Chapter Fifteen
INTRODUCTION
The offshore coral reefs of the Indonesian Archipelago exhibit a high degree of geomorphological complexity. Reef types present range from the classical Darwinian barrier reefs and atolls, to a variety of shelf reefs (e.g., patch and barrier) and oceanic platform reefs. The vast majority of these reefs are found in the central and eastern parts of the archipelago. With the exception of the Kepulauan Seribu patch reef complex (Sunda Shelf) and the reefs of the Spermonde Archipelago (i.e., Spermonde Barrier Reef), it appears that little is known about the offshore reefs of Indonesia. A review of Indonesian coral reef literature reveals that barrier reefs have received relatively little attention since the early works by the Dutch scientists (Molengraaff 1929; Kuenen 1933a, 1947; umbgrove 1947). It seems that coral reef research in Indonesia has been focused mainly on the more accessible near-shore shelf reefs, which usually have high economic value. However, coral reef research is slowly beginning to expand into the more remote areas of the archipelago (e.g., Taka Bonarate Atoll; Komodo; Lombok; Irian Java). Perhaps the most important (i.e., economically and scientifically) coral reef areas in Indonesia are the three major barrier reef systems along the southwest coast of Sumatra, the Great Sunda Barrier Reef [note that the Snellius Expedition (1929-1930) passed through the Great Sunda Barrier Reef, but time constraints did not permit a stop], and the barrier reefs along the east (e.g., Gulf of Tomini; Banggai Islands) and south (e.g., Gulf of Bone) coasts of Sulawesi.
HISTORICAL PERSPECTIVE
The confirmation of the existence of barrier reefs and atolls in the Indonesian Archipelago is generally attributed to Niermeyer (1911), who used new hydrographic charts from the hydrological office of the Royal Dutch Navy Department to support his findings. The first comprehensive review on the distribution and formation of barrier reefs is that of Molengraaff (1929), with subsequent review by umbgrove (1947). During the Snellius Expedition, Kuenen (1933a) focused mainly on atolls and raised reefs. Since these early publications, very little has been written about Indonesian barrier reefs until Salm and Halim (1984) provided the latest available information on their distribution. Because of scale problems, a map of the barrier reefs in this publication would not be very meaningful, therefore the reader is urged to refer to one of the above-mentioned publications for visual presentations.
Figure 15.1. Classical barrier reefs are difficult to recognize from small vessels, and therefore it is no surprise that they remained unrecognized until new charts became available in the early 1900s. The 131-m-high Pulau Sago, south of Banggai Platform, rises steeply from a depth of about 1100 m. Because of calm conditions the encircling barrier reef of this oceanic high island is difficult to recognize from a passing vessel.
Photo by Tomas and Anmarie Tomascik.
Before the new charts became available it was generally assumed that barrier reefs were not well represented in the Indonesian Archipelago, mainly because of the perception that these systems would not develop in areas experiencing tectonic uplift. However, it is interesting to read the following comment:
This reef is sometimes only marked by a line of breakers when there is a little swell on the sea; in other places there is a ridge of dead coral above water, which is here and there high enough to support a few low bushes. This was the first example I had met with of a true barrier reef due to subsidence, as has been so clearly shown by Mr. Darwin.—WALLACE 1869
Since Wallace was well aware of Darwin's theory of atoll and barrier reef formation due to subsidence, he correctly identified one of the first documented barrier reefs in the archipelago. Wallace was referring to a small barrier reef located in the Watubela Island chain in the east Banda Sea. Figure 15.1 clearly illustrates the difficulty that the early naturalists had in recognizing classical barrier reefs which surround many high islands, such as Pulau Sago off the Banggai Platform on the east coast of Central Sulawesi. Subtidal reef crests and calm seas frequently hid the presence of these reef systems.
Barrier Reef Types
Detailed geomorphological studies of Indonesian barrier reefs have still to be conducted, but based on the antecedent foundation upon which they have formed we may recognize two basic types. The first type is the classical Darwinian barrier reef that develops from fringing reefs surrounding slowly subsiding oceanic high islands. Depending on the local geological and tectonic setting, oceanic barrier reefs may encircle the entire high island (e.g., Bora Bora, Pulau Besar), or develop only along some sections of its coastline (fig. 15.2). The size and shape vary, depending on the topography of the antecedent foundation and the geologic history of the area: Some barrier reefs form a continuous rim around the entire island, and lagoonal flushing occurs mainly over the reef crest and through the reef matrix itself (e.g., Pulau Sago). Other barrier reefs are fragmented into smaller sections and breached by a series of shallow and deep passages, referred to as hoas and avas,respectively (Guilcher 1988). Small barrier reefs running parallel along short stretches of coastlines are very numerous, and frequently very difficult to distinguish from fringing reefs based on casual observations (fig. 15.3).
