The world’s largest rhodolith beds have been reported from the Brazilian continental shelf. Highly biodiverse beds are located in Southeast Brazil, but ecological aspects of these beds remain unknown. Despite their ecological importance, rhodolith beds (RBs) have recently been subjected to a severe threat, when more than 35 million cubic meters of mining residues slid down a mountainside on 5 November 2015, after a collapse of a gigantic dam upstream (the Mariana disaster), causing a huge impact on the Doce River. Our aim is to assess rhodolith beds and adjacent coralline formations on the Doce River Shelf (DRS) after the dam collapse. This paper describes the distribution, abundance, vitality, size and shape, as well as unmapped bryozoan rich sediment formations in this area, serving as baseline knowledge for environmental monitoring. Four distinct biogenic sea bottom habitats (bryozoan bottoms, rhodolith beds, carbonate concretions, and reefs) were recognized at different depth ranges with distribution indicated to be mostly related to the local sedimentary regime. Mud sediments dominated the seafloor up to 35 m depth. On the mid shelf, bryozoan bottoms were recorded from 35 to 45 m depth. Crustose coralline algae (CCA) occurring as rhodoliths and carbonate concretions extend over 1953 km² in the mid and outer shelf. Rhodolith beds predominate in these areas, totaling 1521 km² of sea bottom and were more abundant at depths between 45 and 65 m, occupying an extensive area south of the Doce River mouth. Northward, rhodolith beds are less abundant or absent likely due to the long-term deposition of fine sediments in this region. Carbonate concretions and reefs covered by CCA occupy sparse areas on the outer shelf (65–105 m depth). Differences in rhodolith features recorded, including coverage, density and size, may be related to the Doce River sedimentation and related factors (e.g., hydrodynamics, depth, and light). However, since there are no previous detailed studies on RBs along the DRS, we could not assess the impact of sedimentation of dam wastes on RBs’ abundance and density. In any case, these are valuable results for the further monitoring of long-term effects. Considering that the growth of these rhodoliths is relatively slow, and that they are affected by the sedimentation from the Doce River, the implementation of a management and conservation plan for this area is necessary in order to preserve this ecosystem.

Rhodolith beds are biogenic benthic habitats mainly formed by unattached, non-geniculate coralline algae, which can be inhabited by many associated species. The Brazilian continental shelf encompasses the largest continuous rhodolith bed in the world. This study was based on samples obtained from seven sites and videos taken by a Remotely Operated Vehicle (ROV) at four transects off the Sergipe-Alagoas Coast on the northeast Brazilian shelf. ROV operations and bottom trawl sampling revealed the occurrence of rhodolith beds between 25 and 54 m depths. At the shallower depths, fruticose (branching) rhodoliths (maërl) appear in troughs of ripples, and other non-branching rhodoliths occur associated with corals and sponge patches surrounded by bioclastic sand. Rhodoliths also occur in patches from 30 to 39 m depth; some are fused, forming larger, complex tridimensional structures. At deeper depths, from 40 to 54 m, the abundance of rhodoliths increases and occur associated with fleshy macroalgae on a smooth seafloor; some rhodoliths are fused into complex structures, locally some are fruticose (maërl), and others are partially buried by fine-grained sediment. The collected rhodoliths vary from fruticose in two sites to encrusting to lumpy, concentric and boxwork nodules in the rest; their size ranges from small (<1.5 cm) to large (~6 cm) and are mostly sub-spheroidal to spheroidal. A total of 16 red algal morpho-taxa were identified in the study sites. Two phases of growth can be distinguished in some rhodoliths by changes in color. The brownish inner cores yielded ages of 1600–1850 cal years before the present, whereas outer layers were much younger (180–50 years BP old). Growth layers appeared to have been separated by a long period of burial in the seafloor sediment. Other rhodoliths have ages of hundreds of years.