Geological and Paleontological Sites of Brazil - 090
ZELINDA MARGARIDA DE ANDRADE NERY
Universidade Federal da Bahia, Centro de
Pesquisa em Geofísica e Geologia, Instituto de Geociências, Laboratório de Estudos
© Leão,Z.M;A.N. 1999. Abrolhos - The south Atlantic largest coral reef complex. In: Schobbenhaus,C.; Campos,D.A.; Queiroz,E.T.; Winge,M.; Berbert-Born,M. (Edit.) Sítios Geológicos e Paleontológicos do Brasil. Published 22/11/1999 on Internet at the address http://www.unb.br/ig/sigep/sitio090/sitio090english.htm [Actually http://sigep.cprm.gov.br/sitio090/sitio090english.htm]
(The above bibliographic reference of author copy rights is required for any use of this article in any media, being forbidden the use for any commercial purpose)
The Abrolhos reefs, off the coast of southern Bahia, are the largest and the richest coral reefs of Brazil, and they are significantly different from the well-studied coral reef models. These differences are in the reef morphology, surrounding sediments and reef-building organisms. The reefs form two arcs that occupy a total area of approximately 6,000 km2. The basic element of most reefs is the "chapeirão", a mushroom-shaped pinnacle, 5 to 25 m high and 5 to 50 m in diameter. In the Coastal Arc, the top of adjacent "chapeirões" coalesces to form bank reefs, 1 to 20 km long, with varied shapes. These bank reefs do not display the well-marked zones of the North Atlantic reefs. Well-developed algal rims developed on the windward edges of the reefs, like those in the Pacific reefs. The Outer Arc has fringing reefs surrounding volcanic islands and "chapeirões" that do not coalesce. Corals, millepores and coralline algae are the major framebuilders of the reefs. The number of coral species is four times less than that of the North Atlantic reefs and they are dominantly archaic, endemic species that are the combined result of isolation of a late Tertiary community and the stress of periodically high turbidity of the Brazilian waters. In contrast with the predominance of carbonate sediments surrounding most reefs in other tropical seas, the coastal reefs of Abrolhos are surrounded by muddy sediments, which contain 40 to 70% quartz sand and clay minerals.
The Abrolhos reef complex is the most extensive area of coral reefs in Brazil and of all the South Atlantic Ocean, and this aqueous part of the planet harbors less than one percent of the reef ecosystems around the globe. Thus, besides of being rare in the world and the most exuberant in Brazil, the Abrolhos reefs have an unchallenged scientific importance. Because they present distinctive characteristics with respects to growth form and morphology, reef building fauna and depositional setting, they differ in some aspects from the well-studied Caribbean reef systems. But it is also clear that the Abrolhos reefs have some common characteristics with the North Atlantic reefs, and this duality, distinctly different in several aspects but similar in others, means that they can provide important insights into the variations of the North Atlantic reefs, as well as further comparison with the Pacific examples. The reefs form structures with a characteristic growth form of mushroom-shaped pinnacles called "chapeirões", which are build by a coral fauna rich in endemic species that thrive in an inhospitable muddy environment.
The characteristics of the sedimentary setting of the Abrolhos reef area, an example of association of siliciclastic and carbonate deposition, the dynamic interaction that governs the coexistence of a coral reef system with an active terrigenous sedimentation, and the fact that the first National Marine Park of Brazil (Fig. 1) offers protection to a unique community of marine beings, allow us design Abrolhos as a Geobiological Site.
The first scientific reports on the coral reefs of Abrolhos date from the nineteen-century and resulted from visits of pioneer naturalists to Brazil. Among them there are the reports from the Darwin visit to the reefs around the islands of the Abrolhos Archipelago. However it was the Thayer Expedition, leaded by Louis Agassiz, that produced the seminal work by Charles F. Hartt, Geology and Physical Geography of Brazil, which is the first detailed description of coral zonation on the reefs of Abrolhos (Hartt 1870). The corals collected during Hartts expeditions to the reefs were described and classified by Verrill (1868). In the sixties the French Biologist Jacques L. Laborel published several papers on the biology of the Brazilian corals, and produced a complete list of the major reef organisms along the whole coast of Brazil, including the Abrolhos reefs (Laborel 1969a, 1969b). In the last decade an increasing number of scientists were engaged on more detailed surveys of the reef environments of Brazil, particularly mapping the reef areas and providing data on several aspects of the reef communities. Several works described geologic aspects of the reefs such as the papers by Leão (1982), Leão & Ginsburg (1997) and Leão et al. (1988). Contemporaneously, numerous specific taxonomic groups of the Abrolhos ecosystem were described, particularly the coral and hydrocoral fauna, in Amaral (1994), Belém et al. (1982, 1986), Castro (1994) and Pitombo et al. (1988), octocorals in Castro (1989, 1990a, 1990b), reef fishes in Nunam (1979) and Telles (1998), mollusks in the papers from Petuch (1979) and Rios and Barcellos (1980), and the marine flora in Amado-Filho et al. (1997), Coutinho et al. (1993) and Figueiredo (1997).
Figure 1 Aerial view of the Abrolhos Archipelago illustrating the five islands with finging reefs. At the left top the Santa Barbara Island with the small Guarita to the west. At the bottom the Redonda (left) and the Siriba (right) islands. The right top illustrates the Sueste island (photo courtezy of Marcelo Skaf)
The Abrolhos reef complex herein refers to a group of coral reefs, volcanic islands, sand shoals and surrounding channels which occupy an area of approximately 6,000 km2 in the northern part of the Abrolhos Bank (between the coordinates 17o20-18o10S and 38o35-39o20W) (Fig. 2). The Abrolhos Bank is an enlargement of the southern part of the Eastern Brazilian continental shelf, which is irregular in width and generally narrow (average width 50 km). Only off Caravelas does the shelf extend up to 200 km.
