Continental Slope and Submarine Canyons: Benthic Biodiversity and Human Impacts

Abstract

The Brazilian Continental Margin hosts a diverse deep seafloor landscape including vast areas of sedimented slopes, submarine canyons, reef-forming and solitary cold-water corals, methane seeps and pockmarks, seamounts, and guyots. The vast biodiversity associated with deep-sea benthic habitats remain largely undescribed with only a few, mostly descriptive, research programs established in the S and SE margins beginning in the late 1990s. These programs, in particular the REVIZEE -- Evaluation of the Sustainable Potential of Living Resources in the Exclusive Economic Zone -- focused primarily on inventorying species lists and standing stock biomass of commercially exploited species to generate guidelines for a nationwide resource management strategy for Brazil's EEZ. The poorly described nature of Brazil's deep-sea habitats and benthic fauna can be clearly illustrated if we consider that only 4 (2.5{\%}) out of 161 existing submarine canyons had benthic communities sampled and investigated to this date. Another indication of a poorly described deep-sea biodiversity along the Brazilian margin is verified by the limited number of species occurrence records for the SW Atlantic in the Ocean Biogeographic Information System (OBIS) database, representing only 0.06{\%} of the global deep-sea species occurrence records below 500 m. Most deep-sea biodiversity currently described in Brazil comes from sedimented slopes and from upper to mid slope depths in the S-SE margin. Currently, sampling efforts remain largely concentrated on Campos and Santos Basins, and in the context of baseline and environmental impact assessments (EIAs) contracted by the oil and gas industry. Very few studies have dealt with cold-water coral communities, and virtually none has investigated seamount faunas deeper than 150 m. Only very recently, reports have described deep-sea faunas associated with pockmarks, methane seeps, and naturally occurring organic falls (all focus of other chapters in this book). Therefore, the current knowledge on Brazil's deep-sea fauna comes largely from soft-bottom benthic meio- and macroinfaunal communities. Overall depth-related distributional patterns of benthic assemblages on the slope are in relatively good agreement with patterns found for the N Atlantic, with high species diversity and biomass found on the upper slope as well as near upwelling areas on the SE margin. Also, species richness of combined macro- and megabenthos decreases from the SE towards the S margin, spanning 15{\textdegree} of latitude South (i.e., 21--34{\textdegree}S). The peer-reviewed scientific literature describing Brazil's Margin deep-sea environments remains scant, with the majority of recent findings and data repositories still inaccessible from the general public, and available only through gray-literature reports from various baseline and EIA assessments. Currently, the human impact footprint on Brazil's Margin is derived mostly from the oil and gas and deep-water fishing industries, with no systematic studies to date addressing the negative impacts of these activities on the various deep-sea habitats. Future human-related impacts on Brazil's Margin deep-sea biodiversity will also come from long-term climate change effects (predominantly ocean acidification) in accordance with modeling studies, from the offshore oil and gas industry, and potentially from deep-sea mining.