NGERUANGEL AND KAYANGEL ATOLLS
Ngeruangel and Velasco Reefs are the northern most reefs in Palau sharing the same reef formation. Ngeruangel is a small atoll that has only one small unvegetated coral islet at the Southwest side. The reef that forms the barrier of Ngeruangel atoll is nearly continuous except for some small passes in the west and the north facing Velasco Reefs. The lagoon measures 2.7 km along the eastwest axis and 4.6 km along the north-south axis. A total of 145 coral species from 52 genera were reported for Ngeruangel and Velasco Reefs during the REA (Maragos et al. 1994b
Kayangel atoll is made up of four low coral islets with only one inhabited island. The lagoon measures 2.3 km along the east-west axis and 5 km along the north-south axis. The lagoon of Kayangel atoll generally has low coral cover and diversity. The channel (Ulach) along the western rim supports healthy marine communities including corals. During the REA surveys in 1992, the average coral cover for the outer walls was 70%, with 50% on the inside. Coral cover on the reef slope averaged 20-25% with the number of coral species varying from 40-50 at different sites along the ocean slopes. A total of 126 species of corals belonging to 47 genera have been reported for Kayangel atoll. Marine invertebrates other than coral observed around the atoll totalled 144 species. Seagrass beds are found on the lagoon shorelines of Kayangel and Ngeriungs. Small patches are found in the other islets of the atoll chain.
In 1998, surveys of Ngeruangel were conducted using a Manta Tow and revealed 50-60% of corals were bleached at the south and west side of the islands (PCS unpublished data). At the western side of the atoll (barrier reef), live coral cover ranged from 10-30%, with an average cover of 5-10%. Recently dead corals accounted for 25-35% of the cover, and all table Acropora were dead. An estimated 75-85% of the soft corals in the west and south side of Ngeruangel were bleached. On the eastern side of the atoll, coral cover on the slopes ranged from 1-10%, with an average around 1-4%. About 20% of the live corals were bleached and all staghorn Acropora were dead. Live corals consisted mainly of massive corals. Recently dead coral cover averaged 10-25%. The lagoon patch reefs showed very low live coral coverage. Only a few Heliopora and massive corals were found in the patch reefs. All of the staghorn Acropora were dead; about 50-70% of the corals on lagoon patch reefs were dead. The southern side of the main channel also showed similar results as the patch reefs. Most of the corals on the southern side of the channel were dead. The few live corals consisted mainly of blue and brain corals. (PCS, unpublished data).
No surveys have been conducted in Kayangel Atoll since the 1998-bleaching event.
NORTHERN BARRIER REEF AND LAGOON
The west barrier reef and the east fringing reef extend further north to form the northern barrier reef and lagoon that lies south of Kayangel Atoll and north of Babeldaob. The northern lagoon measures 325 sq. km and contains numerous patch reefs, pinnacles and reef holes. Ngerkeklau and Ngerechur are the only two high islands in the northern lagoon.
One mound in the lagoon had a coral cover of 35% with 50 coral species. The reef flats at the northern barrier reefs average 1.7 km in width with the widest margins reaching 5.8 km. Fleshy algae, coralline algae and corals dominated the seaward side of the reef flat. Numerous reef holes were found in the middle of the reef flat. Micro-atolls could be found on the lagoon side of the reef flat.
The ocean reef slopes reach a total length of 62.7 km. Coral cover at the northeast slopes averaged 10% and diversity of coral species was 35. The protected bight at the southern end Ngkesol had a higher coral cover at 25% and coral diversity with 45 species of corals. Coral cover on western facing ocean slopes ranged from 60-70% with coral diversity at 35 species. Northwest facing reefs had lower coral cover ranging from 10-20%, but have higher species diversity with 50 species encountered during the REA. The northern reef slopes, which are protected by Ngerael, Ngkesol and Kayangel reef had higher coral abundance and diversity.
There are seven passes at the northern reef complex. Coral cover in the passes ranged from 10-30%. The diversity of corals in the passes ranged from 35-40 species. Relatively rare corals such as Siderastrea, Psuedosiderastrea and Merulina are present in the passes. The total number of coral species that have been reported for the northern lagoon complex is 161 in 62 genera. There are 107 species of macro-invertebrates other than corals observed at the northern lagoon complex.
