Ecology of Indonesian Papua Part Two. Andrew J. Marshall
2,000 km long, covers an area of about 128,000 km2, and contains about 2,500 individual reefs. Coral reefs can also be very thick. At Enewetak Atoll in the Marshall Islands, a hole was drilled through over 1,400 m of coral rock before reaching the underlying volcanic rock. Thus, some reefs are major geological features of planet earth. The coral rock near the bottom of the coral at Enewetak was found to be 65 million years old, indicating that some coral reefs are also very old.
Three of the most widely recognized coral reef shapes are the fringing reef, barrier reef, and atoll. A fringing reef grows along the shore of a landmass, much like a fringe along the edge of a coat. A barrier reef parallels a coastline, with a lagoon between the barrier reef and the shore. These lagoons are relatively shallow (about 10–50 m deep) and usually have a sandy bottom. Barrier reefs are named because if they are continuous and reach the surface, they are a barrier to navigation. An atoll is a ring of coral with no land other than some low sand islands within the ring of coral. The center of the atoll is a lagoon. Other reef shapes include bank or platform reefs where an offshore reef does not reach the surface, and patch reefs, which are small patches of coral in lagoons. There are a wide variety of other shapes and intermediates between categories as well (e.g., Andréfouët 2004; Guilcher 1988; Hopley 1982; Tomascik et al. 1997).
Evolution of Reefs
Charles Darwin proposed a theory of the evolution of coral reefs (Darwin 1842; Tomascik et al. 1997). He proposed that coral reefs begin as fringing reefs, then become barrier reefs, and finally become atolls. Darwin’s theory applied to volcanic islands. He suggested that after an oceanic volcano erupts and builds an island, fringing coral reefs will grow around the shoreline of the volcanic island and that the island will then slowly subside or sink. As the island sinks, the reef will grow upward, and if it can grow upward as fast as the island sinks, eventually the reef will be separated from the island by a lagoon. The reef will then be a barrier reef, and the reef will mark the location of the original shoreline of the island. Eventually the island will sink under water and out of sight under the lagoon sand. The result is an atoll (Figure 5.2.1). Darwin knew that the way to test his theory was to drill into an atoll. He predicted that the drill would reach volcanic rock under the coral. The technology to drill such a deep hole was not available in his time, but eventually atolls such as Enewetak were drilled and volcanic rock was found underneath the coral rock, confirming Darwin’s hypothesis. The full sequence of evolution of coral reefs can be seen in Hawai’i, where the Big Island has an erupting volcano and fringing reefs. Small barrier reefs can be found on older islands such as Oahu and Kaui, and the oldest islands (which are found in the northwestern Hawai’ian Islands) are all atolls (Grigg 1982, 1997; Scott and Rontondon 1983)
Figure 5.2.1. Schematic diagram of the creation of a coral atoll. Charles Darwin proposed that fringing reefs growing on volcanic islands changed over time to produce barrier reefs and ultimately formed coral atolls. See text for details.
Papuan Reefs
Papua Province has many fringing reefs, some barrier reefs, and very few atolls. So, for instance, in the Raja Ampat Islands, reefs are mostly fringing or platform reefs with 36 fringing reefs and nine platform reefs reported in one study (Mc-Kenna, Boli, and Allen 2002). Several maps show coral reefs in the Raja Ampat Islands at the west end of Papua, around the islands in and along the western shore of Cenderawasih Bay, on the south shore roughly across from Cenderawasih Bay, and around the Aru Islands south of the main landmass (UNDP/FAO 1988; Spalding, Ravilious, and Green 2000; Burke, Selig, and Spalding 2002). The south coast of the Vogelkop Peninsula has narrow fringing reefs, with 450 km of coastline suitable for reefs. Tomascik et al. (1997) list nine barrier reefs in Papua, totaling 601 km in length and having an area of 2,366 km2. They also list one atoll in Irian Bay, nine in the Halmahera Sea, and five in the Pacific Ocean.
