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Marine Environment and Coral Reefs
Palau has an abundance of coral reef habitat types, as well as complex marine habitats associated with coral reefs, including mangroves, seagrass beds, deep algal beds, mud basins, current swept lagoon bottoms, and rich tidal channels. According to Yukihira et al. (2007), the total area of coral reefs in Palau is approximately 203 square miles (525 km²) and includes barrier reefs (102 square miles or 265 km²), fringing reefs (75 square miles or 195 km²), and atoll habitats (25 square miles or 65 km²) with 1,457 patch reefs scattered throughout the lagoons. An effort to map Palau’s benthic habitats using high resolution satellite imagery was completed by NOAA’s Center for Coastal Monitoring and Assessment’s Biogeography Branch in 2007. The project classified marine habitats for 571 square miles (1,478 km2) and estimated that coral reef and hard bottom areas cover 344 square miles (892 km2).
Palau’s marine habitats stretch from Ngaruangel Atoll in the north to Helen Reef Atoll in the south. A barrier reef surrounds much of the main island cluster from the northern tip of the island of Babeldaob down to the southern lagoon, merging into the fringing reef with the island of Peleliu in the south. The barrier reef is well-developed on the west coast of the main archipelago – extending for 89.5 miles (144 km) – and less developed and discontinuous on the east. There are also fringing reefs along several island coastlines and numerous patch reefs scattered throughout the archipelago. There are 14 Marine protected Areas (MPAs) with coral reefs, which comprise 40% of the coastal zone area. A national monitoring program annually monitors 22 coral reef sites.
There are more than 70 marine lakes in Palau, each with its own unique physical, chemical and biological characteristics. The lakes were formed both by rising sea level inundation of the land and by erosion of the limestone islands, which created depressions, cracks, and crevices into which marine water seeped over time. Palau’s marine lakes are their own individualized ecosystems, containing atypically small and isolated populations of unrelated marine organisms that have inhabited and independently evolved for many thousands of years. These heterogeneous environments are natural laboratories for adaptive evolution and speciation.
Some lakes in Palau are comprised of a mixture of fresh rainwater and marine water. Others are vertically stratified and less dense, with lighter rainwater resting on top of denser salt water to form two distinct layers. Some lakes have an anoxic, noxious layer at deeper depths. In addition, certain lakes have completely lost their connection to a lagoon, while others show a continuum from completely isolated to almost lagoon-like. The largest lake in Palau is called the Metukercheuas Uet Lake. This oxygen-rich lake measures over 1.5 miles (2.4 km) long and 200 feet (60.96 m) deep.
The Kayangel Atoll consists of four low coral islets with only one inhabited island. The lagoon measures 1.4 miles (2.3 km) east to west and 3.1 miles (5.0 km) along the north-south axis. Seagrass beds are found on the lagoon shorelines of Kayangel and Ngeriungs (a small island in the southeast of the atoll), and small patch reefs are found in the other islets of the atoll chain. The lagoon of Kayangel Atoll generally has low coral cover and diversity, although 126 coral species belonging to 47 genera have been reported there. In addition, 147 species of other marine invertebrates have been accounted for.
Ngaruangel (Ngaruangl) Reef is an incipient atoll northwest of Kayangel, which is separated from it by the Ngaruangl Passage. Ngaruangel Reef is protected by the Ngaruangel Reserve. The lagoon floor is covered with thick sand deposits and thickets of staghorn coral.
Velasco Reef, in the northernmost reef area of Palau, is a sunken atoll north of Ngaruangl Reef, rising steeply from the surrounding seafloor. It is not clearly separated from Ngaruangl Reef and appears as its large but submerged northern extension. Heavy wave exposure limits coral diversity and cover on Velasco Reef. The lagoon is 131-164 feet (40-50 m) deep with scattered patch reefs. Beds of the seagrass, Thalassodendron ciliatum, uncommon in the rest of Palau, are found in the western rim and northern tip of the lagoon.
