THE BENTHIC ECOSYSTEM IN THE 3 EUMELI SITES IN THE NORTHEAST TROPICALATLANTIC - GENERAL PERSPECTIVES AND INITIAL RESULTS ON BIOLOGICAL ABUNDANCE AND ACTIVITIES

The benthic component included in the French EUMELI JGOFS program in t he Northeast tropical Atlantic has several objectives: to identify and measure benthic processes, to describe spatial and temporal variabili ty of the benthic fauna and to relate these on variations to the physi cal and chemical environment and the trophic input. The general aim is to understand factors governing the cycle of carbon and the role of t he various benthic communities in the recycling of organic matter. The multidisciplinary investigations and the various types of sea-bottom operations undertaken during the Eumeli 2 cruise are described. The st udies were undertaken north of Cape Verde Rise on three localities at 20-21-degrees-N latitude under contrasting natural regimes of differen t primary productivity in the photic layer, considered to be EUtrophic , MEsotrophic and oLIgotrophic. The high number of successful benthic operations with box corer, multiple corer, trawl and free vehicles at the three sites at mean water depths of 1700 m, 3 100 m and 4600 m res pectively has yielded an exceptional set of ecological data for compar ison with other environmental characteristics obtained simultaneously at the three EUMELI sites. The first results in benthic biology mainly from the Eumeli 2 cruise are presented; they reveal a great variabili ty between the stations. The study of the distribution of bentho-pelag ic necrophagous amphipods living above the bottom carried out with bai ted traps at the oligotrophic and mesotrophic sites, shows counter-int uitive results: the mesotrophic area appears very poor with only 1 ind ividual/24 h period whereas in the oligotrophic area, 32 ind/24 h were trapped with a maximum observed at 5 m above the sea-bottom. The numb er of captured amphipods decreased rapidly with distance from the sea- bottom. Very seldomly one amphipod was captured at 1000 m above the se a-bottom. The high abundance of carnivorous megafauna at the deepest s tation remains an unresolved problem. The low food availability at gre at depth, if it consists of a steady rain of sedimenting organic parti cles, cannot sustain such biological activity. By contrast, the sedime nt bacterial activities are greater at sites receiving a more signific ant flux of carbon to the sea-floor; bacterial abundances in the sedim ents of the mesotrophic site were 3-4 times greater than those observe d in the oligotrophic sediments. The comparison of mean densities of b enthic assemblages, according to their size category, indicates distin ct benthic responses for each faunal group with lower densities at the oligotrophic site. Meiofauna and macrofauna densities were 8 and 28 t imes higher at the eutrophic site than in the oligotrophic one, wherea s the megafauna was 260 times higher at the eutrophic site than in the oligotrophic one. Thus, we observed a relatively smaller influence of the change in magnitude of trophic input on the smaller organisms (ba cteria, meiofauna and small macrofauna), and a sharper decrease in lar ge organism abundance related to food availability in the form of smal l sinking particles.