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REPORT
J.F. Bruno áC.E. Siddon áJ.D. Witman áP.L. Colin
M.A. Toscano
El Nin
Äo related coral bleaching in Palau, Western Caroline Islands
Received: 3 January 2000 / Accepted: 8 January 2001 / Published online: 4 July 2001
ÓSpringer-Verlag 2001
Abstract Mass coral bleaching is currently viewed as a
major threat to the long-term health of coral reef
communities. Here we quantify coral bleaching in Pa-
lau coincident with the 1997/1998 El Nin
Äo Southern
Oscillation event and with local sea surface tempera-
tures of 31 °C, which were 1.0±1.25 °C higher than
long-term, satellite-derived climatological maximum
monthly means for the region. We sampled nine sites,
including protected lagoon and fringing reefs, vertical
reef walls, and exposed barrier reefs. The percentage of
living scleractinian coral tissue that was bleached was
53.46.2 (range: 32.3±79.3, n=8 sites) at 3±5 m depth
and 68.96.2 (45.7±91.7, n=6 sites) at 10±12 m and
did not dier signi®cantly between depths. The overall
mean percent cover of bleached scleractinians was
18.91.5 (mean1 SE, n=9 sites), while the cover of
healthy corals was only 15.62.0. Nearly half (48%) of
946 surveyed colonies belonging to 20 scleractinian
taxa were totally bleached, while 15% were partially
bleached. Overall, the results indicate that the 1998
coral bleaching episode in Palau was relatively severe
and widespread across depths, sites, habitats, and coral
taxa.
Keywords Coral bleaching áDisturbance áEl Nin
Äoá
Palau áSea surface temperature
Introduction
Coral reefs throughout the world are currently experi-
encing accelerated degradation (Wilkinson 1992, 1999;
Sebens 1994). The changes generally comprise reduced
coral cover, ®sh abundance, and overall species diversity
(Wilkinson 1992; Hughes 1994; Edmunds and Bruno
1996; Jackson 1997). Although there are certainly many
causal factors (e.g., over®shing, outbreaks of coral dis-
eases and predators, sedimentation and nutrient inputs;
Sebens 1994; Jackson 1997; Wilkinson 1999), coral
bleaching is currently viewed as a major agent of change
in coral reef communities (Brown 1997; Hoegh-Guld-
berg 1999). Bleaching refers to the loss of symbiotic di-
no¯agellates (zooxanthellae) from the host tissue of
scleractinians and other cnidarians, a reduction in zoo-
xanthellae pigmentation, or both (Hoegh-Guldberg
1999). Bleaching is considered to be a response to en-
vironmental stresses including elevated seawater tem-
perature (Hoegh-Guldberg and Smith 1989; Jokiel and
Coles 1990; Gates et al. 1992), high irradiance (Lesser
et al. 1990; Gleason and Wellington 1993), calm weather
(Jaap 1979), and decreased salinity (Goreau 1964). Ef-
fects on coral populations range from total recovery in 1
or 2 months to mortality rates of nearly 100% (Glynn
1984, 1990; Harriott 1985; Fitt et al. 1993). On a scale of
months to years such high mortality rates may not aect
many other reef inhabitants since the dead coral skele-
tons remain in place and should continue to provide
spatial refuge. However, prolonged recovery and sub-
sequent bioerosion can result in habitat loss with cas-
cading eects on numerous ®sh and invertebrate species
(Glynn 1993; Sebens 1994).
Although biologists have been aware of localized
bleaching for over a century (Glynn 1993), mass
bleaching episodes that result in large-scale coral mor-
tality were ®rst recorded in the early 1980s (Glynn 1983).