Figure 15.2. Barrier reefs come in all shapes and sizes and may develop only along certain sections of a subsiding coastline. South coast of Buru, the Moluccas.
Photo by Tomas and Anmarie Tomascik.
Figure 15.3. Small barrier reefs in the Gulf of Tomini are unusual structures with unknown origins. This small barrier reef along the southern coastline of the Gulf of Tomini may be of karstic origin. The lagoon is 20-30 m deep. Note the large "bluehole," lower right.
Photo by Tomas and Anmarie Tomascik.
Barrier reefs are, however, not restricted to oceanic islands, but occur along the seaward margins of continental shelves. These barrier reefs are analogous to the Great Barrier Reef along the northeast coast of Queensland. The largest continental shelf barrier reef in Indonesia is the Great Sunda Barrier Reef, which runs along the east coast of Kalimantan, closely following the 200 m isobath (fig. 15.4). The barrier reef is over 600 km long and up to 140 km offshore. The lagoonal and outer barrier reefs are extensive, but poorly known. The origins of the outer reefs on the Great Sunda Barrier Reef may date to the Holocene transgression, however, the geologic history of the shelf is considerably older. Drilling projects on some of the outer and lagoonal reefs are needed to understand the geologic history of the least-known reefs in the archipelago.
Figure 15.4. Two depth profiles across the Sunda Great Barrier Reef
Barrier Reef Zonation
The general geomorphic/ecologic zonation of oceanic barrier reefs is not too dissimilar to that of fringing reefs. However, sufficient differences exist, such as a deep lagoon, that a brief mention of some profiles is warranted. Since there have been no formal studies conducted on the geomorphic zonation of Indonesian barrier reefs, our presentation is purely qualitative, based on information obtained from numerous dives from a number of these reefs. While the geomorphic zonation is very similar among the various reefs surveyed (e.g., Pulau Sago in Banggai and Pulau Besar in Maumere Bay), their size and ecologic zonations are different. The most striking feature of the barrier reefs that were observed was the absence of a well-defined spur-and-groove zone, which is a characteristic feature of most Pacific and Indian Ocean barrier reefs and atolls. The absence of spur-and-groove zones in the survey sites is probably related to the sheltered nature of the environment, since well-developed spur-and-groove zones were found on high-energy fringing reefs. Unfortunately, we did not have the opportunity to visit a barrier reef under similar high-energy conditions.
Figure 15.5. Generalized zonation of a typical oceanic barrier reef in the Banggai Islands, Central Sulawesi. Not to scale.
Nine general geomorphic/ecologic zones can be recognized for the oceanic-type barrier reefs, namely: 1) outer fore-reef slope (less than 35 m); 2) outer reef slope; 3) reef crest; 4) outer reef flat; 5) inner reef flat; 6) inner reef slope; 7) lagoon; 8) fringing reef; and 9) the beach (fig. 15.5). The fringing reef zone is quite ephemeral, and in many instances (e.g., Pulau Bangkulu), the lagoon bottom shallows-up gently towards the beach without an intermediate fringing reef. In all instances where fringing reefs were absent, the bottom was covered by dense sea-grass beds, Thalassia hemprichii and Cymodocea spp. being the most common species.
1. The outer fore-reef slope of Pulau Sago barrier reef begins at a depth of about 30 m, from a 10-15-m-wide coral-rubble-covered submarine terrace. The terrace is topped by a dense growth of sponges and deep-water coral genera such as Favia, Favites, Lobophyllia and Plerogyra. From the seaward edge of the terrace the outer fore-reef slope drops to depths greater than 200 m (based on charts) at an angle of about 45°-65°. The slope below the 30 m terrace is covered by sponges, octocorals and antipatharians (fig. 15.6). It consists of a solid coral matrix as well as coral rubble. Large densities of Odonus niger were quite startling, as was the abundance of large serranid (groupers) predators such as Plectropomus leopardus, P. laevis, and P. areolatus. Large Cheilinus undulatus (>100 cm) were also seen at the edge of the outer fore-reef slope.
2. The outer reef slope can vary in profile from vertical walls, as was the case at Pulau Sago, to slopes with angles between 45°-75°, as at Pulau Bangkulu and many other reefs in the archipelago (fig. 15.7). With sufficient time it would be possible to identify various zones on both types of slopes; unfortunately, only generalizations can be offered at this time. The vertical wall at Pulau Sago has two clearly defined zones. The upper zone is covered by a luxuriant growth of branching and foliaceous coral species, with massive Acropora palifera and tabulate acroporids forming the uppermost tier. Dispersed among the acroporids are large colonies of massive Pontes, Millepom spp., Heliopora coerulea, Favia and Favites. At a depth of c: 5 m, just before the slope sharply drops, is an area of exceptional high diversity (fig. 15.8). Strong currents, sweeping over this zone, bring a continuous supply of nutrients to a species-rich mixed scleractinian and octocoral community. Below 5 m, in the deeper zone of the outer reef slope, massive Diploastrea heliopora, Favia, Merulina, Echinopora, Hydnophora and many other genera can be found in high abundance. There is a visible loss of scleractinian coral diversity and cover as the outer slope reaches the 30 m terrace. However, octocorals are abundant and diverse. In the upper slope Sinularia, Lobophytum, and Sarcophyton are the most abundant alcyonaceans, whereas Echinigorgia and Euplexaura are the most common Gorgonacea genera. In the deeper zone of the outer reef slope, alcyonaceans such as Nephthya, Stereonephthya, and Dendronephthya become more common, and gorgoneans such as Subergorgia form large colonies. In contrast, the outer reef slope at Bangkulu was about 35° from the shallow upper zone to a depth of about 15 m, where it dropped to about 30 m at an angle of 70°-80°. However, the outer fore-reef terrace was, in terms of structure, as well as biota, very similar to Pulau Sago.
