Heterogeneity of rhodolith beds expressed in backscatter data

Rhodoliths play a major role as benthic habitats and are important calcium carbonate bio-factories. Several studies have acoustically mapped rhodolith beds as a homogeneous habitat but spatial changes in nodule coverage, or density, commonly occur. Here, the objective is to acoustically recognize heterogeneities in rhodolith bed structure using the response of backscatter to map changes in nodule coverage and density. High-resolution multibeam sonar (MBSS) data were acquired on April 2018, in the Marine Protection Area (MPA) Costas das Algas, located in the Espirito Santo continental shelf, Eastern Brazilian coast. Three areas were selected based on available data, totalizing 73km(2) of data acquisition. Ground truth was obtained at 80 stations, using a drop camera system. Seabed classification was determined by applying an image segmentation process on multibeam sonar backscatter mosaic. Ground truth data was used to statistically show the relation between backscatter and seabed types. Results identified three classes related to rhodoliths in different coverage densities: low rhodolith coverage (inferior to 25% of rhodoliths over a determined area), moderate rhodolith coverage (between 25% and 35% of rhodoliths), and high rhodolith coverage (> 35%). Classes associated with sediment, bioconcretions and red algae genus Peyssonelias were also identified. A map showing changes in rhodolith coverage across the shelf depth gradient were produced. The seabed classes had a statistically significant correlation with the segmented backscatter classes. In general, higher rhodolith coverage correlates with higher backscatter. The backscatter mosaic over the area is marked by an extensive presence of the red algae Peyssonelias, resulting in a lower correlation index, when compared to the areas with no Peyssonelia. The seabed mapping approach used in this work provided an efficacious tool to map benthic habitats and to detail nodules distribution across a rhodolith bed. Moreover, the recognition of heterogeneities in rhodolith bed structure provides important information for monitoring and protecting these vulnerable marine ecosystems. The main driver of the coralligenous algae nodule coverage, in the research area, seems to be the morphology of the area and the influence of mesoscale circulation.