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Rocas (Southwestern Equatorial Atlantic, Brazil): An atoll built primarily by coralline algae

Authors:
Ruy Kikuchi at Universidade Federal da Bahia
Ruy Kikuchi
Zelinda Leao at Universidade Federal da Bahia
Zelinda Leao
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Abstract

Rocas is an ellipsoid atoll with an internal area of 5.5 km2. Its largest axis (E-W) is 3.7 km long, and the shortest (N-S) is 2.5 km long. An algal ridge limits the reef flat, that is dominated by a coralline algae-vermetid gastropods association growing as small linear ridges. In the reef front (in some Grooves), in the pools and in the lagoon, corals (Siderastrea stellata, Montastraea cavernosa and Porites sp) are found. Seismic refraction profiles revealed the presence of two subsurface strata. A 11.6 m long drill core on the western part of the reef, with the recovery rate of 40%, shows that the Holocene sequence of Rocas was primarily built by coralline algae and, subordinately, by corals, along with some encrusting foraminifer Homotrema rubrum and vermetid gastropods. The reef growth began before 4.8 ky BP with the accretion rate varying from 1.5 to 3.2 m/ky. Subaerialy exposed old reef spits, elevated above tidal range, and a beach rock cliff in one of the cays present in the atoll are evidences of an equal to or higher than present sea level in Rocas, earlier in the Holocene. Low degree of competition for space and low grazing pressure may be the ecological reasons that promoted such a strong growth of coralline algae in Rocas.
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733
An Atoll Built by Coralline Algae
The internal structure of the reef
The results of the seismic exploration, allowed the
detection of 3 strata. A reevaluation of the data
presented in Kikuchi (1994) and Kikuchi and Leao (1994),
allowed the estimation of the mean velocities, shown
bellow:
Vo = O.33m/ms
v, = 2.50m/ms
V2 = 4.70m/ms
from the shallower to the deeper strata
Thus, the layers thickness are:
coralline algae associated with vermetid gastropods.
most of which parallel to the reef edge (Fig. 3) .The
sandy deposit is composed mainly by a coralline algae
debris, medium to fine sand-grained. More than 50% of
the fragments are algal debris. with foraminifera tests
and mollusk fragments appearing subordinated (the mean
frequency reaches about 10% each) .A shallow lagoon is
seen on the northwestern side of the reef and it
communicates with the open sea through the northern
channel (Fig. 2) .The maximum depth of the lagoon is 6
m. Residues of a former higher reef building (Fig.4)
appear at the windward arch of the atoll. more
frequentlyon the eastern part of the reef flat, but can
be found even on its southwestern part. This feature,
named old reef spits or feo in the literature
(Battistini et al. 1975) stands up 2 to 3 m above the
surface of the reef flat, in Rocas (Fig. 4) .Notches on
the spits feet (Fig. 4) indicate that the mean high
water springs (MHWS) is about 0.5 m above the reef flat
surface. There are two sand cays on the western part of
the atoll (Fig. 2) .The southern cay is called Cemiterio
and has a cross bedded beachrock cliff 1.5 m high. on
its northeastern side. The height of this cay is about 2
m above the reef flat level. A lighthouse was built on
the northern cay called Farol. This cay is about 3 m
higher than the reef flat level and there is not
beachrock developed on it.
The reef surface is mainly covered by soft algae and an
association of living coralline alga and vermetid
gastropods. A study of the corallines by Gherardi (1995)
indicates the occurrence of the genera Spongites, and v.
Testa (personal communicatien) mentioned. still. the
occurrence of the genera Lithophyllum and Sporolithon.
Massive corals. such as Siderastrea stellata, Montastrea
cavernosa and Porites sp. only occur in protected areas.
mainly in the lagoon. within the pools and in some
grooves of the reef front.
Zo = 1.70m
z, = 10.0m
were Zo refers to the thickness of the upper low velocity
layer and z, is the thickness of the reef itself.
Consequently, the depth of the bedrock (V2), in the
investigated site, has a minimum value of 11.7m (Fig.
5) .
composition and aqe of the reef
A core hole was drilled to a depth of 11.69m, with a
recovery rate of 40\ (Fig. 5) .Encrusting coralline
algae was the most abundant organism found in the core
samples, forming more than 60\ of the recovered rock
(Figs. 5 and 6) .Subordinated, the corals Siderastrea
stellata, Favia gravida, Mussismilia hispida, Agaricia
sp and Porites sp made up about 10\ of the rock.