Coral Reefs (2001) 20: 127±136
DOI 10.1007/s003380100151
J.F. Bruno (&)áC.E. Siddon áJ.D. Witman
Department of Ecology and Evolutionary Biology,
Brown University, Providence, Rhode Island 02912, USA
P.L. Colin
Coral Reef Research Foundation, P.O. Box 1765,
Koror, Palau 96940, Western Caroline Islands
M.A. Toscano
NOAA/NESDIS/ORA/ORAD E/RA31,
SSMC3 Room 3608, 1315 East-West Highway,
Silver Spring, Maryland 20910, USA
Present address: J.F. Bruno
Department of Marine Sciences,
University of North Carolina, Chapel Hill,
North Carolina 27599±3300, USA
Tel.: +1-919-9620263
e-mail: jbruno@unc.edu
Since then there have been several similar bleaching
events on reefs around the world (e.g., Roberts 1987;
Lang et al. 1992; Hoegh-Guldberg and Salvat 1995;
Brown 1997), and there is strong evidence that the fre-
quency and severity of bleaching have increased (Glynn
1991, 1993; Goreau 1992; Hoegh-Guldberg and Salvat
1995; Brown 1997; Winter et al. 1998; Hoegh-Guldberg
1999). A leading explanation for such mass bleaching
events is the interactive eect of increased tropical sea
surface temperatures with El Nin
Äo Southern Oscillation
events (ENSO) (Hoegh-Guldberg 1999), which also may
be increasing in intensity (Stone et al. 1999; Fedorov and
Philander 2000). The most recent widespread bleaching
event occurred in association with the 1997/1998 ENSO,
which resulted in sea surface temperatures 1±4 °C above
normal summer highs over a broad geographic range,
and by some measures was the strongest ENSO on re-
cord (McPhaden 1999). Coincident mild to catastrophic
bleaching was reported from many locations worldwide
including the Caribbean, Indian Ocean, East Africa,
Southeast and East Asia, and the Eastern and Western
Paci®c (Baird and Marshall 1998; Berkelmans and Oli-
ver 1999; Hoegh-Guldberg 1999; Wilkinson 1999;
Aronson et al. 2000).
The purpose of this study was to document wide-
spread coral bleaching in the Republic of Palau, Western
Caroline Islands, that was associated with the 1997/1998
ENSO. We assessed the 1998 bleaching event in Palau
using several methods, including line transects at nu-
merous sites, and qualitative in situ and aerial surveys
over a larger area. We quanti®ed the percentage of living
coral cover that was bleached at one or two depths (3±5
and 10±12 m) at nine sites to determine the spatial extent
of the bleaching across sites, depths, and habitats. A
variety of habitat types were sampled, including highly
protected lagoon and fringing reefs, vertical reef walls,
exposed barrier reefs, as well as reefs with both high and
low tidal current velocities. We also surveyed 964 colo-
nies from ®ve sites and 20 coral taxa to determine how
general the bleaching was within the Scleractinia.
Initial qualitative observations
The ®rst indications of coral bleaching in Palau were
seen in late June 1998, as small portions of healthy coral
colonies became light in color. By mid-July it was evi-
dent that numerous species were starting to bleach, and
through August the extent of bleaching increased
markedly. In September 1998 aerial surveys undertaken
to assess the geographic extent of bleaching indicated
that bleaching was evident throughout the Palau archi-
pelago. Bleaching of large colonies, particularly Porites
lutea and P. lobata, could be seen from 2,500 m altitude.
Qualitative observations suggested that bleaching in-
tensity peaked during September and October, with
large numbers of colonies of numerous species totally
bleached or already dead. In situ surveys using SCUBA
in lagoon areas indicated bleaching was common to
depths of at least 35±40 m. On outer reef drop os,
bleaching was observed to depths of 55±60 m, the lower
limits of most colonial scleractinians in Palau (P. Colin,
unpublished data). Many other coral reef organisms
were bleached including a majority of colonies of com-
mon Alcyoniid soft corals such as Sarcophyton spp. and
Lobophyton spp., and a large number of giant clams
(Tridacna gigas).
