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Extensive coral bleaching Occurred intertidally in early August 2003 in the Capricorn Bunker group (Wistari Reef, Heron and One Tree Islands; Southern Great Barrier Reef). The affected intertidal coral had been exposed to unusually cold (minimum = 13.3degreesC; wet bulb temperature = 9degreesC) and dry winds (44% relative humidity) for 2 d during predawn low tides. Coral bleached in the upper 10 cm of their branches and had less than 0.2 x 10(6) cell cm(-2) as compared with over 2.5 x 10(6), Cell cm(-2) in nonbleached areas. Dark-adapted quantum yields did not differ between symbionts in bleached and nonbleached areas. Exposing symbionts to light, however, led to greater quenching of Photosystem 11 in symbionts in the bleached coral. Bleached areas of the affected colonies had died by September 2003, with areas that were essentially covered by more than 80% living coral decreasing to less than 10% visible living coral cover. By January 2004, coral began to recover, principally from areas of colonies that were not exposed during low tide (i.e., from below dead, upper regions). These data highlight the importance of understanding local weather patterns as well as the effects of longer term trends in global climate.
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Limnol. Oceanogr., 50(1), 2005, 265–271
2005, by the American Society of Limnology and Oceanography, Inc.
Coral bleaching following wintry weather
Ove Hoegh-Guldberg
and Maoz Fine
Centre for Marine Studies, University of Queensland, St Lucia 4072, Queensland, Australia
William Skirving
NOAA/NESDIS/ORA/ORAD—E/RA3, NOAA Science Center, Room 601, 5200 Auth Road, Camp Springs, Maryland
Ron Johnstone and Sophie Dove
Centre for Marine Studies, University of Queensland, St Lucia 4072, Queensland, Australia
Alan Strong
NOAA/NESDIS/ORA/ORAD—E/RA3, NOAA Science Center, Room 601, 5200 Auth Road, Camp Springs, Maryland
Extensive coral bleaching occurred intertidally in early August 2003 in the Capricorn Bunker group (Wistari
Reef, Heron and One Tree Islands; southern Great Barrier Reef). The affected intertidal coral had been exposed to
unusually cold (minimum
C; wet bulb temperature
C) and dry winds (44% relative humidity) for 2 d
during predawn low tides. Coral bleached in the upper 10 cm of their branches and had less than 0.2
as compared with over 2.5
cell cm
in nonbleached areas. Dark-adapted quantum yields did not differ
between symbionts in bleached and nonbleached areas. Exposing symbionts to light, however, led to greaterquench-
ing of Photosystem II in symbionts in the bleached coral. Bleached areas of the affected colonies had died by
September 2003, with areas that were essentially covered by more than 80% living coral decreasing to less than
10% visible living coral cover. By January 2004, coral began to recover, principally from areas of colonies that
were not exposed during low tide (i.e., from below dead, upper regions). These data highlight the importance of
understanding local weather patterns as well as the effects of longer term trends in global climate.
The majority of Scleractinian coral live in a mutualistic
endosymbiosis with single-celled dinoflagellate algae of the
genus Symbiodinium. Together, these two organisms are re-
sponsible for a major component of the structure (frame-
work) and function (primary productivity) of tropical reef
systems. In recent years, the abundance of coral colonies on
reefs worldwide has been rapidly declining under the pres-
sure of human-derived stresses (Bryant et al. 1998; Wilkin-
son 2000; Hughes et al. 2003). Among these stresses, cli-
mate change has assumed a major importance as ocean
temperatures have warmed and major periods of symbiotic
dysfunction, called mass bleaching events, have been trig-
gered. These events are undocumented prior to the 1970s
but have been expanding since then in frequency, severity,
and geographic scale (Hoegh-Guldberg 1999). Since their
advent, there have been six major periods of bleaching
across the planet. The frequency in some areas is even high-
er. Mass coral bleaching events, which have been reported
as the worst yet in each case, have affected coral populations
on the Great Barrier Reef twice since 1997 (1998, 2002). In
Corresponding author (
We thank Efrat Fine for laboratory assistance; Mark Davy, Paul
Fisher, Sascha Thyer, and the Stanford Australia Program class of
2003 for field assistance; and David Logan, Michael Phillips, and
Collette Bagnato for collecting meteorological and field data.
each case, coral bleaching has spread to more than 50% of
the Great Barrier Reef Marine Park (Berkelmans and Oliver
1999; Berkelmans 2002; Dennis 2002).
Understanding how variability in the physical environ-
ment affects coral reefs is a priority if we are to truly un-
derstand the ramifications of the changing global climate.