Vermetid gastropods and the encrusting foraminifer
Homotrema rubrum are responsible, each of them, for
about 6\ of the core and occur associated with the
coralline algae (Fig. 6) .Thin sections revealed primary
porosity filled with peloidal, fibrous and palisade
submarine cements.
The '.C ages obtained from the core are 4.86 ky BP at the
depth of 11.2 m, 4.41 ky BP at the depth of 10.5 m, 3.06
ky BP at the depth of 7 m and 0.84 ky BP at the surface
(Fig. 5, Table 1) , and they indicate that the 2.5 m/ms
layer corresponds to a Holocene reef sequence.
Consequently reef growth, at the cored site, must have
begun at about 5 ky BP and grew up to present sea level
with an average accretion rate of 2.8 m/ky (from 1.5
m/ky to 3.2 m/ky, Table 2) .The age of a skeleton of S.
stellata, of 2.02 ky BP, found in life position on one
small old reef spit on the southwestern part of the
windward arch (Fig. 2), 0.5 m above the level of the
reef flat surface, indicates that parts of the reef
reached the present level of the sea at about 2.0 ky BP.
The dates yielded by the beachrock ranging from 1.91 ky
BP to 2.83 ky BP (Table 1), are concurrent with the age
of the coral on the reef spit, what reinforces the
hypothesis that some parts of the reef have already
reached the present position of sea level between 3.0 ky
BP and 2.0 ky BP.
~: Photography showing the linear feature of algal
crests on the reef flat
Table I: C"ages of coral skeletons ( Ss = Siderastrea
stellata, Fg = Favia gravidaJ and mollusk shells (mol)
from the reef surface, the core samples and the
Cemiterio island beachrock.
Location Material Conventional
,.
Cage
(ky BP)
L
reef front, 10 m deep, surface Ss recent
reef flat, 0.5 m above surface Ss 2.02 f 0.16
reef flat surface Ss 0.94 f 0.14
core top Ss 0.84 f 0.14
core, 7.0 m deep Ss 3.06 f 0.18
core, 10.5 m deep Ss 4.41 f 0.20
core, 11.2 m deep Ss 4.86 f 0.21
beachrock, 0.5 m above reef flat Fg 2.63 f 0.15
beachrock, 1.5 m above reef flat Ss 1.91 f 0.15
beachrock, 1.8 m above reef flat mol 2.51 f 0.17
beachrock, 2.0 m above reef flat Fg 2.83 f 0.16
DISCUSSION
Reef morphology
Rocas is composed of zones frequently found in the
Caribbean atolls (Kornicker and Boyd 1962; Stoddart
1962; Mil1iman 1967; Milliman 1969, table 2), despite
its reduced dimensions and formation of an semi-closed
~: Photography showing the old reef spits. on the
eastern part of the reef flat
Kikuchi and Leao
736
Reef accretion and sea level position
The oldest l.C date obtained from the core (4.8 ky BP,
Table I) may not represent the very beginning of reef
development in Rocas. There may be a difference in the
reef age between the windward arch and the leeward arch,
where the core was bored. This difference is suggested
by the following: i) the continuos and well developed
algal ridge on the windward arch, contrary to what
happens in the leeward arch, where the algal ridge is a
feeble and discontinuous feature; ii) the presence of
reef residues (old reef spits) higher than today's sea
level, on the windward arch (Fig. 4) ; iii) the ages of
the skeletons collected on the reef flat, on the old
reef spit and in the beachrock indicate a possible
difference of about 2.0 ky (Table I) between the time
when the reef surfaced at the windward part and the time
when it surfaced at the leeward part. Considering that
the reef accreted at the same average rate (2.8 m/ky,
Table 2) in the two arches, reef growth should, thus,
have begun earlier on the windward arch, say at about
6.0 ky BP (Fig. 5) .It developed as an open atoll and
the heights of the old reef spits indicate that at 2.0
ky BP the reef should have reached up to 3 m above the
today reef flat surface (Table I) .The leeward arch
began to develop at about 5.0 ky BP, with an accretion
rate increasing from 1.5 m/ky to 3.2 m/ky. Consequently,
the reef may have attained its semi-closed shape only
recently, after the leeward arch reached its present
level, after 1.0 ky BP (Table I) .