Figure 15.6. Sponges and soft corals and platy scleractinians are abundant on deep fore-reef slopes.
Photo courtesy of Coral Cay Conservation Ltd.
Figure 15.7. The steep seaward reef slopes of many barrier reefs are covered by octocorals and sponges. Large Petrosia testudinaria are among the most conspicuous sponges, especially in the deeper parts of the fore-reef slope.
Photo by Tomas and Anmarie Tomascik.
Figure 15.8. Mixed scleractinian and octocoral communities on current-swept upper fore-reef slopes, support high biomass and diversity of reef fish.
Photo by Tomas and Anmarie Tomascik.
Figure 15.9. The reef crest at Pulau Sago, Banggai Islands. The inner reef crest is dominated by a diverse Acropora community.
Photo courtesy of Coral Cay Conservation Ltd.
3. The shallow sub tidal reef crest at Pulau Sago was covered by a luxuriant assemblage of acroporids (fig. 15.9), with large areas dominated by Acropora hyacinthus. Large colonies of massive Porites spp. were very abundant along the entire reef crest, but the absence of microatolls was striking. All available space among the acroporids was occupied by dense beds of Thalassodendron ciliatum,which seem to be important nursery grounds for siganids, since large schools of juvenile Siganus guttatus and S. lineatus (2-5 cm) were observed along the reef crest. At the seaward margin of the reef crest (2-3 m), massive Acropora palifera and Porites spp. form large aggregations. Other massive and sub-massive genera present were Favia pallida, Favites abdita, and Goniastrea spp. Coralline algae seem to play a minor role in this zone, most likely as a result of the relatively sheltered location of the reef. In contrast, the reef crest at Bangkulu was covered by branching acroporids, notably Acropora humilis, A. nobilis, and other corals such as Seriatopra hystrix and Montipora spp. (fig. 15.10). At both reef sites the reef crests were clearly subtidal low-energy habitats, even though the reefs are facing the open sea. However, tidal currents running along the reef slope can be considerable, which may partly explain the relatively high abundance of the tabulate acroporids. The long-held view that Acropora is one of the key reef-builders can easily be seen on these reefs.
Figure 15.10. The seaward margin of gently sloping reef crest cover dominated by an Acropora community.
Photo courtesy of Coral Cay Conservation Ltd.
4. The outer reef flat, which may be intertidal, is dominated by small branching acroporids (e.g., Acropora aspera, A. pulchra, A. millepora, A. valida) as well as tabulate Acropora hyacinthus. Thalassodendron ciliatumis abundant, but other seagrasses (Thalassia hemprichii) are common. Surprisingly, the pocilloporids were not in great abundance, as they usually are on the outer reef flats of the more exposed reefs in the Banda or Flores Seas.
5. The inner reef flat is about 100 m from the reef crest and was covered by 0.5 m of water at low tide. Mixed seagrass beds (e.g., Thalassia hemprichii, C. rotundata, C. serrulata and Halophila ovalis) are the dominant benthic community. Corals present were Goniastrea spp., Leptoria sp., Psammocora sp., as well as small colonies of the massive Porites spp. and branching acroporids.