Methods
Study location and sites
The Palau archipelago in the Western Paci®c (07°N, 134°E) is a
group of hundreds of small and large islands and lagoons sur-
rounded by an extensive barrier reef. Palau has the highest shallow
water marine species diversity found in Micronesia, with nearly 400
species of scleractinian corals reported (Veron 1986, 1995; Maragos
and Cook 1995), plus at least 200 species of other anthozoans.
Six of the nine sites that were sampled quantitatively (Fig. 1,
Table 1; Risong Lagoon, Ot Lagoon, No Name, Ngerikuul Pass,
Wonder Pass, and Big Drop O) were located in the southern rock
island region of Palau which consists of hundreds of small lime-
stone islands that are often surrounded by a narrow (5±30 m),
shallow (1±5 m depth) shelf/fringing reef that ends at a near-ver-
tical drop-o that usually extends to 20±30 m in depth. The rock
island sites are all protected from open ocean swells but experience
a range of local tidal ¯ow conditions (J. Witman, unpublished
data). The other three sites (West Pass, Siaes Reef, and Short
Drop O) were located on the 120-km-long, exposed outer barrier
reef.
Quantitative survey techniques
The spatial extent of scleractinian bleaching was quanti®ed in
November 1998 using the point-intercept technique (Lang et al.
1992) in two depths: at 3±5 m on the horizontal fringing reefs or
reef crests at eight sites, and at 10±12 m on vertical rock and reef
walls at six sites. Three non-overlapping, horizontal 20-m transect
lines were haphazardly placed at each sampled depth/site combi-
nation. The sessile organisms and substrate directly beneath the
point of each 10-cm increment were characterized as one of four
categories: healthy coral (coral tissue with apparently ``normal''
coloration), bleached coral (coral tissue that was obviously pale or
white in appearance), recently dead coral (corals that had appar-
ently died in the last few weeks ± this category includes both the
``just dead'' and ``recently dead'' categories of Lang et al. 1992),
and other (which included bare substrate, dead coral skeletons, and
space occupied by other organisms). Means and standard errors
were calculated from the three replicate transects at each depth/site
combination.
We examined variation in bleaching at the colony level within
and among coral taxa in ®ve 1´30-m video transects placed along
the 10- to 12-m depth contour (one per site at ®ve sites: West Pass,
Ot lagoon, No Name, Siaes, and Ngerikuul Pass). Video transects
were analyzed in the laboratory by scoring each coral colony (or
individual coral polyp in the case of solitary corals) that fell within
the transects as healthy, partially bleached, or totally bleached
(>90% of the coral tissue was pale or white). These data were
pooled across sites for analysis.
Sea surface temperature measurements
Sea surface temperature (SST) was measured bi-weekly at an o-
shore reef area (Short Drop O) using a hand-held mercury ther-
mometer, beginning >1 year prior to the bleaching event. We also
128
consulted the large-scale (50-km) SST Coral Bleaching ``HotSpot''
anomaly images, provided by the United States National Oceanic
and Atmospheric Administration, National Environmental Satel-
lite Data and Information Service (NOAA/NESDIS, http://
psbsgi1.nesdis.noaa.gov:8080/PSB/EPS/SST/climohot.html), as a
forecasting tool for potential bleaching conditions. These images
are based on multi-channel, night-time-only satellite AVHRR
[advanced very high resolution radiometer on NOAA polar orbit-
ing environmental satellites (POES)] SST data (Walton et al. 1998),
and highlight SST anomalies that are greater than 1°above the
maximum monthly mean (MMM) climatological SST at each pixel.