Around the Abrolhos reefs the shelf is very shallow. Depths do not exceed 30 m and the shelf edge is only about 70 m deep. The average shelf slope is in the order of 0o08. Depths between the coastal reefs and the coastline are less than 15 m. A deeper channel (Abrolhos Channel) (20-30 m) separates the coastal reefs from the Abrolhos Archipelago and the outer reefs. Sand shoals and isolated coral pinnacles surround the reefs and the islands, and sandbanks and sandbars are present at the mouths of the rivers (Fig. 2).
Figure 2 Location map of Abrolhos coral reefs.
Climate and oceanography
The coastal belt along Eastern Brazil has a tropical humid climate with average air temperature ranging from 24oC during winter to 27oC in summer. July is the coldest month of the year and March is the warmest. Annual mean precipitation in the coastal region facing the Abrolhos area is 1750 mm. March, April and May are the rainiest months, concentrating 35% of yearly precipitation (612 mm) (Nimer 1989).
The Abrolhos site is located at the southern part of the trade wind area. This wind system has two main directions: NE and E during spring/summer (October to March) and SE during fall/winter (April to September). This is caused by the northward migration of the South Atlantic anticyclone cell in the summer and southward in winter (Nimer 1989). During winter, the northward advances of polar cold fronts enhance the SE winds and add a SSE component to the atmospheric circulation.
Average monthly sea surface temperature ranges from 24.5° C in August
to 27.5° C in March (US Navy 1978). An analysis of the monthly anomaly of SST during the
years of 1980-1984 by Servain et al. (1987) showed that during the strong El Niño
event of 1982-1983, temperature anomalies were up to 1.5° C. Data provided by Dr. Alan
Strong, from the National Oceanic & Atmospheric Administration, National Environmental
Satellite Data, and Information Service (NOAA/NESDIS), for the years of 1997 and 1998,
available at the website:
show that during the 1998 El Niño event, daily temperature anomalies raised up to 1° C in the Abrolhos area.
There are two main wave trains in this area that coincide with the wind regime. The spring/summer, from October to February, is dominated by NE-E waves with significant height of 1 m and period of 5 s (US Navy 1978), which produces a longshore sediment transport in the shoreface toward the south, north of Baleia Point (see Fig. 2). The fall/winter, from March to September is dominated by SE-SSE waves, with significant heights of 1.5 m and period of 6.5 s (US Navy 1978), and this wave train produces a northward longshore sediment transport in the shoreface, south of Baleia Point.
Tides are semidiurnal, with maximum height of 2.3 m during spring tide and minimum of 0.5 m during neap tide. Tidal wave travels from south to north, with a time lag of 1 hour and 45 minutes from Mucuri (30 km south of Nova Viçosa Town) to the Abrolhos islands and Cumuruxatiba (about 30 km north of Prado Town).
The Brazil Current waters flow over the Abrolhos Bank with a general north to south direction. During a short time experiment, Meyerhöfer & Marone (1996) show the importance of the tidal signature superimposed on the Brazil Current flow. At the Abrolhos Channel the average velocities during this experiment were 19 cm/s at the surface and 13.1 cm/s near the bottom, flowing along-channel (SSW). Cross channel components have the same magnitude. On the other hand, a station at the Caravelas Channel (between the coastline and the Coastal Arc), shows that although tidal signature is conspicuous, as well, the along-channel velocity component is much more important than the cross-channel. The former may reach 55 cm/s while the latter does not exceed 10 cm/s, this suggests that material exchange between the two reef arcs is more significant than between the coast and the reefs.
Morphology of the coastline
The northernmost part of the coastline adjacent to the reefs is cut by a long stretch of vertical low cliffs, alternating with steep green slopes and occasional patches of sand or marsh environments. The Jucuruçu River (Fig. 2), taken as the northern limit of the described area, reaches the coast by flowing southward behind a beach-ridge. The sediment at the mouth of this river is a mixture of unsorted quartz grains with mollusk shells from different environments: marine, brackish and mangrove associated communities. From the mouth of this river to the Baleia Point (middle part of the area) the shore is a long monotonous sandy beach, broken by one river (the Itanhem River), which bends abruptly to the south, flowing for a few kilometers almost parallel to the coast. The Baleia Point is the result of coastal outbuilding, which was probably produced by the confluence of the longshore drift currents with the complicated hydrographic pattern, resulting from the presence of the reefs close to shore. Extensive sandbanks are visible even at high tides in front of Baleia Point (Fig. 2). Southward, in the area between the Caravelas and Peruipe rivers, tidal channels extend parallel to the coast through an extensive mangrove swamp. The water nearshore is shallow, calm and rather turbid. Reefs adjacent to this section of the coast dissipate the energy of the ocean waves.
The shelf sedimentary facies
The surface of the inner shelf of the Abrolhos Bank is flat and smooth. Narrow channels cut the middle and outer shelves and sandbanks are common. The former topographic features were built during the last Pleistocene Regression when the Abrolhos shelf was subaerially exposed and its surface was deeply eroded by a fluvial drainage system discharging into the Abrolhos Depression (southern part of the bank), and terrigenous sediments were deposited. Later, this terrigenous sedimentation was replaced by a marine biogenic carbonate deposition (Vicalvi et al. 1978). These carbonate sediments are concentrated mainly on the middle and outer shelves and extend into the inner shelf around the reefs. Encrusting coralline algae dominate this sediment composition. Coarse sediments dominated by bryozoan debris are abundant in the southern part of the shelf. A well-developed fauna of mollusks and benthonic Foraminifera is present in the muddier areas. Coral fragments occur in the coastal zone. Siliciclastic sediments are confined to the inner shelf (Leão 1982).