The age of the Siderastrea stellata found in life
position in an old reef spit is an evidence that the sea
level reached a position at least 0.5 higher than the
present situation at about 2.0 ky BP, what is supported
further by the ages of the beachrock sediments. The ages
of Siderastrea Btell,3ta and Favia gravida found in the
beachrock implies that there were reef building at a
position near the present sea level between 1.9 and 2.8
ky BP, in such a manner that they could be swept by
erosion and added to the beachrock.
The maximum height of the old reef spits on the windward
arch of Rocas gives an approximate idea of the maximum
relative sea level height during the Holocene. Assuming
that the tidal range of 2.7 m observed at the region
today has not changed in the past 6.0 ky BP, the maximum
Holocene mean sea-level (MSL) may have had the same
height of the old reef spits, minus half of the tidal
range, that is, around 1.2 m above the reef flat
surface. This height must have been reached before the
age yielded by the Siderastrea stellata found in life
position on the old reef spit, therefore, before 2.0 ky
BP.
ACKNOWLEDGMENTS
We would like to thank Mr. Gilberto Sales, former Chief
of the Reserva Biol6gica do Atol das Rocas, for the help
in the field work, Mrs. Myriam Abdon for the help in the
digital processing of the TM/LANDSAT images, Dr. Joaquim
Xavier for the execution of the seismic lines and Dr.
Antonio E. Azevedo for the l.C dating (Laborat6rio de
Fisica Nuclear Aplicada, Universidade Federal da Bahia) .
The TM/LANDSAT images was acquired in Instituto Nacional
de Pesquisas Espaciais (INPE) .Dr. Robert Steneck and
Dr. Viviane Testa and an anonymous reviewer provided
invaluable suggestions for the final version of this
paper. The Centro de Apoio ao Desenvolvimento Cientifico
e Tecnol6gico (CADCT) of the State of Bahia, Brazil,
funded the senior author participation on the 8th ICRS.
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Full-text available
  • May 2024
The Southwestern Atlantic (SWA) harbors a relatively species poor but highly endemic coral assemblage due to historical processes, environmental and ecological drivers. Despite its low to moderate cover, corals still have a disproportionate contribution to ecosystem function and stability in this region. In the context of global change, it is imperative to know corals’ diversity and biogeographic patterns, yet a comprehensive approach is still missing for SWA corals. We integrated occurrence data from 21 sites and nine functional traits across 20 coral (scleractinian and hydrozoan) species to explore the taxonomic and functional diversity of coral assemblages in the SWA (1°N-27°S). We identified eight regions based on coral species composition, and then described their functional diversity using four metrics: functional richness (FRic), functional dispersion (FDis), functional evenness (FEve), and functional originality (FOri). Taxonomic and functional diversity peak between latitudes 13°S-20°S, decreasing with increasing distance from this diversity center, known as the Abrolhos Bank that harbors a wide continental platform. Our findings reveal a prevalent pattern of high functional redundancy across these eight regions (indicated by low functional originality), with species occupying the edges of the trait space (high functional evenness) and converging around few trait values (low functional dispersion). Such patterns resulted in low taxonomic and functional beta diversity and increased nestedness among regions caused by dispersal barriers and environmental filtering. The Southernmost region (24°-27°S) has the lowest taxonomic and functional diversity and comprises only two species that share similar traits, with these corals being: hermaphrodites, brooders and depth-tolerant, and having a wide corallite. As this region might become critical for corals in a future tropicalization scenario, tropical corals that share similar traits to those of the southernmost region can be more likely to thrive. Knowledge on taxonomic and functional diversity patterns can offer critical information to conservation by helping prioritizing areas with higher diversity and species with traits that enhance survival under climate change.
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... Rocas Atoll (3˚52'S; 33˚48'W) (Figure 1) is located at 269.5 km east from Natal city, Northeast coast of Brazil (Granville et al., 2012), in the South Atlantic Ocean. It is located in the mid-Atlantic ridge and rises 4,000 m from the ocean floor (Kikuchi and Leão, 1997). This atoll presents two sand islands, Farol Island (FI) and Cemitério Island (CI), which are influenced by marine and terrestrial biota, waves, currents, and winds (Soares et al., 2011). ...