One hundred forty-five coral species belonging to 52 genera have been reported for Ngeruangel and Velasco Reefs.
Northern Barrier Reef and Lagoon
Northern Barrier Reef of the Republic of Palau
The Northern Barrier Reef and Lagoon lies south of Kayangel Atoll and north of Babeldaob. It encompasses an area of approximately 200 km² of enclosed reefs and lagoons and is a complex system of rich marine biodiversity and fish spawning aggregations. The Northern Lagoon, which is home to only two high islands – Ngerkeklau and Ngerechur, contains numerous patch reefs, pinnacles, and reef holes. More than 107 species of macroinvertebrates have been reported from within this complex.
Deep-water channels situated along Palau’s Northern Barrier Reef are spawning areas for groupers and other commercially important fishes. Ebiil Channel, only a few miles north of Ngarchelong, is one of the most important grouper spawning sites in Palau, prompting the community of Ngarchelong to make it a specially managed conservation area.
Western Babeldaob has an extensive fringing reef that extends 118 miles (190 km) from Ngarchelong in the north to the southernmost part of Babeldaob. It is continuous, except where a channel cuts through the entrance to Ngermeduu Bay, and is comprised of numerous partial channels, fingers, and indentations. It provides habitat for more than 200 species of corals and approximately 200 species of macroinvertebrates. Three main passes connect the western barrier reef to the open ocean. These passes are important migratory routes for many fishes and other marine organisms.
Ngermeduu Bay in western Babeldaob is the largest estuary and bay in Micronesia,
containing mangroves, seagrass beds, and coral reefs. Several rivers including the Ngermeskang – the longest river in Micronesia, drain into the bay. Ngermeduu Bay is a unique area in terms of overlapping fish communities. One fish community is the rich coral reef assemblage typical of other areas in Palau and Micronesia, and the other fish community is dominated by planktivores, which are typical of islands in Indonesia. These ecosystems support a highly productive fishery for mangrove crabs, sea cucumbers, rabbitfish, snappers, groupers, surgeonfishes, jacks, parrotfishes, and many more.
The Toachel Mlengui channel, north of the Ngermeduu Bay entrance, is the most important pass connecting the open sea with the western and southern lagoon. The next pass, Sengelokl, located 53 miles (85 km) south, only offers subtidal connection between the western ocean and the southern lagoon. During 1992 Ngermeduu Bay Natural Resource Surveys, 200 species of corals, 170 species of other macroinvertebrates, and 277 species of fishes were found.
In eastern Babeldaob, the reefs run from Ngos to Airai and end at Toachel El Mid, a deep east-west channel separating Babeldaob from Koror. Fringing reefs cover the entire coastline of eastern Babeldaob, while protective barrier reefs lie toward the southern part of the island. Those areas that have fringing reefs with protective barrier reefs have extensive seagrass beds and mudflats. In areas without protective barrier reefs, the inner fringing reef seagrass beds are not as extensive and sand replaces mudflats. Two hundred species of scleractinian (stony) corals have been reported from eastern Babeldaob and approximately 69 species of non-coral invertebrates. Sea squirts, sponges, sea stars, and sea cucumbers are also common on the reef flats.
Southern Barrier Reef and Lagoon
Palau's famed "Rock Islands" are a collection of rounded, foliage-covered isles which seem to float above the surface of the water.
The Southern Barrier Reef and Lagoon is the largest area in Palau covering 193 square miles (500km2) from Toachel El Mid, through the pass between Babeldaob and Koror, and extending south to the island of Peleliu. It is also home to a 53.4 mile-long (86 km) barrier reef. The Southern Lagoon has a large number of small islands, including the Rock Islands, low coral islands on the barrier reefs, and raised platform islands. Palau’s nine unique marine lakes are found in the interior of some Rock Islands. The Southern Lagoon also has more patch reefs than any other region of Palau.