Reef growth form
Coral reefs in Abrolhos grow over the bottom like towers, and often attain the low water level. These coral pinnacles are usually very irregular in shape, and their tops expand laterally like mushrooms. Hartt (1870), who used the local name "Chapeirão" (pl. chapeirões) first, described these coral structures. There are chapeirões of all heights and dimensions (height <1 to >25 m, diameters <1 to >50 m) and in all stages of growth. In plain view they are roughly circular or, less often, elongate. In three dimensions they are mushroom-shaped, with their overhangs often more pronounced in the windward side (Fig. 3). Small columns have a mushroom form from their initial stage of growth and may be smaller than 1 m in height and 1-2 m in diameter across the top, as well as a single colony of the coral Mussismilia braziliensis with diameters of no more than 20 cm already presents the form of a small mushroom. In a fully developed chapeirão that can reach a height of about 25 m with a surface area of about 50 m, the growth of the mushroom head is accentuated when these reefs columns reach sea level. This mushroom-like growth form of the Abrolhos reefs is not a common growth form of coral reefs in other parts of the tropical seas.
Figure 3 Sketches of the mushroom-like growth forms of the "chapeirões". Left a cross section of an isolated "chapeirão" and to the right coalescent "chapeirões" (illustration courtezy of Viviane Testa).
Major reef types
The coral reefs in Abrolhos are distributed into two arcs almost parallel to the mainland shore. The Coastal Arc is located from about 10 to 20 km off the cost, and is formed by a complex of bank reefs and isolated coral pinnacles of varied dimensions. The Outer Arc, which borders the east side of the Abrolhos Islands, is formed by isolated giant coral pinnacles, located circa 70 km from the coast (see Fig. 2).
COASTAL ARC Where the chapeirões are closely spaced they can fuse together at their tops, forming large compound reef structures called bank reefs. The smaller bank reefs are the result of the coalescence of only few chapeirões, whereas the larger structures, as for example the Pedra Grande Reef of the Parcel das Paredes (see Fig. 2), which is about 17 km long, are formed by the coalescence of numerous chapeirões and the infilling of open spaces at the upper part of the reefs by biogenic sediment. In the deeper parts, though narrow channels are still open. These reef structures do not form a classical barrier reef, they are instead isolated shallow bank reefs with varied shapes and dimensions (Fig. 4).
Figure 4 Aerial view of a bank reef from the coastal arc of Abrolhos, surrounded by turbid waters after a storm.
FRINGING REEFS OF THE ABROLHOS ISLANDS The fringing reefs are not remarkable coral constructions, but are rather a veneer of reef organisms growing on a stable substrate (volcanic or sedimentary rocks) (see Fig. 1). They are formed by the growth of corals and cementation by coralline algae and other encrusting organisms, as well as the infilling of the cavities with carbonate cemented sediment. There is a reef flat that extends about 50-60 m from shore, and their height do not exceed 5 m, from the bottom to low water level. Most of the island slopes, however, are covered with an encrusting reef community that is composed with the same organisms that build a reef, but the limestone does not exceed a few centimeters.
OUTER ARC About 5 km east of the Abrolhos Archipelago, there is an area of abundant coral pinnacles called the Parcel dos Abrolhos, which extends about 15 km N-S and 5 km E-W (see Figs. 1 and 2). These reefs are isolated coral pinnacles surrounded by water depths usually exceeding 20 m. Thus they do not form the bank reefs as it is seen in the coastal arc of reefs. Moreover they are not exposed during low tides. The vertical distribution of corals on the chapeirões walls show a zonation from photophile forms in their upper parts to the shade-loving species (sciaphile) in their lower parts, which attain maximum development under the chapeirões overhang.
Three zones are distinguished in the reefs: the top, the edge and the wall. In this aspect they are quite different from the Caribbean reefs. Their patchiness, reduced dimensions and position on the shelf did not produce a typical back reef zone with a lagoon and a fore reef slope.
REEF TOP It is the upper part of a pinnacle (chapeirão) and of a bank reef. On isolated chapeirões it may occur at different depths within the shallower 10 m of the water column. The dominant organisms are massive corals. On bank reefs it is the intertidal reef flat, which is completely uncovered during low waters. These reef tops are somewhat irregular, with numerous small pools, some shallow and sandy, and others deep and rocky. Some are isolated from the surrounding waters during low tides, but others are open to the sea. These ponds probably mark the intervals between chapeirões that were not filled completely. Irregular, meandering channels cut the surface of these bank reefs and may connect the tidal pools. Life in the tidal pools is very prolific. Carbonate sand of skeletal debris accumulates in mounds of all sizes. A carpet of calcareous, fleshy red and green algae, as well as zoanthids mostly cover the rest of the exposed surface of the reefs.
REEF EDGE It is extremely irregular. In isolated chapeirões, it is a 2-3 m thick overhang projected from the pinnacle. In depths lesser than 10 m, impressive colonies of the hydrocoral Millepora alcicornis are seen. On banks, small isolated chapeirões border the reef structures and some of them will eventually become part of the main reef body through lateral growth and trapping of sediment in the open spaces. At the windward side, an irregular rim, built by encrusting organisms, rises about 30 cm above the flat floor of the bank reef top. Coralline algae and vermetid gastropods are the major components of this algal rim, particularly on the northernmost reefs, but millepores are also an important constituent of the border of some reefs, specially the southern reefs. The leeward side lacks the algal rim. Its irregular contour is a function of the reef growth and is not a product of erosion. Hartt (1870), describing Lixa Reef (see Fig. 2 for location) reckons the dominance of Millepora sp on the windward (east) side of this reef.
REEF WALL The endemic coral Mussismilia braziliensis, and the hydrocoral Millepora sp dominate the upper parts of the reef walls. Under the overhangs only the small sciphiale forms of corals develop, the species that either prefer or thrive in dark environments. The deepest part of the reef is dominated by infilled spaces that form interconnected underwater galleries that are large enough for even a diver to pass through. Towards the bottom, the muddy floor falls away to depths of 10 to 20 m. On the muddy bottom between the chapeirões, marine grass grows prolifically and alternate with the calcareous green algae Penicillus, Halimeda and Udotea and the free form of the coral Meandrina braziliensis.
