Nature and condition of outer shelf habitats on the drowned Açu Reef, Northeast Brazil

CHAPTER 34

Nature and condition of outer shelf habitats on the drowned Ac¸u Reef, Northeast Brazil M.P. Gomes1,2, H. Vital1,2, L.L. Nascimento Silva1,2, P.B. Eichler2,3, D. Rovira4,5 and G.O. Longo4,5 1

Post-Graduated Program of Geodynamics and Geophysics, Department of Geology, Federal University of Rio Grande do Norte, Natal, Brazil 2Laboratory of Marine Geology and Geophysics and Environmental Monitoring, University of South Santa Catarina, Palhoc¸a, Brazil 3Post-Graduated Program of Environmental Sciences, University of South Santa Catarina, Palhoc¸a, Brazil 4PostGraduated Program of Ecology, Center of Biosciences, Federal University of Rio Grande do Norte, Natal, Brazil 5Laboratory of Marine Ecology, Department Limnology and Oceanography, Federal University of Rio Grande do Norte, Natal, Brazil

Abstract This study presents the environmental boundaries and quality of the outer shelf Ac¸u Reefs in northeastern Brazil by integrating multiple approaches including satellite images, bathymetry and backscatter imagery, surficial sediments, benthic foraminifera, and underwater surveys. The outer shelf is narrow, shallow, and steep (B6 km wide; 25 80 m deep), with sand bodies, valleys (B500 m wide and 15 m of relief), reef knolls, and ridges (B4 m height). Hard coral cover can reach B17% (6 species; genus Montastrea, Siderastrea, Mussismilia, Madracis) and sponge cover was B10% (16 species). Endemic Brazilian parrotfish were common (e.g., Scarus zelindae, Sparisoma amplum), but top predators were absent. Soft sediment habitats are well correlated with 65 foraminiferal species, which comprise a Caribbean-type reef community, including Buccella peruviana evidencing upwelling and nutrient enrichment. Although the benthic habitat is good, exhibiting suitable conditions for developing reef communities, reef fish assemblages indicate a declining trend based on quantitative data and an assessment of current threats.

Keywords: Foraminifera; mixed sedimentation; hard coral; reef fish; incised valley; shelf edge; ecotone

Introduction The study examined the outer shelf environment of the Brazilian Equatorial Margin, where the Ac¸u Reef and Ac¸u Incised Valley occur (Fig. 34.1). The Ac¸u valley formed during the Seafloor Geomorphology as Benthic Habitat. DOI: https://doi.org/10.1016/B978-0-12-814960-7.00034-8 © 2020 Elsevier Inc. All rights reserved.

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Figure 34.1 LANDSAT 8 image (path-row 215/063, August of 2013) with digital processing (e.g., band math, atmospheric correction, RGB color composition). (A) Main features of the continental shelf, including the Ac¸u Incised Valley (AIV) and Apodi Incised Valley (ApIV); the compartments of Inner shelf (I.S.), middle shelf (M.S.), and outer shelf (O.S.); (a) longitudinal dunes; (b) transversal dunes; (c) beachrock chain; (d) isolated marine sand bodies; (e) Ac¸u Reefs. (B) Delimitation of the fields of the Ac¸u Reef in the outer shelf.

Pleistocene regression (Gomes et al., 2016) and the Ac¸u Reefs flourished during the subsequent Holocene transgression, with rapid vertical accretion and lateral distribution (Nascimento Silva et al., 2018). This outer shelf inherited a bedrock terrace morphology, underfilled incised valleys, and entrances of submarine canyons heads. During the shelf inundation, sediment-mixing processes started with a significant increase of carbonate production leaving only thin bodies of relict siliciclastic sands covering the modern seafloor (Gomes et al., 2014, 2015). This outer shelf physiography and the Ac¸u Reef habitats are a key area to understand far-field responses of the last sea level rise and the establishment of modern conditions for habitat dynamics. The continental shelf studied (Fig. 34.1) is the narrowest ( , 40 km) and shallowest (B70 m at the shelf break) part of the Brazilian Equatorial shelf. The shelf is the submerged part of the Potiguar Basin located in the Rio Grande do Norte State, Brazil. The distributions of sediments, seabed features, and general physiographic breaks divide the shelf into inner, middle, and outer sections, which have a clear seaward trend of increasing carbonate content (Table 34.1; Vital et al., 2008, 2010a; Gomes et al., 2014). The shelf has two underfilled incised valleys, the Ac¸u Incised Valley and the Apodi Incised Valley (Fig. 34.1), both of which have significant expression on the seafloor. The Ac¸u

