Community structure and spatial heterogeneity of the deep-sea macrofauna at three contrasting stations in the tropical northeast Atlantic

A study of macrofauna community structure and spatial patterns was undertaken at three deep-sea stations during three EUMELI cruises in the tropical northeast Atlantic Ocean. The benthic response to contrasting regimes of organic carbon supply at the eutrophic (E), mesotrophic (M) and oligotrophic (O) stations (1700 m, 3 100 m and 4700 m depth, respectively) was evaluated through qualitative and quantitative analysis of the composition and structure of benthic macrofauna (> 250 μm) and discussed with regard to the biological and physical conditions. The mean densities of the total macrofauna, dominated by the Polychaeta, Tanaïdacea, Isopoda and Bivalvia, decreased with increasing depth from 5403 ind. m−2 at station E to 231.5 ind. m−2 at station O. From the 16 taxonomic groups represented at the E and M sites, only eight could be sampled at the O site. The density gradient can be related to the decreasing surface production, food supply and hydrographic conditions between the three stations. The comparison of the high densities encountered at station E with other studies underlines the particular feature of this station subjected to intense upwelling activity resulting in high organic carbon input. The patchy distribution of the macrofauna at stations E and M suggests that both physical and biologically-mediated disturbances create environmental heterogeneity favouring the aggregation of organisms. This process occurs at different observation scales. At a large scale, bottom current and upwelling activity induce high densities and faunal aggregations, displaying a degree of density-dependence. At the local scale, the complexity of the food web induces biologically-mediated disturbances, and the activity of major deposit-feeding taxa, like polychaetes, create sediment microhabitats favouring patchy distribution. Faunal patch sizes are variable among the taxa considered. Most are distributed in patches less than 1 m z. Our work contributes to the understanding of how the deep benthic communities are structured, particularly under the influence of environmental conditions in tropical latitudes.