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Coral Reef Bleaching In The South Central Pacific During 1994
Coral Reef Initiative U.S. Department of State Washington, D.C. October 30, 1995
Thomas J. Goreau President Global Coral Reef Allianceand
Raymond L. Hayes Professor Department of Anatomy Howard University College of Medicine Washington, DC 20059
NOTE: This report presents detailed descriptions of coral reef bleaching at 19 sites across the South Pacific during 1997. The analysis of 15 environmental variables at each site shows that while many environmental factors that are linked to human population density all have strongly negative effects on live corals, none are related to the amount of bleaching. Only high temperatures are correlated with bleaching. Analysis of satellite sea surface temperatures at sites in the Pacific, Indian Ocean, and Atlantic show clear relationships between high temperatures and coral bleaching. The implications of these findings for environmental policy to protect reefs at national and global scales are outlined. This book, based on a study undertaken for the Coral Reef Initiative of the US State Department, was published by the Global Coral Reef Alliance in 1995. Copies containing large numbers of color photographs and graphs can be ordered from GCRA.
TABLE OF CONTENTS Foreword
Summary
Introduction
Methods
Results:
A. Site descriptions and reef assessments
1. French Polynesia
a. Tahiti 1
b. Moorea
c. Rangiroa
2. Cook Islands
a. Aitutaki
b. Rarotonga
3. American Samoa
B. Interviews with Other Observers
Statistical Analysis and Discussion
A. Bleaching and local environmental factors
B. Bleaching, sea surface temperature and anomalies
Conclusions
Recommendations
Glossary of terms
Literature cited
Acknowledgments
FOREWORD The coral reef bleaching episode which appeared in the central south Pacific region in early 1994 evoked considerable concern among local marine scientists. In response to that concern, this project was conceived. A field survey was necessary to attempt to document the impacts of this event upon the reef communities at several accessible sites within that region.
The International Coral Reef Initiative of the U.S. Department of State approved the travel of two US-based scientists with several years of field experience working on coral reef bleaching in the Caribbean. Their objective in this study was to survey directly several representative coral reefs in two French Polynesian archipelagos, in the southern Cook Islands, and in Samoa. In their travels, however, they were to interview local residents whose experiences over time and into other areas of the region might allow reliable data to be added to the study beyond areas actually visited. This effort would allow them to assemble records from a broad geographical area, to trace back in time the local history of these reefs, and to develop a comparative database on reef vitality. This study was designed to document both acute and long term influences of stresses upon the region's coral reef ecosystems.
Our report is offered as a working resource to marine laboratories, scientific libraries, and individual researchers with interests in regional characterizations of coral reef and associated ecosystems. We expect that the observations and information documented herein will be of value to other marine scientists working in the central south Pacific region as well as in large scale regional studies within the tropical near shore marine environment. Therefore, we are pleased to contribute this report as a practical reference and as a complement to further coral reef investigations. Thomas J. Goreau and Raymond L. Hayes December 1994
SUMMARY A mass coral reef bleaching event was first reported across the South Central Pacific Ocean during March, 1994. Because bleaching may be related to climate change and because of the importance of coral reef protection for sustainable development of tropical marine resources, the Bureau of Oceans and International Environmental and Scientific Affairs of the U. S. Department of State sponsored a rapid field survey of the bleaching event as one of the first action programs funded under the International Coral Reef Initiative.
Nineteen reef sites were surveyed between August 3 and 22, 1994. This study covered sites within an area of 2,500 by 1,000 km and included a variety of reef types in lagoons, bays, and open water conditions in French Polynesia (Tahiti, Moorea and Rangiroa), the Cook Islands (Aitutaki and Rarotonga), and American Samoa (Tutuila). This was the first attempt to compare bleaching intensity and history across the area affected. Previous field investigations by the same team of researchers have been limited to one island or country, so that local impacts could not be readily distinguished from regional stresses.
The types of hard and soft corals present, percent living coral cover, the degree of bleaching, recent coral mortality, calcareous and fleshy algal covers, and effects of hurricanes, storms, sedimentation, eutrophication, fishing practices, crown of thorns starfish, and extreme climatic variability were assessed by visual and photographic surveys. In order to determine the history of changes in the reef ecosystem, interviews were conducted with locally-based professional divers, government officials, environmental agencies, non-governmental organizations, journalists, and scientists. These individuals had first-hand knowledge of the extent, duration, intensity, and start of bleaching, and provided information about other areas which were not examined directly by the authors.
Evidence of coral bleaching was found at all sites examined in August. Since the 1994 event had begun six months previously, bleaching to whiteness was relatively rare; most bleached corals were recovering or had already recovered, but many branching corals were recently dead. Water temperature, salinity, dissolved oxygen, and pH were measured at these sites. All values were in the normal range for the time of year, the cool season in the southern hemisphere. While there were significant differences from place to place in these parameters, the pattern was different for each variable. Coral mortality was seen at all sites, but these patterns were site-specific and did not correlate with bleaching. Reefs in American Samoa were most damaged, followed by lagoonal reefs in Tahiti and Moorea. The highest living coral cover was observed in the southern Cook Islands of Rarotonga and Aitutaki.
The only common factor at all sites had been exposure to abnormally high sea surface temperatures (more than 1 °C above the average value in the warmest months of the year) which affected the region between February and March of 1994. Corals in lagoonal habitats near human populations were less subject to bleaching than those from outer reef slopes. Bleached corals on outer slopes in American Samoa that had been subjected to sediment and pollution stresses were the slowest to recover. The results of this survey indicate that reefs in the South Pacific are highly vulnerable to a 1 °C elevation above average sea surface temperature. Similar patterns also appear to have been present during coral bleaching episodes which took place off the coasts of East Africa and Brazil in 1994. This suggests that corals around the world are near their upper temperature tolerance limits.
MAP 1. The Pacific Ocean. Relative locations of the Society Islands, Tuamotu Islands, Cook Islands and American Samoa (map adapted from Stanley, 1993)
INTRODUCTION
Coral reef bleaching (please see the Glossary at the end of the volume for detailed explanations of this and other scientific terms used in this report) is caused when environmental stress causes corals to lose the symbiotic inter-cellular algae from which corals derive their color, most of their food, and their ability to rapidly grow their skeletons and form the coral reef structure. Without their microscopic algae, coral tissues turn transparent, revealing the white limestone skeleton beneath. This apparent whitening is called 'bleaching" because it superficially resembles dead corals killed by chemical bleaching agents, but in fact the tissue is still present and the coral is alive. However it is starving, unable to grow its skeleton, and under increased risk of succumbing to other stresses such as algae overgrowth or excessive sedimentation. Death of the corals ultimately results in the erosion the growing wave-resistant coral reef structure on which all other reef creatures rely on for shelter and for food.
Prior to the 1980s all known cases of bleaching were geographically limited in extent and caused by clear local stresses such as restricted circulation (Mayor, 1918; Yonge & Nicholls, 1931; Jaap, 1979) or hurricane floods (Goreau, 1964a). Large scale coral bleaching, affecting huge areas of reef both near to and far away from known stresses, was unknown before the 1980s, but has since become more frequent and intense (Williams et al., 1987; Williams & Bunkeley Williams, 1990; Glynn, 1991, 1993; Jokiel & Coles, 1992; Goreau et al., 1993; Goreau & Hayes, 1994). While most field researchers have attributed mass bleaching since the 1980s to unusually high ocean temperatures, there has been controversy over whether coral reefs ecosystems as a whole are undergoing climate-related stress because a number of local stresses are also potentially capable of contributing (Salvat, 1992).
