Macrofaunal community structure and biodiversity patterns based on a wood-fall experiment in the deep South-west Atlantic

Wood falls provide locally and temporally restricted inputs of organic material to the deep sea supporting heterotrophic and chemotrophic fauna. These habitats also play a significant role in the diversity, abundance, and evolution of deep-sea macrofauna. Despite the importance of wood falls to the global deep-sea biodiversity, there is a large gap in understanding these patterns in the South Atlantic, and especially along formerly densely forested continental margins. In this paper, we assessed bathymetric variations in macrofaunal community structures and the biodiversity of wood falls. We deployed experimental wood logs at 1500 m and at 3300 m at six stations along the South-eastern Brazilian continental margin between 21 and 28 degrees S. In total, 13,803 individuals belonging to 44 taxa were identified, displaying local and bathymetrical differences in assemblage abundance, biomass, and diversity. Wood-boring bivalves (Xylophagidae) were the dominant taxa in mid-slope stations (1500 m), whereas polychaetes and gastropods dominated the abyssal stations (3300 m). Total macrofaunal biomass (wet weight) was markedly higher in all mid-slope stations (Avr = 0.0053 g cm(-3)) compared to abyssal stations (Avr = 0.0001 g cm(-3)). Rarefaction diversity (ES) revealed that abyssal wood falls at Espirito Santo (ES_3300) and Sao Paulo (SP_3300) respectively had the highest and lowest diversity among all the stations (LC95%, P < 0.05). Wood-fall macrofauna taxa composition was distinct in terms of depth (not significant) and among stations (significant), with xylophagid bivalves and polychaetes (Strepternos didymopyton and Pleijelius cf. longae) mainly responsible for these patterns. We found similar species of bivalves (Xyloredo nooi and Spiniapex gilsonorum) and polychaetes (Streptemos didymopyton) previously reported from the North-west Atlantic and North-east Pacific. Our experiment thus supports the idea that some wood-fall species might have wide distribution ranges. Since our wood logs were all the same size, our data supports the hypothesis that niche dynamics are the most important factor controlling the (beta) diversity of bathyal and abyssal wood-fall assemblages in the South-west Atlantic.