Seascape Structure and Benthic Habitat Heterogeneity of Santos Basin Slope, SW Atlantic

The Santos basin is a prominent offshore sedimentary feature of the SW Atlantic, extending 350,000 km² off the southeastern Brazilian coast. The deep sectors of Santos Basin comprise the slope and the São Paulo Plateau, which, in the last decades, have been subject to fishing and oil and gas exploration. Seafloor is mostly covered by sediments, interspersed by hard substrate patches associated with pockmarks, carbonate mounds and ridges, and salt diapirs. These features increase the diversity of benthic habitats that remain mostly unexplored, but are expected to host cold-water corals and cold-seeps communities. We analyze benthic seascape structure and habitat heterogeneity in a 4,854 km² section of Santos Basin slope region between 259 and 1,275 m depths (seascape area). Seascape analysis was based on a high-resolution digital bathymetry model (14 m grain size) produced during deep-sea surveys conducted in 2011 and 2022. Using a Benthic Terrain Modeler tool, seabed was segmented into seven classes, including: (a) the dominant and highly interconnected sediment “matrix”, (b) the hard-substrate pockmarks, mega pockmarks, depressions, salt diapirs and carbonate ridges, and (c) the human-made “trawl furrow”. Seascape structure was quantified considering all 1,430 mapped patches of all classes (patch types), except the sediment matrix, and 16/22 patch class/seascape scale metrics that described: patch area and edge, aggregation, shape, core area and diversity. Patches tended to become denser (3 – 5/ km²) and more irregular at depths greater than 500 m. Between 600 and 700 m, patches were also more diverse and evenly distributed. Pockmarks included the most abundant and dense patches (2.7/km2), with their core covering 34.4% of the seascape area. They exhibited extensive edge sectors and were relatively small, disaggregated, regular-shaped and little interspersed among other classes. Pockmarks density was highest between 300 and 400 m depths and declined towards deeper slope sectors. Between 400 and 600 m their core area was smaller, but overall, they covered the highest proportions of the depth strata seascape (48.8 – 53.9%). Because cold-water coral and cold-seeps communities are likely to occur on the edges and core of pockmarks, respectively, chances of their occurrence may be increased at the 400 – 600 m depth stratum. In the 600 – 700 m stratum a more heterogeneous mix of hard bottom structures occur, some of large sizes (e.g., carbonate ridge), potentially harboring more diverse benthic communities. The definition of trawl furrows as habitat patches, represents an important evidence of human activities modifying deep seascapes in Santos Basin. Whereas their core area was little significant, they tend to create extensive edge sectors amid the sediment matrix, potentially affecting soft bottom benthic communities.