In early April 1994, Thomas Goreau noticed, while scanning the Climate Diagnosis Bulletin from the National Oceanic and Atmospheric Administration (NOAA), that a warm water mass exceeding 30 °C was displayed on sea surface temperature (SST) maps of the South Central Pacific (Goreau et al., submitted). Having just completed an analysis of SSTs and coral reef bleaching in that region during 1991 (Goreau and Hayes, 1994), he immediately alerted several colleagues by facsimile, including Bernard Salvat, Director of the E.P.H.E. Iaboratory in Moorea, French Polynesia. A response from Salvat indicated that coral reef bleaching had begun in late March in both the Society and Tuamotu Islands of French Polynesia. Dr. Goreau also alerted several academic colleagues, including Raymond Hayes of Howard University, and the Coral Reef Initiative of the U.S. Department of State about this event. The Coral Reef Initiative of the U.S. Department of State is designed to foster partnerships among federal government agencies, state and local governments, and non-governmental organizations. The specific objectives of these partnerships are to promote resource conservation and sustainable development in coral reefs and associated ecosystems as well as to strengthen management of these sensitive and fragile coastal marine environments (U.S. Department of State, 1994). Through domestic and international networking, the Coral Reef Initiative has focused upon research, assessment and monitoring of coral reefs on a global scale (Crosby et al., 1995).
Deputy Assistant Secretary Rafe Pomerance encouraged Hayes and Goreau to submit a research proposal to the Coordinator of the Coral Reef Initiative. In June, that proposal was submitted, recommending that they survey the areas influenced by this event. After review and identification of funds, this project was approved by the State Department and the two scientists went into the field to survey the coral reef bleaching episode from August 3-22, 1994. The major objectives of this research trip were to evaluate the reliability of the NOM satellite sea surface temperature data as a predictor of coral reef bleaching, to determine the current bleaching status of the reefs, and to assess the role of local factors influencing reef health.
METHODS Only non-destructive methods were used in this underwater survey, mainly still photography, visual estimates of reef cover and stresses, and direct measurements of water quality. Interviews were also conducted with long-term residents familiar with the local near shore marine environment. To maximize coverage of the reef, which is necessary because of its high spatial variability, rapid visual estimates were made throughout the course of as long a swim as possible given constraints of compressed air supplies or wave conditions
Both shore diving and boat diving were utilized, with breath-hold or SCUBA technique depending on depth and accessibility. Line transects were not used because they would have severely limited the area of reef covered (Carleton and Done, 1995). Live coral and algae cover estimates refer to the average value seen on each dive on hard bottom only, and do not include sandy or muddy bottom. Comparisons of our visual cover estimates were made with local divers. Al1 estimated values were either identical or differed by no more than 10%. We are, therefore, confident that our estimates of cover and percentage bleaching are valid within a 10% range of error.
All photographs were made using a Nikonos camera (V or IVB) with a 15mm lens or a 35mm lens with a close-up attachment for macro-photography. Fields were selected for photography to represent the reef profiles or to represent the appearance of bleached coral colonies observed on the reefs. Images with the 15 mm lens were shot from a foreground distance of 5-8 feet; close-up images were taken at a constant distance of 12 inches from the camera lens. Strobe lighting was provided for all photographs using a Nikon 103 unit. All original photographs were shot with Fuji 100 ASA slide film. Unless otherwise indicated in the figure legends as photos by Pete Ely, all pictures used to document this survey were taken by Hayes.
Bleached coral tissue was distinguished from corals which appeared white because of recent predation by Acanthaster planci (Crown-of-Thorns starfish) and from dead coral skeleton, and live tissue was differentiated from filamentous algae overgrowth, which can be similar in color. This was done by close visual examination and whether coral tissue was bleached and intact or dying was determined by fanning the water by hand just above the coral surface to see if it was covered by filamentous algae or had streamers of necrotic tissue after predation.
Timing of coral mortality was estimated from the sequence of algae succession and overgrowth of dead skeleton. Coral which had just died within days had white skeleton without intact tissue. This was apparently colonized by filamentous algae within days to weeks. This either grew into filamentous turf algae communities or cyanobacteria, or was succeeded by fleshy algae. After several months encrusting calcareous red algae, particularly Porolithon, had colonized the projecting tips of coral skeletons which had died early in the 1994 bleaching event. Coral which had died in the 1991 bleaching event were structurally intact, but completely covered by Porolithon, or by fleshy algae in eutrophic lagoonal habitats. The skeletons of coral which had died in earlier bleaching events were significantly bio-eroded by boring clams and worms and scrapers of calcareous algae such as parrotfish, snails, and sea urchins.
Stresses to reefs were estimated using relative values, except for Acanthaster, which was based on counts of the number of individuals seen per dive. Water temperature, salinity, pH, and dissolved oxygen were measured using a calibrated submersible electronic probe. Six small samples (2-3 cc per sample) of coral mucus were taken from the water column as well as from bleached, unbleached and necrotic portions of coral colonies by gentle suction into clean syringes held near the coral tissue. These samples were analyzed by Garriett Smith, a microbial ecologist at the University of South Carolina, for bacterial taxonomic content based upon carbon utilization assays (Jindal et al., 1994, 1995).
Many corals and algae were identified to the species level and many to only the family, or genus level, using primarily Veron's "Corals of the Indo-Pacific", and Magruder and Hunt's "Seaweeds of Hawaii". The very large number of species in certain genera, and the lack of good species level guides for the area, makes identification of species very difficult in genera like Acropora, Montipora, and families like the Faviidae and Fungiidae. Since samples of coral and algae were not taken in this study, and since many species in certain families can only be identified by microscopic examination of dead skeletons by a handful of taxonomists, we believe that identification to genus was the best compromise to allow extensive coverage in a very brief time without destructive sampling.
Some of the sites studied have been collected in much more detail by taxonomic experts, and their reports should be regarded as definitive with regard to species distributions. Most genera are effectively monotypic (only one species occurring in a particular habitat or region), and most genera with many species tended to be bleached in a similar way (our observations and Paulay, personal communication). However a few genera showed a wide range in the bleaching response of different species, and it is clear that more detailed work is required by experts in both coral and symbiotic algae taxonomy before these differences can be fully understood.
Direct field measurements of conductivity (salinity), water temperature, pH, and dissolved oxygen concentration were taken at each site surveyed using a Daigger portable water analysis meter. This battery powered unit was outfitted with a multiple probe assembly suspended from an eight meter waterproof insulated cable. Although this unit had been factory calibrated, it was re-calibrated and field validated against other instrumentation before each application. Digital readings were made at 1 meter and 7 meter depths for each site. As these values were very similar, they were averaged. All data were recorded manually, including date and time of day. These data were also logged automatically in memory for eventual downloading directly into a computer for tabular retrieval and analysis. Routine statistical methods were applied in subsequent analysis of numerical data.
Ocean sea surface temperatures and their anomalies were based on NOAA data largely derived from global satellite advanced very high resolution radiometer (AVHRR) measurements as reported in the Oceanographic Monthly Summaries for 1994, based on the climatology of Reynolds et al. (1993, 1994).
At the 19 South Pacific sites where field work was conducted in August 1994, 15 environmental parameters were evaluated Other environmental factors were also identified at different sites which were not included in this comparative assessment because they usually affected only one or a few sites. These include factors such as dredging, damage to corals from fish collectors, use of dynamite or poisons, factory effluents, diver damage, overfishing or heavy grazing by herbivorous fish. Those factors are discussed in the individual site descriptions section. Physical damage from hurricanes is important in structuring reef communities (Rogers, 1993), but it was not possible to get good before and after assessments of hurricane damage to reefs at many sites from local observers. Although sediment stress to corals (Rogers, 1990) was evident at several sites we did not estimate water turbidity directly because it is so extremely variable that long time series of measurements are needed to establish significant differences. The factors which were assessed statistically for all of the sites are as follows:
1. Habitats were classified according to degree of protection from storm waves. A value of (1 ) was assigned to all exposed fore reef sites, (2) to semi enclosed bays, (3) to lagoon and reef crest habitats. This parameter is a proxy for average wave energy.