The MMM climatology was developed from satellite SST spanning
the years 1984±1993, exclusive of 1991/1992 (due to aerosol con-
tamination from the Mt. Pinatubo eruption). Degree-heating week
(DHW) accumulations of these SST HotSpot anomalies (http://
psbsgi1.nesdis.noaa.gov: 8080/PSB/EPS/icg/dhw/dhw_new.html)
commence at the 1 °C threshold and provide an estimate of the
residence time of anomalously warm water in the region and are
included in this analysis. In addition, we incorporated 15-year SST
time series data for several sites around Palau using NASA/JPL
Fig. 1 Map of study sites in
Palau. Circles Quantitatively
sampled sites; stars qualita-
tively surveyed sites
Table 1 Coordinates and
descriptions of sites used in
quantitative bleaching surveys
in Palau. Flow data (Witman,
unpublished) were obtained
using the dissolution block
technique (Thompson and
Glen 1994)
Site Coordinates Characteristics
Big Drop O 07°06.32¢N, 134°15.25¢E Rock island, med. ¯ow
Ngerikuul Pass 07°19.26¢N, 134°29.78¢E Rock island, high ¯ow
No Name 07°14.80¢N, 134°23.02¢E Rock island, low ¯ow
Ot Lagoon 07°09.48¢N, 134°20.53¢E Rock island, low ¯ow
Risong 07°18.45¢N, 134°28.85¢E Rock island, low ¯ow
Wonder Pass 07°10.88¢N, 134°21.65¢E Rock island, high ¯ow
Short Drop O 07°16.47¢N, 134°31.50¢E Barrier reef, med. ¯ow
Siaes 07°18.79¢N, 134°13.43¢E Barrier reef, high ¯ow
West Pass 07°32.52¢N, 134°28.25¢E Barrier reef, high ¯ow
129
Oceans Path®nder Best SST (AVHRR) data at 9-km resolution
(http://podaac.jpl.nasa.gov/sst/; see also Kearns et al. 2000; Kil-
patrick et al. 2001). High (9-km) resolution HotSpot maps were
also prepared from the Path®nder Best SST data and a 9-km
Path®nder-based MMM climatology [1985±1993 inclusive (aero-
sols corrected); Liu et al. 2000; Toscano et al. 2000, unpublished
data). These are presented to illustrate the detailed water heating
around the Palau islands.
Results and discussion
The results of our surveys indicate that the 1998 coral
bleaching in Palau was relatively severe and widespread
across depths, sites, habitats, and coral taxa. The mean
percent cover of bleached scleractinian corals was
18.91.5 (mean1 SE, n=9 sites), and the cover of
healthy corals was only 15.62.0 (when pooled across
sites and depths). The overall percentage of living scle-
ractinian coral tissue that was bleached (i.e. bleaching
severity) was 53.46.2 (range of site means=32.3±79.3,
n=8 sites) at 3±5 m and 68.96.2 (45.7±91.7, n=6 sites)
at 10±12 m and did not dier signi®cantly between
depths (Fig. 2; t=1.741; df=1,12; P>0.05). Although
bleaching severity varied signi®cantly among sites
within depth strata (3±5 m, F
7, 15
=6.28, P<0.01;
10±12 m, F
5, 12
=3.25, P<0.05), the lowest recorded
percentage at any site/depth combination was 32.34.8
(n=3 transects, Fig. 2). Among-site variation in the
degree of bleaching could have been due to variability in
the susceptibility of locally dominant species to thermal
stress and might explain why bleaching intensity did not
correspond to any obvious site characteristics. For ex-
ample, bleaching severity did not vary signi®cantly be-
tween barrier reef and rock island sites (Kruskal±Wallis
test of 3- to 5-m transect data: v
2
=1.088, df=1,
P=0.296).
The percent cover of recently dead scleractinians
ranged from 0±6.6 among sites. Our point-intercept
sampling took place 12±16 weeks after the beginning of
severe bleaching and it is possible that many coral col-
onies died and were overgrown weeks before we quan-
ti®ed the occurrence of recently dead corals. For
example, dead skeletons of tabular Acroporids (e.g.
