LOCATION

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 17^o20-18^o10’S and 38^o35’-39^o20’W) (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 0^o08’. 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).

Climate and oceanography

The coastal belt along Eastern Brazil has a tropical humid climate with average air temperature ranging from 24^oC during winter to 27^oC 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:
http://psbsgi1.nesdis.noaa.gov:8080/PSB/EPSSST/climo&hot.html
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 reef’s 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.

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).

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.

Reef morphology

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.

Reef organisms

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 Verrill’s taxonomy with contemporary forms and Tertiary fossils and corroborated Verrill’s 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 3^oS of latitude to as south as 30^oS. 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.

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).

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.