ArticlePDF Available

El Niño related coral bleaching in Palau, Western Caroline Islands

Authors:
  • coral reef research foundation
Article

El Niño related coral bleaching in Palau, Western Caroline Islands

Abstract and Figures

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 Palau coincident with the 1997/1998 El Nio Southern Oscillation event and with local sea surface temperatures 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 53.4&#456.2 (range: 32.3-79.3, n=8 sites) at 3-5 m depth and 68.9&#456.2 (45.7-91.7, n=6 sites) at 10-12 m and did not differ significantly between depths. The overall mean percent cover of bleached scleractinians was 18.9&#451.5 (meanǃ SE, n=9 sites), while the cover of healthy corals was only 15.6&#452.0. 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.
Content may be subject to copyright.
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
53.46.2 (range: 32.3±79.3, n=8 sites) at 3±5 m depth
and 68.96.2 (45.7±91.7, n=6 sites) at 10±12 m and
did not dier signi®cantly between depths. The overall
mean percent cover of bleached scleractinians was
18.91.5 (mean1 SE, n=9 sites), while the cover of
healthy corals was only 15.62.0. 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 aect
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 eects 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 eect 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 os,
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
18.91.5 (mean1 SE, n=9 sites), and the cover of
healthy corals was only 15.62.0 (when pooled across
sites and depths). The overall percentage of living scle-
ractinian coral tissue that was bleached (i.e. bleaching
severity) was 53.46.2 (range of site means=32.3±79.3,
n=8 sites) at 3±5 m and 68.96.2 (45.7±91.7, n=6 sites)
at 10±12 m and did not dier 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 32.34.8
(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 aected 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 means1 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 unaected
(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
dierences 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 dicult 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
sucient 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 eects of the 1998 bleaching in Palau are dicult
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 eects 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 unaected
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 aected
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 aected 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
References
Aronson RB, Precht WF, Macintyre IG, Murdoch TJT (2000)
Coral bleach-out in Belize. Nature 405:36
Baird AH, Marshall PA (1998) Mass bleaching of corals on the
Great Barrier Reef. Coral Reefs 17:376
Berkelmans R, Oliver JK (1999) Large-scale bleaching of corals on
the Great Barrier Reef. Coral Reefs 18:55±60
Birkeland C (1982) Terrestrial runo as a cause of outbreaks of
Acanthaster planci (Echinodermata: Asteroidea). Mar Biol
69:175±185
Brown BE (1987) Worldwide death of corals ± natural cyclical
events or man-made pollution? Mar Pollut Bull 18:9±13
Brown BE (1997) Coral bleaching: causes and consequences. Coral
Reefs 16:S129±S138
Brown BE, Dunne RP, Chansang H (1996) Coral bleaching relative
to elevated seawater temperature in the Andaman Sea (Indian
Ocean) over the last 50 years. Coral Reefs 15:151±152
Colgan MW (1987) Coral reef recovery on Guam (Micronesia)
after catastrophic predation by Acanthaster planci. Ecology
68:1592±1605
Edmunds PJ (1994) Evidence that reef-wide patterns of coral
bleaching may be the result of the distribution of bleaching
susceptible clones. Mar Biol 121:137±142
Edmunds PJ, Bruno JF (1996) The importance of sampling scale in
ecology: kilometer-wide variation in coral reef communities.
Mar Ecol Prog Ser 143:165±171
Fedorov AV, Philander SG (2000) Is El Nin
Äo changing? Science
288:1997±2002
Fitt WK, Spero HJ, Halas J, White MW, Porter JW (1993) Re-
covery of the coral Montastrea annularis in the Florida Keys
after the 1987 Caribbean ``bleaching event.'' Coral Reefs 12:57±
64
Gates RD (1990) Seawater temperature and sublethal coral
bleaching in Jamaica. Coral Reefs 8:193±197
Gates RD, Baghdasarian G, Muscatine L (1992) Temperature
stress causes host cell detachment in symbiotic cnidarians: im-
plications for coral bleaching. Biol Bull 182:324±332
Gleason DF, Wellington GM (1993) Ultraviolet radiation and
coral bleaching. Nature 365:836±838
Gleason MG (1993) Eects of disturbance on coral communities:
bleaching in Moorea, French Polynesia. Coral Reefs 12:193±
201
Glynn PW (1983) Extensive `bleaching' and death of reef corals on
the Paci®c coast of Panama. Environ Conserv 10:149±154
Glynn PW (1984) Widespread coral mortality and the 1982±83 El
Nin
Äo warming event. Environ Conserv 11:133±146
Glynn PW (1990) Coral mortality and disturbances to coral reefs in
the tropical Eastern Paci®c. In: Glynn PW (ed) Global eco-
logical consequences of the 1982±83 El Nin
Äo-Southern Oscil-
lation. Elsevier, Amsterdam, pp 55±126
Glynn PW (1991) Coral reef bleaching in the 1980s and possible
connections with global warming. Trends Ecol Evol 6:175±179
Glynn PW (1993) Coral reef bleaching: ecological perspectives.