2. Water temperature measurements were made with a recording meter during field work in August. This period corresponded to near the coolest time of year at all sites.
3. Salinity was measured by a recording conductance sensor as total density anomaly in parts per thousand. The period of measurement corresponded to the dry season at all sites. Tutuila had the highest rainfall and river runoff into the coastal zone, followed by Rarotonga and Tahiti.
4. Dissolved oxygen was measured by calibrated recording meter. Values would also be expected to be seasonal, dependent on temperature, algae photosynthesis, reef respiration, and the rate at which oxygen consuming pollution and sewage contributes to the coastal zone.
5. pH was measured by calibrated recording meter.
6. Live coral cover was estimated visually to the nearest 10%. These estimates were confirmed by review of wide-angle lens photographs. All bottom cover estimates refer only to hard bottom, and do not include sand and mud bottom.
7. Proportion of corals bleached was estimated visually over the course of each dive. These were confirmed through wide angle photographs.
8. Fleshy algae cover at each site was estimated visually.
9. Calcareous algae cover was estimated visually.
10. Human population density in the watersheds inland from each site was ranked on a relative scale. The lowest values refer to areas with no shore population, and the highest densities are for the capital cities of Papeete, Tahiti, and Pago Pago, Tutuila. This parameter serves as a proxy for census data, which was not available at all sites at the required scale.
11. Mud cover of corals was estimated visually by the two scientific divers and through wide-angle photographs.
12. Acanthaster planci (Crown-of-Thorns starfish) seen were counted on each dive. At only one site were there too many to count, and the value was estimated.
13. Years since last cyclone or major storm was based on interviews with local divers, scientists or government officials.
14. Previous bleaching events were based on local interviews.
15. Pollution was ranked on a relative comparative scale, based on the amount of debris (bottles, paper and plastic items, etc.) and artifacts (junked construction materials, munitions, etc.) seen.
Statistical regressions were run between all pairs of variables. Factors were compared using non-parametric statistics. Some measurements were measures of the status of the entire ecosystem (such as the bottom cover), but other factors vary strongly on a seasonal basis and were probably different at the time when bleaching began (such as measurements of water temperature, salinity, and oxygen content) than when they were measured in August. Some factors were estimated only on a comparative, rather than a quantitative basis (for example habitat was represented by relative wave exposure rather than wave height measurements, and population by relative density ranks rather than from census figures). Since many assessments were based on comparative rankings we have contrasted them using qualitative rank order statistics rather than parametrically. The Spearman correlation coefficients were calculated between all pairs of variables. This examines the degree to which the 15 factors are ranked by their relative magnitudes in the same order at all 19 sites. This method has been previously used to analyze 13 environmental factors at 11 coral reef sites in Jamaica (Goreau 1992).
Monthly mean sea surface temperatures and their anomalies were taken from maps published in the Oceanographic Monthly Summary by the National Oceanic and Atmospheric Administration, the National Ocean Service, the National Weather Service, and the National Environmental Satellite, Data, and Information Service, Ocean Products Branch, of the United States Department of Commerce. This data set provides a consistent data base with complete global coverage, based on a combination of satellite-derived measurements and those from oceanographic vessels.
In this report we have analyzed all available data from January, 1993, through September, 1994. Twenty-two sites were evaluated in the Pacific Ocean, six in the Indian Ocean, and five in the South Atlantic (see Maps 8 and 9; also see Table 7A and 7B for geographical coordinates). Values at each site were determined by preparing an overlay transparency on which all sites were marked, superimposing it on the published maps and reading the values right below marks. Values were interpolated to the nearest 0.1 degrees Celsius from the slope of marked contours. This generally assumes that slopes are linear between contours, and it is estimated that non-linear contour slopes could provide inaccuracies of the order of plus or minus 0.1 degrees Celsius in most cases. However, where there are very sharp spatial temperature gradients, contours are close together and precision errors could be larger, up to 0.3 degrees Celsius in extreme cases. Sites near near warm or cold currents and western boundary current areas are especially likely to be affected. A different source of uncertainty occurs where temperature gradients are flat in the core of the warmest regions. The maximum values are not given, and thermal contours within these areas have to be estimated from those outside. Generally conservative assumptions were made, so that the values given are likely to be under-estimates of the true values. This problem affected only a few sites very near the equator.
MAP 2. Tahiti, Society Islands.
The Aquarium (lagoon) to the north and The Gap (outer slope) to the south were the two sites surveyed (map adapted from Stanley , 1993)
SITE DESCRIPTIONS AND REEF ASSESSMENTS:
TAHITI, SOCIETY ISLANDS, FRENCH POLYNESIA
Background: Bleaching was first reported in March, 1994 (Henri Poliquen, personal communication). Almost all corals were affected, but the majority had either recovered, as indicated by tissue pigmentation, or were dead by August. Mortality of all lagoon corals had occurred after bleaching in 1984, which eliminated all dominant large plate-shaped Acroporas. Some bleaching took place in 1987, but before that time there had been recovery from the previous event. High mortality and tissue fluorescence of small branching Acroporas were seen after bleaching in 1991. A very minor bleaching event took place in 1993 (Drollet et al., 1994). Tahiti was in the direct path of several hurricanes in the 1980s, with the most recent in 1987.
1. Lagoon, "The Aquarium", northwest Tahiti off-shore from the end of the Faa'a airport runway (August 6 1994)
a. Physical setting: Patch reefs surrounded by sand in depths of 3 to 5 meters.
b. Corals and bleaching: The majority of corals were dead, and those live corals (Porifes lobata, Porites lutea, Pocillopora verrucosa, Pocillopora eydouxi, Pocillopora damicornis, Acroporas, brown and purple encrusting Montiporas, and Pavona) were physically overgrown by algae. Also, several bleached and recovering anemones (Heteractis) were observed (Figures 1-3). Synarea (Porites rus) colonies up to 10 meters across were in good condition. Large thicket-like colonies of one branching Acropora species (possibly A. robusta) up to 10 meters across were recovering from bleaching and were growing at the rate of 2-3 cm per month (Poliquen, personal communication).
c. Algae: Turbinaria ornata, Sargassum, and 1 cm high branching brown turf algae were growing on the tops of dead coral heads. Turf algae predominate, including Sphacelaria. Some Chaetomorpha were seen, but all were cryptic due to grazing. Algal cover was approximately four times that of stony coral cover.
d. Local reef stresses: High density of the grazing sea urchin Diadema, but they appear to have little impact on algae because no bare grazing halo is visible around the coral heads where they are found in dense swarms. High eutrophication was indicated by overgrowth of corals by fleshy algae (Figure 2). There was evidence of recently suspended sediment on living coral. The lagoon is subjected to high human population-dependent stresses, including shipping activity, industrial waste and human sewage, in the Fa'aa area and the western suburbs of the capital, Papeete (population of around 75,000).
Figure 1. Site #1. "The Aquarium" in lagoon of Tahiti. Extensive patch of new growth of Acropora in recovery from recent bleaching (1994). The living coral is emerging from algal overgrowth on the dead coral frame. Depth 2-3 meters.
Figure 2. Site #1. 'The Aquarium'' in lagoon of Tahiti. Algal overgrowth on bleached and dying (center) and old dead (left) Acropora in lagoon. Depth 3-5 meters.
Figure 3. Site #1. ''The Aquarium'' in lagoon of Tahiti. Bleached anemones (Heteractis) with clownfishes in shallow lagoon site. Depth 3-5 meters.