Mass coral bleaching events are triggered by periods in
which sea temperatures rise above the long-term averages
for a particular region. Plant–animal endosymbioses are very
sensitive to changes in temperature, which result in an in-
creased sensitivity of the dinoflagellate symbiont to photo-
inhibition (Iglesias-Prieto et al. 1992; Warner et al. 1996;
Jones et al. 1998), cellular damage, and eventually disinte-
gration. The mechanism that underlies the earliest stages of
coral bleaching is very similar to that seen during thermal
stress in higher plants (Jones et al. 1998; Salvucci and
Crafts-Brandnera 2004), leading to the conclusion that coral
bleaching is at least in part a result of thermal stress occur-
ring within the photosynthetic processes of the dinoflagel-
lates. Warm seas, often only 1–2
C above the long-term av-
erages, can be detected by satellite measurements and used
to predict (with more than 95% accuracy) mass coral bleach-
ing events several weeks in advance (Strong et al. 2000).
Reduced temperatures also intensify photoinhibition in
higher plants (Aro et al. 1990; Lyons 1973; Greer and Laing
1991; Long et al. 1994) in a similar way to that that occurs
at elevated temperatures. Reduced temperatures lead to a re-
266 Hoegh-Guldberg et al.
duction in the rate at which the quenching of Photosystem
II (PSII) develops (Krause 1992), leading to an accumulation
of active oxygen species and cellular damage. Cold temper-
atures have also been observed to trigger the loss of dino-
flagellate symbionts from anemones and coral (Shinn 1966;
Coles and Jokiel 1977; Steen and Muscatine 1987). In a
previous study, Saxby et al. (2003) established that cold
stress creates similar physiological symptoms in coral to
those seen when they are heat stressed. Saxby et al. (2003)
demonstrated that water temperatures of 12
C for 12 h or
more led to the complete loss of photosynthetic efficiency
by PSII and death of exposed coral. Exposure of coral to
C revealed a light-dependent response, in which (as with
elevated temperature) thermal stress in low light had little
effect while coral exposed to 14
C and full sunlight were
heavily impacted. In the latter case, photosynthetic efficiency
was reduced and the coral became bleached in 24 h. These
observations match those of Jones et al. (1998) and other
research groups for elevated thermal stress, with the en-
hancement of the effect by light giving insight into where
the damage due to low or high thermal stress occurred.
This paper investigates cold stress on reefs that manifested
itself as a mass bleaching event in the intertidal areas of the
Capricorn–Bunker group (southern Great Barrier Reef) in
the austral winter of 2003. The measurements made during
this event confirm the conclusions of Saxby et al. (2003) and
previous workers. In addition, this study highlights the im-
portance of understanding the impact of the variability in
weather patterns as well as the overall shift that has been
occurring due to climate change.
Materials and methods
Heron Island is a platform reef (approximately 8 km
located within the Capricorn–Bunker group of reefs on the
southern Great Barrier Reef (23
S, 151
E). Large
areas of the intertidal flat at Heron Island were found to be
bleached in the austral winter, being first observed on 5 Au-
gust 2003. In addition to the exploration of environmental
data, measurements were made of the condition of the coral
and their fate over 5 months following the event.
Changes in coral cover—With the advent of bleaching in
the reef flat at Heron Island in early August 2003, the Cap-
ricorn–Bunker group of reefs was surveyed from the air to
assess the scale of the bleaching event.
Two 50-m permanent belt transects were set to follow the
progression and patterns of bleaching and recovery on the
reef flat on the northern side of Heron Island. Permanent
belt transects were established that were oriented north to
south (perpendicular to the shore line) or east to west (par-
allel to the shore line). These sites were photographed using
a digital camera (Nikon Coolpix 5000) over a measuring
tape. Photography was carried out during high tide to have
sufficient distance between the camera and the studied sub-
strate and in order to minimize observer impact on the sur-
veyed reef. Each exposure covered 1 m
of substrate. The
pictures were analyzed using the point-intercept method by
placing a transparency marked with 16 randomly marked
points over each image on the computer screen. The sub-
strate underlying the points was scored according to three
categories: healthy, bleached, or dead coral. We then calcu-
lated percentage of each of the categories for each survey
date. The same procedure was repeated during September
and October 2003 (6 and 10 weeks after the first set of pho-
tography, respectively) and during early February 2004 (6
months after the first set of measurements).
Environmental data—Temperature data for Heron Island
was obtained from three sources.
1. Average sea surface temperature for the period 30 July–
4 August (1600 h) of the Capricorn Islands group was re-
trieved from NOAA-16 AVHRR Satellite (downloaded at the
Australian Institute of Marine Science), pixel size
1 km.
2. A set of data loggers that were deployed at Heron Island
reef crest. One logger was positioned approximately 10 m
from the low-tide mark on the southern side of Heron Island.
A second logger was deployed midway across the intertidal
flat and a third was deployed off the reef crest in 5-m depth
at low tide. In each set of data loggers, there were sensors
for temperature, light, and pressure (depth/tides).
3. A log of weather observations by the Heron Island Re-
search Station staff, performed twice daily. These included
meteorological parameters, such as sea temperature, air tem-
perature, humidity, wind speed, precipitation, cloudiness,
and tides. These parameters have been logged since 1967.