An incomplete channel of the Southern Barrier Reef, Ngerumekaol is an important fish aggregation site, which has been established as a conservation area by the Palau government. According to a 1991 survey, coral cover at Ngerumekaol was 52% with very high coral diversity (90 coral species). Nearby, at Rebotel, another incomplete channel, coral cover was 50% with diversity as high as 55 species.
On the ocean side of the barrier reef, coralline algae are common with seaweed beds along the outer margins. Sand deposits and coral knolls dominate the lagoon side. The southeastern walls of the Southern Barrier Reef have coral cover around 50% (40 species), whereas ocean slopes off Ngederrak, just south of Koror, have 50% coral cover (45 species).
The isolated island group, known as Ngerukewid or Seventy Islands, in the Southern Lagoon is a protected Wildlife Preserve. These inner reefs are characterized by sand and rubble interspersed with branching Acropora, Porites, and micro-atolls. Coral cover is variable, ranging from 3–51%.
Peleliu and Angaur
Seagrass meadow in Peleliu (photo by Brad)
Peleliu supports the largest seagrass beds in Palau. Coral cover and diversity on the reef flat terrace is low, but the reef walls are more diverse with higher coral cover. In Angaur, coral cover ranges from 50-80% along the western coast and 25-40% along the other areas of the island.
Prior to the 1998 coral bleaching event, Helen (or Helen Reef), the smallest of the remote Southwest Islands, was home to 248 species of coral, making it the most diverse coral reef known to exist in Palau or in the Pacific Islands. Fanna, the third smallest island of the Southwest Islands, has the least amount of reef habitat and fewest coral species at 94.
The coral reefs of Sonsorol, the largest island in the southwest of Palau, showcase about 130 species of corals, while Merir, the second largest island, is home to 132 species. Tobi, the third largest island in the Southwest group, is surrounded by healthy fringing reefs that support 174 coral species.
Staghorn coral in Palau (Photo: Jane Thomas, IAN Image Library)
Not only is the Palauan archipelago home to the greatest diversity of terrestrial plant and animal species in Micronesia, it also provides habitat for greater than 10,000 marine species. Situated close to the global center of biodiversity, Palau has the highest recorded levels of marine biodiversity in Micronesia, especially among corals. The archipelago is also thought to have the highest density and variety of tropical marine habitats of comparable geographic areas around the world, including the Philippines, Indonesia, and Australia, which has served to attract divers and snorkelers from all parts of the globe. While known endemism among Palauan marine organisms is low, many groups of organisms are not well documented; therefore, true levels of endemism in the marine environment are difficult to estimate.
Numerous marine ecosystems within Palau, including fringing, patch, and barrier coral reefs, seagrass beds, mangrove forests, and marine lakes, support about 300 species of sponges (Kelly-Borges and Valentine 1995), 185 species of sea slugs, 21 species of sea lilies, more than 100 species of sea squirts, 1,300 species of reef fishes, 350 species of stony coral, and greater than 200 species of soft coral. Palauan waters also provide habitat for endangered and threatened species such as the dugong (a relative of the manatee), saltwater crocodile, hawksbill and green turtles, and giant tridacna clams (which include seven of the world’s nine giant clam species). However, of the more than 10,000 species in Palau’s marine environment, only two major groups of marine organisms can be considered “well-studied”: the scleractinian (stony) corals and coral reef fishes.
Palau contains extensive seagrass beds, which are comprised of 10 species of seagrasses. Seagrass beds are important nursery grounds that provide food and shelter for juvenile fishes, invertebrates, sea turtles, and dugongs. They also assist in sediment accumulation and stabilization. Thalassia hemprichii and Enhalus acroides are the most abundant and dominant species of seagrasses in Palau. They serve as the main food source for herbivorous fishes, sea turtles, and dugongs and provide habitat for rabbitfish, pipefishes, juvenile wrasses and parrotfish, and various invertebrates.