2. Outer reef slope, "The Gap", near Tuputu Pass, off Punaauia, west Tahiti (August 6 1994)
a. Physical setting: Steep, nearly vertical wall with a slope 75° to 90° was examined from 3 to 26 meters depth.
b. Corals and bleaching: Live coral cover averages 70%, with values up to 80-90% in deeper water below (16 meters) and less than 50% in the wave breaking zone (upper 5 meters). Pocillopora was dominant, followed by Acropora, and Montipora. All corals are small except for some shelf-like Montiporas on edges of canyons, up to 1-2 meters across. Most corals appear to be only a few years old, and may have settled since the 1991 bleaching event. Around 10% of live corals showed bleaching, with very few colonies bleached to complete whiteness. Almost all bleached corals were partially bleached and in a recovery phase. Bleaching was observed in Acropora, Pocillopora, Fungia, Pavona, Montipora, and Heteractis anemones (with clownfish). Coral fluorescence, when observed, was confined to the oral disk of normally colored Acropora. Around 10% of corals were recently dead and covered with filamentous algae; almost all dead corals were of the genera Acropora and Pocillopora (Figures 4-5)
c. Algae: Encrusting red algae (mostly Porolithon) cover about 20%, with parrotfish grazing bite marks. Branching algae ranged between 1% and 5%. these algae were primarily wave resistant Turbinaria ornata (shallower than 6 meters). There are few Halimeda, and all are cryptic (in crevices protected from parrotfish grazing). Blue-green algal growth (probably Lyngbya) and Dictyota grew on recently dead and physically undamaged corals which had died after bleaching during the previous months. Corals which had died from bleaching in 1991 were covered by Porolithon.
d. Local reef stresses: Intense grazing by abundant surgeonfish and damselfish was observed. The area does not appear to be affected by pollution or sediments and the steep slopes keep it well exposed to open ocean waters. The major cause of coral mortality since hurricanes in the mid-1980s appears to be bleaching, since corals are dead and overgrown in place, without physical damage. Diving activities nearby use moorings and are carefully controlled with no evidence of damage due to anchors or diver contacts.
MAP 3. Moorea, Society Islands. Lagoon and outer slope survey sites on the north coast near Tiahura motu (map adapted from Stanley, 1993).
MOOREA, SOCIETY ISLANDS, FRENCH POLYNESIA Background: Previous mass coral reef bleaching had occurred in 1984 and 1987. In 1991, bleaching occurred again in Moorea and was studied by Salvat (1991), Gleason (1993), and Goreau and Hayes (unpublished observations). That episode was remarkable because of the fluorescence of the bleached corals (Jardin et al., in preparation). A few cases of bleaching were seen in 1993.
Bleaching began in March, 1994, according to Bernard Salvat, Catherine Jardin, and Yannick Chancerelle. The reports of Houegh-Guldberg (1994) and Houegh-Guldberg and Salvat (1995) are based upon transect analyses during April, 1994. Most corals were affected, but they had already either recovered or died by August. Hurricane history for the area is identical to the neighboring island of Tahiti, as are ocean temperatures. However Moorea has a far smaller population.
Figure 4. Site #2. ''The Gap'' on outer slope of Tahiti. One completely bleached colony of Acropora surrounded by recovered assortment of Pocillopora and Acropora corals. Depth 20 meters.
Figure 5. Site #2. "The Gap" on outer slope of Tahiti. Partially bleached Acropora and Pocillopora corals in recovery from bleaching of 1994. Depth 20 meters.
3. Lagoon, "Barrier reef", on inside edge of reef flat, 200 meters southwest of Tiahura pass, near site of Salvat thermograph (August 7 1994)
a. Physical setting: Patch reefs were interspersed with sand bottom cover at about 2 meters of water depth. Long-term surveys of species composition and bleaching have been undertaken near this site by Salvat since 1991, along with water temperature measurements.
b. Corals and bleaching: The area is dominated by 2 to 3 meter diameter heads of Porites lobata, P. Iutea, and P. rus (Synarea), with about 20% Pocillopora verrucosa and very few Acropora. Very little bleaching was seen, with only a few small pale patches on Porites heads (Figures 6-73. The soft coral, Sarcophyton, has become very abundant, covering between a fourth to a half as much bottom as hard corals. Soft corals were virtually absent in 1991, with only one colony being seen on the back reef and two on the fore reef.
c. Algae: Algal cover and species diversity have increased greatly in the last three years. In 1991 only Turbinaria ornata and Caulerpa racemosa were common
in the back reef, with Chaetomorpha and blue-green algal mats confined to the area immediately in front of Club Med where sewage was soaking through the beach sand. In 1994 there was extensive algal overgrowth on the top of most large Porites and Synarea heads in the lagoon. Algae cover as much of the hard bottom as corals. Common algal species seen in 1994 were Turbinaria ornata, Sargassum (vast increase, almost absent in 1991), Chlorodesmis, Enteromorpha cryptic but common, Padina, Dictyota, large mats of blue green algae of various colors, fine and coarse grained Halimeda and possibly Siphonocladus. No Caulerpa was seen in 1994.
d. Local reef stresses: A rapid increase of fleshy algae and soft corals has taken place since 1991, indicating that the north lagoon is undergoing rapid eutrophication, although not yet as bad as in the Tahiti lagoon. This is thought due to rising nutrients associated with increasing sewage inputs from rapidly expanding tourism and shore hotels. Additional stress from resuspended sediments are due to high motorboat traffic, largely tourism-related. There is no industry on Moorea except a pineapple juice canning plant.
4. Outer reef slope, north Moorea, 200 meters east of Tiahura pass, near site previously studied in 1991 (August 7 1994)
a. Physical setting: Reef slopes gently at about 30° from the surface to 20 meters. It was examined between 10 and 20 meter depths. Low buttresses (around 1 meter high) exist with sand channels in between. This site is near one which was examined by Hoegh-Guldberg in April, one which has been studied continuously by Salvat since the previous bleaching event in 1991, and one at which Acropora survivorship and fluorescence were studied by Jardin et al. in 1991 (in prep.).
b. Corals and bleaching: Live coral cover was about 60%. Pocillopora was dominant, about twice as common as Acropora species. P. verrucosa was roughly 2 to 3 times more common than P. eydouxi. About 20-30% of all live coral was bleached, almost all partially, with few colonies showing full bleaching. About 80% of live Pocillopora was normal, about 20% was partially bleached, and an additional 10% was dead. About 20% of Acropora were normal in color, about 60% were partially bleached and were recovering from their bases upwards to their tips. About 20% were completely bleached to yellow or white, and about 20% were dead. Mortality and algal overgrowth were most pronounced on bushy species (poss. A. subglabra) and on very thin knobby yellow species (pose. A. aculeus). Most Acropora were in the A. humilis, A. aspera, A. nobilis, or A. formosa groups and were at various stages of recovering. More than 80% of Fungias and Montastrea curta were partially bleached. About 20% of Montipora were partially bleached. Some partial bleaching was seen in green encrusting Plesiastrea. Very little bleaching was evident in Porites. Overall hard coral bleaching was roughly 2 to 3 times more abundant than on the fore reef in Tahiti. This feature might be due to the shallower slope and higher light intensity on the bottom. Almost all coral colonies were small, less than 50 cm diameter, and probably had settled since the 1991 bleaching event. Dead coral cover ranged from 10-20%, made up mostly of Acropora and Pocillopora. Bare rock and rubble constituted about 30%, mostly covered by Porolithon.
c. Algae: Porolithon was encrusting around 20% of bottom. Around 10% fleshy algae, largely Turbinaria, Sargassum, and fuzzy brown turf algae are present. Algal overgrowth on corals is mostly fine Dictyota. Two kinds of Halimeda (fine and large plates) are seen, but all are cryptic due to grazing, with exposed portions grazed flat.
d. Local reef stresses: Coral cover was fairly low, and algal cover and algae species diversity were higher than in 1991. Only Turbinaria ornata was common in 1991. There is a high incidence of dead coral and rubble. Corals which died in 1991 are covered with Porolithon; in contrast, dead corals from 1994 are covered with fleshy algae and filamentous algae turf. The last hurricane hit the area in 1987. There is a high level of anchor and diver damage here, due to the large number of sport divers from hotels on the northwest of the island.