Physiological measurements—Twelve fragments, approx-
imately 5 cm long, from four colonies of Acropora aspera
were sampled for area density of symbiotic dinoflagellates
on 5 August 2003. They were taken as follows: Four were
taken from the upper 10 cm (bleached section) of the colo-
nies, four from 10 cm under the bleached part, and four from
20 cm under the bleached section. In the lab, the tissue of
each fragment was removed using an air gun. The resulting
slurry was homogenized and diluted to a total volume of 50
ml. A volume of 10
l was sampled out twice from every
test tube and put onto a hemocytometer (upper and lower
fields). Two hemocytometer fields were photographed using
a fluorescent microscope (Olympus BX5) and a digital cam-
era. Counting of the fluorescent symbiotic dinoflagellates
was performed from the digital images using Blob Analysis
in MATROX 2.1 software (Matrox Electronic Systems). The
software counts the number of fluorescent objects in the pho-
to under predefined conditions of size and area. The number
of cells was calculated for the total 50-ml volume.
Surface area was measured by dipping each fragment in
hot wax (65
C) for 10 s. It was then cooled to room tem-
perature to allow the wax to solidify. After weighing the
fragments, they were dipped for 10 s in wax again at the
same temperature and weighed again after it solidified. The
net weight (weight 2
weight 1) was multiplied by a co-
efficient of 0.038, which was obtained by using the same
coating method on cylinders with known surface area and
calibrating against a regression coefficient. The number of
cells was then calculated per square centimeter coral surface
Photosynthetic efficiency—The portable Diving-Pulsed
Amplitude Modulated (PAM) Fluorometer (Walz Gmbh)
267Coral bleaching and wintry weather
Fig. 1. Changes in coral cover over the period August 2003–
January 2004.
Fig. 2. Photograph taken on 24 January 2004 of areas severely bleached in early August 2003. Regrowth of coral out of crevices is
evident, as indicated by arrows. Transect tape is shown running down middle of photograph.
was used to explore the photosynthetic capacity of dinofla-
gellates inhabiting the tissue of A. aspera colonies during
the bleaching event. The PAM light meter was precalibrated
against a quantum sensor of a Li-Cor LI-189 light meter. In
each measurement, the tip of the PAM main optical fiber
was placed on the coral surface. In situ measurements were
performed on 5 August, right after the initial observation of
bleaching and on 26 August (3 weeks after the onset of
bleaching). Both measurements were undertaken at 2200 h
(low tide; total darkness). Dark-adapted maximal quantum
yield (Fv/Fm) was measured for three sections of each of
five branches (note: branches were taken from five separate
colonies): (a) upper 10 cm (bleached section) of the colonies,
(b) 10 cm under the bleached section, and (c) 20 cm under
the bleached section. Four saturation pulses were done on
each of these sections. Quantum yields were also measured
after incubating branches in light (1 min continuous actinic
light before saturation pulse; 1,000
mol m
). This was
done on five bleached and five nonbleached fragments of A.
aspera. At the end of these measurements, rapid light curves
of photosynthesis versus irradiance were done on five
bleached and five nonbleached fragments of A. aspera. Rap-
id light curves were done by measuring the quantum yield
of symbionts after illuminating branches from each of the
colonies for 10 s at each one of a series of eight irradiances.
Inspection of bleaching at Heron Island revealed that most
of the visual impact on coral was restricted to the upper
portions of colonies, which had been presumably exposed at
low tide (for photographs, see Hoegh-Guldberg and Fine
2004). Affected areas stretched as far as the eye could see
along the reef crest at Heron Island. Aerial inspection by
helicopter revealed extensive bleaching around the entire
reef crest at Heron and the intertidal areas of neighboring
Wistari Reef and One Tree Island.
Changes in coral cover—Monthly surveys done after first
observing mass bleaching in the intertidal areas of Heron
268 Hoegh-Guldberg et al.
Fig. 3. (A) Average sea surface temperature (30 July–4 August
2003). The coolest water of the Great Barrier Reef during this pe-
riod is along the southern coast. (B) Average sea surface tempera-
ture of the Capricorn (30 August–4 September 2003). Note the cool
water engulfing Heron Island. Data sets in both A and B from
NOAA-16 AVHRR data downloaded at the Australian Institute of
Marine Science.
Fig. 4. Data retrieved from loggers placed in the intertidal areas
of Heron Island. Water temperatures for loggers positioned (A)
within 10 m of the shoreline of Heron Island on the intertidal flat.
Missing data in July are due to logger failure. Arrows indicate the
extremely cold days of 31 July and 1 August 2003. (B) Midway
across the intertidal flat, and (C) off the reef crest in 5-m depth at
low tide.
Island revealed that the affected areas of bleached coral
mostly died (Fig. 1). By September, areas that had greater
than 80% coral cover prior to the bleaching event (due main-
ly to one species, A. aspera) had less than 30% living coral
cover. By October, living coral represented less than 12%.
