Benthic megafauna habitats, community structure and environmental drivers at Rio Grande Rise (SW Atlantic)

Abstract

The Rio Grande Rise (RGR) is a large and geomorphologically complex feature located in the Southwest Atlantic, with great commercial and scientific interest due to its potential for mining rare earth elements that are critical for low-carbon technologies. Brazilian interest in this area led to the submission of a petition to the UN Commission on the Limits of the Continental Shelf in 2018 to include RGR on the limits of its continental shelf beyond 200 nautical miles. However, mining activities are potentially harmful to deep-sea ecosystems and will likely cause some extent of biodiversity loss. Thus, baseline and continuous environmental studies in the RGR are important to address potential conflicts between mineral extraction and the conservation of deep-sea biodiversity. The RGR is characterized by a series of summit plateaus of ∼600 m deep divided NE-SW by a rift valley, up to 2000 m deep. In 2018, the plateaus and rift of a small area in RGR (30°35′S – 31°03′S, 35°36′W – 36°16′W) were explored through 13 dives of the robotic underwater vehicle (RUV) HyBIS. Videos were analyzed for the description of structuring factors (topography and habitat types) and to record benthic megafauna occurrences. Video transects revealed highly heterogeneous and rapidly changing habitats. Eleven habitats, five in the rift and six in the plateaus are proposed based on geomorphology, slope, and substrate textures. We recorded 17,008 megabenthic organisms classified in 83 morphotypes and six different phyla, from which Porifera (42.7%) and Cnidaria (41.5%) were the most representative. Samples were characterized by a high dominance and the dissimilarities result chiefly from differences in abundance scores. PERMANOVA tests indicated that Habitat and Region variables were the most important to explain structure within the community data, followed by depth and slope. The rift floor exhibited a low abundance of megabenthic epifauna, except in a sinkhole in the northern part of the rift. The lower and upper rift wall were characterized by different communities delimited by the transition between the Antarctic Intermediate Water and the Upper Circumpolar Deep Water. The habitats formed by Fe–Mn deposits were dominated by distinct communities, which were rarely observed elsewhere. Additionally, we found variations in community structure at regional scales (20–30 km), with distinct communities on each side of the rift and at the southwest of the study area. Our results contribute toward understanding the diversity, biogeography, and environmental drivers of the RGR. Fauna distribution is patchy, linked to habitats with potential mining resources, and dominated by Vulnerable Marine Ecosystems (VMEs) indicator taxa. Extensive community analysis should occur at a given site prior to consideration for the exploitation of natural resources.