Outer shelf habitats on the drowned Ac¸u Reef 573 Table 34.1: Physiography and sediments of shelf compartments. Shelf Depth compartments range (m)

Width (km)

Gradient 1:400 1:000

Inner shelf Middle shelf

0 15 15 25

20 15

Outer shelf

25 70

6

Upper slope

70 300

2

Sediment/features

Siliciclastic predominates; longitudinal dunes Mixed carbonate and siliciclastic; transversal dunes and isolated sand bodies, algal gardens 1:250 Carbonate predominates; reefs, terraces, sand banks, canyon heads 1:4 1:11 Pleistocene outcrops; submarine canyons

valley cuts across the shelf in a N NE direction. The valley is approximately 8 km wide and 12 m deep on the inner shelf and narrows to less than 1 km wide and 15 m deep on the outer shelf. Tidal currents, up to 1 m s21, are channelized within the valley and produce a large amount of suspended sediment. The valley floor is covered by terrigenous and carbonate marl, and the mud and carbonate content increase toward the outer shelf (Gomes et al., 2016). The Apodi Incised Valley exhibits two axes of NW SE and NE SW orientations and is 2 km wide and 15 m deep (Vital et al., 2010b). This valley is observed only in the inner and middle section of the shelf and is buried in the outer shelf. The region has easterly alongshore drift that is consistent with dominant shelf currents (Vital et al., 2008; Vital 2009; Gomes et al., 2016). Nearshore and offshore tidal currents have an average speed of 50 cm s21 toward the W NW direction (Vital, 2009; Knoppers et al., 1999). Near-bottom currents within the Ac¸u Valley are tide-induced, with an alongvalley orientation and a mean speed of approximately 25 cm s21. The east region of Ac¸u Reef field presents the strongest bottom flows of 42.2 cm s21, which coincides with the large (up to 3 m in height) flow-transverse dunes on the middle and outer shelves (Gomes et al., 2016). The semidiurnal, mesotidal regime dominates with a maximum range of 3.3 m and minimum range of 1.2 m during spring and neap tides, respectively. The shelf waters are generally warm with temperatures of 27 C 30 C. Oblique wave fronts, with annual mean of 60 cm, strike the shoreline from the east and the mean wave height offshore is 200 cm (Vital, 2009). The inner and middle shelves exhibit mainly soft sediment ecosystems with less abundant and less diverse macroalgae cover. The biogenic sediments consist mainly of fragments of coralline red algae and green algae (Halimeda), mollusks, foraminifers, bryozoans, echinoderms, arthropods, and sponges (Costa, 2015). Wave resuspension of nearshore sediment, together with the channelization of currents within the incised valleys, produces a large amount of sediment in suspension during most of the year and causes disturbance of the benthic biota (Gomes et al., 2016).

574 Chapter 34 This study aims to understand the role of the outer shelf physiography and features on the Holocene evolution of the Ac¸u Reefs and the surrounding soft sediment habitats. We used LANDSAT 8 images, single-beam and multibeam bathymetric charts and 3D models, maps of acoustic backscatter patterns from sidescan sonar mosaic, grab sediment samples, benthic foraminifera, and underwater surveys using transects and photoquadrants to assess reef fish assemblages and benthic cover. The outer shelf is a hotspot of biodiversity with respect to soft sediment communities and the presence of the Ac¸u Reef. According to our observations on coral and benthic communities in this present study, the Ac¸u Reef appears to be healthy. In addition, foraminiferal analysis indicates suitable soft sediment habitats with nutrient enrichment and upwelling processes. Moreover, the fishing pressure and oil exploration in the region appear to have a significant impact on the reef fish assemblages, which lack common fish schools and large-sized individuals. Therefore based on Ward (2011), we score the naturalness and conditions of the Ac¸u Reef and the inter reef habitats as “good.” According to several video records of SCUBA dive inspections since 2012, we can state with “high confidence” that the condition of the reef environment is “declining.”