Coral Reefs 12:1±17
Glynn PW, D'Croz L (1990) Experimental evidence for high tem-
perature stress as the cause of El Nin
Äo-coincident coral mor-
tality. Coral Reefs 8:181±191
Goreau TF (1964) Mass expulsion of zooxanthellae from Jamaican
reef communities after Hurricane Flora. Science 145:383±386
Goreau TJ (1992) Bleaching and reef community change in Ja-
maica: 1951±1991. Am Zool 32:683±695
Harriott VJ (1985) Mortality rates of scleractinian corals before
and during a mass bleaching event. Mar Ecol Prog Ser 21:81±88
Hoegh-Guldberg O (1999) Climate change, coral bleaching and the
future of the world's coral reefs. Mar Freshwater Res 50:839±
866
Hoegh-Guldberg O, Salvat B (1995) Periodic mass-bleaching and
elevated sea temperatures: bleaching of outer reef slope com-
munities in Moorea, French Polynesia. Mar Ecol Prog Ser
121:181±190
Hoegh-Guldberg O, Smith GJ (1989) The eects of sudden changes
in temperature, light and salinity on the population density and
export of zooxanthellae from the reef corals Stylophora pistil-
lata Esper and Seriatopora hystrix Dana. J Exp Mar Biol Ecol
129:279±303
Hughes TP (1994) Catastrophes, phase shifts, and large scale de-
gradation of a Caribbean coral reef. Science 265:1547±1551
Jaap W (1979) Observations on zooxanthellae expulsion at Middle
Sambo Reef, Florida Keys. Bull Mar Sci 29:414±422
Jackson JBC (1997) Reefs since Columbus. Coral Reefs 16:S23±S32
Jokiel PL, Coles SL (1990) Response of Hawaiian and other Indo-
Paci®c reef corals to elevated temperature. Coral Reefs 8:155±
162
Kearns EJ, Hana®n JA, Evans R, Minnett PJ, Brown OB (2000)
An independent assessment of Path®nder AVHRR sea sur-
face temperature accuracy using the Marine-Atmosphere
Emitted Radiance Interferometer. Bull Am Meteor Soc
81:1525±1536
Kilpatrick K, Podesta GP, Evans R (2001) Overview of the
NOAA/NASA Path®nder algorithm for sea surface tempera-
ture and associated matchup database. J Geophys Res
106(C5):9179±9197
Lang JC, Lasker HR, Gladfelter EH, Hallock P, Japp WC, Losada
FJ, Muller RG (1992) Spatial and temporal variability during
periods of ``recovery'' after mass bleaching on Western Atlantic
coral reefs. Am Zool 32:696±706
Lesser MP, Stochaj WR, Tapley DW, Schick JM (1990) Bleaching
in coral reef anthozoans: eects of irradiance, ultraviolet radi-
ation, and temperature on the activities of protective enzymes
against active oxygen. Coral Reefs 8:225±232
Liu G, Toscano MA, Strong AE, Guch IC, Casey KS (2000) Ex-
amination of satellite-derived SST ®elds for improved early
Table 3 Continued
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
color (bleaching) variation
Favia/Favites spp. Variable, high in some, moderate
mortality
Goniastrea spp. Variable bleaching among species,
high mortality in some
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 unaected
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
135
warning of coral reef bleaching events. In: Proc American
Geophysical Union Fall Meeting, 15±19 Dec
Maragos JE, Cook CW Jr (1995) The 1991±1992 rapid ecological
assessment of Palau's coral reefs. Coral Reefs 14:237±252
McPhaden MJ (1999) Genesis and evolution of the 1997±98 El
Nin
Äo. Science 283:950±954
Roberts L (1987) Coral bleaching threatens Atlantic reefs. Science
238:1228±1229
Sebens KP (1994) Biodiversity of coral reefs: what are we losing
and why? Am Zool 34:115±133
Stanley GD (1981) Early history of scleractinian corals and its
geological consequences. Geology 9:507±511
Stone L, Huppert A, Rajagopalan B, Bhasin H, Loya Y (1999)
Mass coral reef bleaching: a recent outcome of increased El
Nin
Äo activity? Ecol Lett 2:325±330
Thompson TL, Glen EP (1994) Plaster standards to measure water
motion. Limnol Oceanogr 39:1768±1779
Toscano MA, Strong AE, Guch IC, Casey KS (2000) Improved
prediction of coral bleaching using high-resolution HotSpot