6. The inner slope consists of loose rubble covered by seagrasses and branching acroporids and poritids. Acropora yongei and A. formosa were very abundant. Halimeda spp. become abundant as do other macrophytes
Shelf Barrier Reefs
The most comprehensive study of scleractinian coral communities on an Indonesian shelf barrier reef is that of Moll (1983), who studied coral community structure of eight coral reefs on the Spermonde Barrier Reef (figs. 15.11 and 15.12). His study supported umbgrove's (1947) and Kuenen's (1933a) earlier views that the coral reefs on the Spermonde Shelf were monsoonal-driven systems, which was reflected in their geomorphology and ecological zonation. Umbgrove (1947) pointed out that the formation of shingle ramparts (geomorphological features) on the west side of the Spermonde reefs was very similar to those in Kepulauan Seribu. This is not altogether surprising since both reef systems, while different in origins, are affected by a strong Northwest Monsoon. Moll (1983) obtained quantitative data which suggest that disturbances associated with the Northwest Monsoon have a much greater effect on coral community structure of the outer rim reefs than on the inner, more sheltered lagoonal reef communities of the Spermonde Barrier Reef. The lowest number of coral species were found on the outer rim reefs, while dominance increased (i.e., Simpson's [1949] measure of concentration). However, in the outer rim high-energy environment, Acropora rotumana, A. diversa, Pocillopora eydouxi and Goniastrea aspera provided higher coral coverage than on any other reef in the study (Moll 1983). High wave-energy conditions along the outer barrier reef are also reflected in the greater abundance of red calcareous algae (Moll 1983). Clear morphological and ecological differences among reefs seem to be related to their distance from the mainland, which clearly suggests an environmental gradient. Moll (1983) stated explicitly that environmental factors likely to be responsible for the apparent zonation were sedimentation, which decreases from shore seawards, and exposure (i.e., water movement due to waves and currents), which decreases from the outer rim landwards.
Figure 15.11. Map of Spermonde Archipelago (left) showing Moll's (1983) division of the barrier reef (right).
From Moll 1983.
Figure 15.12. Generalized depth profiles across northern (upper figure) and southern (lower figure) parts of the Spermonde Shelf, illustrating the double barrier reef system.
Moll (1983) also found that scleractinian community structure varied more within, than between, reefs studied. The within-reef variation is at two levels: 1) geographic orientation with respect to the island (i.e., north, east, south, west), which also corresponds to the degree of exposure; and 2) within-reef variations due to differences in habitats. The reefs of the Spermonde Barrier Reef have three distinct geomorphic/ecologic zones, the reef flat, reef edge and reef slope. Of the 224 scleractinian species, 110 are common to all three zones, 24 species are restricted to the reef flat, 17 to the reef edge and 17 to the reef slope. Ten species are common to reef flat and reef slope, but absent from the edges, while reef flat and reef edge have 23 species in common, and reef edge and reef slope share 19 species. The least diverse area of individual reefs is the lagoon (moat), with highest diversities on the reef edge and slope. Based on species dominance and exclusivity (i.e., corals found only at certain reefs), Moll (1983) identified seven coral associations that reflect hydrodynamic conditions on the reefs, namely: 1) Very exposed habitats dominated by Acroporaformosa/Seriatopora hystrix association, which accounts for 44% live coral cover; 2) Exposed habitats where A. aspera/A. pulchra association contributes 27% to live coral cover; 3) Faviidae association in moderate exposures (24% coral cover); 4) Montipora digitata association in moderate exposures (21% coral cover); 5) Poritesassociation in moderate exposures (21% coral cover); 6) Pachyseris rugosa association in sheltered environments (16% coral cover); and 7) Heliopora coerulea association in lagoons (22% coral cover). This zonation scheme differs from those of Geister (1977) for the Caribbean, Rosen (1971a,b) for the western Indian Ocean and of Done (1982) for the Great Barrier Reef in Australia. However, Moll's (1983) zonation scheme seems to be similar to that of Head (1980) for the Sudanese coast of the Red Sea. The important difference that separates Moll's and Head's zonations is the degree of symmetry. The occurrence of the same association at both extremes of the exposure spectrum in the Spermonde reefs makes the zonation more symmetrical than that of the Red Sea (Moll 1983). The difference is attributed to the higher influence of hydrodynamic factors in Spermonde reefs than in the Red Sea, where Head (1980) stresses the importance of light-energy effect over hydrodynamics
General Distribution
Barrier reefs, unlike atolls, occur from the westernmost regions of the archipelago (i.e., Aceh) to the east (i.e., Biak). Based on recent hydrographic charts, we identified 72 barrier reefs in the archipelago, with a combined length of approximately 4823 km (table 15.1). The combined length of known Indonesian barrier reefs is roughly twice the length of the Great Barrier Reef in Australia. The length, or size, of the individual barrier reefs varies from hundreds of kilometres (for example, the great Sunda Barrier Reef in the Makassar Strait) to smaller barriers only a few kilometres long and a few hundred metres wide. However, the small barrier reefs all have deep lagoons that clearly distinguish them from large fringing reefs. The variety of small barrier reefs is impressive, but, because of their remoteness and small size, they are generally overlooked. A case in point is an unusual barrier reef off the northeast coast of Binongko Island (fig. 15.13). The island, part of the Tukang Besi chain, has a rather impressive scientific history. It was visited by three major expeditions to the archipelago, namely the Siboga Expedition (1899-1900), the Snellius Expedition (1929-1930) and most recently by the Snellius-II Expedition (1984-1985). In the past, the island's unique geology and topography has attracted the attention of numerous geologists, notably Verbeek (1908). Ph. H. Kuenen devoted three days to unravel the mystery of Binongko's unique topography during the Snellius Expedition. However, he was so engrossed in the fascinating sequences of uplifted coral terraces that the presence of the Palahidu barrier reef went unreported. During the Snellius-II Expedition, all research once again focused on the uplifted terraces, while marine and coastal surveys were restricted along the north and northwest coasts of the island, thus the Palahidu barrier reef remained undescribed. It is also surprising that another very unusual barrier reef running continuously along the east and northeast coast of Wangiwangi, just to the north of Binongko, went undescribed as well. The Wangiwangi submerged barrier reef (25 m below sea level) is 100-150 m wide and runs parallel to the coastline for about 25 km. It is separated from the mainland by a narrow lagoon whose width varies from 200 m (north sector) to 1 km (south sector). The lagoon is,however, exceptionally deep, registering 52 m and 43 m at its south and north sectors, respectively. The barrier is continuous, without breaks, and the reef flat becomes intertidal for about 3 km at its northwest margin. The 200 m isobath is only a few metres off the seaward slope of the reef. Pulau Wangiwangi has an impressive assortment of reef types, with geomorphic features such as large blue-holes or dolines (800 m x 1000 m), fringing reefs with enclosed lagoons and large coral cays (fig. 15.14). Wangiwangi also illustrates an example where a barrier reef was formed by the joining of two fringing reefs of neighbouring high islands.
