density of amphipods and mysids, an increase in density of isopods and cumaceans, plus the appearance of tanaids were detected. The second observed pattern was the increasing diversity starting at 650 m depth and reaching its highest level in station M 950 (i.e. at 950 m of depth). In both cases the effect was very distinct, the stations that displayed the highest diversity also showed the largest percentage of mud (i.e. clay and silt), and the station where the species composition started to change was also the depth were the percentage of mud started to increase. Based on the community analysis a clear difference was made between the fauna inhabitating the water layer adjacent to the bottom (i.e. 0 cm to 50 cm above the sediment (lowernet) and the water layer right above, i.e. 50 cm to 1 m, sampled by the uppernets. Another division could be observed in the multivariate analysis namely the separation between samples obtained from the stations on the upper-slope and those of the deeper stations of the gradient. Again the sediment seemed to be one of the determining factors. |