No surveys have been conducted in the Northern Lagoon since the 1998-bleaching event. Observations at some of the reef areas and passes in the northern lagoon reveal high coral mortality, reaching about 90% in certain areas. A survey of 490 Porites heads on a shallow reef near Ngeregabal Island, central lagoon found 32% were totally bleached (with over 90% of the surface dead), 27% were partially (20-80% dead) bleached, and 41% had only minor bleaching. This area was resurveyed in March 1999 and it was found that 100% of Porites heads seen as bleached from the air, subsequently had complete or partial mortality. Re-photos of the same reef, also revealed that the bleached coral heads had died and become covered with algae (CRRF, unpublished). Other surveys in the rock islands area indicated of 161 colonies of Lobophyllia spp. 35% were dead, 58% partially dead, 6% with slight damage and 1% or less unaffected by bleaching (CRRF, unpublished).
Western Babeldaob has an extensive fringing reef that extends 190 km from Ngarchelong in the north to the southern most part of Babeldaob in Airai; longer than all the other fringing reefs of the Palau main islands combined. The fringing reef is continuous except the entrance to Ngermeduu Bay where a channel cuts through it. Western Babeldaob fringing reefs are complex, exhibiting numerous partial channels, fingers and indentations. A total of 220 corals have been reported from western Babeldaob. The number of macroinvertebrates other than corals that have been reported from western Babeldaob is 200 species.
The inner fringing reefs contain thick seagrass beds. Large populations of numerous species of sea cucumbers were found among these seagrass beds. Toward the outer part of the fringing reefs, seagrass becomes less common and coral heads and micro-atolls and soft corals become more common. Outer slopes of the fringing reefs have 30-40 different coral species with coverage of 33- 50%.
The western Babeldaob lagoon has over 500 patch reefs. Coral cover on the slopes of the patch reefs was around 50%. Coral diversity ranged from 45-70 species in different sites. Several rare corals were found in these patch reefs including Cynarina, Zoopilus, and Siderastrea.
Three main passes cut the western barrier reef. Since only three passes connect west Babeldaob to the open ocean, these passes are very important in that they serve as migratory routes for many fish and other marine organisms. Coral cover and diversity was high on the walls of these passes. Coral cover ranges from 50-70%. Coral diversity was moderate to high with numbers ranging from 45-95 species at different sites. Coral cover at the lagoon slopes of the barrier reef was 60% and diversity of 45 species. The rare coral Podabacia was found on the barrier reef slopes.
The Ngermeduu Bay region in west Babeldaob is one of the most significant areas in the whole of Palau. Ngermeduu Bay is the largest estuary and bay in Micronesia. Several rivers including the Ngermeskang, the longest river in Micronesia drain into the bay. Several important ecosystems are found surrounding Ngermeduu Bay including mangroves, seagrass beds, and coral reefs. These ecosystems support a highly productive fishery for mangrove crabs, sea cucumbers, rabbitfish and numerous other desirable fishes such as snappers, groupers, surgeonfishes, jacks and parrotfishes.
During the 1992 Ngermeduu Bay Natural Resource Surveys, 200 species of corals (Maragos, 1992
), 170 species of invertebrates other than corals (Richmond, 1992
) and 277 species of fish (Amesbury, 1992
) were found. Areas in Ngermeduu Bay region that are away from freshwater and sedimentation stress, averaged 50 coral species per site with several sites having as many as 60 species of corals. The sites with high species diversity also had high coral coverage averaging 50% or higher cover with some sites as high as 70%. Suspension feeders dominated the mouth of the bay. The seagrass beds supported numerous species of echinoderms including edible sea cucumbers. Ngermeduu Bay is a unique area in terms of fish communities with two overlapping fish communities. One is the rich coral reef assemblage typical of other areas in Palau and Micronesia, and the other fish community is dominated by planktivores more typical of islands in Indonesia (Amesbury, 1992
). The Toachel Mlengui channel slightly north of the Ngermeduu Bay entrance and 15 km southwest of Daimechesengel pass in Ngardmau, is the most important pass connecting the open sea with the western and southern lagoon. The next pass Sengelokl located 85 km south only offers subtidal connection between the western ocean and the southern lagoon.
During the period from December 1998 to January 1999, two patch reefs located northwest of Kamesang Dock on the west side of the Rael Edeng were surveyed. The larger patch reef known as Oruaol Libuchel was declared as a clam conservation area in late 1998 and Ngatpang State Law restricts entry into the site. The patch reef immediately to the south of Oruaol Libuchel was also surveyed as a control site. Corals cover at Oruaol Libuchel was little over 17%, and almost 21% of the corals were dead, probably due to the recent La Nina bleaching and the subsequent high seawater temperatures. The east side of the conservation area showed higher coral cover at 26% and lower dead coral cover at 11%. A nearby patch reef was also surveyed as control for the conservation area monitoring. The reference site had a higher coral cover at 35% and 13% dead coral cover (Golbuu, unpublished data).