Table 15.1. Barrier reefs of the Indonesian Archipelago. Location: main landmass associated with the barrier reef; numbers in brackets are total number of barriers. Approximate position: latitude or longitude coordinates given when landmarks cannot be identified from the hydrographic charts; the names indicate approximate position of the beginning and end of the barrier system. Lagoon width indicates the maximum distance from shore to the outer barrier reefs. Lagoon depth is the maximum depth of the lagoon. Length is the total length of the barrier complex. Reefal area is based on the formula: Reefal area (km2) = [Lagoon width (km)/2] x [Length of the barrier reef (km)]. Based on hydrographic charts with scales ranging from 1:500,000 to 1:15,000.
Fig. 15.13. A) Map of the Palahido barrier reef along the northeast coast of Binongko, Tukang Besi. B) Depth profile across the Palahido barrier reef just south of Palahido.
A. Drawing by B. Rahmad.
Fig 15.14. Map of Wangiwangi, Tukang Besi, with a narrow continuous subtidal barrier reef along the west and south coast of the island. Timor and Sumanga Islands are fringing reef coral islands. Four large "blue holes" are present. Note the barrier reef between Kapetan and Kampenaune Islands. Depths in metres.
Small barrier reefs seem to be very common predominantly in the central and eastern parts of the archipelago. Their combined length may be considerable. Unfortunately, to distinguish these coastal features one needs either an access to maps on the scales of at least 1:50,000, which are not available for much of the archipelago, or satellite images that are now widely available. The 22-km-long barrier reef along the south coast of Sumbawa was identified only by the use of remote sensing technology, which is now being successfully applied in development projects by the Research and Development Centre for Oceanology (Pusat Penelitian dan Pengembangan Oseanologi) in Jakarta (Siswandono et al. 1993; Siswandono 1994) (colour plate 15.1). Figure 15.15 clearly illustrates that hydrographic charts with scales of 1:200,000 are inadequate to identify these important coastal features. The image in colour plate 15.1 is based on the Landsat Thematic Mapper (TM) data combined with ground- and sea-truthing data (Siswandono 1994). The image was produced by the microBRIAN system version 3.2 using Landsat TM of Path 144, Row 66, taken on October 8, 1991 (Siswandono 1994). The separation of the outer rim from the shallow coastal reef was possible by using spectral values of Landsat TM channels 2, 3, 4 and 5, which correspond to depth of penetration (i.e., depth). Comparing colour plate 15.1 to figure 15.15, we can see the great potential of remote sensing technology in coastal development projects.
Figure 15.15. Waworada Peninsula, southeast Sumbawa, West Nusa Tenggara. Waworada barrier reef along the southwest sector of the peninsula here shown as a fringing reef. Original scale 1:200,000.
Even though large barrier reefs are present along the seaward margin of the Sunda Shelf, they have not been studied or discussed since umbgrove's (1947) review. In fact, it is quite surprising that none of these large barrier reef systems have been mentioned in any of the latest international conservation reports, even though they are a significant component of the regional biodiversity potential. The Indian Ocean coast of Sumatra is particularly neglected in this respect. The four large barrier reefs have a combined length of 660 km, roughly 35% of the coastline. The northernmost 85-km-long barrier reef runs along the west coast of Aceh, roughly between Ujung Glumpang and Teluk Ritteng. This is a unique submerged or drowned (sensu Guilcher 1988) barrier reef under 13 to 20 m of water, and is separated from the mainland by a 20-km-wide, and up to 62-m-deep, lagoon. The depth of the barrier is still well within the euphotic zone, and thus coral growth and reef development are possible. The barrier is breached by deep passages that seem to correspond to old rivers, for example, off Teluk Daya the passage is 71 m deep. To the south, roughly between Ujung Raja and Ujung Mangki, runs a 119-km-long barrier reef 10 to 20 km offshore with a 14-50-m-deep lagoon. The longest barrier reef along the west coast of Sumatra runs between the Tanahmasa Islands and Ujung Indrapura, a distance of about 280 km. Recent work by the staff of Bung Hata University and Dr. A. Kunzmann of the Coral Reef Assessment and Monitoring Project West Sumatra, has shed new light on this fascinating region of the archipelago.