Seafloor mapping methods LANDSAT 8 image (path-row 215/063, August of 2013) with digital processing (e.g., band math, atmospheric correction, RGB color composition) and a broadscale bathymetric map were used to map the broad shelf morphology (Fig. 34.1 and contour on Fig. 34.2). Regional bathymetric data were collected using a 200 kHz single-beam echosounder (Odom Hydrotrac) on the whole shelf and upper slope along N S profiles regularly spaced at 1 km. A grid cell of 200 m 3 200 m was interpolated with the ordinary kriging method (Geostatistical Analyst tool from ArcGIS). Acoustic seafloor images from 2008 to 2012 were collected along 720 km of survey track lines covering an area of 500 km2 (Fig. 34.2). A towed sidescan sonar operating at 100 kHz (EdgeTech 272-TD) was used. The classification of acoustic patterns was based on the backscatter intensity, texture, heterogeneity, or homogeneity. Based on results from sonography 76 sites were chosen for sediment sampling along 11, 10-km-long transects, aligned perpendicular to the shelf break (Fig. 34.2). The sediment samples were processed for grain size, carbonate content, organic matter content, and foraminiferal studies. Sedimentary facies classifications are based on Vital et al. (2008). Additionally, a finescale multibeam bathymetry data set was acquired over the eastern Ac¸u Reef field (Fig. 34.3). We used a RESON SeaBat 8124 echosounder (200 kHz), with survey lines aligned parallel to the shelf break. The swath width was 70 m and an area of 35 km2

Outer shelf habitats on the drowned Ac¸u Reef 575

Figure 34.2 Map of acoustic backscatter patterns (P1 P7), including the Ac¸u Reef spatial distribution (P1, green) (modified after Nascimento Silva, L.L., Gomes, M.P., Vital, H., 2018. The Ac¸u Reef morphology, distribution, and inter reef sedimentation on the outer shelf of the NE Brazil equatorial margin. Continental Shelf Res. 160, 10 22), and color symbols representing the 11 sedimentary facies. Arrows indicates submarine canyon heads. Sonographs “a” “d” are different forms of reef occurrence, “e” is a shelf ravinement, and “f” sand ripples.

was mapped. A final grid cell was 1 m 3 1 m, achieved via inverse distance weighted interpolation, and was used in a preliminary classification of the benthic zones derived from Benthic Terrain Modeler (BTM) analysis.

Geomorphic features and habitats The outer shelf of the study area is narrow, 6 km in width, with a steep gradient of 1:200. This zone is limited in a landward direction by discontinuous beachrock chains submerged at depths of from 20 to 25 m and in a seaward direction by the shelf break at the 70 m water depth (Fig. 34.1). Terraces are observed at depths of between 25 and 55 m mainly in

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Figure 34.3 Maps and 3D models outer shelf bathymetric expressions: (A) bathymetric slope map showing the main breaks on reliefs, outer shelf steps, valleys borders, and shelf break. Survey areas of sidescan sonar (Fig. 34.2), multibeam (B and C), and bathymetric profile A B are indicated; (B) digital terrain model from multibeam data showing the eastern reef field; (C) bathymetric map from multibeam. Bathymetric profile A B, insights of (C) and (D) are indicated. (D and E) Preliminary application of Benthic Terrain Modeler (BTM) analysis.

the eastern region (profile A B in Fig. 34.3). Transverse dunes on the outer shelf present crest-to-crest wavelengths varying from 500 m up to 2 km. Dunes are not apparent in between the reef fields. The dunes are low relief features with an average height of 3 m. Well-sorted sand sediments occur on dune crests, while gravelly sediments, rhodoliths, and algal gardens occur in the troughs.