The overall biotic communities comprising the Abrolhos reef complex are not well known, yet. Among the most representative components of this system are the cnidarians and belonging to them, the hermatypic corals have a fundamental importance. Besides being the major reef framework building organisms, they are also producers of organic material, along with their simbiont zooxanthelae micro algae. The reef macro algae are important too, because as primary producers they are on the beginning of the energy flow within the system. Reef fishes, marine turtles and man himself are at the highest levels of the reef food chain. Besides cnidarians, algae and fishes, other easily observed components of the reef ecosystem can yet be found, such as sea anemones, sponges, worms, mollusks, crustaceans and echinoderms, for example.
CNIDARIANS The cnidarians are among the best studied group of organisms in Abrolhos. Eighteen species of stony corals (scleractinians), four of hydrocorals, four of antipatharians, and eleven of octocorals constitute the so far identified cnidarian fauna of Brazil, most of them present in the Abrolhos reefs. Verrill (1868), whom elaborated the first descriptions of the cnidarians from Brazil, remarks that the ahermatypic corals are all related to Caribbean species and among the reef framebuilders (hermatypic species), the endemism is rather strong. Later on, Laborel (1969a, 1969b) compared Verrills taxonomy with contemporary forms and Tertiary fossils and corroborated Verrills remarks. More recently, Belém et al. (1982, 1986) and Castro (1989, 1990 a, 1990 b, 1994) confirmed and expanded the Brazilian cnidarians list.
Scleractinians Of all eighteen species of corals identified in the Brazilian reefs, seventeen are reported in Abrolhos. Six of them (Mussismilia braziliensis, Mussismilia hispida, Mussismilia hartti, Siderastrea stellata, Favia gravida and Favia leptophylla,) are endemic species, and are the commonest corals in modern Brazilian reefs. Among these endemic species, Mussismilia braziliensis is the coral that shows the greatest geographical confinement. It is the most common coral in the Abrolhos reefs and occurs only along the coast of the state of Bahia. It can be considered as the "big star" of Abrolhos (Fig. 5). On the other hand the species Mussismilia hispida has the largest spatial distribution, because it occurs from the 3oS of latitude to as south as 30oS. According to Laborel (1969 a), Siderastrea stellata and Favia gravida present close similarity to the Caribbean species and are the most common corals in shallow intertidal pools of the reef tops, being resistant to variations in temperature, salinity and water turbidity. Among the archaic corals Mussismilia hispida is referred in the fossil record of the Pinecreast Sandstone, Pliocene of Florida (Meeder 1987 apud Budd et al. 1994). The cosmopolitan species Porites astreoides, Porites branneri, Agaricia agaricites, Agaricia fragilis, Montastrea cavernosa and Madracis decactis have a secondary role in the construction of the reefs in Abrolhos. Most of the framebuilder corals from the Brazilian reefs are massive. Encrusting forms are commonly present along the edges of the reefs. All reefs lack the branching forms that are dominant in the reef crest and fore reef slopes on most of North Atlantic reefs. Depth can be a controlling parameter in the coral morphology as for example in the coral Montastrea cavernosa (Amaral 1994). The shallow forms are hemispheric and the ones found on the lateral walls of the reefs at depths greater than 5 m are fringed and deeper than that, they may be flattened and encrusting. The species Meandrina braziliensis has two morphological variations: a fixed form attached to the reef walls and a free-living form that inhabits sandy bottoms. The small coral Scolymia welsii, Phyllangia americana, Astrangia braziliensis, and Stephanocoenia michelini, are rare and do not contribute significantly to the construction of the reef structures.
Figure 5 Colonies of the endemic coral Mussismilia braziliensis, the major reefbuilding coral of the Abrolhos reefs (photo courtezy of Carlos Secchin).
Hydrocorals Two of the three species of millepores reported from the Abrolhos reefs are considered as endemic. They exhibit the branching and the encrusting growth forms. Delicate, finger-like branches are characteristics of low energy environments. Irregular, short, rounded branches are common on the edges of the reefs, with the thick, massive branches, in zones of higher energy. Encrusting forms are seen on reef top surface or on the axes of gorgonians. The ubiquitous species Millepora alcicornis dominates the windward borders of the reefs, and it is described along the whole tropical Brazilian coast. Verrill (1868), first described the endemic species Millepora braziliensis. More recently Amaral (1997), using biochemical studies, has proved it to be a valid species. In zones of high energy, the colonies of this hydrocoral are more massive, but on protected zones their branches are flattened. Laborel (1969b) located the zone of Millepora braziliensis immediately below the zone of Millepora alcicornis. The species Millepora nitida, endemic from Brazil, is common in Abrolhos. Besides the millepores, a small hydrocoral, Stylaster roseus is observed among branches of Millepora on the outside walls of the chapeirões (Castro 1994). It forms small colonies, few centimeters high, that have a thick base covered with small pointed branches. Another very rare hydrozoan found in the deeper, shaded zones of the Abrolhos reefs, is the species Solanderia gracilis, which was registered for the first time in the South Atlantic Ocean, in 1982 (Belém et al. 1982).