By early January 2004, coral cover had begun to recover
(17.8%), with coral tissue growing out of regions that were
shaded as well as not being exposed at low tide (Fig. 2).
Environmental data—Waters along the inshore region of
the southern Great Barrier Reef were cool by comparison
with other regions within the Reef and ranged between 19
and 21
C. Average sea surface temperature (30 July–4 Au-
gust 2003) retrieved from the NOAA-16 AVHRR dataset
revealed that the coolest water for the Great Barrier Reef lay
along the southern coastal regions. A tongue of relatively
cool water (21
C) extended out from the coast to the seaward
side of the platform reef on Heron Island (Fig. 3A,B). This
patch of cool water bathed Wistari and Heron Island but did
not reach as far as One Tree Island, which was approxi-
mately 0.5–1.0
C warmer.
The satellite data were validated by data loggers that were
located in intertidal and reef crest areas associated with
bleached areas (Fig. 4A–C). Sea temperatures just off the
reef crest (5 m; Fig. 4C) averaged 21.0
C, with a
maximum value of 22.7
C and a minimum of 18.1
C. These
values match the values reported within the satellite data
(Fig. 3). The two loggers based in the intertidal also revealed
similar average temperatures of 21.0
C and 21.0
C for inshore and offshore intertidal locations, re-
spectively (Fig. 4A,B). The range of values was much larger
for both than sea temperatures measured by the logger lo-
cated off the reef crest. In this case, minimum values of
C and 14.5
C and maximum values of 26.3
C and
C were recorded for inshore and offshore intertidal
sites, respectively. Importantly, the minimum values for sea
temperature off the reef crest occurred 3–4 d before bleach-
269Coral bleaching and wintry weather
Fig. 5. Detail of (A) water temperature and (D) light data taken
from data loggers positioned midway across the reef crest at Heron
Island (29 July–5 August). Meteorological data collected by Heron
Island Research Station for the same period is also shown. (B) Wind
speed (left axis, solid line) and tidal height (right axis, solid high-
frequency sinusoidal line), and (C) relative humidity (left axis, solid
line) and air temperature (right axis, dashed line). Accompanying
data and comments for records are shown in Table 1.
Fig. 6. Symbiont density as a function of distance along branch-
es of Acropora aspera (shown) on 5 August 2003. Upper portions
(left hand in this diagram) were visibly bleached relative to lower
portions (right hand in this diagram).
Table 1. Daily weather data collected by Heron Island Research Station over the period 26 July–5 August.
Date Rain (mm) T
C) T
C) Remarks from research personnel
26 Jul 03
27 Jul 03
28 Jul 03
29 Jul 03
30 Jul 03
31 Jul 03
Little cumulus to the north
No cloud
Cumulus and alto cumulus on horizon
Scattered cumulus and stratus
Cold, minute amount of cumulus
Freezing, minute amount of cumulus
1 Aug 03
2 Aug 03
3 Aug 03
4 Aug 03
5 Aug 03
Freezing, tiny amount of cumulus
Freezing, tiny amount of cumulus
Freezing, mostly cumulus
Varied cloud cover, rain surrounding
Mostly alto cumulus
ing was first reported on the intertidal regions of Heron Is-
Detailed examination of dates revealed that the coldest
days (Fig. 5A) coincided with predawn low tides and were
typified by high winds (Fig. 5B), low humidity and air tem-
peratures (Fig. 5C) on cloudless days (Table 1, Fig. 5D).
Significantly, this is the only time in the history of daily
weather records on Heron Island (from 1967 to 2004) in
which observers used the word ‘‘freezing’’ to describe the
conditions (Table 1). Air temperature ranged from 13.3
C over the period 26 July–5 August 2003. From these
data, it is possible to calculate a wind chill of 9
C for coral
exposed at low tide during these conditions (based on the
coldest day, 1 August, in which air temperatures dropped to
C and wind speed increased to 0.040 km h
). This
would have been accentuated by the very low humidities
(44%) that accompanied these conditions.
Physiological condition of cold-stressed coral—Bleaching
was most pronounced in affected colonies in the upper 10
cm of the branch tips. Symbiont density decreased in the
upper 10 cm to less than 0.2
cell cm
and increased
away from the affected zone to over 2.5
cell cm
(Fig. 6). The lower values were similar to the values ob-
tained by Dove (2004) for control A. aspera. Dark-adapted
photosynthetic efficiency did not correlate with the extent of
bleaching down the branches (Fig. 7A). A major difference
in quenching behavior was revealed when quantum efficien-
cy was measured at successively higher light levels (Fig.
7B). This confirmed the observation of low-light–adapted
yields in the bleached regions as compared with those mea-
sured in the nonbleached regions (Fig. 7C). In this case,
symbionts remaining in the bleached areas quenched the
photosynthetic efficiency of PSII by a much larger amount
270 Hoegh-Guldberg et al.
Fig. 7. Physiological state of symbionts in Acropora aspera as a function of distance down the branches. (A) Dark-adapted yield at
three different sections of A. aspera branches measured soon after 2200 h on 5 August 2003. (B) Light-adapted yield (after 1 min of 1,000
mol photons m
) for bleached and unbleached A. aspera. (C) Quantum yield of PSII of symbionts in A. aspera as a function of light
level on 5 August 2003. (D) Measurements repeated for symbionts in A. aspera, 3 weeks after the onset of bleaching (26 August 2003).