Outer shelf habitats on the drowned Ac¸u Reef 577 The sediment cover on the outer shelf is predominantly bioclastic sediments, with smaller zones of siliciclastics sands (Fig. 34.2). It was mapped into seven patterns of backscatter (P1 P7) which are associated with the grain size grading of 11 sedimentary facies (Fig. 34.2). Reef (P1) displays the highest acoustic reflectivity, rough texture, and is associated with three-dimensional rocky features (Fig. 34.2A). Backscatter patterns P3 and P2 are well associated with bioclastic coarse sands with granules, nodules, and shells, while dunes and megarriples are widespread on these patterns (Fig. 34.2F). P4 is associated with bioclastic, fine and medium sands with granules on flat bottoms (Fig. 34.2E). P5 depicts the occurrence of banks of macroalgae gardens, such as Halimeda, on a generally flat bottom with fine to very fine sands of mixed carbonate siliciclastic sediments. They produce a low and heterogeneous backscatter pattern. Backscatter pattern P6 occurs on flat bottoms with ripples and megaripples and is to some extent associated with sedimentary facies consisting of bioclastic coarse sands with granules, siliciclastic medium and fine to very fine sands, and biosiliciclastic fine to very fine sands. P7 represents the flat bottom of the valley floor, which is covered by bioclastic fine- to very fine-grained sands. The Ac¸u Incised Valley (AIV) was formed in the last fall in sea level (Gomes et al., 2016) and it extends over 43 km from the present-day Ac¸u River to the submarine canyon at the continental slope (Fig. 34.1). On the outer shelf the drowned valley is 8 km long, very straight, S N oriented, less than 600 m in width, V-shaped, and with relief up to 30 m. It is asymmetric in cross-section with a gentler gradient on the west flank (Fig. 34.3A). The AIV channelizes tidal currents having speeds of up to 50 cm s21, constrains the sediment within the valley from escaping, mainly carbonate fine to very fine sands, and serves as a barrier to regional easterly transport of shelf sediment (Gomes et al., 2016). The Ac¸u Reefs occur at the transition of photic to mesophotic zones in the depth range of 25 55 m (Fig. 34.2). Reefs cover 100 km2 of the study area. The Ac¸u Reefs are distributed in three fields on either side of the Ac¸u Valley (Fig. 34.1B). The multibeam bathymetry displays two terraces, one at approximately 30 m water depth and other at 40 m (profile A-B in Fig. 34.3C). The terrace margins occur as linear ridges, orientated parallel to the shelf break and extending for a distance of tens of km. They are also associated with scattered mounds/knolls having various diameters of several tens of meters and average heights of about 4 m above the level of surrounding seabed. Some reef pinnacles reach up to 10 m in height (Fig. 34.3B). A number of reefs show evidence of sediment erosion with depressions surrounding the reef. Reef mounds aggregate forming large tables with an irregular surface (Fig. 34.3). Preliminary results from BTM analysis classified the reefs into five relief classes: very flat bottom, flat bottom (associated with a variety of soft sediments), reef steep flanks, sharp reef edge, and rugged reef top (Fig. 34.3D and E).

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Biological communities Underwater surveys of reef habitats We assessed reef fish assemblages and benthic cover across nine reef locations within the incised valley, at water depths varying between 17 and 29 m. Diving in the area is restricted to the months of June to October, when navigation is safer and water visibility increases (up to . 20 m). We assessed the reef fish assemblages using underwater visual census (40 m2 belt transects; sensu Floeter et al., 2007) where a diver swam unrolling a 20 m measuring tape while counting, identifying, and estimating the size of every fish observed within 1 m of each side of the tape (20 m 3 2 m transect). A second diver swam along the same transect and took standardized 25 cm 3 25 cm photoquadrats of the benthic substrate every meter. All photos were analyzed using PhotoQuad 1.4 software to distribute 30 points on each image (Trygonis and Sini, 2012) and identify the benthic organisms under each point. Organisms were identified to the highest taxonomic resolution whenever possible and grouped in functional categories (Littler and Littler, 1984; Foster et al., 1991; Steneck and Dethier, 1994; Bell and Barnes, 2001).