Octocorals Before 1980, only three species of octocorals were described on the Abrolhos reefs, all of them belonging to the group of the gorgonians, and two of those were considered endemic from Brazil. They are the abundant Brazilian blade sea fan Phyllogorgia dilatata, the large octocoral Plexaurella grandiflora, commonly found in the shallow and lit reef areas, and the species Muriceopsis sulphurea, which colonies have a characteristic yellow color. Nowadays this group of organisms is one of the best known in the Abrolhos reefs due, mainly, to the latest reviews of the Brazilian octocorals by Castro (1989, 1990a, 1990b), that revealed eight new species: Plauxaurella regia, Muricea flamma, Neospongodes atlantica, Lophogorgia punicea, Carijoa risei, Heterogorgia uatumani, Ellisella barbadensis and Ellisella elongata. Among these new described species two are only recorded on the reefs located along the coast of the state of Bahia, Plauxaurella regia and Muricea flamma. Two other species were considered endemic from the Brazilian reefs too, Neospongodes atlantica and Lophogorgia punicea. The others are also registered in the reefs from the North Atlantic Ocean.
Antipatharians According to Castro (1994) four species of corals from the group of the antipatharians are registered from the Abrolhos reefs: three of the genus Antipathes and one from the genus Cirripathes. The former shows flat, fan-shaped colonies, or have branches arranged like brush bristles; the latter, also known as wire coral, forms long branched colonies up to several meters long. The occurrence of these black corals in Brazil is of little knowledge for the scientific community yet.
Three characteristics distinguish the Abrolhos coral fauna from the reefs of the tropical Atlantic, the low diversity, the strong endemism, and the lack of branching scleractinians, and this can be a result of two probable factors: (i) the isolation of the Brazilian reefs from the Caribbean, due to the west and northward flow of the north arm of the Equatorial current, which is and has been a barrier to the propagation of the planula larval stage of many modern Caribbean coral species, and (ii) the environmental conditions of the Brazilian reef areas. In Abrolhos, for example, temperature, salinity and depth of water afford optimum conditions for the development of a diverse coral fauna, but there are at least two environmental aspects that may have deterred colonization by Caribbean coral species and limited the development of new indigenous ones: the consistently high turbidity of the reef surrounding waters, which may have some influence on the zonation of the photophile species (require more intense light), and the limited variation in reef habitats, because the Abrolhos reefs lack some of the well marked zones of Caribbean reefs.
ALGAE The algal flora is one of the most abundant constituents of the Abrolhos reefs area, found in various conditions on the reef structures, particularly covering the reef bottoms.
The simbiont zooxanthelae, unicellular algae that live in the tissue of reef-dwelling corals, is of fundamental importance for the coral nutrition, either producing organic compounds, or expelling oxygen that is absorbed by them. Because of this symbiosis, the zooxanthelate corals are found in shallow well illuminated waters, given the need of light for the algae photosynthesis.
The crustose coralline algae (calcareous red algae) are among the major reef-framework builder organisms in Abrolhos. According to data from Figueiredo (1997) its abundance among the benthic organisms of the Archipelago reefs varies between 32 to 79%, and the surveyed community is apparently represented by four genera: Lithothaminion, Lithophyllum, Sporolithon and Porolithon. Among them Porolithon pachydermum is the dominant species. In the internal structure of the coastal reefs, the percentage of crustose coralline algae to build the reef rigid frame reached up to 20% of a drilled core from the Coroa Vermelha Reef (Leão 1982).
The foliose-type algae (macro algae) were also surveyed in both, the coastal arc of reefs and the offshore fringing reefs of the Abrolhos Archipelago. The brown algae dominate in some reefs of the coastal arc covering more than 90% of the reef surface (Amado Filho et al. 1997), but on the offshore reefs the percentages of this type of algae diminishes, possibly due to a higher herbivore activity (Coutinho et al. 1993). Among the identified species Sargassum sp dominates, followed by Padina sanctae-crucis, Dictyota cervicornis, Lobophora variegata and Dictyopteris plagiograma. Those types of algae are used as food for many groups of reef animals, but if their growth is fast enough to restrain coral development, the reef structures are at risk. This situation may occur if the large algae consumers (herbivore fishes) are not around due to overfishing, and/or if there is an increase of nutrients in the water, caused by the dumping of organic sewage.
In a recent survey Figueiredo (1997) found that filamentous-type algae (turf algae) that overgrow coralline substrates, can cover up to 80% of the Archipelago fringing reefs surface, and the most common species are Sphacelaria tribuloides and Ceramium sp. But according to Coutinho et al. (1993) the presence of this type of algae in the Abrolhos Archipelago reefs is ephemeral, an indication of a high herbivorous pressure, given the fact that fishing is prohibited in the area.
The genus Halimeda is the most abundant calcareous green algae, and one of the major sediment producers of the inter-reefal bottoms. It can reach up to 20% of the coarse sand fraction of the sediment around the coastal reefs, and around 70% of the sediment surrounding the fringing reefs of the Abrolhos Archipelago (Leão 1982). The genus Udotea and Penicillus, also important constituents of the Abrolhos reefs flora, contribute to the production of the fine fraction (mud size) of the inter-reefal sediment.
FISHES The description of ninety-five species of fishes belonging to two different reef communities that inhabit the reefs and the coastal region was the first published scientific data, by Nunam (1979), on the fishes from the Abrolhos area. Only three species were common to both regions. According to the author, most of the identified species are related to the Caribbean fish fauna. Thus, the Abrolhos bank is the southernmost area of the Atlantic Ocean that is inhabited by a large permanent population of such a coral reef fish fauna. The ecological model developed by Telles (1998) for the fringing reefs that border the islands of the Abrolhos Archipelago, inside the limits of the National Marine Park, showed that these reefs present a high proportion between fish biomass and total biomass (Bf/Bt), when compared with reefs from other areas. He suggests that this is maybe an effect of the park management program that prohibits fishing in the area. Among the identified fish species 39% are herbivorous (Scaridae, Acanthuridae, Kyphosidae) 54% are omnivorous (Haemulidae, Balistidae, Pomacanthidae, Lutjanidae, Pomacentridae) and 7% are carnivorous (Serranidae, Carangidae, Sphyraenidae).