In east Babeldaob, the reefs run from the northern end of Ngarchelong at Ngos to Airai in the south ending at Toachel el Mid. Fringing reefs cover the entire coastline of east Babeldaob and from Ngarchelong to Ngaraard, the fringing reef lacks a protective barrier reef. The fringing reef in Ngiwal is fronted by a submerged barrier reef. Melekeok does not have a barrier reef, but further south in Ngchesar and Airai, there are protective barrier reefs. The inner margins of the fringing reef contain seagrass beds. 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, with sand replacing mudflats.
Stony and soft corals dominated the outer slope of the fringing reefs in east Babeldaob. Coral cover was 10-50% while diversity was 65-75 species on the slopes of fringing reefs not protected by barrier reefs. The barrier reefs along the east coast are not well developed and discontinuous. Ngiwal, Ngchesar and Airai have protective barrier reefs, with coral species diversity ranging from 70-90 species at different sites with 50% or higher cover. Thein slopes of lagoon patch reef at Ngchesar had 40% coral with 40 different species of corals (Maragos et al. 1994b
There are 11 passes and channels at east Babeldaob, with coral cover and diversity higher on the pass walls compared to the inner channel slopes, which are exposed to more sedimentation. The coral cover at the inner channel slopes is around 20% with diversity at 40 species (Maragos et al. 1994b
A total of 200 species of stony corals have been reported from eastern Babeldaob. Ascidians, sponges, sea stars and sea cucumbers are common on the reef flats at Babeldaob e.g. a survey of three sites in east Babeldaob found 69 species of invertebrates (Maragos et al. 1994b
Manta Tow surveys of the slopes of Ngemai reef in Ngiwal, east Babeldaob before and after the massive bleaching event suggest no significant change. Live coral cover ranged from 0-30% in 1997 and 10-30% in 1998. In 1998 survey of the same sites, coral cover was at 10-30% and dead coral cover was 0-10%. At the seaward edge of the reef, soft coral cover was higher than that of hard corals, ranging from 50-75% (PCS, 1999).
Ngchesar in east Babeldaob was seriously hit by the 1998 bleaching, with most of the corals on the barrier reef recently dead. The Ngchesar fringing reefs were not as badly affected by the bleaching as the barrier reef, with some surviving corals, mainly in the genus Porites. Most of the Acropora on the fringing and patch reefs killed as a result of the bleaching. Only few Acropora at Ngchesar survived the 1998 bleaching.
SOUTHERN BARRIER REEF AND LAGOON
From Toachel El Mid, the boundary between Babeldaob and Koror, and extending south to gemelis and Peleliu, the southern lagoon area is the biggest in Palau covering 500 sq. km. The barrier reef is extensive, measuring 86km along the ocean side. The southern lagoon has the largest number of islands, many of which are referred to as 'rock islands' and also has more patch reefs than any other region of Palau. Three types of islands can be found in the southern lagoon: rock islands; low coral islands on the barrier reefs; and raised platform islands. The unique marine lakes are found in the interior of some rock islands in the southern lagoon.
Pre Pre-1998 bleaching
In 1976 and 1991, quantitative surveys were conducted on the fringing reef at the southern tip of Malakal. In 1976, the coral cover at the reef margin was 60% and 74% on the slope. During the survey of the same site in 1991, coral cover was 56% on the reef margin and 82% on the slope (Birkeland et al. 1976
; Birkeland et al. 1993
). There were no significant differences in coral cover between 1976 and 1991.
Several passes connect the southern lagoon to the open ocean, but they are concentrated at the south east side of the barrier reef. Sengelokl at the southwest side offers only limited tidal exchange with the open ocean. Denges channel is located at the south east before Sar Passage, and includes several passes being the main connection between the lagoon and the open ocean. Coral cover at the pass walls ranged from 25% to 50%. Ngemelis bight has high coral diversity at 70 species while diversity at Denges ranges from 40-45 species at different sites (Maragos et al. 1994b
There are several dead end channels at the south barrier reef. The most notable is Ngerumekaol a known aggregation site for reef fish and a conservation area established by both the Palau National Government and recently, the Koror State Government. During the Ngermeduu Bay Natural Resource Survey conducted in 1991, coral cover at Ngerumekaol was 52% with very high coral diversity (90 coral species; Maragos, 1992
). Nearby, at Rebotel, another dead end channel, coral diversity was 55 species and coral cover was 50%.