mapping and DHWs. In: Proc 9th Int Coral Reef Symp,
Abstract
Veron JEN (1986) Corals of Australia and the Indo-Paci®c. Angus
and Robertson, Sydney
Veron JEN (1995) Corals in space and time. Cornell University
Press, Ithaca
Walton CC, Pichel WG, Sapper JF, May DA (1998) The development
and operational application of nonlinear algorithms for the
measurement of sea surface temperatures with the NOAA polar-
orbiting environmental satellites. J Geol Res 103:27,999±28,012
Warner ME, Fitt WK, Schmidt GW (1999) Damage to photo-
system II in symbiotic dino¯agellates: a determinant of coral
bleaching. Proc Natl Acad Sci USA 96:8007±8012
Wilkinson CR (1992) Coral reefs of the world are facing wide-
spread devastation: can we prevent this through sustainable
management practices? In: Proc 7th Int Coral Reef Symp Publ
1, pp 11±21
Wilkinson CR (1999) Global and local threats to coral reef func-
tioning and existence: review and predictions. Mar Freshwater
Res 50:867±878
Williams EH Jr, Bunkley-Williams L (1988) Bleaching of Carib-
bean coral reef symbionts in 1987±1988. In: Proc 6th Int Coral
Reef Symp Publ 3, pp 313±318
Winter A, Appeldoorn RS, Bruckner A, Williams EH Jr, Goenaga
C (1998) Sea surface temperatures and coral reef bleaching o
La Parguera, Puerto Rico (northeastern Caribbean Sea). Coral
Reefs 17:377±382
136
... Page 3/21 reefs 11,13 . Even with more severe anomalies, during the 1997-1998 ENSO (the most devastating bleaching in Palau 9,11 ) only ~25% of Rock Island corals bleached, compared to nearly 60% of corals in Palau's outer reefs; Rock Island reefs also recovered faster [9][10][11]13 . This pattern was repeated during the shorter and less severe 2010 ENSO, when Rock Island temperatures were ~0.6°C warmer than on outer reefs, but corals showed lower bleaching: ~15% vs. 25-30% in outer reefs 11,13 . ...
... The Palauan archipelago harbors robust coral communities with a demonstrated tolerance to high temperatures [9][10][11][12][13][14][15] . South of Palau's mainland, hundreds of small islands --the Rock Islands --form a series of semi-enclosed bays. ...
Preprint
Full-text available
Ocean warming is killing corals, but heat-tolerant populations exist; if protected, they could replenish affected reefs naturally or through restoration. Palau’s Rock Islands experience chronically higher temperatures and extreme heatwaves, yet their diverse coral communities bleach less than those on Palau’s cooler outer reefs. Here, we combined genetic analyses, bleaching histories and growth rates of Porites cf. lobata colonies to identify thermally tolerant genotypes, map their distribution, and investigate potential growth trade-offs. We identified four P cf. lobata genetic lineages. On Palau’s outer reefs, a thermally sensitive lineage dominates. The Rock Islands harbor two lineages with enhanced thermal tolerance and no consistent growth trade-off. One of these lineages also occurs on several outer reefs. This suggests that the Rock Islands provide naturally tolerant larvae to neighboring areas. Finding and protecting such sources of thermally-tolerant corals is key to reef survival under 21st century climate change.
... Coral bleaching is defined as a breakdown of coral-algae symbiosis, when the corals are stressed by a variety of factors, in which the increased atmosphere temperature (thus seawater temperature) is the major triggering cause, and the endosymbiotic algae (in particular photosynthetic dinoflagellate Symbiodinium) expel (Hoegh-Guldberg and Smith, 1989;Bruno et al., 2001;Eakin et al., 2010). This results in the loss of coral color (become white) and of food that is supplied through photosynthesis by the dinoflagellates for coral growth, reproduction, resistance to diseases, and survivability (Baker et al., 2008). ...