The longest barrier reef in the archipelago runs along the east seaward margin of the Sunda Shelf. Both Molengraaff (1929) and umbgrove (1947) speculated on its origins, but since their review, the reef has generally been neglected. It seems that barrier reefs may have developed along the coasts of some large continental islands on the Sunda Shelf proper. Their development and geologic history, however, remains obscured. Considering the fact that much of the Sunda Shelf was exposed during the last glacial period, it is likely that the development of these barrier reefs was greatly influenced by die subaerial exposure. The development of barrier reefs around or parallel to coastlines of stable continental islands may conform to the karstic-saucer theory of barrier reef formation, as suggested by Purdy (1974a,1974b) for the Belize Barrier Reef in the western Caribbean.
Figure 15.16. The number and total length (km) of barrier reefs in the provinces.
A reef complex that may fit this classification is a small barrier-like reef along the southwest coast of Mendanau Island, located in the Gelasa Strait that separates Bangka and Belitung Islands. The entire coastline of Mendanau (i.e., c. 75 km) is fringed by a well-developed Holocene fringing reef. The island is part of a mountain range, which, according to van Bemmelen (1949), has been largely base-leveled and partly abraded. However, the Sagoweel Mountain range on the southwest coast reaches 193 m above sea level. The numerous small islands in the Gelasa Strait, and on much of the Sunda Shelf, represent drowned topography. The barrier reef is about 10 km long, with a lagoon that is 1 to 2 km wide and up to 10 m deep. The lagoon is dotted with numerous pinnacle and patch reefs, while the entire shoreline is fringed by a well-developed fringing reef with a reef flat that extends approximately 50 to 100 m offshore. However, at the southeast margin of the barrier reef, the intertidal reef flat of the fringing reef extends up to 1 km offshore. The largest offshore reef is 2 km long and about 150 m wide. Whether there are comparable reefs elsewhere on the Sunda Shelf remains to be seen.
While barrier reefs are found throughout the archipelago, they occur most frequently in the central and eastern regions (fig. 15.16). There are 34 individual barrier reefs in Sulawesi with a total length of 2084 km. This represents about 43% of Sulawesi's coastline. As mentioned earlier, the Spermonde Barrier Reef is the best known, and in terms of its length (c. 143 km) ranks fourth in Sulawesi. However, in terms of reefal area covered, which in our definition includes the entire lagoon and all its habitats, and fisheries value, it is the largest and economically most important in Sulawesi. The most unique barrier reef occurs along the south margin of the Banggai Platform, which is the western extension of the Sula Spur (van Bemmelen 1949). To the east the Banggai Platform is separated from the Sula Islands by the Bote Strait (-614 m) that connects the Molucca Sea (north) with the Banda Sea (south). To the west, the platform is separated from the East Arm of Sulawesi by the Peleng Strait (-920 m, 15-30 km wide) (van Bemmelen 1949). Coral reefs on the Banggai Platform date to the Plio-Pleistocene, with their uplifted remains forming much of Peleng, the largest island in the group. Raised Plio-Pleistocene coral reef terraces (200-600 m thick) in southwest Peleng are found at an altitude of 1000 m above sea level (van Bemmelen 1949). The absence of folding in the Peleng limestone suggests that the Banggai Archipelago sits on a stable platform. Paleontological evidence (i.e., the lack of Paleogene marine fossils) indicates that during the Paleogene the Banggai Platform was dry land.
The present 175-km-long, but not clearly delineated, Banggai Barrier Reef has developed along the south and west (south of Peleng) margins of the platform facing the Banda Sea. It is one of the most unique barrier reefs in the archipelago, since along its south and west margin we find a number of large faros. Kuenen (1933a) mentioned one of these (Karang Merpati; fig. 105, p.Ill) as evidence against Gardiner's (1931) thesis that atoll and barrier reef lagoons are being excavated. As is clearly illustrated in figure 15.17, the adjacent barrier reef lagoon is deeper than the faro (Karang Merpati) lagoon, thus largely invalidating Gardiner's arguments. Further to the south is a double faro, Karang Maringki, with extremely shallow lagoons (i.e., 1-5 m deep). Note that Darwin's theory as well as Daly's glacial-control theory assume lagoonal aggradation (i.e., infilling over time), which is clearly occurring in Banggai.
