Utilização de sonar interferométrico de alta resolução para identificação e diferenciação de fases construtoras e não construtoras de recifes na Baía de Todos os Santos

Abstract

Coral reefs are highly relevant ecosystems due to their role from biogeochemical cycles to the protection of the coastline and their tourist value. One of the most impactful threats to the building capacity of a reef is phase shift, a phenomenon in which hermatypic corals are largely replaced by non-calcifying organisms, mainly macroalgae and soft corals. Geophysical mapping was carried out in 3 reef areas of Todos os Santos Bay, namely Pedra Alva, Poste IV and Mangueira. Bathymetric, sonographic and acoustic backscatter level maps were generated. The reef outcrops vary from 3m deep in the shallowest reef (Poste IV) to 10m at the top of the deepest reef (Pedra Alva), where depths in the sedimentary surroundings reach up to 34m. The elevation profile of the Poste IV reef was the one with the least topographic irregularity and also where the greatest coverage of non-building organisms such as algae and soft corals was calculated. Through sonographic imaging it was possible to delimit the reef formations and detect textural changes in the sedimentary areas due to the difference in reflectivity in the mosaic. From the Backscatter mosaic it was possible to segment the areas into different acoustic classes, from which validation points were carried out, including underwater photos, video transects and sediment collection. With this ground truthing it was possible to calculate the percentage of coverage of building and non-building organisms, as well as determining the predominant grain size classes in the sedimentary areas. Despite all reefs having high percentages of coverage of non-building organisms (above 70%), the Pedra Alva reef still has a significant coverage of hermatypic corals, which may be indicative of a construction phase. The levels of acoustic backscatter in the reef areas did not show greater intensity than some sedimentary points in their surroundings, most likely due to the large coverage of non-building organisms, which directly affects the roughness and hardness of the bottom.