A.tenuis and A. hyacinthus), still in growth position,
were very common at depths of 1±10 m at many of the
sites we surveyed. However, Palau was aected by a
poorly documented Acanthaster planci outbreak during
Fig. 2 Percentage of the substratum that was covered by Alive
scleractinian corals (including bleached and non-bleached tissue),
and Bthe percentage of that live coral tissue that was ``bleached.''
Data are from quantitative point-intercept sampling at nine sites in
Palau. Bars represent untransformed means1 SE, n=3 transects/
site; asterisks site/depth combination was not sampled
Table 2 Percentage of colo-
nies (or individual polyps in the
case of solitary species) of each
scleractinian taxa that displayed
``normal'' or ``healthy'' colora-
tion, and those that were
partially or totally bleached (i.e.
>90% of surface area). Data
were pooled from 1´30-m-band
video transects at ®ve sites
Taxa Percent healthy Percent bleached Percent partially
bleached
Total no.
Acropora sp. 62 32 6 47
Astreopora sp. 41 50 9 22
Favia sp. 16 71 13 80
Favites sp. 23 61 17 168
Fungia sp. 28 51 21 121
Galaxea sp. 60 20 20 5
Goniopora sp. 95 0 5 20
Heliofungia actiniformis 100 0 0 11
Lobophyllia sp. 8 83 9 88
Montipora sp. 92 5 4 83
Pachyseris speciosa 17 58 25 12
Pavona sp. 65 10 25 20
Pectinia paeonia 18 53 29 17
Physogyra lichtensteini 14 86 0 7
Platygyra sp. 41 55 5 22
Pocillopora sp. 60 20 20 25
Porites sp. 35 42 23 168
Psammocora contigua 0 100 0 25
Scolymia sp. 88 13 0 8
Turbinaria sp. 47 13 40 15
Totals 37 48 15 964
130
the mid to late 1990s which caused near total coral
mortality at some sites (P. Colin, personal observations).
Because plating species of Acropora are a preferred prey
of A. planci (Birkeland 1982; Colgan 1987), it is unclear
whether predation or bleaching (or some other factor)
caused the mortality of these normally common species.
Nearly half (48%) of the 946 colonies surveyed in the
video transects (pooled across 20 taxa) were totally
bleached, and 15% were partially bleached (Table 2).
However, there was considerable variation among taxa
and a few (e.g. Goniopora spp. and Montipora spp.) dis-
played much lower bleaching frequencies. Such taxa-
speci®c bleaching susceptibility could result in a major
shift in species composition on reefs that have been se-
verely or repeatedly bleached (Glynn 1993). Interesting-
ly, at the generic level, the relative bleaching frequencies
we recorded in the quantitative surveys do not corre-
spond to the order of susceptibility reported from pre-
vious Paci®c bleaching episodes in which Acropora spp.
was the most susceptible and Porites spp. was the least
susceptible taxa (Gleason 1993; Hoegh-Guldberg and
Salvat 1995). Variation within genera may account for
the discrepancies between our results and previous re-
ports. For example, qualitative surveys (Table 3) indi-
cated considerable variation in bleaching susceptibility
among Acroporid species as some had apparently expe-
rienced nearly 100% mortality (e.g. A. echinata and
Fig. 3 SST data at the Short Drop O site in Palau for 1997±1999.
In situ measurements were made by hand and 9-km night-time
satellite SST data are from the NASA/JPL AVHRR Oceans
Path®nder program
Fig. 4 Portion of NOAA/
NESDIS Degree Heating
Weeks (DHW) chart for the
eastern hemisphere, covering
the period 1 Jul±30 Sept 1998.
DHW accumulations begin
when the HotSpot anomaly (at
each pixel) reaches the 1 °C
level. One DHW is equivalent
to 1 week of SST 1 °C above
the MMM climatological value.
Two DHW are equivalent to
2 weeks of SST 1 °C above the
MMM, or 1 week of SST 2 °C
above the MMM. Four to seven
DHW had accumulated around
the Palau region during this
period, mainly during August
and early September (see
HotSpot anomaly maps in
Fig. 5)
131
A. hyacinthus), while other species appeared unaected
(e.g. corymbose Acroporids). Despite strong variation
among taxa, gross colony morphology was not obviously
related to bleaching as some species with massive (e.g.