Faros (or faroes) are usually found on the edges of large atolls, such as the Tiladummati Atoll in the Maldives (Guilcher 1988). In almost all instances, faros in the Maldives are oriented with their convex sides seawards. A similar orientation is found on the Banggai Barrier Reef, where the convex sides of the faros face the Banda Sea along a southeast-northwest trending axis. The four largest faros are each about 10 km long, and up to 5 km wide. Karang Merpati has an exceptionally deep lagoon (c. -29 m), while the lagoon depths of the other faros are 3-10 m. For centuries these shallow lagoons have offered shelter where the Bajau people have built their villages.
A striking feature of the Banggai faros is that they closely follow the 200 m isobath, and thus their morphology may be largely due to the topography of the antecedent foundation. However, Guilcher (1988) suggested that surface geomorphologies of faros in the Maldives developed in response to surf, generated by alternating monsoon winds. Similar climatic conditions (i.e., alternating monsoon winds) exist in the Banggai Archipelago. The faros in Banggai are, however, protected from the Northwest Monsoon by the large Peleng and Banggai Islands, and thus have only one weather side - the Banda Sea side. The general geomorphology of the faros corresponds very closely to the southwest exposure during the Southeast Monsoon. The outer rim of these reefs is unbreached either by shallow channels or deep passages, and the reef flats and reef crests support flourishing Acropora dominated coral communities. Suharsono et al. (1995) recorded 125 scleractinian species from Karang Merpati during a brief survey of the area.
While barrier-reef-associated faros are not uncommon, and do occur in other parts of the world (e.g., New Caledonia, Mayotte Island, Great Barrier Reef, etc.) (Guilcher 1988), the Banggai faros have the strongest resemblance to the faros in the Maldives (where the name originated), perhaps not surprising for two regions with monsoonal climates. Note that Guilcher (1988) has incorrectly stated that Suvadiva Atoll, discussed by Kuenen (1947), is found in Indonesia. In his review of atoll formation, Kuenen (1947) used Suvadiva Atoll to illustrate deep passages of large atolls, but did not mention its location. However, the latitude (73° E) and longitude (0°45' N) given by Kuenen (1933a; fig. 10, p. 22) corresponds to the Maldives, where the large Suvadiva Atoll is found. The Snellius Expedition passed through the Maldives during their out-bound and home-bound legs.
Figure 15.17. Faroes of the Banggai Barrier Reef.
Drawing by B. Rahmad.
The Gulf of Tomini, located between the North and East Arms of Sulawesi, shelters some of the most diverse coral reef habitats and reef types in the archipelago. The entrance to the gulf, between Tanjung Tambalilatu on the south coast of the North Arm and the Bualemo Peninsula of the East Arm, is the narrowest (i.e., 95 km) part of the gulf. Further to the west, the gulf widens and stretches some 200 km between Poso and Tomini. Based on the detailed bathymetric chart prepared by the Snellius Expedition (1929-1930), it is evident that the Gulf of Tomini is a gently upward-sloping western extension of the Gorontalo Trough. The Gorontalo Trough begins just to the east of the gulfs entrance between the south coast of the eastern part of the North Arm and the northeastern part of the East Arm. The maximum depth is about 4180 m at the northeastern margin of the trough, but gradually decreases to about 1500 m in the western part of the gulf.
The Togian Islands, volcanic in origin, are a part of a subsiding submarine ridge that runs through the southeastern Gulf of Tomini along an east-west trending axis (umbgrove 1939c). The islands are a subaerial extension of a ridge that branches off the Bualemo Peninsula, at the easternmost part of the East Arm. The volcanic history of the Togian Islands is also suggested by the presence of an active volcano, Unauna, just northeast of Batudaka Island, which rises steeply from a depth of about 1800 m and has an altitude of 508 m above sea level. The Togian Ridge can be broadly divided into two distinct regions, each with its own unique set of coral reef habitats. The submarine ridge extends from the northwestern tip of the East Arm in a northwesterly direction to Teloga Island. These islands are the westernmost extension of a large barrier reef running from Tanjung Batuhitam to Teloga Island, and from there in an easterly direction to Tanjung Pasir Panjang with a total length of about 165 km. The barrier reef encloses a wide lagoon that is up to 100 m deep.
The Togian Islands rise steeply from a depth of about 1200-1500 m. The Togians consist of several islands, however, the five largest are, from west to east, Batudaka, Togian (Puncak Togian +542 m), Talatakoh, Walea Kodi, and Walea Bahi. The easternmost large, island Puah, is separated from the main group by the Walea Strait (-459 m, 2.5 km wide). Boasting impressive habitat diversity, from various fringing reefs to a number of different barrier reefs and atolls, the Togian Islands are located in a very sheltered environment. One of the most unique features is the absence of visible shingle ramparts that are very characteristic features of all monsoonally-influenced sand cays and shallow patch reefs in the archipelago (e.g., Pulau Seribu and Spermonde Archipelago). The best-developed barrier reef occurs off the west and north coasts of Batu Daka (fig. 15.18). Much of Batu Daka is capped by limestone, indicating a complex tectonic history. Umbgrove (1947) studied the lagoonal profiles and morphology of shorelines along the west and northwest coasts of Batu Daka and concluded that they are features of drowned landscapes, which suggests that subsidence must have occurred. He speculated that the submarine relief is at most uppermost Neogene (Umbgrove 1947).