Article
Full-text available
As seawater temperature rises, repeated thermal bleaching events have negatively affected the reefs of the Andaman Sea for over decades. Studies on the coral-associated microbial diversity of prokaryotes and microbial eukaryotes (microbiome) in healthy and bleached corals are important to better understand the coral holobionts that involved augmented resistance to stresses, and this information remains limited in the Andaman Sea of Thailand. The present study thereby described the microbiomes of healthy (unbleached) and bleached colonies of four prevalent corals, Acropora humilis , Platygyra sp., Pocillopora damicornis , and Porites lutea , along with the surrounding seawater and sediments, that were collected during a 2016 thermal bleaching event, using 16S and 18S rRNA genes next-generation sequencing (NGS). Both prokaryotic and eukaryotic microbes showed isolated community profiles among sample types (corals, sediment, and seawater) [analysis of similarities (ANOSIM): p = 0.038 for prokaryotes, p < 0.001 for microbial eukaryotes] and among coral genera (ANOSIM: p < 0.001 for prokaryotes and microbial eukaryotes). In bleached state corals, we found differences in microbial compositions from the healthy state corals. Prevalent differences shared among bleached coral genera (shared in at least three coral genera) included a loss of reported coral-beneficial microbes, such as Pseudomonadales, Alteromonadales, and Symbiodinium ; meanwhile an increase of putative coral-pathogenic Malassezia and Aspergillus . This difference could affect carbon and nitrogen availability for coral growth, reflective of a healthy or bleached state. Our findings in part supported previously microbial dysbiosis knowledge of thermal bleaching coral microbiomes around South East Asia marine geography, and together ongoing efforts are to support the understanding and management of microbial diversity to reduce the negative impacts to corals in massive thermal bleaching events.
... This result is in agreement with other studies that have documented coral mortality and consequent declines in coral cover following the temperature-induced mass-bleaching events on the BBR in 1998 and 2005 [50,58]. Many other studies have documented the role of ocean heatwaves in coral decline around the world [21,22,67,80,91,92,[101][102][103][104]. ...
Article
Full-text available
Disease, storms, ocean warming, and pollution have caused the mass mortality of reef-building corals across the Caribbean over the last four decades. Subsequently, stony corals have been replaced by macroalgae, bacterial mats, and invertebrates including soft corals and sponges, causing changes to the functioning of Caribbean reef ecosystems. Here we describe changes in the absolute cover of benthic reef taxa, including corals, gorgonians, sponges, and algae, at 15 fore-reef sites (12–15m depth) across the Belizean Barrier Reef (BBR) from 1997 to 2016. We also tested whether Marine Protected Areas (MPAs), in which fishing was prohibited but likely still occurred, mitigated these changes. Additionally, we determined whether ocean-temperature anomalies (measured via satellite) or local human impacts (estimated using the Human Influence Index, HII) were related to changes in benthic community structure. We observed a reduction in the cover of reef-building corals, including the long-lived, massive corals Orbicella spp. (from 13 to 2%), and an increase in fleshy and corticated macroalgae across most sites. These and other changes to the benthic communities were unaffected by local protection. The covers of hard-coral taxa, including Acropora spp., Montastraea cavernosa , Orbicella spp., and Porites spp., were negatively related to the frequency of ocean-temperature anomalies. Only gorgonian cover was related, negatively, to our metric of the magnitude of local impacts (HII). Our results suggest that benthic communities along the BBR have experienced disturbances that are beyond the capacity of the current management structure to mitigate. We recommend that managers devote greater resources and capacity to enforcing and expanding existing marine protected areas and to mitigating local stressors, and most importantly, that government, industry, and the public act immediately to reduce global carbon emissions.
... Reyes-Bonilla, Carriquiry, Leyte-Morales y Cupul-Magaña (2002) reportan que los blanqueamientos ocurrieron en la costa oeste de México luego de que la temperatura superficial del mar fuera igual o superior a los 30 °C por más de 60 días. En otros lugares como las Islas Carolinas, el blanqueamiento y la mortalidad de los corales ocurrió alrededor de 31 °C (Bruno, Siddon, Witman, Colin, & Toscano, 2001). Durante la ocurrencia del blanqueamiento la temperatura estuvo por debajo de los 29 °C, lo que sugiere que está no fue la causante de la mortalidad observada. ...