OTHER COMPONENTS OF THE REEFS ASSOCIATED COMMUNITIES There are not many published references about other biotic components of the Abrolhos system. The most recent one, from Castro (1994), refers to some organisms that live in these reefs, as the soft bodied sea anemones and zoanthids, the sponges, polychaete worms, mollusks, crustaceans, echinoderms, marine turtles, whales and the sea birds.
Two species of corallimorpharian sea anemones, also known as false corals, are described in Abrolhos, Discosoma carlgrenic and Discosoma sanctithomae, both found attached to the reef structures. True sea anemones are plentiful and so far identified species are Condylactis gigantea, Bellactis ilkalyseae, Alicia mirabilis, Lebrunia danae and Lebrunia coraligens.
Among the zoanthids the endemic species Palythoa caribaeorum is commonly found covering large areas of the reef substrate.
Sponges are not a dominant organism in shallow coral reefs where its most common activity is the bioerosion of the coral skeleton, through the dissolution of the calcareous material by boring, having as a result the production of very fine sediment that is accumulated into the interreefal bottoms. Belonging to this group the genus Cliona is the most common. Some polychaete worms have also an important bioeroding activity in coral reefs, functioning thus as sediment producers. But other types can either construct calcareous tubes where they live protruding from the surface of living corals, as for example several species of the genus Spirobranchus, very common in Abrolhos, or they may live as errant on the reefs, as the species Eurithae complanata, which diet includes coral polyps. Bioeroding mollusks from the genus Lithophaga bore into coral skeletons and produce sediment to the Abrolhos interreefal areas. Castro (1994) also points out the presence of other mollusks found in Abrolhos that feed in cnidarians, as for example the species Cyphoma macumba that inhabits colonies of the Brazilian blade sea fan Phylogorgia dillatata. The encrusting mollusk vermetid gastropod is widespread on the reef borders. Together with crustose coralline algae form a rim on the reef edges. Species of the genus Spiroglyphus (=Dendropoma) was identified on the edges of several reefs of the coastal arc, as well as on the fringing reefs of the archipelago (Leão 1982).
Among the echinoderms, two herbivore groups sea urchins and sea stars, play an important role on the reefs, because they open space for corals to grow. The sea star Oreaster reticulatus is abundant in Abrolhos, feeding basically on the algae carpets that cover large areas of the reef bottom.
Marine turtles come to the reefs for feeding and reproduction. During summer (between November and February) the species Caretta caretta (the loggerhead turtles) and Chelonia midas (the green turtles) lay their eggs on the sandy beaches of the Abrolhos islands, and the species Eretmochelys imbricata (the hawksbill turtles) is seen feeding on invertebrates that inhabit the reefs.
The Abrolhos Bank is the largest area for the reproduction of the humpback whale Megaptera novaeangliae, in the whole Southwest Atlantic Ocean. From June to November (winter and spring in the southern hemisphere) this very active and acrobatic whale, which has coastal habits, migrate from the subantartic waters to the warmer shallow waters that surround the Abrolhos Archipelago (Fig. 6). The enhanced presence of this humpback whale in the reef area witnessed by the Marine National Park Authorities is an indication that this species is recovering from past overhunting.
It is worth of note, yet, the presence of many sea birds that are seen on the Abrolhos environs. Most of them came there for nesting, but migrating birds are also attracted to this area, for resting and for feeding. Among the species that were observed to nest on the islands there are Sula dactylatra (the blue-faced booby), Sula leucogaster (the brown booby), Fregata magnificens (the frigate bird), Sterna fuscata (the sooty tern), Anous stolidus (the brown nooddy), and Phaeton aethereus (the red-billed tropic-bird).
Figure 6 The acrobatic Humpback whale (Megaptera novaeangliae) visiting the warm waters of the Abrolhos Bank (photo courtezy of Mia Morete).
THE INTER-REEFAL SEDIMENTS
In Abrolhos, the reef organisms are responsible by the presence of a transition from siliciclastic dominant sediments, on the nearshore environment, to carbonate reefal facies seawards. There are, thus, three distinctive types of sediment: (i) quartz sands prevalent along the coastline, (ii) biogenic material that dominate in all reef areas, and (iii) mixed sediments in the intermediate area between the coastal arc and the outer arc of reefs (Fig. 7).
The siliciclastic content
According to data from Leão (1982) and Leão and Ginsburg (1997), quartz, mica, rare feldspar, and the clay minerals kaolinite and illite are the most common terrigenous constituents of the sediments surrounding the coastal reefs. They have two major origins: reworked sediment originating from the erosion of the Tertiary Barreiras Group which covers most of the hinterland and outcrops along the coast, and the river loads transported to the reef area, by longshore currents. This siliciclastic content dominates along the coastline (>70%) and it ranges from 30 to 60% around the coastal reefs (Fig. 7). Quartz grains are the most common constituents of the coarse fraction of the sediment of the coastal area. Mica flakes occur in the muddy sediments in the deepest parts of the nearshore area. In this mud fraction (silt and clay sizes) the amount of the terrigenous material reaches more than 60% in the leeward side of the nearshore reefs. During seasonal storms this muddy sediments is resuspended and plumes of turbid waters occur in the reef areas, as it is seen in the reef illusrated in figure 4.
The carbonate components
The biogenic constituents of the sediment surrounding the reefs are predominantly skeletal in origin. Part of this material has a detrital origin and part is composed of grains formed in situ by the various organisms of the reef-associated fauna and flora. The detrital material is derived from the breakdown of the reef structure, being most commonly fragments of coral, milleporids and coralline algae, which reach their maximum occurrence on top of and in areas bordering the reefs. The in situ grains originate from the skeletal parts of organisms living on or around the reefs and they include the hard skeleton-bearing animals such as mollusks, echinoderms, forams, ostracods and calcareous algae, particularly the green alga Halimeda sp, Penicillus sp and Udotea sp, the brown algae Padina sp, and the articulated red algae Amphiroa sp and Jania sp, which are particularly abundant on the Abrolhos area. The fine fraction of the carbonate sediments is the result of the organic disintegration of the calcareous parts of fragile red and green algae, and the bioerosion of the reef structures.