Symbols indicate different positions on the colony: top section, 10 cm down from tip and 20 cm down from the tip are shown.
as light levels increased. Three weeks later, this behaviorwas
much less pronounced (Fig. 7D).
Sea temperature has a major influence over survivorship
of reef-building coral and their symbionts and is considered
to be a major determinant of their latitudinal distribution
(Kleypas et al. 1999). While coral can sustain seasonal var-
iability in sea temperature that may be more than 12
(Kleypas et al. 1999), small excursions above geographically
associated maxima or thresholds leads to syndromes asso-
ciated with stress (Harvell 1999; Hoegh-Guldberg 1999).
One of these syndromes, coral bleaching, has increased enor-
mously in magnitude and frequency over the past 30 years.
A combination of elevated sea temperature and exposure
time predicts mass coral bleaching with great certainty
(Hoegh-Guldberg 1999; Strong et al. 2000; Hoegh-Guldberg
2001), with high values leading to mass mortality events.
Estimates of mortality from mass coral-bleaching events
range from zero to an almost total loss of reef-building coral
from affected areas. An average of 17.7% of the living coral
in six major coral reef regions of the world was killed during
one of the warmest years on record, 1998. The range of
mortality estimates is perhaps the most interesting detail hid-
den within the average. While some regions (e.g., Australia
and Papua New Guinea) lost small amounts (3%), regions
like the Arabian Gulf and the Wider Indian Ocean lost 33%
and 46%, respectively during the single event in 1998.
Cold temperatures can also be a problem for reef-building
coral. While some coral are adapted for colder conditions
(e.g., down to 11.5
C in the case of the temperate hermatypic
coral Plesiastrea versipora; Kevin and Hudson 1979), down-
ward excursions have a similar impact on reef-building coral
in laboratory experiments. The massive coral Montastrea an-
nularis is killed by water temperatures of 14
C for more than
9 h (Mayer 1914), while many coral species appear able to
tolerate 15
C for short periods (Roberts et al. 1982). Saxby
et al. (2003) revealed similar responses from the hardy in-
tertidal coral Montipora digitata at Heron Island. In this
case, 16
C and 12
C lead to a progressive reduction of pho-
tosynthetic efficiency (Fv/Fm) of algal symbionts in M. dig-
itata over6hofexposure to these temperatures in the light.
In both cases, incubation in the dark over the next 12 h did
not lead to recovery of Fv/Fm, suggesting chronic photo-
damage had occurred. In subsequent experiments, Saxby et
al. (2003) showed that coral became bleached and did not
recover when exposed to temperatures as low as 12
C for
over 12 h.
The lower threshold of M. digitata from the Heron Island
in the laboratory was 12–14
C for exposure times of 1–2 d
(Saxby et al. 2003). In the light of these conclusions for what
is considered to be a fairly hardy coral, the observation of
a bleaching event on 5 August is not surprising. Coral were
exposed at low tide to reduced temperatures at night (down
to 13.3
C) with 0.030–0.035 km h
, low humidities, and
clear sunny conditions during the day. Affected areas of the
colonies (upper 10 cm) were exposed to air at night time
during these periods. Combining the effect of high winds
and low humidities, coral tissue was exposed to surface tem-
peratures of 9
C (wet bulb temperature) for several hours.
Based on the observations of Saxby et al. (2003), exposure
to the sunlit conditions of the intertidal while experiencing
these low temperatures would have led to chronic photo-
271Coral bleaching and wintry weather
inhibition as light levels increased. This damage appears to
have been repaired by 5 August, as measurements of the
quantum efficiency of PSII under darkness revealed similar
values for symbiotic dinoflagellates in bleached and un-
bleached coral (Fig. 7). The quantum efficiency of PSII in
symbionts left in bleached areas was considerably lower
when measured after incubation in the photosynthetically ac-
tive radiation (1,000
mol m
). In this case, symbionts
left in the bleached areas exhibited extensive quenching
when exposed to the light. One explanation for this is that
light levels within the skeletons were much higher due to
greater reflection of light within the polyp calices after the
majority of symbionts had left (Roberto Iglesius-Prieto,
UNAM, pers. comm.). This highlights the important obser-
vation that light stress (and hence the effect of anything that
affects the speed at which excitations are processed) will
have proportionately higher effects on coral that are partially
or fully bleached.
