Habitat mapping in Abrolhos inter-reefal inner shelf seabed, Brazilian coast, southwestern Atlantic
Authors: Longo L L , Malachias F Z , Bastos A C 
Published: September 2017

In the Abrolhos Bank, coral reefs communities, mainly at 25 m depth, have been largely studied. On the other hand, the inter-reefal regions remain unknown in what concerns to their benthic habitats dynamics, i.e., sedimentary facies and biotic communities.
Previous studies revealed a geomorphologic and sedimentary facies mosaic of benthic habitats under terrigenous input and biogenic formations interferences in these inter-reefal regions. How the faunal composition of benthic communities in inter-reefal areas of Abrolhos Channel and external adjacent
areas respond to these interferences constituted the main goal of the present study.
Previous studies using sidescan sonar evidenced five potentials habitats: rhodoliths beds, pinnacles, reefal banks, coarse sand and fine sand, in the Abrolhos Channel and in the offshore area surround the reefs. Complementary sampling was conducted using sidescan sonar and a 3 m3 van Veen grab for
ground truthing surveys, including bottom sediment biological characterization and granulometric surficial sediment analyses. Each potential habitat proposed previously was adopted as samples domains and at least six sample units (replicates) were established inside each one for sediment grab
sampling. From the total sediment sample volume, 500 ml were utilized for particle size analyses and 1 liter was washed over 0.5 mm square mesh sieves to retain macrofauna and maintained in 4% formaldehyde solution. In laboratory, 50 ml were sorted, the specimens identified in high taxonomic
groups and quantified, using a stereoscopic microscope and taxonomic keys. The community structure of each potential habitat was analyzed using ecological descriptors and the similarity among sample units were calculated by cluster analyses. Relations among sidescan sonar backscatter, benthic
community and particle size data for each sample unit were examined using non-metric Multi-dimensional Scaling (nMDS). The analytical procedures were conducted using the PAST and Primer 7 software packages.
A total of 52 samples were analyzed and 52,075 specimens from 13 different taxonomical groups were identified. The Phylum Mollusca comprised 94.6% from the total number of individuals registered, with 60.5% for the Class Bivalvia, followed by the Class Gastropoda, with 32.18%, and
Scaphopoda, with 1.94%. For the Phylum Annelida was registered 2.04% of the total individuals and for the Phylum Arthropoda, SubPhylum Crustacea was registered 1.63% from the total individuals.
The faunal community distribution showed a very close association with sediment granulometry inside each potential habitat analyzed, nevertheless, differing in faunal groups abundances among them. These results evidenced that the inter-reefal Abrolhos Bank area is comprised of a benthic mosaic
habitat, where each potential habitat identified is composed by different granulometric sediment fractions, given similar environments among them, highlighted in faunistic composition, and in the other hand, they present peculiarities, evidenced by the faunal group abundance differences.
The granulometric analyses showed that a sedimentary facies mosaic occurred inside each potential habitat identified in Abrolhos inter- reefal area, where each habitat was composed of different granulometric sediment fractions. The faunal community composition showed a very close association with
sediment granulometry, and same taxonomical groups occurred in several potential habitats, according to the presence of their favorable granulometric fractions. Nevertheless, besides the common occurrence of some taxonomical groups in different potential habitats, their abundance differed among
these habitats, highlighting faunal groups preferences for others possible environmental features (not studied here) of each habitat. Based on these results, it was possible to conclude that, although the faunal taxonomical group's composition remained similar among the Abrolhos inter-reefal
potential habitats, their abundance differed and allowed the identification of peculiarities of each potential habitat faunal community.