Submerged channels of the Eastern Brazilian Continental Shelf: can the slope value be used as potential surrogates of reef environments?

Abstract

The Eastern Brazilian Continental Shelf (EBCS) is narrow, shallow and predominantly covered by biogenic sediments. Bathymetric surveys have revealed the common occurrence of submerged channels, related to the largest watersheds in the region and probably carved during periods of marine regression. With its break located between 40 and 80 metres deep, the EBCS was subjected to sub-aerial erosion for long periods, which justifies the occurrence of these erosive features related to continental drainage. Based on a DEM with spatial resolution of 10 metres, the slope values of a section of the continental shelf of the state of Pernambuco were extracted. This initiative indicated an average slope value of 0.38°, consistent with that expected for a continental shelf. However, along the edges of three submerged channels the slope is more pronounced, with values generally between 3 and 45°. Therefore, in order to verify the characteristics of these steep reliefs, 03 video-transects were performed in two sectors of the Zieta Channel with a GoPRO 3 Black. Video-transects I and II were situated about 18 kilometres off the coast, while video-transect III was about 30 kilometres. The data analysis involved the integration of the three parameters: substrate, geomorphology, and biota. The seascapes in each video-transect were defined such as: predominant, secondary or occasional. The record of combinations of substrate characteristics, geomorphology and biota occurrences also contributed to the definition of an associative pattern between the physical characteristics and the biological assemblages recorded. Extreme values of slope (20-45°) are related to seascapes dominated by large rocky outcrops, cut by small channels and with relative structural complexity, as well as fish, sponges, algae and corals. On the other hand, values of slopes between 3 and 8°, composed seascapes dominated by small rocky outcrops, with irregular surface, usually covered by algae, and seascapes dominated by sub-outcrops, associated with rocky bottoms, sponges and corals. The flat reliefs (0-3°), related to the thalweg, harboured seascapes dominated by unconsolidated substrates, with bioturbation and occurrence of dense spots of algae of the genus Halimeda. From these preliminary results, it can be suggested that the slope presented potential to be considered a surrogate for seascapes dominated by consolidated substrates and associated reef environments. This potential should be better investigated through new video surveys, contributing greatly to the consolidation of this relationship between slope, seascapes and benthic habitats, which, in turn, may guide the most appropriate design of Marine Protected Areas, as well as management measures of their fishing resources.