MAP 4. Rangiroa, Tuamotu Islands. Sites surveyed were Avatoru Pass, Tivaru (outer slope), and a patch reef within the lagoon (map adapted from Stanley, 1993).
RANGIROA, TUAMOTU ISLANDS, FRENCH POLYNESIA
Background: Bleaching began in March 1994 (Yves Lefevre, personal communication) and this was the worst episode seen here in ten years. Bleaching occurred in 1991; mild bleaching occurred in both 1992 and 1993. Many coral species have been affected, with the order of severity being greatest in Acropora, intermediate in Pocillopora, and lower in Porites, which bleached only in 1994. Heferactis anemones have been bleached during past bleaching episodes (Lefevre, personal communication). The last hurricane took place in 1983, but very strong wave activity and coral damage have occurred along exposed shores following rough weather in 1991 and 1993. Mortality from bleaching is most obvious in areas which suffered heavy breakage from waves in the 1993 storms.
Figure 6. Site # 3. Lagoon on north side of Moorea. Bleached Pocillopora showing recovery from bleaching. Depth 3-5 meters.
Figure 7. Site # 3. Lagoon on north side of Moorea. Close-up of Porites with recovering, bleached and dying zones. Dead coral is overgrown by algae (right). Depth 3-5 meters.
5. Interior of northwest quadrant of Rangiroa lagoon (August 9 1994)
a. Physical setting: This patch reef of 100 meters across is surrounded by deep water in the interior of the lagoon and was several kilometers removed from uninhabited atoll islets. High wave energy was indicated by pounding surf and abundant rubble, which was cemented together by encrusting red algae and beach rock below 7-10 meter depths. Rangiroa is the world's second largest atoll, and is so large that the lagoon is extremely well mixed by wind and waves, with conditions inside similar to the open ocean. Well developed reefs fringe both the inside and outside of the lagoon, and coral species found are similar on both sides
b. Corals and bleaching: Living coral cover was about 50%, with a lot of rubble. Acropora is the most common genus, containing three species (one with rounded heads, one with bushy heads, one with flat heads) up to 1 meter across. Pocillopora and Porites are subdominant and roughly equal in numbers, with the latter 1 to 2 meters in diameter. All Acroporas showed partial bleaching, but only a minority of Pocillopora and Porifes. Brain corals (Leptoria) were uncommon and also partially bleached. Less than 10% of Acropora colonies were recently dead or dying. (Figure 8)
c. Algae: Porolithon covered most hard ground and rubble. Halimeda was present, but all were cryptic and grazed. No fleshy algae were seen.
d. Local reef stresses: Physical damage from breaking waves was observed.
6. Outer reef slope, near Tivaru Pass, west end of Rangiroa atoll (August 9 1994)
a. Physical setting: The reefs face open ocean along the outer slope of the atoll. The slope was about 15° out to 10-12 meter depth. There are no inhabitants within several hours by boat, but the area is occasionally visited by fishermen. Tivaru Pass is entirely filled by sand and rubble, and water flows through it only during hurricanes.
b. Corals and bleaching: Live coral cover was about 50%. Pocillopora made up half of all coral colonies, Acropora 30%, and others (mainly Porites, Montipora, Montastrea curta, Favia, and Millepora) make up 20%. About 90% of all Acropora colonies were partially bleached. This included all of one bushy species, which were recovering brown color from the bases. Two of the least common species (one corymbose, one with large rounded polyp tips) were entirely unaffected. Less than 10% were recently dead. About 50% of Montipora colonies were partially bleached, and others were completely bleached. 10 to 20% of Pocillopora verrucosa were bleached, most partially bleached to pale colors, but about 1% bleached white. Coral cover was highest in shallow fringing reefs from 1 to 5 meters depth, and below this depth largely consisted of coral rubble and small flattened colonies. (Figures 9-10)
c. Algae: Porolithon encrusted limestone cover was roughly 20%. Halimeda of 2 or 3 species covered about 10% of the bottom, forming 5 cm high bushes which did not appear to be grazed. Fleshy algae were not seen.
d. Local reef stresses: Wave damage is evident from previous hurricanes and storms. Parroffish herbivory is clearly very low based on lack of Halimeda grazing and bite marks on Porites. This could be due to low fishing pressure and high abundance of predatory fish. Both blacktip and whitetip sharks were seen, and sharks (and manta rays) are so abundant in Rangiroa to be a major attraction to sport divers. Small black encrusting sponges were a unique feature of the site.
Figure 8. Site #4. Patch reef in lagoon of Rangiroa. Bleached Acropora on small patch reef in middle of lagoon of the atoll. Depth 4-7 meters.
Figure 9. Site #5. Tivaru pass, outer reef slope of Rangiroa. Bleached Acropora surrounded by clusters of calcareous algae, Halimeda. Depth 10 meters.
Figure 10. Site #5. Tivaru pass, outer reef slope of Rangiroa. Bleached Acropora and soft coral on a spur formation in outer reef slope. Depth 10 meters.
Figure 11. Site # 6. Avatoru pass in Rangiroa. Bleached Pocillopora and Acropora attached to coral rock along edge of pass. Depth 4-5 meters.
Figure 12. Site # 6. Avatoru pass in Rangiroa. Bleached and recovering zoanthid soft coral, Palythoa in pass. Depth 5 meters.
7. West side of Avatoru Pass, north Rangiroa (August 9 1994)
a. Physical setting: A strong current flows northward out of the atoll in response to the prevailing trade winds, which had been blowing with unusual force from the southeast for three days before, eroding up to 6 meters from beaches along the interior of the atoll. These conditions, known as "maurua", occur around 10 days a year, in the coolest season. High wave energy was also indicated by complete absence of sand in the cobble to boulder-sized outer beaches, primarily composed of hurricane tossed Pocillopora corals, and protected by beach rock. The wall of the pass was examined between the surface and the bottom at 7-10 meters along most of its length.
b. Corals and bleaching: Live hard coral cover was about 50%. The predominant species was Pocillopora verrucosa up to half a meter to a meter in diameter, with roughly equivalent area occupied by large Porites heads up to 1 or 2 meters in diameter with a very high density of parrotfish bite marks. Neither species was bleached. Only one Acropora was seen, and it was partially bleached. Alcyonarian soft corals (Lobophytum or Sinularia) up to 1 meter across were abundant, up to 20% cover. Around 10% was made up of the zoanthid soft coral Palythoa, which was partially bleached and fully contracted. (Figures 11-12)
c. Algae: Porolithon covered most limestone buttresses and coral rubble. Halimeda was present in small amounts, but only in cryptic habitats with projecting growth bitten flat by parrotfish. No fleshy algae or turf algae were seen.
d. Local reef stresses: High currents and high wave energy are obvious at this site. Possible oil pollution by ships and boats using the pass to get to the atoll's main dock constitute additional stress to the reef.
MAP 5. Aitutaki, Cook Islands. Two outer slope sites on west side of the atoll were surveyed: Shipwreck and Fish Spot near Maina motu. Lagoon site near the airport was also surveyed (map adapted from Stanley, 1993).
AITUTAKI, COOK ISLANDS
Background: Bleaching began in Aitutaki in March of 1994 (Neil Mitchell, personal communication). The only major bleaching event in the previous 10 years was in 1991. That event was much more severe than 1994, water temperatures reached 31 degrees Celsius, and all bleached corals died. Water conditions were very hot and calm in 1991, and Crown-of-Thorns starfish were not responsible. A major Crown-of-Thorns infestation took place in 1982 and 1983, and swarms were still working their way around the island, leaving patchy damaged trails behind. Other stresses to reefs were coral death in the lagoon in 1987 as a result of exposure and stagnant water due to exceptionally low sea levels, possible minor bleaching in 1986, 1987, and 1989, and wave damage from storms in the early 1990's. The most recent devastating hurricane, Cyclone Sally, hit Aitutaki on December 25 (Christmas Day) of 1986, but no bleaching followed it. Detailed line transect surveys of coral species abundance were conducted around Aitutaki around the time of bleaching by Dr. Gustaf Paulay and the staff of the Cook Islands Conservation Service (in preparation).