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downward trends in local sea and air temperatures (IPCC
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... Mass mortalities of corals following extreme low tides and high solar radiation have been reported in Guam (Yamaguchi, 1975), Eastern Pacific (Glynn, 1976;Eakin, 1996;Zapata et al., 2010;Mejıá-Renterıá et al., 2019), the Red Sea (Fishelson, 1973;Loya, 1976), Hawaii (Krupp, 1984), Thailand (Brown et al., 1994) the Central Great Barrier Reef (GBR) in Australia (Anthony and Kerswell, 2007), and Indonesia (Ampou et al., 2017). Armstrong et al. (2007) report a cold weather emersion event for Ningaloo in Western Australia, and Hoegh- Guldberg et al. (2005) describe extensive coral bleaching and subsequent partial mortality following a low water level event at Heron Island in the GBR in which corals were exposed to unusually cold conditions and dry winds. Similarly, Fadlallah et al. (1995) notes mass coral mortality in the western Arabian Gulf coincident with winter low tides. ...
... Potential stressors include a range of factors such as rainfall and wind, however for this this preliminary study analyses focused on high solar irradiance and low atmospheric temperature. These were selected due to published events related to these factors (Anthony and Kerswell, 2007;Armstrong et al., 2007 andHoegh-Guldberg et al., 2005;Buckee et al., 2019). Threshold values were defined for emersion (ƟWL), high solar irradiance (ƟSI) and low atmospheric temperature (ƟT). ...
... Low atmospheric temperature stress was estimated using BOM hourly wet-bulb thermometer temperature data from the nearest weather station (Table 2). This metric was selected since we were unable to identify cold air emersion related coral bleaching events reported by Hoegh-Guldberg et al. (2005) at Heron Island (HER) in early August 2003, or at Ningaloo reef (NIN) in July 2006 (Armstrong et al., 2007) using observed air temperature data. However, using wet-bulb air temperature (consistent with the approach taken by Hoegh-Guldberg et al. (2005)), we were able to identify the published events at these sites. ...
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Sea level exerts a fundamental influence on the intertidal zone, where organisms are subject to immersion and emersion at varying timescales and frequencies. While emersed, intertidal organisms are exposed to atmospheric stressors which show marked diurnal and seasonal variability, therefore the daily and seasonal timing of low water is a key determinant of survival and growth in this zone. Using the example of shallow coral reefs, the coincidence of emersion with selected stressors was investigated for eight locations around the Australian coastline. Hourly water levels (1992 – 2016) from a high-resolution sea level hindcast (, were linked to maximum surface solar radiation data from the Copernicus ERA5 atmospheric model and minimum atmospheric temperature observations from the Australian Bureau of Meteorology to identify seasonal patterns and historical occurrence of coral emersion mortality risk. Local tidal characteristics were found to dictate the time of day when low water, and therefore emersion mortality risk occurs, varying on a seasonal and regional basis. In general, risk was found to be greatest during the Austral spring when mean sea levels are lowest and a phase change in solar tidal constituents occurs. For all Great Barrier Reef sites, low tide occurs close to midday during winter and midnight in the summer, which may be fundamental factor supporting the historical bio-geographical development of the reef. Interannual variability in emersion mortality risk was mostly driven by non-tidal factors, particularly along the West Coast where El Niño events are associated with lower mean sea levels. This paper highlights the importance of considering emersion history when assessing intertidal environments, including shallow coral reef platform habitats, where critical low water events intrinsically influence coral health and cover. The study addresses a fundamental knowledge gap in both the field of water level science and intertidal biology in relation to the daily timing of low tide, which varies predictably on a seasonal and regional basis.
... Corals here are experiencing warmer summers and winters, year on year, resulting in longer periods of summer-like temperatures and shorter winter reprieves, a pattern detected for many reefs worldwide (Heron et al., 2016). Minimum temperatures are also increasing faster than maximum temperatures, so stressful winter cold snaps that may have limited recovery or resulted in cold-water bleaching in the past (Hoegh-Guldberg et al., 2005;Hoegh-Guldberg & Fine, 2004;Howells et al., 2013), may be less problematic in the future (Schlegel et al., 2021). Shorter or less frequent winter reprieves from heat stress may also reduce the recovery capacity of corals in the future. ...
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Natural bleaching events provide an opportunity to examine how local‐scale environmental variation influences bleaching severity and recovery. During the 2020 marine heat wave, we documented widespread and severe coral bleaching affecting 75%–98% of coral cover throughout the Keppel Islands in the southern inshore Great Barrier Reef. Acropora, Pocillopora, and Porites were the most severely affected genera, while Montipora was comparatively less susceptible. Site‐specific heat‐exposure metrics were not correlated with Acropora bleaching severity, but recovery was faster at sites that experienced lower heat exposure. Despite severe bleaching and exposure to accumulated heat that often results in coral mortality (degree heating weeks ~4–8), cover remained stable. Approximately 94% of fate‐tracked Acropora millepora colonies survived, perhaps due to reduced irradiance stress from high turbidity, heterotrophic feeding, and large tidal flows that can increase mass transfer. Severe bleaching followed by rapid recovery and the continuing dominance of Acropora populations in the Keppel Islands is indicative of high resilience. These coral communities have survived a 0.8°C increase in average temperatures over the last 150 years. However, recovery following the 2020 bleaching was driven by the easing of thermal stress, which may challenge their recovery potential under further warming.