Just to the southwest of Batu Daka is a classical high-island barrier reef at Pulau Taupan (fig. 15.19). The volcanic island is capped by a thick limestone deposit that has been raised to an altitude of 83 m above sea level. A barrier reef surrounds the entire island. The barrier forms a closed, mostly subtidal, rim that encloses a 27-m deep lagoon. Absent from the barrier reef are features such as an algal ridge, shingle ramparts and spur-and-groove formations, reflecting the sheltered nature of the region. Along the south coast, the lagoon becomes very narrow (<50 m) and shallow (3-5 m). On hydrographic charts, the southern barrier reef is depicted as a fringing reef. From aerial observations it is clear, however, that the lagoon extends around the entire island, even though it-is greatly reduced along the south side of the reef. The depth of the,narrow south lagoon is unknown.
The variety of reef types offer diverse opportunities for coral development, however, an important common feature to the region in general is the sheltered nature with regards to monsoonal influence. This relatively unique condition in such a large region is reflected in the presence of large Acropora-dominated communities, where fine arborescent and delicate "bottlebrush" species dominate. UNDP/FAO (1982) report on Togian Islands lists 59 genera and 115 species, which according to a UNEP/IUCN (1988) report is relatively low. Recent surveys by coral taxonomists have shown that the report has greatly underestimated taxonomically difficult groups such as Acropora, and most likely other groups as well. Dr. C. Wallace and J. Wolstenhome, from the Museum of Tropical Queensland, have found at least two endemics, as well as a number of acroporids with very peculiar distributions (see box 7.1). This fact alone gives the Togian reefs high biodiversity and conservation value. Table 15.2 lists the preliminary Acropora species checklist for the Togian Islands. Acropora caroliniata and A. multiacuta are very common on all five reefs surveyed in the Togians, which is very unusual, since they have been reported as very rare on reefs in other parts of the world (C. Wallace, pers. comm.; Veron 1986). Even more striking was the presence of species previously reported only from eastern Papua New Guinea and Western Australia. Wallace (1994) described A. turaki, a small hispidose species from the Rowley Shoals off Western Australia, where it occurs sporadically in lagoonal settings on unconsolidated substrates. It has recently been found in the Togian Islands, on reefs along the south coast of Talatokoh, where it is one of the dominant coral species (C. Wallace and J. Wolstenhome, pers. comm.). Another interesting find was A. jacquelineae Wallace (1994), first described from Huon Gulf in eastern Papua New Guinea. This extremely delicate side-attached plate species occurs on walls. In the Togians it was a very common species on the walls of Pasir Tengah Atoll, just north of Batu Daka.
Figure 15.18. Map of Batu Daka in the Togian Islands, Gulf of Tomini, illustrating a well-developed barrier reef system with an extensive lagoon.
Figure 15.19. Chart of Pulau Tapuan in the Togian Islands, Gulf of Tomini, illustrating a well-developed barrier reef system with an extensive lagoon. Tapuan Island is a 83-m-high volcanic island capped by limestone.
Drawing by B. Rahmad.
Table 15.2. Preliminary species checklist of genus Acropora in the Togian Islands, Gulf of Tomini, Central Sulawesi.
Another striking feature of the Acropora communities in the Togian Islands is the absence of species such as A. humilis, A. aspera and A. gemmifera (C. Wallace, pers. comm.). These three species are very common everywhere else in the archipelago, and are characteristic of high energy (i.e., wave related) reef flat habitats. The presence of A. palifera and A. hyacinthus, which are also very common species in high-energy reef crest and upper reef slope environments, suggests that strong tidal currents are the dominant environmental factor controlling the development of these reef systems.
Oceanic barrier reefs occur throughout the archipelago, but very little is known about them. One of the relatively few barrier reefs in East Nusa Tenggara occurs at Pulau Besar, just outside of Maumere Bay. However, well-developed barrier reefs are found along the northwest coast of Sumbawa and along the north coast of Flores. The absence of barrier reefs along many of the larger islands in the eastern parts of the archipelago may be related either to lack of suitable foundations or to rapid tectonic shifts. Considering the rapid tectonic uplift measured at Alor (Hantoro et al. 1994), the paucity of barrier reefs in this region is not all that surprising, and one can easily sympathize with Darwin for his hesitancy to classify Indonesian reefs as anything but fringing reefs.