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... RAMBR provides full protection of all marine species, where fishing and all other anthropic impacts are banned other than for research purposes carried out under license and causing the least possible interference. Rocas Atoll (RA) is the only atoll found within the whole Southwestern Atlantic Ocean and was the first Brazilian no-take MPA, created in 1979 (Kikuchi & Leão, 1997). It is a Ramsar Site and, together with the National Marine Park of Fernando de Noronha, forms the complex of the Atlantic Islands, declared by UNESCO as a World Heritage Site. ...
Evidence of the Atlantic nurse shark (Ginglymostoma cirratum) population shrink at Rocas Atoll, Southwestern Atlantic
Article
  • Jun 2023
  • AQUAT CONSERV
The Atlantic nurse shark, Ginglymostoma cirratum , was recently listed as Vulnerable by IUCN Red List, however, as highlighted, little is known about its populations in the Southwestern Atlantic. Therefore, population studies are essential to propose status evaluations and new conservation measures. Natural marks present on the animals' bodies are used as an alternative non‐invasive method rather than the usual tagging system for individual identification, thus avoiding the need for animal handling. This study aimed to evaluate the population size trend of G. cirratum at the Rocas Atoll Marine Biological Reserve, the only atoll located in the South Atlantic. Data were collected through underwater filming, photo‐identification, and visual censuses. Three expeditions were carried out in 2018 and the abundance of sharks was sampled in the inner part of Rocas Atoll Marine Biological Reserve. Sharks were identified individually with the aid of the software Interactive Identification System – Contour (I3S). Based on the re‐sightings of identified animals, two capture–recapture estimators of population size were used (Petersen–Bailey; Jolly–Seber). The estimates were then compared to the only previous estimate made 20 years before, which employed the same number of expeditions and estimators. A total of 139 sharks were identified from 444 sightings in 63 h of diving effort. Population estimates varied from 200 to 205 sharks, which are significantly lower than the previous estimate (339–368 sharks) made 20 years ago, suggesting a demographic decline in Southwestern Atlantic populations. Population studies and movement of G. cirratum are essential to inform the implementation of additional protective measures, including the establishment of protected corridors between marine protected areas and the recovery of G. cirratum populations. It is crucial to consider that the distribution of individuals may extend beyond the borders of the non‐take marine protected area. Therefore, the observed decline in the local population could potential indicate a broader decline across the Southwestern Atlantic.
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... The reef ring is open on its western and northern faces. Despite its small dimensions, all of the characteristic features Kikuchi et al. (2023) of a reef can be distinguished, such as the reef front, reef flat and a shallow lagoon (Kikuchi and Leão 1997). ...
Reefs of the Western Tropical South Atlantic Ocean: Distribution, Environmental Impacts and Trends on Environmental Suitability Due to Climate Changes
Chapter
  • Mar 2023
Reefs that grow in the western tropical South Atlantic are known as marginal reefs because they thrive in environmental conditions (e.g., elevated turbidity) far from those considered to be the optimal conditions for framework builders (calcareous skeleton secreting organisms, such as corals). This is assumed to be one reason for the lower taxonomic richness in coral species and relatively higher endemism compared to reefs in other regions of the world, such as the Caribbean and the Pacific. These reefs, considered hot spots of biodiversity and home to turbidity-tolerant corals, are increasingly affected by local anthropogenic-driven impacts (pollution, dredging-related sedimentation, overfishing, unregulated tourism, and bioinvasions), global climate changes (warming, heatwaves, acidification, and sea-level rise) and their synergic effects. As a consequence, ecosystem goods and services are severely affected. Reef location in relation to the coastline and to urbanized areas are key points of vulnerability to the impacts of climate change. In this chapter, we synthesize the main characteristics of the distribution of reef and coralline ecosystems of the tropical southwestern Atlantic. Finally, we use three endemic coral species (Mussismilia hispida, Mussismilia harttii and Mussismilia braziliensis) as proxies of the reef ecosystem to evaluate the trend of environmental suitability across the Brazilian Tropical Marine region in the RCP8.5 scenario.KeywordsMarginal reefsCoralline environmentsCoral distributionFish diversityGlobal change
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... It has 7.5 km 2 in area and is located 145 km west of Fernando de Noronha (Almeida, 2015). Rocas is unique in the world for the predominance of coralline algae as the main reef builder, but developing typical atoll structures such as reef front, reef flat (with channels, sand cays and pools) and lagoon (Kikuchi & Leão, 1997). Along the Ceará State coast, we sampled the Mundaú reef, which is a ferruginous sandstone (beachrock) reef that emerges during low tides (Matthews-Cascon & Lotufo, 2006). ...