8. Lagoon off the northern tip of the island, (August 12 1994)
a. Physical setting: The lagoon was very shallow in this area, around 1 meter deep, and was around 80% Halimeda sand-covered hard limestone bottom, with large coral heads which have grown up to sea level and died on top due to air exposure, but have continued to grow outward to form "micro-atolls". There was an extremely strong current driven by waves pounding over the reef flat by the prevailing winds. Large numbers of blue Linckia starfish and sea cucumbers were in the sandy areas.
Figure 13. Site #9. Shipwreck site and adjacent "Fish Spot" site on extreme southwestern outer slope reef of Aitutaki, Cook Islands. Panoramic reef scene showing coral cover and a single bleached Acropora in foreground. Depth 20 meters.
Figure 14. Site #9. Shipwreck and "Fish Spot" on outer slope of Aitutaki. Edge of shallow reef zone with assorted Acropora showing partial bleaching. Depth 10 meters.
Figure 15. Site #9. Shipwreck and 'Fish Spot" on outer slope of Aitutaki. Upper segment of reef with Acropora of several species, small Pocillopora and Pavona. Depth 10 meters.
Figure 16. Site #9. Shipwreck and "Fish Spot" on outer slope of Aitutaki. Partially bleached Acropora showing recovery from base to tips. Depth 12-15 meters.
Figure 17. Site #9. Shipwreck and ''Fish Spot" on outer slope reef of Aitutaki. Encrusting Psammocora surrounded by Acropora and Pocillopora (Photo by Ely).
Figure 18. Site #10. Mainatai Point on outer slope of Aitutaki. Panoramic profile of reef indicating numerous small Acropora corals amidst coral rubble and a few other coral genera. (Photo by Ely)
b. Corals and bleaching: Large Porites heads formed micro-atolls up to 10 meters across, covering around 20% of the bottom. Many of the larger ones were fused together if sufficiently near. Around 40% consisted of live coral, with flat tops algae covered. A few Acropora species and Millepora dichotoma were found. Less than 5% of corals showed bleaching. However higher levels of bleaching, up to a third of the iive corals, had occurred in Acropora stands in lagoon sites in somewhat deeper and more protected waters around half a kilometer south of this site (Flora, personal communication), and other lagoon sites had complete mortality of Millepora and 80% mortality of Acropora (Paulay, personal communication). More detailed results will be published from his transect data at sites around the island (Paulay, in preparation). Coral diversity in the lagoon was observed by Mitchell to have been much greater before exposure and stagnant conditions in 1987 and before coral mortality from bleach in 1991.
c. Algae: Algal cover was high, consisting around 50% Halimeda opuntia and around 10% Turbinaria ornata on the dead areas covering the top of microatoll coral heads.
d. Local reef stresses: This area is protected from all but hurricanes by the reef flat, but the last hurricane had broken an enormous amount of coral rubble which formed a steep rubble beach. The shallow reef lagoon is extensively walked on by Aitutakians during reef gleaning activities. Those observed included group gill netting using sticks to beat the water to drive fish and use of crowbars to turn over rocks and shovels to dig clams out of rubble and sand. Fish poisons, derived from traditional plant materials, are also used, especially "ore papua" derived from the Barringtonia asiatica tree seed pods, Tephrosia purpurea, or Derris trifoliata (Whistler, 1992)
Giant clams have been overharvested, and the Cook Islands Fisheries Service has instituted a seed farm to restock the area. From 1970 until the market collapsed in the mid 1980's, most of Aitutaki was cultivated with banana for export to New Zealand, and the lagoon may have been subjected to higher levels of fertilizer, chemicals, and soil erosion at that time.
There is an old tradition of quarrying live Porites heads from the lagoon to be burned for quicklime, and the oldest houses are made from coral cemented with lime. However such houses have not been built for a long time since concrete became available, and the old communal lime kilns closed by the 1950's or 1960's. Only small scale lime production now takes place for whitewashing houses.
9. Outer reef west of southernmost point of Aitutaki island, near the shipwreck at the mid point of the outer atoll fringing reef (August 11 1 994)
a. Physical setting: Reef flat slopes moderately steeply, 45° to 60° with strong vertical zonation, and has numerous canyons and buttresses.
b. Corals and bleaching: The upper 10-20 feet have about 50% live coral cover with large amounts of red calcareous algae-encrusted bare rock and dead coral. Corals in this area are all small, largely less than 50 cm across, and appear to be recent recruits which settled after the last hurricane had made a near complete sweep of previous corals. Live coral cover increases sharply with depth and is 60-70% between 40 to 80 feet and deeper. The species diversity and evenness were very high, but Acropora was the most common genus, with around twice as many species as French Polynesia. Full bleaching was relatively uncommon, less than 5% of corals, but around 20% were partially bleached. Species observed to be partially bleached were many Acropora species including A. humilis, digitifolia, verweyi, tenuis, hyacinthus, and numerous others; Pocillopora verrucosa, eydouxi, and meandrina; perhaps 6 Montipora species, Astreopora, Fungias, Lobophyllia, Favias, Goniastrea, Leptoria, Montastrea, Merulina, and others. See Figures 8-13. Detailed species surveys were made nearby by Paulay and the staff of the Cook Island Conservation Service. (Figures 13-15)
c. Algae: The upper 3-7 meters are about 50% composed of lilac and pink Porolithon covered reef framework and dead corals, and below this it covers about 30%. Halimeda was common, but everywhere cryptic. Other algae were very rare, and some blue green algae were seen.
d. Local reef stresses: There has been severe coral mortality in the past from coral bleaching and from wave stress by hurricanes and tropical storms. The are just north of the site had very heavy ongoing mortality from an Acanthaster planci swarm, and around 50 Crown-of-thorns starfish were seen on the dive. Acanthaster predation is easily distinguished from bleaching because it leaves corals with large rounded white patches, having a sharp edge between dead skeleton with wisps of necrotic tissue and the completely normal-colored coral tissue beyond range of the starfish stomach which is extruded through the mouth. In contrast, bleached corals have intact tissue with a gradation in colors. Bottom cover estimates at this site refer to the area which had not yet been reached by the southward-migrating Acanthaster swarm, which did not appear to discriminate between bleached and normal corals.
10. Outer reef slope at the southwest corner of the Atoll (August 11 1994), Mainatai Point.
a. Physical setting: The habitat is very similar to the previous site, sloping around 45° to 60°, but is more remote from land. Abundance of fish was very high, especially around canyons and crevices in the reef.
b. Corals and bleaching: Coral species found were very similar to the previous site, as was the vertical zonation: young corals predominated in shallower areas with larger corals deeper down. However while the live coral was also 50% in shallow water, it became even higher with depth, being more than 80% from 10-30 meters and deeper. While full bleaching was less than 5%, only around 10% of the corals were partially bleached. Not only was bleaching less marked at this site than the previous one, most Acroporas at this site showed no signs of bleaching, and were distinguished by extremely healthy colors, with exceptionally strong natural light fluorescence. See Figures 14-31. Other sites along the western coast of the atoll are pictured in Figures 32-44. These resemble in composition the reef as described at Mainatai. (Figures 16-24)
c. Algae: Algae were similar in type to the previous site but less abundant as a result of the higher coral cover. Porolithon decreased to around 10 to 20% below 10 meters. It appears that this site had been a little more protected from the hurricane which had reduced coral cover in shallower areas.
d. Local reef stresses: This reef, while subject to similar general historical conditions as the previous site, appeared to be in better condition in that coral cover was higher and bleaching lower. The area is near the corner of the semiatoll most remote from human influence. While the previous site was infested with crown of thorns, only one small juvenile was seen at this site.