Porites lutea), branching (e.g. Acropora formosa), tabular
(e.g. Acropora hyacinthus), plating (e.g. Pachyseris spec-
iosa), and free-living (e.g. Fungia fungites) morphologies
all exhibited high frequencies of bleaching (Tables 2 and
3). Hoegh-Guldberg (1999) has suggested that variation
among taxa in bleaching susceptibility may be caused by
dierences in tissue thickness. However, the absence of a
repeatable susceptibility hierarchy indicated by this and
previous studies (e.g. Williams and Bunkley-Williams
1988) seems to preclude a universal explanation.
The exact causes and mechanisms of coral bleaching
have proved dicult to elucidate (Brown 1987; Edm-
unds 1994) and remain controversial (Warner et al.
1999). Nonetheless, a number of experimental labora-
tory studies have demonstrated the importance of in-
creased water temperature (Hoegh-Guldberg and Smith
1989; Glynn and D'Croz 1990; Gates et al. 1992; review
in Brown 1997), and many past mass bleaching episodes
were correlated with sea surface temperatures 1±4 °C
above normal summer highs (Glynn 1984; Gates 1990;
Hoegh-Guldberg and Salvat 1995; Brown et al. 1996;
Winter et al 1998). For example, the 1983/1984 ENSO
caused 2±3 °C SST increases and was related to espe-
cially severe bleaching and high rates of coral mortality
in the Eastern Paci®c (Glynn 1983, 1984, 1990). Glynn
(1993) suggests that SST increases of 3±4 °C for 1±
2 days or 1±2 °C for several weeks are required to cause
severe thermal bleaching. Local temperature records
indicate that surface water temperatures at the Short
Drop O site in Palau were 31 °C for a period of at least
30 days, during the late summer of 1998 (Fig. 3), when
ENSO-related increases in SST peaked in the Western
Paci®c (McPhaden 1999). NOAA/NESDIS 50-km
DHW maps indicated that the equivalent of 4±7 weeks
of thermal stress had accumulated over the 12-week
period spanning 1 July through 30 September 1998
(Fig. 4). High-resolution (9-km; Toscano et al. 2000; Liu
et al. 2000) HotSpot maps focused on Palau (Fig. 5)
show temperature anomalies above the MMM SST of
29.55 °C beginning in late July 1998, with warmth
Fig. 5 Time series of 9-km
HotSpot anomalies for the
waters surrounding Palau
during 1998. Landmasses
and shallow water areas
are masked in black.Gray
areas indicate no satellite
retrievals due to cloud cover.
The scale indicates the level
of SST anomaly above the
satellite-derived maximum
monthly mean (MMM) SST.
The 1 °C threshold level,
above which coral bleaching
is likely to occur, is indicated
in yellow
132
peaking in August and remaining around the Palau is-
lands into September and October. Long-term satellite
SST for sites around Palau (Fig. 6) indicate that SST
between July and November 1998 were above the
29.55 °C, 9-km MMM, and exceeded that threshold by
at least the 1 °C anomaly level for periods considered
sucient to induce coral bleaching.
Although correlative, the results of this study sug-
gest that ENSO-related STT increases are likely to be
the cause of the 1998 bleaching in Palau. However, it is
not possible to rule out other factors in either the
bleaching or recent coral mortality documented in this
study. For instance, ENSO events can also be associ-
ated with unusually calm periods that can enhance
shallow subtidal irradiance levels (Lesser et al. 1990;
Gleason and Wellington 1993). Although, somewhat
controversial (Warner et al. 1999), it is thought that
increased visible and UV light can induce bleaching or
increase its severity (Lesser et al. 1990; Gleason and
Wellington 1993; Hoegh-Guldberg 1999). Thus, in-
creases in irradiance may have contributed to this mass
bleaching event.