Two new species of Haliclona (Demospongiae: Haplosclerida: Chalinidae) from north-eastern Brazilian coastal waters and oceanic islands
Article
  • Mar 2023
Two new species of Haliclona (Chalinidae, Haplosclerida) are described here from Brazilian oceanic islands and the coast of Ceará State. They derive from collections undertaken between 1999 and 2014, and materials are deposited in the Porifera collection of Museu Nacional in Rio de Janeiro (MNRJ). Haliclona ( Halichoclona ) insularis sp. nov. can be distinguished from other Haliclona spp. in the western Atlantic by the combination of surface with light to deep red colour intermixed with white to beige areas (occasionally completely beige), white to beige choanosome, small fistular outgrowths, fragile consistency and blunt oxeas (90–175 × 1–5 μm) with irregular tips. Haliclona ( Soestella ) moraesi sp. nov. is the only western Atlantic species of Haliclona with the combination of thickly encrusting to cushion-shaped habit, purple or light pink colour, and a dense (sub)isotropic skeleton with rounded meshes, and without microscleres. Several attempts to amplify COI mtDNA and 28S C2–D2 rRNA for both species were unsuccessful, in spite of the initial successful extraction of genomic DNA. The number of Haliclona spp. known from the Brazilian coast is now 17. The long gap between species collection and formal description supports the concept of the taxonomic impediment, which is particularly marked in speciose taxa with little morphological disparity, as observed in Haplosclerida.
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... Rocas Atoll (3 50 0 S, 33 49 0 W, 7.5 km 2 ) is the only atoll in the South Atlantic, located 266 km off the Brazilian coast (Kikuchi & Leão, 1997). The semidiurnal tides with high tides reaching up to 3.8 m lead to only two small sand islands permanently emerged (Gherardi & Bosence, 2001), where the present surveys have been conducted. ...
Ocean temperature and density dependence as key drivers of the population dynamics of an intertidal crab at the Brazilian oceanic islands
Article
Full-text available
  • Apr 2022
  • POPUL ECOL
Understanding how climatic and density-dependent processes affect demography is crucial for predicting population responses to climate change. For marine invertebrates with complex life cycle such as decapod crustaceans, increasing temperatures might affect survival and development of early pelagic stages, whereas high density can increase competition and thus reduce growth and fecundity of older life stages. In this study, we investigate the effects of warm ocean events, body size and density on the population dynamics of the intertidal Sally lightfoot crab (Grapsus grapsus) at the Brazilian oceanic islands. Firstly, we assessed the trends of marine heatwaves (MHW) and positive temperature anomalies (ΔSST+) at the equatorial St Peter and St Paul (SPSP) Archipelago and Rocas Atoll and the subtropical Trindade Island. We then jointly analyzed short-term count, capture-recapture and fecundity data, and long-term population monitoring data (2003-2019) using an integrated population model. Warm ocean events have become more frequent and intense only at the equatorial islands. Increasing MHW frequency positively influenced recruitment in the high-density SPSP population, while MHW intensity and ΔSST+ frequency had negative impacts. Conversely, no climatic effects were observed for the low-density Rocas population, which has the largest crabs. Despite a lack of warming in Trindade, this subtropical population with intermediate density and body size was negatively affected by ΔSST+. Our findings revealed population-specific responses to climate change when accounting for local life history and ecology. Thus, environmental and density-dependent effects should be broadly considered in future conservation studies regarding ocean warming impacts on marine invertebrate populations.
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Bulletin JML, the of The role of and drift Southof and 43:97-145 a chegada Anais 10:362-369 of Marine de doa Hogsty Scienceof JD 1969 Cays
  • De
  • Hidricos
De Recursos Hidricos and history atoll. Bulletin JML, the of The role of and drift. Southof and 43:97-145 a chegada Anais 10:362-369 of Marine de doa Hogsty Scienceof JD 1969 Cays, Bank. LL (1940) McGraw-Hill, (1963) Tropical