Article
Full-text available
Introducción: Los arrecifes coralinos son afectados negativamente por diferentes causas antropogénicas y ambientales en el Golfo de Papagayo. Objetivo: En esta investigación, la condición del arrecife coralino de Playa Blanca fue evaluado. Métodos: Entre agosto de 2010 y setiembre de 2011 el porcentaje de coral vivo por especie y del alga Caulerpa sertularoides fue determinado; el crecimiento anual de P. damicornis y P. elegans fue medido y relacionado con variables ambientales. Resultados: Se encontraron 14 especies de corales, dos de ellas, F. curvata y L. papyracea, únicamente como esqueletos. La zona con la mayor cobertura fue en la zona media de la playa. La especie con mayor CCV en todos los ámbitos de profundidad fue P. damicornis, seguido de P. elegans. La CCV descendió de 29 % en septiembre 2010 a 1 % en octubre 2010. La mortalidad masiva fue causada por una Floración Algal Nociva (FAN). La tasa de crecimiento de P. damicornis (26.0 ± 8.9 mm año-1) y de P. elegans (27.9 ± 4.5 mm año-1), no mostraron diferencias significativas entre profundidades ni por ubicación en el arrecife. Conclusiones: El exuberante arrecife de coral de Playa Blanca experimentó una regresión repentina de la CCV similar a las descritas por Glynn et al. (1983). Esto sugiere que ocurren ciclos de mortalidad y recuperación natural de los arrecifes de coral, aunque las presiones antropogénicas reducen la capacidad de resiliencia de estos. El ordenamiento espacial, tanto en tierra como en el mar, es vital para un adecuado desarrollo humano, en el que se incorpore en el análisis el aumento de la capacidad de recuperación de los ecosistemas coralinos.
... Reyes-Bonilla, Carriquiry, Leyte-Morales y Cupul-Magaña (2002) reportan que los blanqueamientos ocurrieron en la costa oeste de México luego de que la temperatura superficial del mar fuera igual o superior a los 30 °C por más de 60 días. En otros lugares como las Islas Carolinas, el blanqueamiento y la mortalidad de los corales ocurrió alrededor de 31 °C (Bruno, Siddon, Witman, Colin, & Toscano, 2001). Durante la ocurrencia del blanqueamiento la temperatura estuvo por debajo de los 29 °C, lo que sugiere que está no fue la causante de la mortalidad observada. ...
Book
Full-text available
The North Pacific and the South Pacific of Nicaragua is a region of great biological, geological, economic and social wealth. There the dry forest mixes with the rain forest, the sea with the islands and nature with the people. It is a region influenced by the Papagayo upwelling, the emergence of cold marine waters during the dry season, which generates an abundance of life in the sea. As a sample of this marine wealth, in this Special Issue, more than half of the contributions are dedicated to advances in the knowledge of the marine biodiversity of the region. Contributions from the social sciences, geology and physics of the region are also included. Within these areas, the publications provide information on maritime border management, archaeology and sustainable tourism, coastal geology, projected climate changes, as well as various oceanographic aspects of the area. We hope that this Special Issue on the North Pacific of Costa Rica and the South Pacific of Nicaragua will promote more research in the region and help inform decision-making processes and educational activities. We thank the authors for their manuscripts, as well as the more than sixty reviewers who with their comments and suggestions helped to improve the quality of the manuscripts.
... Coral bleaching occurred in Palau in 1998 and 2010. Bruno et al. (2001) quantified the extent of the coral bleaching caused by the elevated SSTs in 1998 in Palau, and found widespread bleaching at numerous sites across the main islands of Palau, with total bleaching mortality of nearly 48% of coral colonies belonging to 20 scleractinian taxa, while partial bleaching affected 15% of all colonies surveyed. Golbuu et al. (2007) assessed the recovery rates of coral communities in Palau based on data from 2001 to 2005. ...
Article
Full-text available
Marine heatwaves can have devastating impacts on marine species and habitats, often with flow-on effects to human communities and livelihoods. This is of particular importance to Pacific Island countries that rely heavily on coastal and ocean resources, and for which projected increases in future marine heatwave (MHW) frequency, intensity, and duration could be detrimental across the Pacific Island region. In this study, we investigate MHWs in the tropical western and central Pacific Ocean region, focusing on observed MHWs, their associated impacts, and future projections using Coupled Model Intercomparison Project phase 6 (CMIP6) simulations under a low (SSP1–2.6) and a high (SSP5–8.5) greenhouse gas emissions scenario. Documented impacts from “Moderate” mean intensity MHW events in Fiji, Samoa, and Palau, that were categorised as “Strong” at their peak, included fish and invertebrate mortality and coral bleaching. Based on CMIP6 multi-model mean estimates, and relative to current baselines, “Moderate” intensity MHWs are projected to increase from recent historical (1995–2014) values of 10–50 days per year (dpy) across the region to the equivalent of >100 dpy by the year 2050 under the low emissions scenario, and > 200 dpy nearer the equator. Under the high emissions scenario, 200 dpy of Moderate MHW intensities are projected across most of the region by 2050, with >300 dpy nearer the equator. For the most intense “Extreme” category of MHW, estimates range from <1 dpy under the current climate to >50 dpy projected under the high emissions scenario by 2050. In contrast, “Extreme” MHWs are projected to increase to <5 dpy by 2050 under the low emissions scenario, highlighting the importance for Pacific Island nations that global emissions more closely follow the low emissions scenario trajectory.