The eects of the 1998 bleaching in Palau are dicult
to assess in more detail because of the lack of quanti-
tative baseline data. A qualitative rapid ecological
assessment of Palau's reefs in 1992 reported that ``coral
reefs in Palau are in excellent condition supporting di-
verse and abundant coral reef, seagrass, mangrove, and
lagoon ecosystems,'' and ``only a few coral reef areas
have been subjected to anthropogenic impacts'' (Mara-
gos and Cook 1995). Ultimately, the eects of the 1998
bleaching will be determined by rates of coral recovery
and mortality and subsequent regrowth and recruitment.
Moderate to severe mortality of corals that were
bleached in 1998 could reduce coral cover to <10±20%
at a number of rock island and barrier reef sites.
As of September 2000, there appears to have been
very little recovery of bleached corals following the re-
turn of SST to normal levels (P. Colin, personal obser-
vations). Preliminary estimates of mortality based on
reefs that were originally sampled with low-altitude
(300 m or less) aerial photographs and resampled using
SCUBA indicate that bleaching-related mortality was
relatively high for some massive reef-building species.
For example, at one patch reef in the central lagoon near
the small island of Ngeragabal (Fig. 1), colonies of
Porites spp. that were bleached in September were dead
by December 1998 in nearly all cases (n=100 colonies).
However, not all Porites spp. colonies on this reef
bleached. Non-bleached colonies were not evident in the
Fig. 6 Fifteen-year night-
time satellite SST records for
northern and southern areas
of the outer barrier reef on
the western side of Palau.
Satellite data are from the
NASA/JPL AVHRR
Oceans Path®nder program.
Night-time SST were above
the HotSpot temperature
threshold at both sites for
almost all of the latter half
of 1998, and exceeded the
1°C level during August
and September 1998
133
aerial photographs, and of 491 Porites spp. colonies
surveyed at this site, 40% were found to be alive, 30%
were heavily damaged by bleaching but a portion was
still alive, and 30% were completely dead. These results
are concordant with qualitative observations at numer-
ous other sites during early 1999, which suggested that
bleaching-related mortality, although variable, was high
for some species.
Reef-building corals have inhabited shallow tropical
waters for >200 million years (Stanley 1981). However,
they only generated coral reef habitats intermittently
during periods when water temperature and ocean
chemistry were favorable to high calci®cation rates
(Veron 1995). During numerous periods of non-optimal
environmental conditions, scleractinians experienced
high extinction rates, were largely restricted to habitat
refuges, and generally did not create large structures
(Veron 1995). If tropical sea temperatures continue to
increase or if ENSO events become more frequent or
severe, reef-building corals may not be able to maintain
their current role as habitat providers to numerous reef-
dependent taxa (Sebens 1994; Brown 1997). Because of
the critical importance of coral reefs to tropical marine
species diversity and human economic interests, their
fate should be regarded as a pressing scienti®c, conser-
vation, and social issue.
Acknowledgments We would like to thank the sta of the Coral
Reef Research Foundation laboratory in Palau for their support.
This research was funded in part by a National Science Foundation
grant (no. OCE-9730647) to J.D.W. and a National Science
Foundation dissertation improvement award to J.F.B. (no.
DEB98±01422).
Appendix
The additional Table 3 shows bleaching of various
cnidarian species from qualitative observations at 24
sites in Palau (1998).