... Long periods of cold stress can lead to cold bleaching (Hoegh-Guldberg & Fine, 2004;Yu et al., 2004), which is common in temperate (Suzuki et al., 2013) and tropical reefs (Hoegh-Guldberg et al., 2005), with 18°C considered the minimum SST for coral reef development (Veron & Minchin, 1992). Periodic extreme cold events (SST <14°C) at higher latitudes can lead to the repeated mass mortality of corals (Yu et al., 2004). ...
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As high‐temperature stress due to climate change threatens tropical corals, cooler areas at relatively high latitudes may be potential refuges. Tolerance to low temperatures is critical in determining whether corals can successfully migrate to higher latitudes. However, the physiological and molecular adaptations that protect corals from low‐temperature stress are unclear. In this study, scleractinian Porites lutea samples from the tropical Xisha Islands (XS) and subtropical Daya Bay (DY) in the South China Sea were subjected to a reduction in ambient temperature from 26 °C to 12 °C. Differences in physiological changes and gene expression were analyzed. P. lutea from both XS and DY exhibited physiological bleaching under low‐temperature stress, and the Symbiodiniaceae density, Fv/Fm, and chlorophyll a content were significantly reduced. Symbiosome antioxidative stress and metabolic enzyme activity first increased and then decreased. RNA‐seq analysis showed that the host responded to low‐temperature stress by activating immune, apoptotic, and autophagic pathways and reducing metabolic levels. Nevertheless, Symbiodiniaceae lacked the physiological regulatory capacity to adapt to low temperatures. The lower cold tolerance of XS tropical P. lutea may attribute to lower oxidative stress resistance, lower photosynthetic capacity, worse energy supply, and higher susceptibility to bacterial and viral infections and diseases in XS corals. The difference in cold tolerance may result from genetic differences between the geographic populations and is possibly detrimental to the migration of tropical coral to relatively high‐latitude refuges. This study provides a theoretical basis for anthropogenically assisted coral migration as a response to global change.
... An increase in temperature raises the rates of the biological processes until it reaches a thermal opti-mum beyond which these rates decline rapidly (Huey and Stevenson 1979). Similarly, corals may lose their symbiotic algae and bleach when SSTs exceed the local summer maxima by 1°C (Hoegh-Guldberg 1999) or when SST reaches temperatures low enough to induce cold stress (Saxby et al. 2003;Hoegh-Guldberg et al. 2005). ...
Knowledge of environmental factors is crucial in understanding biological and ecological processes. Yet information on the environment around Sesoko Island, Okinawa, Japan, one of the main locations for coral reef research in Japan, remains scarce. Data of air and sea surface temperature (SST), wind velocity, wave height, and frequency of typhoons have been manually recorded at Sesoko Station, Tropical Biosphere Research Station, the University of the Ryukyus from September 1990 to November 2021. Here we describe the seasonal and long-term trends in these environmental variables at Sesoko Island. Some of the key findings were that the air temperature and SST fluctuated by ~9-12°C throughout the year. A rise in air temperature and SST between 1990 and 2021 was observed in the winter and autumn season, respectively. The Degree Heating Week (DHW) based on the in-situ data reflected the bleaching observations around Sesoko Station. The DHW exceeded the critical bleaching level of 8°C-week in 1998 and the significant bleaching level of 4°C-week in 2001, 2016, and 2017. Weak southerly winds were dominant in summers, while stronger northeasterly winds were dominant in winters. The frequency of winds between 3.4 to 7.9 m/s and northeastern winds have increased through time. Typhoons generally occur between May and October, and the frequency of typhoons has not increased over the past 30 years. Wave heights never exceeded 0.5 m and were highest between July and September. These findings will provide a reliable baseline of the environment at Sesoko Island for further ecological studies.
... Unlike in foraminifera, the photosymbiotic relationship between corals and their zooxanthellae is more specific. Previous studies reported massive bleaching of symbiont-bearing corals after natural exposure to extremely cold temperatures (Hoegh-Guldberg et al., 2005;Lirman et al., 2011), and there are experimental observations of symbiont stress under cold exposure, expressed by decreased photosynthetic efficiency, loss of symbiotic algae and changes in concentrations of their photosynthetic pigments (Nielsen et al., 2020;Saxby et al., 2003). The degree of cold sensitivity controls poleward range expansion of corals both in tropical and temperate regions. ...