... Elevated upper ocean temperature imposes thermal stresses on corals and can lead to their bleaching, a condition where the symbiotic zooxanthellae algae found inside the cells of coral polyps are expelled, leaving the tissue largely colorless and the white coral skeleton visible (Colin, 2018;Hughes et al., 2018;Langlais et al., 2017). At Palau, corals were extensively bleached during summer 1998 with water temperatures >30°C from surface to 90 m depth and the mortality rate was estimated at 90% or more on the outer reef slopes (Figure 2a, Bruno et al., 2001;Colin, 2009). This bleaching episode followed the intense 1997 El QIU ET AL. ...
Article
Full-text available
Plain Language Summary This study investigated the role of upper ocean circulation variability in modulating the ambient environment of coral reef ecosystems in the Western Tropical Pacific Ocean (WTPO), in particular, those leading to warming events that exceed the temperature threshold for coral bleaching around the island of Palau. Reef ecosystems rely on a relationship between the coral polyps and symbiotic algae that lives within the coral. Bleaching occurs when increased temperatures or environmental cause the algae to be expelled and unless the stress is removed the coral will die within weeks. Around Palau, moderate to severe coral bleaching occurred in 1998, 2010, 2016, 2017, and 2020. Combining in‐situ water temperature data with analysis of the output from a data‐assimilated ocean state estimate, we present evidence/results that the coral bleaching severity in the WTPO does not depend simply on the intensities of the conventional tropical El‐Niño and La Niña events. Regional intrinsic ocean variability and upper ocean thermal structures pre‐conditioned by the interannually‐varying WTPO circulation both played critical roles in controlling the observed thermal stress levels for coral reefs around Palau.
Article
Full-text available
Coral reefs are facing unprecedented mass bleaching and mortality events due to marine heatwaves and climate change. To avoid extirpation, corals must adapt. Individual variation in heat tolerance and its heritability underpin the potential for coral adaptation. However, the magnitude of heat tolerance variability within coral populations is largely unresolved. We address this knowledge gap by exposing corals from a single reef to an experimental marine heatwave. We found that double the heat stress dosage was required to induce bleaching in the most-tolerant 10%, compared to the least-tolerant 10% of the population. By the end of the heat stress exposure, all of the least-tolerant corals were dead, whereas the most-tolerant remained alive. To contextualize the scale of this result over the coming century, we show that under an ambitious future emissions scenario, such differences in coral heat tolerance thresholds equate to up to 17 years delay until the onset of annual bleaching and mortality conditions. However, this delay is limited to only 10 years under a high emissions scenario. Our results show substantial variability in coral heat tolerance which suggests scope for natural or assisted evolution to limit the impacts of climate change in the short-term. For coral reefs to persist through the coming century, coral adaptation must keep pace with ocean warming, and ambitious emissions reductions must be realized.
Article
Full-text available
The world’s oceans are warming at an unprecedented rate, causing dramatic changes to coastal marine systems, especially coral reefs. We used three complementary ocean temperature databases (HadISST, Pathfinder, and OISST) to quantify change in thermal characteristics of Caribbean coral reefs over the last 150 years (1871–2020). These sea surface temperature (SST) databases included in situ and satellite-derived measurements at multiple spatial resolutions. We also compiled a Caribbean coral reef database identifying 5,326 unique reefs across the region. We found that Caribbean reefs have been warming for at least a century. Regionally reef warming began in 1915, and for four of the eight Caribbean ecoregions we assessed, significant warming was detected for the latter half of the nineteenth century. Following the global mid-twentieth century stasis, warming resumed on Caribbean reefs in the early 1980s in some ecoregions and in the 1990s for others. On average, Caribbean reefs warmed by 0.18°C per decade during this period, ranging from 0.17°C per decade on Bahamian reefs (since 1988) to 0.26°C per decade on reefs within the Southern and Eastern Caribbean ecoregions (since 1981 and 1984, respectively). If this linear rate of warming continues, these already threatened ecosystems would warm by an additional ~1.5°C on average by 2100. We also found that marine heatwave (MHW) events are increasing in both frequency and duration across the Caribbean. Caribbean coral reefs now experience on average 5 MHW events annually, compared to 1 per year in the early 1980s, with recent events lasting on average 14 days. These changes in the thermal environment, in addition to other stressors including fishing and pollution, have caused a dramatic shift in the composition and functioning of Caribbean coral reef ecosystems.