Reef benthic cover and fish The reefs are dominated by foliose algae (49.93% 6 2.8% of total cover) and algal turfs (27.77% 6 1.78% of total cover; sensu Connell et al., 2014); followed by sponges (9.39% 6 0.73%) and corals (8.29% 6 1.25%). Coralline algae, cyanobacteria, filamentous algae, hydrozoan, zoanthids, and sand spots add up to 5% of total cover (Fig. 34.5A). Reefs located on the flanks of the Ac¸u Incised Valley presented higher coral cover (17.04% 6 1.7%; permutation based ANOVA P , .01) than the adjacent areas, indicating good conditions for hard coral development. A total of six species of scleractinian corals were recorded in the valley (Agaricia agaricites, Agaricia fragilis, Madracis decactis, Montastrea cavernosa, Porites sp., Mussismilia hispida, Siderastrea sp.) and one hydrocoral (Millepora brasiliensis); and 15 species of sponges (e.g., Agelas dispar, Aiolochoria crassa, Amphimedon aff. compressa, Aplysina fulva, Aplysina lacunose, Callyspongia vaginalis, Ircinia felix, Monanchora arbuscula, Plakinastrella microspiculifera, and five others that are pending proper taxonomic identification) (Fig. 34.4). There was a high prevalence of foliose algae within the genera Canistrocarpus and Dictyota. Overall benthic communities and corals appeared healthy with no signs of extensive coral bleaching or diseases. A total of 56 reef fish species within 20 families and eight functional groups (sensu Longo et al., 2014) were recorded. Most fish prefer areas with higher coral cover and deeper waters, avoiding high cover of cyanobacteria. Parrotfish and surgeonfish (Acanthurus chirurgus, Scarus zelindae, Sparisoma amplum, and Sparisoma frondosum) comprised

Outer shelf habitats on the drowned Ac¸u Reef 579

Figure 34.4 Underwater photos of the benthic macrofauna found on Ac¸u Reefs. Corals: (A) Montastraea cavernosa; (B) Porites astreoides; (C) Siderastrea sp. Sponges: (D) Callyspongia vaginalis on right side and, on left, M. cavernosa and Mussismilia hispida; (E) Aiolochroia crassa; (F) Ectyoplasia ferox; (G) Agelas dispar; (H) Ircinia felix; (I) Amphimedon compressa; (J) Monanchora arbuscula. Site view: (K) Halimeda garden; (L) Globose reef top.

most of the fish biomass (scrapers; 49.86% 6 3.85%), followed by mobile invertebrate feeders (mainly the labrid Bodianus rufus and haemulids; 20.75% 6 2.87%), macrocarnivores (Cephalopholis fulva and lutjanids; 13.29% 6 1.94%), planktivores (Clepticus brasiliensis, Chromis multilineata, and Chromis scotti; 5.81% 6 1.22%), and spongivores (Holacanthus ciliaris, Holacanthus tricolor and Pomacanthus paru; 4.84% 6

580 Chapter 34

Figure 34.5 Dispersion plot of (A) benthic cover of the main groups and (B) reef fish biomass per functional group. Black lines indicate the average and the thickness of the shadowed areas represent the density of point in that area of the graph. Abbreviations in (B): mobile invertebrate feeders (Mobile Inv. Feeders); carnivores (Carniv.); planktivores (Planktiv.); spongivores (Spongiv.); piscivores (Pisciv.); sessile invertebrate feeders (Sessile Inv. Feeders); omnivores (Omniv.).