Figure 19. Site #10. Mainatai Point on outer slope of Aitutaki. Panorama showing dead and bleached Acropora, Favites and Millepora. Depth 10 meters.
Figure 20. Site #10. Mainatai Point on outer slope of Aitutaki. Dead and partially bleached Acropora. Depth 15 meters.
Figure 21. Site #10. Mainatai Point on outer slope of Aitutaki. Normal Pocillopora beside cluster of bleached, partially bleached and dead Acropora. Lobophyllia, Montipora and Pavona also included in field. Depth 20 meters.
Figure 22. Site #10. Mainatai Point on outer slope of Aitutaki. Acanthaster damage to Acropora, and assorted corals showing partial bleaching. Depth 20 meters.
Figure 23. Site #10. Mainatai Point on outer slope of Aitutaki. Bleached soft coral species (center) surrounded by assortment of small corals showing normal to bleached color patterns. Depth 15 meters.
Figure 24. Site #10. Mainatai Point on outer slope of Aitutaki. Dead and partially bleached Acropora surrounded by Lobophyllia and other corals. Depth 15 meters.
Figure 25. Site #10. Mainatai Point on outer slope of Aitutaki. View of outer reef slope showing bleached and dead Acropora (center) with bleached Pavona and partially bleached Pachyseris. Depth 15 m.
Figure 26. Site #10. Mainatai Point on outer slope of Aitutaki. Edge of outer slope reef showing bleached Montipora and Acropora. Depth 10 meters.
Figure 27. Site #10. Mainatai Point on outer slope of Aitutaki. Assortment of corals of various genera at base of reef, showing a wide range of colors. Dead Pocillopora overgrown by red calcareous algae also seen. (Photo by Ely).
Figure 28. Site #10. Mainatai Point on outer slope of Aitutaki. Denuded skeleton in wake of Acanthaster (Crown-of-Thorns starfish). (Photo bv EIy).
Figure 29. "The Ridge" on outer slope of Aitutaki. Colony of Pocillopora eydouxi on reef slope. (Photo by Ely).
Figure 30. 'The Ridge'' on outer slope of Aitutaki. Acanthaster grazing over coral with encrusting red algae and coralline algal orange overgrowth. (Photo by Ely).
Figure 31. Atuatane outer stope on Aitutaki. Dead and algal-encrusted corals showing the calcareous algae Halimeda (Photo by Ely).
Figure 32. Maungapu outer slope on Aitutaki. Brown algal growth on dead coral rock. Colony of Coeloseris (Photo by Ely).
MAP 6. Rarotonga, Cook Islands. Black Rock and Pinnacle sites on the outer slope along the west side of the island and the lagoon at Black Rock were surveyed (map adapted from Stanley, 1993).
RAROTONGA, COOK ISLANDS
Background: Coral reef bleaching began in March of 1994 (Greg Wilson, personal communication) and was most noticeable in Pocillopora. This bleaching was not related to the presence of Acanthasfer, physical damage or pollution. Bleaching was highest on the outer slope northeast Rarotonga reefs. Bleaching first was seen in 1991, and had not occurred in the previous 20 years. Massive bleaching in 1991 followed long hot calm periods with low sea level, and only affected corals in the lagoon, not on the outside reefs. Yellow Milleporas were first to bleach, but recovered within weeks. Almost all corals were affected, and mortality was very high. Hot doldrum conditions with no wind, low tides, and poor circulation caused the whole lagoon and reef flat to stink from dead coral. Very good lagoon reef recovery afterwards resulted in higher diversity of corals. 1994 bleaching was much milder, and mainly affected outer slope reefs. A major crown of thorns proliferation took place in the 1970's, causing the government to force everyone to collect them all around the island. On one day huge numbers were killed and buried in pits, and the problem had been minor since. The last devastating hurricane, cyclone Sally, hit Rarotonga from the north on New Years Day, 1987, after passing over Aitutaki, but with less force.
11. Outer reef slope off Black Rock at the northwest of Rarotonga (August 13 1994)
a. Physical setting: Reefs have a medium steep slope, 45° to 60°, but considerable topographic complexity, with many pinnacles and canyons fringed with shelf corals.
b. Corals and bleaching: Coral cover was very high, around 90%, down to more than 30 meters, but with some dead Acropora. No full bleaching was seen, but 20% of colonies were partially bleached. Coral species cover, species diversity, and evenness were the greatest seen, higher than Aitutaki. The same genera were seen, but more species seemed present. Partial bleaching was most noticeable in Pocillopora verrucosa, with around half of the colonies affected. Colonies were large, up to 1 to 2 meters in diameter, but many colonies had segments covering a quarter to half the colony where all branches were pale or nearly white. Very few Acroporas were bleached, in sharp contrast to the relative rankings seen in French Polynesia. Almost all Montastrea curta were bleached very pale to nearly white. Porites was rare, but bleached in spots. Partial bleaching was seen in Favia, Favifes, Leptoria, Acanthastrea, and Pavona species. A Turbinaria coral species with a yellow rim was common, and never seen bleached (Figures 31-33)
c. Algae: Calcareous encrusting red algae, primarily Porolithon, and Halimeda covered about 10% of the bottom. Dictyota were present, but were rare (no more than 1%).
d. Local reef stresses: Reefs appeared to be affected primarily by natural stresses. Cyclone Sally passed directly over Rarotonga on January 1, 1987 (New Year's Day), after hitting Aitutaki several days earlier. Reefs on the north coast were most affected. Acanthaster damage to the reefs has been rare, but a major outbreak did occur in the 1970's.
Current degradation of the reefs of Rarotonga are attributable to commercial aquarium fish collectors. Most large Pocillopora corals have been partially or completely smashed open by aquarium trade tropical fish collectors who routinely break the coral heads to drive flame angel fish out from between the branches into nets. These collectors operate with permits to collect by netting from government, although their presence is actively opposed by some local sport divers. Their activities degrading the coral reef are nearly impossible to document and enforce. There is considerable spear fishing by collectors, sometimes using SCUBA gear and in areas where the fish have been tamed through repetitive feeding. Dynamite use in fishing also has been reported. Ciguatera is a problem with some species of surgeonfish and triggerfish.
Some areas of Porolithon, which were initially thought to be overgrown by a yellowish calcareous algae, were later reported to be affected by Coralline Lethal Orange Disease or "CLOD" (Littler and Littler, 1995). This algal growth was observed on less than 5% of the calcareous algae (Figure 30).
12. Outer reef near "The Pinnacle", off the southwest corner of Rarotonga (August 14 1994)
a. Physical setting: Physical setting consists of mounds and hills on a gradually sloping (30°) coral plain. The reefs here are quite a bit further offshore than those in the northwest, in generally rougher waters because of greater exposure to the trade winds, and are consequently less affected by divers or fishermen. The area was relatively unaffected by the last hurricane
b. Corals and bleaching: Coral cover was very high, around 90%. Acropora was the most common genus, with perhaps a dozen species with a wide range of morphologies. Species diversity and colony size were high, and similar to the previous site. Bleaching was very rare at this site, and only about 1% of colonies were affected. All were Pocillopora verrucosa with bleaching in some branches. (Figures 34-37)
c. Algae: Similar to previous site, except that fleshy algae were not seen.
d. Local reef stresses: Wave stresses are evident. Acanthasfer was rare, only one was seen. Many large Pocil/opora heads had been broken by fish collectors. Other branching corals nearby were undamaged, indicating that wave stress was not responsible.
Figure 33. Site # 11. "Black Rock" on outer slope of Rarotonga. Several corals in panoramic view of reef Pavona. Acropora, and Goniastrea. Depth 10 meters.
Figure 34. Site # 11. ''Black Rock'' on outer slope of Rarotonga. Assortment of corals covering reef: Acropora, Pocillopora, Pavona, Lobophyllia. Depth 20 meters.
