Chapter 30 - Submerged reefs in the Abrolhos Shelf: morphology and habitat distribution

Abstract

Reef morphology varies across the depth gradient of tropical shelves and the controlling drivers operate at different geological timescales. For instance, while Holocene sea level rise drowned wide expanses of offshore reefs, modern disturbance (i.e., hurricanes/storm regime) shapes coral and coralline reef morphology. Here we report a regional scale acoustic mapping using a high-resolution sidescan sonar along the Abrolhos Shelf (South Atlantic, Eastern Brazil) mapping the spatial occurrence of submerged reefs, from 5 to 90m deep. A database comprising photographs and video footage taken by divers, ROV, and drop cameras complemented the acoustic surveys with information on finer-scale reef morphology and benthic communities. Results showed that reef structures were classified as pinnacles, reef banks, and paleovalley edges. An extensive rhodolith bed was also mapped in the outer shelf. Pinnacles and reef banks could also be classified into low- and high-relief structures. Reef morphology follows a cross-shelf trend, with high-relief structures dominating the shallow waters (shallower than 20m), while low-relief structures dominate the offshore area up to 40m deep. The outer shelf is covered by rhodoliths up to the shelf break. The benthic community changes from shallow to deeper reefs. The most significant change is in coral species richness. The transition from a reef to a rhodolith habitat is still a matter of investigation in order to understand the ecological dynamics of this change. Still, rhodoliths are known as hot spots of biodiversity. This means that more than 70% of the Northern Abrolhos shelf encompasses highly complex and diverse habitats that support high biodiversity and provide important ecosystem services, but most of the information about the structure and the dynamics of biological assemblages is restricted to the small portion of emerging reefs.