Geohabitats characterization in areas of dredge sediment disposals on Rio de Janeiro continental shelf, adjacent to Guanabara Bay: Brazil

摘要

Disposal of dredged sediments from harbor areas is often carried out without prior characterization and diagnosis of the benthic ecosystem and its relationships with the physical environment. Geohabitats studies contribute to this previous characterization, showing how geological and geomorphological features are associated with the benthic ecosystems of a given location. To investigate these relationships, acoustic geophysical methods (echobatimetry, sonography, and high-resolution seismic) and direct observation methods (underwater video and geological sampling) were used in two distinct areas on the Rio de Janeiro continental shelf, adjacent to Guanabara Bay: Area I (degraded with dredging disposal) and Area II (control, without disposal). After the acquisition, processing, and integrated data interpretation, it was possible to determine 5 (five) classes of geohabitats: class I, areas with high acoustic reflectivity, with little or any presence of benthic organisms, under the direct influence of discarded material; class II, areas of high acoustic reflectivity, with predominance of quartzose sand mixed with calcareous fragments (shells), with abundance and diversity of benthic organisms; class III, areas of very high acoustic reflectivity (backscattering), with high abundance and diversity of benthic organisms, indicative of semi-consolidated seafloor (articulated algal "reefs"); class IV, areas with high acoustic backscattering, with strong presence of bed forms (megaripples), associated with the quartzose sand; and class V, areas of low acoustic reflectivity, with sand-muddy textures, low abundance, and relative diversity of benthic organisms. It was verified that the use of these noninvasive GEOHAB techniques had an excellent correlation between the obtained results and proved to be an important tool for the characterization of Geohabitats in areas of dredge sediment disposals on Rio de Janeiro continental shelf, adjacent to Guanabara Bay-Brazil.