Deep-sea ecosystem: a world of positive biodiversity – ecosystem functioning relationships?

Abstract. The global scale of the biodiversity crisis has stimulated research on the relationship between biodiversity and ecosystem functioning (BEF) in several ecosystems of the world. Even though the deep-sea seafloor is the largest biome on Earth, BEF studies in deep-sea benthic ecosystems are scarce. In addition, the few recent studies, mostly focus on meiobenthic nematodes, report quite different results spanning from a very clear positive relationship to none at all. If deep-sea BEF relationships are indeed so variable or have a more common nature is not established. In this first BEF study of deep-sea macrobenthic fauna, we investigated the structural and functional diversity of macrofauna assemblages at three depths (1200, 1900 and 3000 m) in seven different open slope systems in the NE Atlantic Ocean (n = 1) and Western (n = 3) and Central (n = 3) Mediterranean Sea. The results demonstrate a positive relationship between deep-sea macrobenthic diversity and ecosystem function, with some variability in its strength between slope areas and in relation to the spatial scale of investigation and environmental conditions. The macrofauna functional diversity did not appear to be more effective than structural diversity in influencing ecosystem processes. Rare macrofaunal species were seen to have a negligible effect on BEF relationship, suggesting a high ecological redundancy and a small role of rare species in providing community services.