Figure 7 Diagram illustrating the distribution of the superficial bottom sediments in the interreefal area of Abrolhos (data according to Leão 1982, Leão and Ginsburg 1997).
THE ABROLHOS ARCHIPELAGO
Five islands form the Abrolhos Archipelago (Fig. 1). Santa Barbara, the largest, is approximately 1 km long (E-W), 300 m wide and rises 35 m above the sea surface. At its northern and southern sides there are gravelly and sandy beaches with a mixture of carbonate debris, quartz grains and rock fragments. Fringing reefs developed at the west point round the corner and toward the south side of the island, along two thirds of its shore. The remaining part of the south side is a steep slope and coral growth is absent. All of its northern side is characterized by the presence of encrusting reef communities growing on basalt boulders.
To the west of Santa Bárbara, across a 4 m deep channel, one finds the Redonda Island. It has about 400 m in diameter and is 36 m high. The reef fringe appears on its southeastern shore. An encrusting reef community occurs on its slope, similarly to the northern side of Santa Bárbara Island.
Siriba Island is 300 m long (E-W), 100 m wide and is 16 m high. It is located south of Redonda, separated from it by a shallow (less than 4 m deep) channel.
The southernmost island of the archipelago is the Sueste Island, which is about 500 m long, 200 m wide and 15 m high. Reef communities grow on its slope. Siriba and Sueste islands lack true fringing reefs but many coral pinnacles are seen around them. These chapeirões can reach up to 15 m high where depths are around 20 m inside the channel that separated these islands.
Two hundred and fifty meters north of Santa Barbara Island there is the Guarita Island, which is about 100 m wide (SW-NE) and 13 m high. This island is made of volcanic rocks with no sandy beaches around it. Reef communities grow on its slope.
The Abrolhos islands are the outcrop of a structural high called the Santa Barbara High (Asmus and Porto 1972), and according to Fisher et al. (1974) and Ponte and Asmus (1976), the sedimentary strata belong to a slope deposition system of Late Cretaceous to Early Tertiary age. The lithology seen on the islands suggest the occurrence of a transgressive phase followed by a regression. This event may be a small oscillation in the Marine Regressive Sequence, defined by Chang et al. (1991) that characterizes this phase of the evolution of the Brazilian Marginal Basins. The basalt (sills and dykes) (Fig. 8) originated from a Tertiary intrusive and volcanic activity (40-52 Ga., Cordani 1970), an accretion to the shelf that is responsible for the formation of the Abrolhos Bank (Asmus 1970). All these basaltic and alkaline intrusions that occurred in the Brazilian Marginal Basins are related to the volcanism of the Paraná Basin (the Serra Geral Formation), or is a late event of the eastern margin of this cratonic basin (Asmus and Guazelli 1981).
Figure 8 A volcanic outcrop on Siriba Island, Abrolhos National Marine Park.
ENVIRONMENTAL IMPACTS AND PRESERVATION
Because the Abrolhos reefs are not located in any hurricane route and are sit upon a passive continental margin, effects of natural disturbances recorded in these reefs are related only to the sea level oscillations that occurred during the last 5 ky, and a recently observed coral bleaching that occurred after a rise in the sea surface temperature. Human induced impacts however have been threaten these reefs by a combination of different kind of environmental stresses.
Sea level oscillations
The history of sea level during late Holocene time, in Brazil, shows that the Brazilian coast experienced considerable relative sea level fluctuations (Martin et al.1996) that had profound effects on the evolution of the coral reefs. The lowering of the sea exposed the reef tops to marine erosion, dissolution and extensive bioerosion, and the reef communities dwelling thereon these tops experienced stress resulting, chiefly, from strong solar radiation and high levels of sedimentation and water turbidity. Small colonies of the endemic species Siderastrea stellata and Favia gravida are the only corals that inhabit the shallow reef top pools, and are able to withstand this stressing environment. The regression period also approximated the reefs to the coastline and subjected them to the influence of a highly siliciclastic sediment influx. Those environmental conditions, such as strong solar radiation, low light levels and high sediment influx must have exceeded the tolerance levels of most of the Brazilian coral species.
Global temperature changes and coral bleaching
Although temperature anomalies related to the El Niño event and coral bleaching has long been observed along the Brazilian coast, only few studies about their relationship has been made up to now. In Abrolhos, two occurrences are registered to be related to a rise of the sea surface temperature. The first one occurred during a sea surface temperature anomaly in the summer of 1994 when 51 to 88% of colonies of the genus Mussismilia were affected (Castro and Pires 1999), and the second one is related to the strong El-Niño Southern event that begun at the end of 1997 in the Pacific ocean, and caused, also, a rise of the sea surface temperature in some regions of the eastern coast of Brazil. This temperature rise started in mid January 1998 (summer in the southern hemisphere), attained its climax in mid March and beginning of April, and faded away at the end of May (data obtained from the Monthly Climatology Charts produced by Dr. Allan Strong - NOAA/NESDIS). During this event the estimated sea surface temperature anomaly, of about 1° C, matched satisfactorily with measured temperatures in the field, which averaged 29.5° C, about one degree higher than the value of 28.5° C, that is commonly found at that time of the year. According to Ruy K.P.Kikuchi (personal communication) the most affected species were Porites branneri and Mussismilia hispida, both with more than 80% of their colonies totally bleached, M. harttii with an average of 75% of its colonies affected, and Porites asteroides with all colonies with some signal of bleaching. Although the species Agaricia agaricites did not show a totally bleached colony, more than 90% of them had a pale color.