1.38%); also there was a low abundance and biomass of sessile invertebrate feeders (mostly Chaetodon ocellatus, but also Chaetodon striatus) and omnivores (e.g., Canthigaster figueireidoi; Fig. 34.5B). Territorial herbivores were abundant but had low biomass due to their small body size (Stegastes fuscus). Piscivores (e.g., Mycteroperca bonaci) were rare and recorded only at deeper reefs on the valley wall, likely because of increased fishing pressure in the region. Despite the complexity and plentiful benthic resources provided by the Ac¸u Reefs, the reef fish assemblages lack large-sized individuals such as sharks, groupers, and snappers. During our field surveys, we found fishing gear entangled in several areas of the reefs, indicating a high fishing pressure in the region. Reef structures are present but there were few fish inhabitants. Despite its recent discovery and novel biological description, the Ac¸u Reefs are already threatened by unregulated fishing activities with the added threat of oil exploration in their surroundings. Therefore, if we aim to understand more about these reefs and their dynamics, the need for conservation and management actions is imperative.

Foraminiferal assemblages and sediments Foraminiferal analysis methods The samples for foraminiferal study were collected at 76 sites. In each sample the uppermost layer of the sediment sample (about 5 mm) was scraped off and stored in ethanol. A solution of Rose Bengal in ethanol was used for staining live specimens. After staining for 48 hours, a fixed volume of 10 cm3 of sediment was washed through a

Outer shelf habitats on the drowned Ac¸u Reef 581 0.063 mm sieve. Quantitative analysis is based on counts of living specimens. Species identification and counting of dry specimens was done under an optical microscope. Absolute and relative abundances were computed for each species. Statistical analyses included (univariate) number of individuals and of species, Shannon diversity, Simpson dominance, and evenness patterns. Multivariate analyses were applied to both environmental [principal components analysis (PCA)] and foraminiferal data [nonmetric multidimensional scaling (MDS)] using a computer program (PRIMER), described in Clarke (1993) and Clarke and Warwick (1994). Biological matrices were constructed using the Bray Curtis similarity measure on log (x 1 1) to normalize foraminiferal counts. Abundance data were calculated for each foraminiferal species, and were subjected to Q-mode cluster analysis to define foraminiferal assemblages. The Bray Curtis distance measured proximity between samples, and Ward’s linkage method arranged samples into a hierarchical dendrogram. Environmental matrices were constructed using the Euclidian distance similarity measure to normalize variables. BIOENV correlation analysis explored relationships between the abiotic parameters with the main species and ecological indices at a P , .05 significance level. The Spearman rank correlation method was used.

Foraminiferal results We observe that Quinqueloculina lamarckiana, a calcareous robust form tolerant of high hydrodynamics, is the most dominant species. The abundance of this species is followed by Amphistegina gibbosa, Archaias angulatus, and Peneroplis carinatus. These three last species together with Amphisorus hemprichii, Heterostegina antillarum, Laevipeneroplis proteus, and Peneroplis sp., are true reefal forms found in carbonate environments. There are also species tolerant to high organic matter levels (Bolivina striatula, Pseudononion atlanticum, Quinqueloculina patagonica, and Elphidium articulatum), or that thrive in oxygenated waters (in the case of Hanzawaia boueana) and the warm water indicator Pyrgo sp. The presence of Buccella peruviana, a known cold-water species (Eichler et al., 2014) in deeper parts of the study area as well as in shallower areas indicate the way that cold waters are upwelled and penetrate the submerged reefal canyons and onto the shallower parts of reefs. The distribution of Textularia earlandi and P. atlanticum are similar, showing a preference for low carbonate and low silt environments. A. angulatus and A. hemprichii prefer medium to very fine sand, A. gibbosa and P. carinatus proliferate where there is more silt, and B. peruviana and Q. lamarckiana share a similar preference for high organic matter, carbonate, gravel, and silt. In general, we observe that A. angulatus is competing with A. gibbosa for organic matter, carbonate, gravel, and silt. However, A. gibbosa proliferates where there is more silt and A. angulatus prefers medium to very fine sand.

