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In recent decades, the value and utility of multibeam bathymetry and backscatter data has been increasingly recognized within the field of seascape ecology (the ocean-centric equivalent of landscape ecology) to map the bio- and geodiversity of the ocean floor. More recently, multispectral multibeam backscatter has emerged as a promising endeavor in seabed classification, and its acoustic response across multiple frequencies has been studied in a range of substrates. Coral reef systems are an under-represented seabed type within this research, as these shallow water ecosystems are more commonly mapped with optical remote sensing techniques. Further investigation is necessary to determine the extent to which backscatter data can contribute to the characterization of these habitats–especially for reef systems that are beyond the reach of optical remote sensing approaches. This study investigates the multifrequency acoustic response of coral reefs in two study areas on the Abrolhos Shelf, Brazil. Backscatter mosaics and angular response curves (ARC) are investigated to explore the potential applications of these data for enhanced reef seascape characterization. In both survey areas, the acoustic mapping revealed extensive reef features, and the assessment of acoustic response was influenced by reef surface roughness and biological attributes (algal cover). Results contribute significantly to the understanding of multi-frequency signatures in reef seascapes and highlight the potential of backscatter for mapping and monitoring the health of these ecosystems. Herein, the higher frequencies were found to be fundamental for detection of subtle variations in texture in the inter-reef region of both study areas; this underscores the value of employing these frequencies in a multispectral framework for discerning heterogeneities in coarse substrate types. Furthermore, the lower frequencies were indispensable for more precise delineation of reef characteristics in the Abrolhos Channel area, whereas the higher frequencies were the most effective in identifying reefs with epibiotic cover (those characterized by flatter ARC associated with lower backscatter values) in the Forgotten Reefs region.

Seabed mapping is currently the baseline information for ocean management and conservation, and is an indispensable item within research and Marine Spatial Planning. Here, we use a case study and published data along the southeast Brazilian continental shelf to illustrate the concepts and ideas of potential benthic habitat mapping and the importance of having maps on different scales and the potential for using a multi-design approach. The case analysis presented here uses a regional map and distinct criteria (seabed geodiversity and biodiversity, and potential influence of a tailings plume) to define three areas along the continental shelf to be mapped in a high-resolution, as part of a marine monitoring program. The background of the case study is a tailing dam failure disaster that led to the input of iron ore tailings in the ocean, leading to a major environmental disaster. Three areas were chosen, including: a Marine Protected Area (MPA Costa das Algas), a major fishing ground (the Doce River prodelta/depocenter), and an understudied reef area (Forgotten Reefs). Potential benthic habitat mapping was carried out using acoustic surveys (multibeam echosounder acquiring bathymetry and backscatter) and ground-truthing (sediment samples and seafloor image). Results detailed the potential habitats throughout the three areas, providing primary information for the study of benthic habitats and their distribution. The multi-design approach indicated that although a regional analysis can indicate potential vulnerability as a first assessment approach, high-resolution potential habitat mapping (herein, 20 m or finer) is beneficial to evaluate and produce a higher-confidence level for decision-makers. Having an overall view of what and where to collect new and finer data would be best to identify vulnerable areas that, in this case study, could be threatened by plume dispersion and tailings accumulation. Our results support the delineation of priority areas aiming for monitoring, protection and guiding for other studies, besides the consolidation of habitat importance and the integration of various datasets through a hierarchical classification scheme. Although Brazil has never produced a regional or national habitat map for the entire continental margin, there are already conditions to plan, execute and maintain a habitat mapping program, partnering with Universities, Government Institutions (e.g., the Brazilian Navy), and the private sector. In the context of the UN Decade of Ocean Science targets, it is urgent to plan and implement a national database capable of supporting a habitat classification effort on a broader national scale, with the flexibility to support a multi-design or a multi-scale approach.