Article
Full-text available
The remotely sensed sea surface temperature (SST) estimated from the 4-km-resolution Pathfinder SST algorithm is compared to a SST locally measured by the Marine Atmospheric Emitted Radiance Interferometer (MAERI) during five oceanographic cruises in the Atlantic and Pacific Oceans, in conditions ranging from Arctic to equatorial. The Pathfinder SST is a product of the satellite-based Advanced Very High Resolution Radiometer, while the MAERI is an infrared radiometric interferometer with continuous onboard calibration that can provide highly accurate (better than 0.05°C) in situ skin temperatures during extended shipboard deployments. Matchups, which are collocated (within 4 km) and coincident (±40 min during the day; ±120 min during the night) data, from these two different sources under cloud-free conditions are compared. The average difference between the MAERI and Pathfinder SSTs is found to be 0.07 ±0.31°C from 219 matchups during the low- and midlatitude cruises; inclusion of 80 more matchups from the Arctic comparisons produces an average global difference of 0.14 ±0.36°C. The MAERI-Pathfinder differences compare favorably with the average midlatitude differences between the MAERI skin SST and other bulk SST estimates commonly available for these cruises such as the research vessels' thermosalinograph SST (0.12 ±0.17°C) and the weekly National Centers for Environmental Prediction optimally interpolated SST analysis (0.41 ±0.58°C). While not representative of all possible oceanic and atmospheric regimes, the accuracy of the Pathfinder SST estimates under the conditions sampled by the five cruises is found to be at least twice as good as previously demonstrated.
Article
Full-text available
The most widespread and intense bleaching ever reported in the tropical and subtropical western Atlantic occurred in 1987—1988. South Florida, the Bahamas, and the northern Caribbean were the most severely affected areas. Bleaching occurred there and in parts of eastern Colombia and off Texas from summer through fall of 1987. In late fall to early winter recovery began in these areas, while bleaching began in much of the remaining Caribbean at that time. More than 80 species of coral reef symbionts, representing 3 phyla and 14 orders, and including the most important reef building corals, bleached in the West Indies. Many coral reef symbionts suffered necrotic damage and/or death. Almost all parameters of the event differ greatly between locations, within locations, and among species. High variability may be the most common factor.
Article
Full-text available
Large-scale discoloration of Middle Sambo reef, 7.8 km from Boca Chica Key, Monroe County, Florida, was investigated on September 26, 1973. Discoloration of organisms was generally confined to the reef flat. The hydrozoan coral Millepora camplanata displayed greatest discoloration. Some Acropora palmata, Montastraea annularis, and Palythoa sp. colonies were mildly discolored. Corals were still viable.Calm weather, high ambient temperatures, and low tides at midday are believed to have caused water temperature elevation sufficient to produce thermal stress, thereby causing expulsion of endosymbiotic algae, Gymnodinium microadriaticum, with consequent discoloration of cnidarian hosts. Most organisms regained normal color within 6 weeks. Reports of similar coral discoloration incidents are reviewed. Short periods of thermal stress appear to have little lasting effect on shallow reef cnidarian communities.
Article
Full-text available
EPISODES of coral bleaching resulting from dissociation of endosymbiotic algae (zooxanthellae) from host coral tissues have occurred with increasing frequency over the past decade on reefs throughout the tropics1,2. These episodes have usually been attributed to increases in sea water temperatures3–10, but the mass bleaching events that occurred throughout the Caribbean during 1987 and 1990 were not readily explained by temperature alone11,12. An additional factor that may have contributed to these bleaching episodes is ultraviolet radiation in the 280–400-nm band. At many localities where bleaching occurred in 1987 and 1990, sea conditions were described as extremely calm with exceptionally clear water13. In the absence of suspended organic and inorganic matter in the water column, higher than average intensities of ultraviolet radiation probably reached all depths within the photic zone for several consecutive months. Evidence for a possible link between ultraviolet radiation and coral bleaching has not been forthcoming2. Here we report results of a field experiment showing that, irrespective of high water temperatures, short-term (three weeks) increases in ultraviolet radiation of a magnitude possible under calm, clear water column conditions can readily induce bleaching in reef-building corals.
