Structural and functional aspects of the benthic communities in the deep Angola Basin

Abstract

During the RV 'Meteor' expedition 'DIVA I' the macro- and megafaunal communities were studied in 6 areas of the deep Angola Basin in July 2000. Water depths varied between 5162 and 5497 m. The macrofauna was dominated by polychaetes, peracarids and bivalves; the megafauna by ophiurids, bivalves and actiniarians. Abundance and biomass of macro- and megafauna were similar to other non-oligotrophic deep sea areas > 4000 m. Predatory polychaetes, peracarids and facultative surface deposit feeders or predatory sipunculids occurred in similar ratios in all areas. Facultative subsurface deposit-feeding or predatory nematodes and oligochaetes increased towards the northern Areas 4 to 6. The percentage of surface-deposit and interface-feeding organisms such as polychaetes and ophiurids, which dominated in Areas 2 and 3, decreased towards the north. For megafauna, the ratio of organisms feeding as suspension feeder or predator as well as surface-deposit feeder increased from south to north. In the northern Areas 4 to 6, high numbers of bivalves occurred, which feed as surface or subsurface deposit feeders. TOC and chlorophyll a contents in sediment, as well as macro- and megafaunal abundance and biomass, were lower in the southern Areas 1 to 3 than in the northern Areas 4 to 6. These differences seem to be connected to the different water masses north and south of the Angola-Benguela Front, which cross the transect of our 6 study sites. In the north, the Angola Basin is a highly productive region fed by the South Equatorial Counter Current and water masses from the Zaire (Congo) River, whereas in the south, the cold Benguela Oceanic Current is less productive. The structure and function of the benthic fauna in connection with sediment data give evidence for differences in food supply north and south of the front.