Sand deposits and coral heads dominate the lagoon side of the barrier reefs. On the ocean side, coralline algae and pavements are common, with dense seaweed beds along the outer margins. The reef points show high coral cover. The southeast walls of the barrier reef have coral cover around 50% and diversity of 40 species, whereas ocean slopes off Ngederrak have 50% coral cover and 45 coral species.
The isolated island group known at Ngerukewid or Seventy Islands in the southern lagoon has been a protected area since 1956. In 1989, Ngerukewid Islands were surveyed using quantitative techniques by scientific teams organized by the South Pacific Regional Environmental Program (Birkeland and Manner, 1989
). For the benthic communities, four transects were surveyed in three different regions: interior reefs; fringing reef around the islands; and patch reefs outside the islands but within the reserve. Coral cover in the interior reefs was variable, ranging from 3 - 51% cover. The inner reefs are characterized by sand, sand and rubble interspersed with branching Acropora, with Porites mounds and micro-atolls common.
In general, the fringing reefs around the perimeter of the islands have higher coral cover and diversity than the inner reefs. On the reef flat, coral cover ranged from 0-20%, but the north and northeast facing reefs had higher cover on the reef flat averaging 10-50%. The reef edge had relatively low coral cover (5-25%) but as the reef starts to curve toward the lagoon Acropora aspera becomes dominant and coral cover reached 40-80% at some places. The slope of the fringing reef had the highest diversity and coral cover of the other reef zones, ranging from 20-80% cover.
The outer patch reefs have higher coral cover and diversity than the inner reefs and perimeter reefs. Coral cover on the patch reef slopes ranged from 40-90%. Acropora dominated the upper reef slopes of the patch reefs, but further down the slopes, massive and other branching corals were more common.
8 In 1998, quantitative surveys were conducted at Ngerumekaol (Ulong Channel) and reef slope by Palau Community College and the Koror State Government revealed that about 48% of coral colonies were bleached, with a high percentage of corals dead (Bruno et al. in review
). Bleaching was widespread across depths, habitats and coral taxa. There were considerable variations among species on their response to the rise in seawater temperature, even within genus. Within the genus Acropora, A. echinata and A. hyancinthus experienced 100% mortality, while the corymbose forms appeared to have fared better. Coral cover on the channel was 24% and the reef slope was 23%. Dead coral cover on the channel was 38% and the reef slope, 33% (Golbuu et al. 1999
). Most dead corals were recently killed with the skeletons still intact and not covered with algae. If the dead coral cover is added with the live coral cover, the total cover would be similar to that revealed in the 1991 REA. The 1999 surveys also revealed that most dead corals were in the genus Acropora. Before the bleaching, Ngerumekaol has healthy and diverse coral community. Coral cover at Ngerumekaol before the bleaching was 52% (Maragos, 1992
In 1999, the fringing reef at the southern tip of Malakal was resurveyed with increased living coral cover compared to previous surveys conducted in 1976 and 1991, but coral diversity had decreased. The increase in coral cover is due to an almost complete takeover of space by Porites cylindrica, P. rus and massive Porites.
A survey of 490 Porites heads on a shallow reef near Ngerchebal Island, in the southern lagoon, found 32% were totally bleached (with over 90% of the surface dead), 27% were partially bleached (20-80% dead), and 41% had only minor bleaching. This area was resurveyed in March 1999 and it was found that 100% of Porites heads seen as bleached from the air, had subsequent complete or partial mortality. Later photos of the same reef also revealed that the bleached coral heads had died and become covered with algae (CRRF, unpublished). Other surveys in the rock islands area indicated that of 161 colonies of Lobophyllia spp. 35% were dead, 58% partially dead, 6% had slight damage and 1% or less were unaffected by bleaching (CRRF, unpublished).
Coral bleaching of Iwayama Bay resulted in up to 100% mortality in some coral genera (Houk 1999
). The most strongly affected were: Pocillopora -100%; Montipora - 98%; Lobophyllia - 61%; Porites - 42%. Whereas there were minor losses in: Fungia - 14%; Favia - 3%; and no mortality in Favites and Goniastrea. Many Acropora species were also severely affected. Bleaching on all colonies was more frequent on horizontal rather than vertical surfaces. Bleaching of corals on line transects showed deceases in live cover averaging 52% (range 38-80%). After the bleaching event, it was difficult to find living colonies of Pocillopora and Montipora at any site. The bleaching at this site coincided with extensive development for a large tourist resort, such that the corals were already stressed by increases in turbidity.