The extent of coral reefs in Papua may be underestimated due to the assumption that coral reefs cannot live along coasts that have mangroves. The muddy shorelines associated with mangroves are often not suitable habitat for coral reefs, yet in New Guinea there are places where coral reefs grow adjacent to mangroves, and mangroves even grow onto the reef platform (P. Dalzell, per. comm.). Some maps of Papua (e.g., Spalding, Ravilious, and Green 2000) show long stretches of coast containing some of the largest mangrove forests in the world. A long stretch of the north coast is normally depicted as being devoid of coral reefs, but fringing reefs are believed to stretch along much of the coast between Sarmi and the border with Papua New Guinea. Fringing reefs are reported on the northern coast of the Vogelkop Peninsula, as well as east from Jayapura to the Papua New Guinea border, and a fringing reef west of Jayapura for 100 km, possibly for an additional 160 km (Tomascik et al. 1977). The coral reefs of Papua New Guinea are much better studied, and yet about half of its coastline has not been explored for coral reefs (Yamuna and McClanahan 2001). Whitehouse (1973) claimed that on the north coast of Papua New Guinea there are no active coral reefs for 1,250 km, but Kojis, Quinn, and Claereboudt (1985) found fringing reefs with a high coral cover and diversity are common along this coast except near the mouths of rivers. The north coast of Papua probably has similar reefs. The total amount of coral reefs in Papua could be several times that presently known. Reefs in Papua are protected from strong wave action. Reefs at 45 sites in the Raja Ampat Islands off the western end of the Vogelkop Peninsula (described in McKenna, Boli, and Allen 2002) vary from those exposed to the open ocean, to those that are in sheltered bays, to one that was so enclosed it was virtually a saltwater lake (Mayalibit Bay within Waigeo Island). Strong currents were not encountered at most sites. The seas are very calm compared to those at oceanic mid-Pacific reefs or the Great Barrier Reef. The reefs do not feature a reef crest with large crashing waves and high cover of coralline algae, nor extensive reef flats and lagoons. Rather, the bottom usually slopes away directly, starting at the shoreline. This is typical of reefs in the region, as the author has seen in northern Sulawesi (Allen and McKenna 2001), eastern Papua New Guinea (Allen et al. 2003), Malaysia (Harborne et al. 2000), and 11 areas in the Philippines (Fenner, under review c; Werner and Allen 2000). Reefs on the southwest side of Cenderawasih Bay include patch reefs with seaward margins that are sheer drop-offs from the crest to a first ledge at 20–40 m depth. Sub-sea level patch reefs have a variable gradient fore reef. Fringing reefs have a variable gradient in bays and sheltered areas but are steeper elsewhere (UNDP/FAO 1982; UNEP/IUCN 1988).
Coral cover has been used as a measure of coral health. Because damage to reefs reduces coral cover, reefs with higher coral cover have been presumed to be in better condition. Data on coral cover for 13 sites in the Padaido Islands in Cenderawasih Bay are available (Tomascik et al.1997). Most commonly they had cover of 25–50%. In 44 sites in the Raja Ampats, McKenna, Boli, and Allen (2002) found that the average coral cover was 28%. A qualitative scale for coral cover was devised by Gomez et al. (1994), where 75–100% cover was considered excellent, 50–75% good, 25–50% fair, and 0–25% poor. Thus, reefs in these two areas would be considered fair. Indonesia as a whole is reported to have 2.6% of its reefs as excellent, 24.2% good, 31.6% fair, and 41.6% poor condition (Ming et al.1994). Such a scale should be approached with caution, since it implies value judgments that are not based on empirical studies. Coral cover varies substantially depending on habitat and sediment dynamics. No clear generalization can be drawn on the relationship of coral cover to reef health (Maragos 1997). Further, reefs that are among the most pristine known rarely have coral cover in the excellent range, and frequently are in the fair range. Examples include the northwest Hawai’ian Islands (Grigg 1983) and the Great Barrier Reefs (AIMS monitoring, www.aims.gov.au). The scale was originally proposed as a measure of reef health and degradation, yet the natural baseline conditions of reefs are not known. Senior ASEAN scientists assessed eastern Indonesia as having 10% of its reefs degraded 50 years ago and 50% in 1993 (Ming et al. 1994), but figures for Papua were not given. The latest Status of Coral Reefs of the World: 2004 Report (Wilkinson 2004) indicates that the overall reef condition of Indonesia has been improving since 1999, with a shift from reefs with less than 25% cover to reefs with 25–50% cover.
SEA LEVEL CHANGES
The area of eastern Indonesia and New Guinea is a geologically active area, due to the collision of the Indo-Australian, Eurasian, Caroline, Philippine,