Chapter
Widespread bleaching and mortality of reef-building corals occurred in the tropical eastern Pacific region during the severe and prolonged El Niño-Southern Oscillation (ENSO) event of 1982-83. At the height of the 10 month sea warming period, Panamanian reefs experienced 2-3 bouts of coral bleaching (loss of symbiotic zooxanthellae), which resulted in coral death 2-4 weeks later. Coral reefs in Costa Rica, Panama and Colombia suffered up to 70-90% coral mortality; in the Galapagos Islands (Ecuador), most coral reefs experienced >95% mortality. The hypothesis that sea warming caused this disturbance is supported by (a) the coincidence between the warming event and stress responses of reef organisms, (b) the correlation between the magnitude of local temperature deviations and the extent of mortality, and (c) the El Niño simulation experiments that resulted in coral bleaching, mortality, and histopathological changes similar to those observed naturally. El Niño warming usually does not extend north of Ecuador where eastern Pacific coral reefs are best developed. A near-decadal (1976-1988) warming trend over much of the tropical and subtropical Pacific Ocean could have affected the susceptibility of reef corals during the short-term 1982-83 warming disturbance. Zooxanthellate corals were affected most severely in nearly all reef areas, however, other organisms – such as benthic algae, non-zooxanthellate scleractinian corals, black corals, molluscs, barnacles, and crustacean symbiotes of corals - often showed local negative responses associated with non-thermal, El Niño-related conditions (e.g., nutrient depletion, low plankton abundance, high sea level, and wave assault). Secondary disturbances included (a) the elimination of coral barriers, allowing the corallivore Acanthaster planci access to formerly protected coral prey, (b) increased external bioerosion of reef surfaces killed in 1983 because of post-El Niño increases in sea urchin densities, and (c) the establishment of damselfish territories on corals that experienced partial mortality in 1983. Such disturbances are currendy causing longer-term changes to their respective local communities. Estimates of the ages of massive corals that were killed or irreparably damaged, and the interruption of reef framework accumulation suggest that a disturbance comparable to that of 1982-83 probably has not occurred in the Galapagos Islands or Panama during the past 200 years. The initial damaging effects of the 1982-83 disturbance to reef coral populations, combined with persistent secondary disturbances and low coral recruitment, could prolong reef recovery for decades or possibly centuries. Periods of intense upwelling cause localized and moderate levels of coral mortality, but persistent ENSO-related sea warming causes widespread and catastrophic coral mortality. El Niño events of extreme severity may limit eastern Pacific reef growth and diversity as much as do distance and isolation of these reefs from the centers of reef development in the western Pacific.
Article
Coral reefs are generally considered to be the most biologically productive of all marine ecosystems, but in recent times these vulnerable aquatic resources have been subject to unusual degradation. The general decline in reefs has been greatly accelerated by mass bleaching in which corals whiten en masse and often fail to recover. Empirical evidence indicates a coral reef bleaching cycle in which major bleaching episodes are synchronized with El Niño events that occur every 3–4 years on average. By heating vast areas of the Pacific Ocean, and affecting the Indian and Atlantic Oceans as well, El Niño causes widespread damage to reefs largely because corals are very sensitive to temperature changes. However, mass bleaching events were rarely observed before the 1970s and their abrupt appearance two decades ago remains an enigma. Here we propose a new explanation for the sudden occurrence of mass bleaching and show that it may be a response to the relative increase in El Niño experienced over the last two decades.
Article
Recent ‘bleaching’ and death of hermatypic (reef-building) corals has occurred extensively in Pacific Ocean waters of Panamá (Gulf of Chiriquí), near the Panamá-Costa Rica border. All hydrocorals ( Millepora spp.) and scleractinian corals (5 genera) have been affected to some degree in the non-upwelling environment of Chiriquí. No other members of the macrobenthos showed signs of stress (lowered activities, morbidity) or reduced abundance. The affected area, including the mainland, nearshore and offshore islands, and adjacent waters, is about 10,000 km ² . Further surveys in the Gulf of Chiriquí may reveal even more extensive mortality. This disturbance began in the dry season (January–April 1983), during a period of clear skies, low rainfall, and minimal river drainage. I first observed large, ‘bleached’ coral patches (up to 100 m ² in area) in mid-March, and observations by others indicate that coral ‘bleaching’ occurred in February and possibly as early as mid-January. Normal and ‘bleached’ corals observed in mid-March were ‘bleached’ and dead, respectively, by the end of April, suggesting that the disturbance is protracted. By the end of the dry season, 80 to 95% of all corals in the affected areas were severely ‘bleached’ or dead.
Article
The massive ‘bleaching’ (loss of zooxanthellae) and death of reef corals that occurred in one area (Gulf of Chiriquí) on the Pacific side of Panamá and in the Galápagos Islands during February—April 1983 continued in these areas until September—October 1983, resulting in a catastrophic disturbance. Similar episodes have been reported subsequently throughout much of the tropical eastern Pacific region (Costa Rica, the entire Pacific coast of Panamá, and Colombia), in the central and western Pacific Ocean, in parts of the western Atlantic Ocean (Caribbean coasts of Costa Rica, Panamá, and Colombia), and in the Florida Keys and Bahama Islands.