PELELIU AND ANGAUR
Peleliu supports the largest seagrass beds in Palau. Coral cover and diversity on the reef flat terrace was low, but the reef walls are more diverse with higher coral cover. In Angaur, coral cover was high along the western coast, ranging from 50-80%. In other areas, coral coverage varied between 25-40% (Maragos et al. 1994b
The western reef of Angaur was briefly examined in January 2000. North of the harbour, the plate corals, particularly Montipora spp. and Merulina spp., which grow down the steep reef slope, were found to be in relatively good condition. There was some evidence of mortality, probably from the 1998 bleaching event, but at a much lower level than has been seen in drop-off areas of the main Palau group. South of the harbour, bleaching mortality appeared more common in a moderately sloping reef environment, and may be a reflection of the different species composition of these two areas (CRRF, unpublished). No surveys were conducted in Peleliu after the 1998-bleaching event.
The Southwest Islands are small oceanic islets that sit on top of table mountains, except for one islet that sits on a perimeter reef surrounding a lagoon. Fana, Sonsorol, Pulo Anna, Merir, Helen and Tobi are the six islets that make up the Southwest Islands located about 340-600 km southwest of the main Palau archipelago. The fringing reefs around these islands are all subjected to strong currents, and were found to be dominated by blue coral, Heliopora coerulea, in dense stands from shallow water to depths of 20-25 meters (CRRF, unpublished).
Helen is the smallest island of the Southwest Islands, but has the largest and most diverse reef of all the other islands combined. Helen Atoll has only one vegetated islet in Helen atoll with 248 coral species reported, more than any other atoll in the Pacific Islands. Good coral development was observed in the ocean and lagoon slopes, patch reefs, and the floor and walls of the pass (Maragos et al. 1994a
Tobi is the third largest island in the Southwest group and has the third most diverse habitats. The island is surrounded by healthy fringing reefs. The corals at Tobi are in excellent Condition, with 174 coral species Tobi (Maragos et al. 1994a
). Merir is the second largest island after Sonsorol and has the second largest reef system. The number of coral species in Merir is high at 132 species (Maragos et al. 1994a
). Pulo Anna is one of the smallest islands in the Southwest Island group. The reef is small in terms of slope length and reef flat areas, with only 99 coral species, has the second lowest total of the six Southwest Islands (Maragos et al. 1994a
). Sonsorol is the largest island and its reefs are diverse, after Helen and Tobi's reefs. A total of 130 species of corals were found in Sonsorol reefs (Maragos et al. 1994a
). Fanna is the third smallest island in the Southwest Islands group and it has the smallest amount of reef habitat and only 94 species of corals (Maragos et al. 1994a
), the lowest diversity of corals among the Southwest Island group. Coral communities were in excellent condition with no signs of stress. Summary results of coral cover before and after the 1998 bleaching.
Observations during an August 1999 synoptic survey of Palau Southwest Island coral reef habitats indicate that these islands have experienced severe bleaching at about the same times as other reefs in nearby areas during the 1998 La Niña events. Suspected coral beaching and associated coral death was high at Tobi Island as observed during diving. Four days of manta tows and other surveys throughout areas of Helen Reef Atoll indicated that suspected bleaching and associated coral death at this location was also extensive, with a slight variance between habitats. At Helen, coral cover was highest on the outer slope, lower in the channel and shallow lagoon, and lowest on the inner slope. The proportion of live coral was highest in shallow lagoon/reef flat areas, while the outer slope was generally lower. The channel and inner slope had the lowest live coral cover (Weng and Guilbeaux, 2000
During surveys of selected sites in Palau, a team from the University of Guam Marine Laboratory in July 1999 found that bleaching was widespread and variable among the different sites in Palau. Bleaching devastated Acropora which had the highest mortality compared to other corals. Corals in estuaries closer to shore survived better than corals farther from shore. This trend was very evident in Ngiwal where several sites were surveyed and survivorship was highest at sites closer to land. Off-shore reefs like Short Dropoff were severely affected, and even at 30m there was approximately 90% mortality of Faviids, Poritids, Fungias and Acroporas. Mortality of Acropora spp. was virtually 100% at all depths at Short Drop-off (Paulay, unpublished data). Out of 3,630 colonies of corals from 52 genera that were surveyed in 1999 at several sites, 48% were living, 31% were dead and 21% suffered various degrees of partial mortality (Paulay, unpublished data).