582 Chapter 34 T. earlandi correlates well with gravel and P. atlanticum and Quiqueloculina patagonica have positive correlation with high organic matter levels. B. peruviana, Q. lamarckiana, and T. earlandi dominate where organic matter, carbonate, gravel, and silt have higher levels. Competition occurs between A. angulatus and A. gibbosa for high organic matter, CaCO3, silt, very fine sand, fine sand, silt, and gravel, whereas A. gibbosa proliferates where there is more silt, and A. angulatus prefers medium to very fine sand (Fig. 34.6). A. hemprichii and A. angulatus share the preference for medium to very fine sand, P. carinatus and A. gibbosa for silt, and T. earlandi and P. atlanticum show the preference for low CaCO3 with low silt environments. P. atlanticum and Q. patagonica are specific correlated with high

Figure 34.6 Distribution of Archaias angulatus, Amphistegina gibbosa, and Buccella peruviana on the principal components analysis (PCA) of the environmental factors.

Outer shelf habitats on the drowned Ac¸u Reef 583 organic matter levels. This assemblage indicates the presence of an upwelling signature on the sediment water interface, sediment nutrient enrichment, and a high potential environmental quality.

Surrogacy A quantitative examination using a foraminiferal approach indicated a strong correlation of soft sediment habitats and benthic microassemblage distribution. The spatial and physical dependencies of soft sediment habitats surrounding the reefs were investigated using a foraminiferal approach through multivariate methods applied to both environmental and foraminiferal data sets, with PCA, and foraminiferal data, with MDS. The abiotic variables explain approximately 70% of the sample variability in the studied area (Table 34.2). The BIOENV and PCA analysis between abiotic and biologic matrices revealed that organic matter, carbonate, gravel, and very coarse sand are positively correlated and coarse, medium, fine, and very fine sand and silt are negatively correlated with the distribution of benthic foraminifers (Table 34.3). Examples for correlation between foraminifer’s species and sediment microhabitats revealed that the distribution of A. angulatus and A. gibbosa are related to organic matter, CaCO3, silt, very fine sand, fine sand, and silt, and B. peruviana is mostly related to organic matter, CaCO3, and silt (Fig. 34.6). Table 34.2: Eigenvalues, % variation and cumulative variation of PCs. PC 1 2 3 4 5

Eigenvalues

%Variation

Cum.% variation

3.49 2.64 0.817 0.736 0.58

38.7 29.4 9.1 8.2 6.4

38.7 68.1 77.2 85.4 91.8

Table 34.3: Coefficients in the linear combinations of variables making up PCs. Variable Organic matter Carbonate Gravel Very coarse sand Coarse sand Medium sand Fine sand Very fine sand Silt

PC1

PC2

PC3

PC4

PC5

0.274 0.255 0.396 0.459 0.329 2 0.066 2 0.441 2 0.366 2 0.225

0.462 0.290 0.248 0.009 2 0.381 2 0.522 0.057 0.278 0.378

2 0.141 2 0.774 0.321 0.147 2 0.056 2 0.183 2 0.166 2 0.177 0.400

0.063 0.076 2 0.197 2 0.086 0.383 0.253 2 0.549 0.434 0.494

0.150 0.050 0.496 2 0.563 2 0.313 0.502 2 0.188 2 0.165 2 0.006

584 Chapter 34 Most of the modern habitats on the outer shelf inherited depth-limited occurrence of the Ac¸u Reefs and large geomorphic features such as the Ac¸u Incised Valley and outer shelf terraces, which constrain the sediment type distribution. Furthermore, macrobenthos and fishes in the Ac¸u Reef are widely variable and complex and these habitats have been under significant anthropogenic pressure from oil exploration and fishing in the last decades. Therefore further studies are needed to properly correlate the macrobenthic marine biodiversity and the geomorphic features at a finer scale.

Acknowledgements Thanks are due for the funding provided by the following projects: Cieˆncias do Mar II-CAPES 23038.004320/ 2014-11, IODP-CAPES 88887.123925/2015-00, and INCT AmbTropic Brazilian National Institute of Science and Technology for Tropical Marine Environments 565054/2010-4, 8936/2011, and 465634/2014-1 (CNPq/ FAPESB/CAPES). Brazilian National Council Research—CNPq as a researcher fellowship to H. Vital (Grant PQ 311413/2016-1). Thanks to the Brazilian Navy for cruise support and the Lab GGEMMA for the equipment and team.

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