There is a great demand to develop new acoustic techniques to efficiently map the seabed and automate the interpretation of acoustic, sedimentological, and imaging data sets, eliminating subjectivity. Here, we evaluate the potential, limitations and complementariety of distinct supervised and automatic classification techniques in the mapping of reefs by comparing these results with a reference map. The study was carried out in the Abrolhos Continental Shelf (Eastern Brazilian Continental Margin) using a multibeam echosounder and side scan sonar (SSS) dataset. Two automatic supervised techniques were applied. A reference map was derived by detailed manual interpretation carried out by three experts. The two supervised classification techniques were: benthic terrain modeler (BTM), a morphometric classification with focus on spatial analyses of the bathymetric grid derivatives, and object-based image analysis (OBIA), a segmentation applied to the backscatter data from the SSS mosaic. Both automatic techniques obtained similar values of reef coverage area, but overestimated the reef area when compared with the reference map. The agreement between BTM and OBIA results and the reference map was 69% and 67%, respectively. Disagreement was mainly due to quantity of reef (both methods over-estimated reef), while the disagreement in spatial allocation was relatively low, it indicates that both methods are reasonable representation of the spatial patterns of reef. Efficient mapping of reef in the wider area of the Abrolhos Continental Shelf will be best achieved by a further development of automatic methods tested against reference maps obained from representative areas of the seabed. By combining the results of the two automatic methods, it was possible to create an ensemble map, which achieved better agreement with the reference dataset.

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.

Shelf morphology and sedimentary regimes are influenced by processes operating at different temporal and spatial scales and are important records of sea level changes and sediment supply and/or carbonate production. The northern continental shelf of Espírito Santo (Brazil) contains evidence of different sedimentary regimes that distribute diverse and complex marine habitats. Herein, seabed morphology, acoustic images of the seafloor (side scan sonar and sub-bottom profiler), and sediment samples were used to investigate the influence of sedimentary regimes on physical marine habitat distribution. Seabed mapping is also integrated with available data on fisheries to understand the influence of shelf morphology and sedimentology in the usage of distinct fishing gears. The results indicate five morpho-sedimentary facies: terrigenous mud, terrigenous sand, rhodolith beds, carbonate gravel with rhodoliths, and hardground. Through an integrated analysis of the geomorphology and sedimentary distribution, two morpho-sedimentary domains were identified: a sediment-fed shelf adjacent to the Doce River associated with a major mud depocenter and a delta front morphology characterized by gentle slopes and low terrain ruggedness, and a sediment-starved shelf dominated by carbonate sedimentation showing an irregular morphology associated with higher slopes and terrain ruggedness. These contrasting morpho-sedimentary domains are a result of sedimentary responses to sea level fluctuation during Late Quaternary, specially, during the deglaciation processes after the Last Glacial Maximum. The morphological and sedimentary contrasts along the area define the physical habitat distribution. The sediment supply regime area is associated with a terrigenous fine/muddy sedimentation bed, which control the local morphology and favors coastal and delta front progradation. This physical habitat is a well-known shrimp-fishing ground where intense trawling takes place, as well as gillnet fisheries targeting weakfish and croakers. The accommodation regime or low sediment influx area is characterized by carbonate sedimentation associated with hardgrounds and rhodolith beds. In contrast, this physical habitat with scarce sediment supply, facilitates extensive benthic colonization by crustose coralline algae (CCA), which is primarily associated to line fisheries, longlines, and spearfishing. Rhodoliths show a high diversity of CCA and the occurrence of an endemic kelp species. Long-term processes such as relative sea level fluctuations and sediment supply are a legacy for the distribution of benthic habitats, and their resulting morphology can be a surrogate for predicting fishing activities or a first-base analysis for marine spatial planning. Available low-resolution bathymetric datasets can be a powerful tool, if applied with caution and in a regional scale approach. Here, terrain variables (terrain slope and ruggedness) derived from an extensive available (low-resolution and interpolated) bathymetric dataset distinguished two contrasting morphological domains characterized by rugged and smooth/flat seabeds.