Figure 35 Site #12. "The Pinnacle'' on outer slope of Rarotonga. Partially bleached coral showing overgrowth by filamentous algae at skeletal tips. Depth 20 meters.
Figure 36. Site #12. "The Pinnacle" on outer slope of Rarotonga. Two partially bleached colonies of Acropora and bleached Montastrea. Depth 20 meters.
Figure 37. Site #12. "The Pinnacle'' on outer slope of Rarotonga. Corals indicating normal colors and growth forms: Acropora, Pavona, Pocillopora. Depth 15 meters.
Figure 38. Site #12. "The Pinnacle" on outer slope of Rarotonga. Bleached, dead and algal encrusted Pocillopora with Porites, Pavona, Acropora. Depth 20 meters.
Figure 39. Site #12. "The Pinnacle" on outer slope of Rarotonga. Colonies of Pocillopora verrucosa broken apart to collect tropical aquarium fish. Because surrounding corals indicate no evidence of physical damage, this bashing of Pocillopora is not attributed to high wave energy, but to specifically targetted and deliberately destructive human activity. Depth 20 meters.
Figure 40. Site #12. "The Pinnacle" on outer slope of Rarotonga. Close-up of Acropora showing bleaching at base of colony. Depth 20 meters.
Figure 41. Site #12. ''The Pinnacle" on outer slope of Rarotonga Close up of Acropora showing green fluorescence Depth 3-5 meters.
13. Northwest Rarotonga Lagoon, off Black Rock (August 14 1994)
a. Physical setting: Lagoon was very shallow, about 2 meters deep. Very large coral heads were abundant, but most of these were dead.
b. Corals and bleaching: Coral heads were largely made up of Porites microatolls, with some young Acropora of around 4 species. Live coral cover was around 30% of reef surface, the rest being dead and algae overgrown, especially on top of heads. No coral bleaching was seen.
c. Algae: Calcareous algae covered about 60% of reef surfaces. This community was very abundant and diverse, including Halimeda opuntia, Galaxuara rugosa, Galaxuara acunimata, Galaxaura fastigiata, Amphiroa fragilissima, and Jania, which were overgrowing live coral and Porolithon. Large mats of the green fleshy bubble algae Valonia aegagropila up to 30 cm. across were overgrowing coral bases and made up about 10% of hard bottom. Fuzzy brown turf algae were found on intertidal rocks. Fragments of the fleshy algae Turbinaria ornata, Sargassum, Dictyota, and Enteromorpha were found washed up on the beach, but were not seen growing in the lagoon.
d. Local reef stresses: Algae were actively overgrowing corals, indicating eutrophication. The fact that branching calcareous algae dominate over fleshy ones indicates that eutrophication is in the early moderate stages and can still be reversed if prompt action is taken to improve water quality by reducing nutrient loading from inadequately treated sewage. The lagoon here is lined with hotels and homes. This area had been badly affected several years ago by being downcurrent from where dredging of sand was done to extend the airport runway. Runoff from juice factory wastes has affected other areas of the Rarotonga lagoon. The sea urchin Echinometra is extremely abundant, and causes high grazing pressure and active erosion of dead coral heads. Reef walking by tourists and reef gleaning by local residents add further stress. Rarotonga has very high rainfall, and erosion of soil causes turbidity after rains.
MAP 7. Tutuila Island, American Samoa. Two sites on the north coast, Masefau Bay and Fagasa Bay as well as two sites on the south coast, Afao Bay and Faga'alu Bay were surveyed (map adapted from Stanley, 1993).
TUTUILA ISLAND, AMERICAN SAMOA
Background: Reef bleaching began in March of 1994 (Peter Craig, personal communication) and up to 70-80% of corals became completely white. Bleaching in 1994 was much more severe than the previous event in 1991. Chuck Brugman (personal communication), who dived Samoan waters since 1975 saw coral reef bleaching for the first time in 1989 and has noted it regularly since. He described the bleaching as always rare and isolated until after the hurricanes of late 1991, when it became frequent in shallow water. Samoan reefs, which had been in excellent condition had been damaged by Cyclone Ofa in 1990 and then devastated by hurricane Val in 1991 (Craig, 1994). This cyclone was very unusual, because after the eye had passed over Samoa, it turned around and passed over it a second time from the opposite direction, staying in the area for six days. Shallow reefs all around Samoa were almost completely destroyed by the hurricane to depths of 20 meters. 1994 was far worse than any previous bleaching, and the only time the Heferactis anemones bleached. Bleaching also took place on the Manu'a islands and offshore bank reefs down to depths of 65-70 meters, but was less pronounced below 40-50 meters where the water becomes cooler by 1 to 2 °C. Crown of thorns starfish began increasing in the early 1970's and reached a peak in early 1979, doing great damage (Craig et al., 1993).
14. Masefau Bay northeast of Tutuila, northwest slope (August 16 1994)
a. Physical setting: Masefau Bay has an axis oriented to the northeast, and fringing reef grows along both sides of the bay. The reef on the northwest side grows on a nearly vertical drop off from a reef flat at less than a meter deep to a depth of 30 meters, where there is a sloping mud and sand plain.
b. Corals and bleaching: Live coral cover is very low, 20% overall, varying between 0% to 40% on various outcrops. There was a large amount of dead coral, both broken fragments and dead corals which are not physically damaged but are covered with mud over part or all of their surfaces. Large amounts of hurricane rubble were piled up everywhere. Coral diversity was fairly high, but there were very few Pocillopora orAcropora colonies. About 50% of all colonies were bleached, and around a third were bleached fully white over all or much of their surfaces. White bleached corals appeared to especially susceptible to sediment accumulation, apparently being able to put less energy into removing sediment. Corals observed with fully white bleached patches included Pachyseris (the most heavily affected species), Porites, Montastrea curta (all uniformly bleached), Pavona clavus, plate-like Pavonas, Favia, Favites, and possibly Leptoseris. Minor or partial bleaching was seen in Diploastrea, (and possibly Plesiastrea and other Favids), Stylonociella, many species of Montipora, Milleporas, Synarea, Goniopora, and Goniastrea. Little bleaching was seen in Astreopora, Lobophyllia, Psammocora, Hydnophora, Coscinarea, Cyphastrea, Gardinoseris, Halomitra, or Oxypora. (Figures 40-44)
c. Algae: Algae dominated the reef framework, with about 50% calcareous algae cover and 30% fleshy algae. Porolithon and encrusting red algae cover the 1991 hurricane rubble. Branching calcareous algae, especially Halimeda, was extremely abundant, and did not appear grazed. Halimeda made up most of the sand component in the sediment, mixed with terrigenous mud. Also in large clumps were Amphiroa fragilissima, Jania, Galaxaura acunimata, Galaxaura fastigiata, Galaxaura rugosa, and Liagora species, plus several kinds of Porolithons, red and violet Lithophyllums, Peysonnelia rubrum and other encrusting calcareous reds. Fleshy algae included large mats of blue-greens similar to Lyngbya, Centroceras, Chlorodesmis, and minor amounts of Dictyota.
d. Local reef stresses: The area was severely damaged by the hurricane, and was subject to high sediment stress from mud and soil which was eroded by the numerous rivers that flowed down steep mountain slopes into the Bay. It was undergoing eutrophication, and algae are overgrowing corals. High suspended matter concentrations appear to support filter feeding and bacterial feeding organisms, indicated by large numbers of small yellow and white vase shaped sponges. Bleaching was seen in anemones, alcyonarians, in one encrusting sponge, and in encrusting Didemnid ascidians.
Figure 42. Site #14. Masefau Bay on outer slope of American Samoa. Panorama indicating severe bleaching and sedimentation on Pachyseris corals. Dead reef frame also evident. Depth 10-15 meters.
Figure 43. Site #14. |