Table 3 Bleaching of various cnidarian species from qualitative
observations at 24 sites in Palau (1998)
Species Bleaching level and estimated
mortality
OCTOCORALLIA
Lobophytum several spp. Extremely high (90%+), high
mortality of large colonies
Sinularia spp. High, in most species mortality
90%+, species speci®c
HEXACORALLIA
Milleporidae Some mortality seen
SCLERACTINIA
Astrocoeniidae
Stylocoeniella High, high mortality
Pocilloporidae
Palauastrea ramosa Low, low mortality
Pocillopora damicornis Bleaching variable (0±50%
bleached), low mortality
Pocillopora eydouxi High with high mortality
Pocillopora spp. High with high mortality
Seriatopora spp. High with high mortality
Table 3 Continued
Species Bleaching level and estimated
mortality
Stylophora spp. High with high mortality
Acroporidae
Acropora echinata Very high, mortality approaching
100%
Acropora formosa High, high mortality
Acropora spp. arborescent Variable by species
Acropora hyacinthus Very high, mortality approaching
100%
Acropora other tabulate High, but one unidenti®ed species
moderate, mortality high
Anacropora spp. Total mortality in limited areas while
others were unaected
Astreopora spp. Moderate, moderate mortality
Montipora spp. Many species involved with heavy
bleaching in many, but not all
Poritidae Moderate, moderate mortality
Alveopora spp. Relatively little bleaching and
mortality seen
Goniopora stokesii Variable by habitat, low mortality?
Goniopora spp. Locally high, moderate mortality,
some species not aected
Porites lobata/lutea Moderate, moderate mortality
(10±40%)
Porites rus Low to moderate, low to moderate
mortality
Porites cylindricus High to moderate, high to moderate
mortality
Porites nigrescens Very high to moderate, high mortality
Siderastereidae
Psammocora contigua Moderate to high, moderate
mortality
Psammocora digitata High, high mortality, tips bleached
®rst
Agariciidae
Leptoseris gardineri High, high mortality
Leptoseris papyracea High, high mortality
Pachyseris rugosa Variable but generally high,
high mortality
Pavona cactus High, high to moderate mortality
Pavona clavus High, high mortality
Pavona minuta Some high bleaching seen, signi®cant
mortality
Fungiidae
Cycloseris spp. No bleaching seen
Diaseris spp. No bleaching seen
Fungia spp. Variable, often high mortality,
habitat and species speci®c?
Podabacia crustacea Moderate, mortality unknown
Other fungiids Variable, habitat dependent
Oculinidae
Acrhelia horrecens Moderate to high, moderate
mortality
Galaxea astreata High, high mortality in all areas, one
of the most aected species
Pectinidae
Mycedium elephantotus Moderate, moderate mortality
Pectinia lactuca High, high mortality
Pectinia peonia High, high mortality
Mussidae
Cynaria lacrymalis High, mortality appears low
Lobophyllia corymbosa High, high mortality
Lobophyllia nataii High, high mortality
Lobophyllia hemprichii High, high mortality
Lobophyllia pachysepta High, high mortality
Symphyllia spp. High, high mortality
Merulinidae
Hydnophora spp. Low, but variable, low mortality
134
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Species Bleaching level and estimated
mortality
Merulina spp. Moderate to high, moderate
mortality
Favidae
Barabattoia amicorum Low to moderate, mortality
unknown
Caulastrea furcata No bleaching seen
Diploastrea heliopora Variable, often distinct
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Favia/Favites spp. Variable, high in some, moderate
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Goniastrea spp. Variable bleaching among species,
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Leptoria spp. Variable, but generally moderate
Oulophyllia spp. High in some areas, mortality high
Platygyra spp. High in some areas, mortality
unknown
Caryophylliidae
Catalaphyllia jardinei Low to none
Euphyllia divisa High, mortality unknown
Euphyllia glabrescens High, mortality unknown
Euphyllia parancora High, mortality unknown
Physogyra lichtensteini Very high, high to near total
mortality of polyps
Pleurogyra sinuosa Generally high bleaching and
mortality, some colonies unaected
Dendrophyllidae
Dendrophyllia spp. Azooxanthellate, no bleaching
Tubastraea spp. Azooxanthellate, no bleaching
Turbinaria bifrons Locally high bleaching, mortality
high
Turbinaria peltata Locally high bleaching, mortality
high
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