Human induced impacts
The most common anthropogenic agents that are threatening the reefs of Abrolhos are directly related to coastal zone development, marine tourism, the exploitation of natural resources, pollution from the installation of industrial projects and the exploration of fossil fuels (Amado Filho et al. 1997, Coutinho et al. 1993, Leão 1994, 1996; Leão et al. 1994).
URBAN DEVELOPMENT AND COASTAL RUNOFF Despite the coastal reefs of Abrolhos have been coexisting with a muddy siliciclastic influx, they seem to be, in recent time, clearly under a higher stress, mainly due to the increased coastal runoff. This can be attributed to the increasing deforestation of the Atlantic coastal rain forest for agricultural purposes, to cultivate eucalyptus for industrial use, and the unrestrained urban development. The disorderly fast growth of urban centers is occurring, mainly, on the municipalities that already offer an infrastructure for tourism, such as the small towns of Prado, Alcobaça, Caravelas and Nova Viçosa, which some of them have multiplied their areas more than tenfold over the last forty years (Leão et al. 1994). This coastal use besides being the main cause of soil erosion, the untreated urban garbage and organic sewage coming from these areas can cause an abnormal increase of nutrients in the reef's biota, with dramatic consequences to the ecological balance of the environment, such as encouraging algae to grow at the expense of corals.
MARINE TOURISM Allied to the disorderly growth of the coastal towns and representing, in some cases, the main reason for their expansion, the marine tourism industry in Brazil has recently experienced expressive progress, particularly in the marine protected areas. An example of this is the number of visitors to the Abrolhos National Marine Park, which during a period of five years (1988/92) increased over four hundred percent (Leão et al. 1994). This activity if not properly controlled, may cause serious impacts to the reef ecosystem, specially on account of the careless anchoring of boats, the dumping of non-biodegradable garbage overboard, leaks from motor boats, the removal of reef organisms as souvenirs and decorating aquariums, the breaking of reef organisms caused by the impacts of diver's gear, unrestrained underwater fishing by amateurs, and even occasional shipwrecks, usually caused by the presence of unexpected giant chapeirões. Another example of reef damage in the area of the Abrolhos Park, due to boat anchoring , is the destruction of seagrass meadows around the Abrolhos islands, where Creed and Amado Filho (1999) reported an average of 0.5% of destruction per year.
EXPLOITATION OF NATURAL RESOURCES Due to their proximity to mainland, the coastal reefs of Abrolhos have been heavily threatened by the coral trading and the exploitation of both artisanal and commercial fisheries. In many coastal towns, particularly in the historical villages of South Bahia, we can verify that for more than a century corals have been mined to be used for building material, either as mortar and/or brick, in the construction of old fortresses dating back to the 17th century, as well as, nowadays, in rustic beach resorts (Leão and Kikuchi 1999). The extraction of the fire-coral Millepora alcicornis, widely used in aquarium decoration, can actually be considered as the major cause of the disappearance of this species from many of our reefs. At the fringing reefs of the Abrolhos Archipelago, Pitombo et al. (1988) cite the depletion of this hydrocoral species before the implementation of the National Marine Park. The authors refer to the millepore low density (relative reef cover between 1.5 11%), in a reef area that was previously called "the Millepora zone" in Laborel (1969b).
INDUSTRIAL POLLUTION The development of industrial projects is a constant threat to coral reefs, if a strict control of the final disposal of their industrial waste is not managed accordingly. An example in the Abrolhos area is the paper plants installed in the nearby continental area. Data from Amado Filho et al. (1997) shows that in the south part of the Abrolhos reefs, there are already signals of contamination by heavy metals, which must probably be due to the chemical effluents of paper plants.
The petroleum exploration in areas where coral reefs exist can cause serious damage to these ecosystems. The offshore drilling in south Bahia is a threat to the reefs of the Abrolhos National Marine Park, either from the impacts caused by their activities, such as boat collision with the reefs, and/or the increasing water turbidity and unpredictable accidents regarding oil spills.
Protection and Management
Though there have been scientific information about the coral reefs of Brazil over a century, the knowledge about the actual condition of the reefs is still scarce, and some areas are virtually unknown. There are only few well preserved reefs. In the coastal zone only the reefs located in areas with low levels of urban development can yet be considered as pristine and, naturally, the reefs located offshore on the continental shelf and on oceanic islands, due to their inaccessibility, and/or their designation as protected areas.
The institutions concerned with preservation of the coral reefs in Brazil have only recently been created. The Abrolhos National Marine Park is one of the oldest. It comprises two units: the area of the Abrolhos Archipelago and the outer arc of chapeirões (between 17° 43-18° 09S and 38° 33-38° 45W), and the Timbebas reef on the coastal arc (between 17° 27-17° 38S and 38° 58-39° 02W). But the area of the Abrolhos National Marine Park represents only one fourth of the total area of reefs in Abrolhos. In the remaining area there is not any restriction for recreational or even commercial use of the reefs, although the Bahia State Constitution declares in its Article 215, Chapter VIII, "On the Environment", that coral reefs are areas of permanent protection. The Abrolhos National Marine Park has a management plan, which conservation programs are already put in action (IBAMA/FUNATURA 1991). Inside the area of the Abrolhos Archipelago landing is only allowed in the Redonda and Siriba islands under the supervision of a Park technician. In the Sueste and Guarita islands landing, anchorage and diving are forbidden. The biggest island, Santa Barbara, belongs to the Brazilian Navy and landing there is only permitted with an official authorization.
The author is grateful to the Book Editorial Committee, in the person of Dr.Carlos Schobbenhaus, for the invitation to write this chapter. She also acknowledges those ones that provide color photographs: Marcelo Skaf, Director of the Abrolhos National Marine Park, Mia Morete from the Baleia Jubarte Institute, and Carlos Secchin. Viviane Testa is acknowledged for preparation of figure 3.
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