Global warming permits range expansions of tropical marine species into mid‐latitude habitats, where they are, however, faced with cold winter temperatures. Therefore, tolerance to cold temperatures may be the key adaptation controlling zonal range expansion in tropical marine species. Here we investigated the molecular and physiological response to cold and heat stress in a tropical symbiont‐bearing foraminifera that has successfully invaded the Eastern Mediterranean. Our physiological measurements indicate thermal tolerance of the diatom symbionts but a decrease of growth for the foraminifera host under both cold and warm stress. The combined (“holobiont”) transcriptome revealed an asymmetric response in short‐term gene expression under cold versus warm stress. Cold stress induced major reorganization of metabolic processes, including regulation of genes involved in photosynthesis. Analyses limited to genes that are inferred to belong to the symbionts confirm that the observed pattern is due to changes in the regulation of photosynthesis‐related genes and not due to changes in abundance of the symbionts. In contrast to cold stress, far fewer genes change expression under heat stress and those that do are primarily related to movement and cytoskeleton. This implies that under cold stress, cellular resources are allocated to the maintenance of photosynthesis, and the key to zonal range shifts of tropical species could be the cold tolerance of the symbiosis.
... • C. We compared temperatures recorded at Bouraké lagoon to those of the reference St R2, which showed the most typical temperature range for shallow water temperatures in the south of New Caledonia (i.e., 22-28 • C; Varillon et al., 2021). We notice that in Bouraké, temperatures were 40 % of the time above 28 • C during the summer of 2020, while winter temperatures were on average 46.5 % of the time lower than 22 • C. While warming is considered the main threat for coral reefs, low temperatures (< 20 • C) can cause coral bleaching by inducing responses similar to high temperatures, including a reduction in the Symbiodiniaceae cell density and chlorophyll a content (e.g., Saxby et al., 2003;Hoegh-Guldberg and Fine, 2004;Hoegh-Guldberg et al., 2005;Kemp et al., 2011;Bellworthy and Fine, 2021). The negative effect of cold temperatures is even more substantial during neap tides when colonies on the reef crest are exposed to air for hours at low temperatures during cold winters. ...
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According to current experimental evidence, coral reefs could disappear within the century if CO2 emissions remain unabated. However, recent discoveries of diverse and high cover reefs that already live under extreme conditions suggest that some corals might thrive well under hot, high-pCO2, and deoxygenated seawater. Volcanic CO2 vents, semi-enclosed lagoons, and mangrove estuaries are unique study sites where one or more ecologically relevant parameters for life in the oceans are close to or even worse than currently projected for the year 2100. Although they do not perfectly mimic future conditions, these natural laboratories offer unique opportunities to explore the mechanisms that reef species could use to keep pace with climate change. To achieve this, it is essential to characterize their environment as a whole and accurately consider all possible environmental factors that may differ from what is expected in the future, possibly altering the ecosystem response. This study focuses on the semi-enclosed lagoon of Bouraké (New Caledonia, southwest Pacific Ocean) where a healthy reef ecosystem thrives in warm, acidified, and deoxygenated water. We used a multi-scale approach to characterize the main physical-chemical parameters and mapped the benthic community composition (i.e., corals, sponges, and macroalgae). The data revealed that most physical and chemical parameters are regulated by the tide, strongly fluctuate three to four times a day, and are entirely predictable. The seawater pH and dissolved oxygen decrease during falling tide and reach extreme low values at low tide (7.2 pHT and 1.9 mg O2 L−1 at Bouraké vs. 7.9 pHT and 5.5 mg O2 L−1 at reference reefs). Dissolved oxygen, temperature, and pH fluctuate according to the tide by up to 4.91 mg O2 L−1, 6.50 ∘C, and 0.69 pHT units on a single day. Furthermore, the concentration of most of the chemical parameters was 1 to 5 times higher at the Bouraké lagoon, particularly for organic and inorganic carbon and nitrogen but also for some nutrients, notably silicates. Surprisingly, despite extreme environmental conditions and altered seawater chemical composition measured at Bouraké, our results reveal a diverse and high cover community of macroalgae, sponges, and corals accounting for 28, 11, and 66 species, respectively. Both environmental variability and nutrient imbalance might contribute to their survival under such extreme environmental conditions. We describe the natural dynamics of the Bouraké ecosystem and its relevance as a natural laboratory to investigate the benthic organism's adaptive responses to multiple extreme environmental conditions.
... Coral bleaching is affected by numerous biological factors including symbiont community composition and environmental responses (e.g., more or less heat-tolerant algal taxa) [58], host heterotrophy (e.g., reliance on the symbiont) [59], the capacity for acclimation and adaptation both genetic and epigenetic (intra-and inter-generational) [60,61] and coral taxonomy (e.g., different life history strategies) [24,62]. In addition, other environmental factors can influence bleaching responses in corals, such as high solar insolation, cloudiness, winds, tidal extremes, thermal variability, cold water stress and nutrient enrichment [63][64][65][66][67][68][69]. Given this suite of biotic and abiotic factors, a perfectly predicting coral bleaching algorithm would need to combine heat stress metrics with other environmental and biological parameters that, in many cases, are often not available at sufficient spatial or temporal scales. ...
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Increasingly intense marine heatwaves threaten the persistence of many marine ecosystems. Heat stress-mediated episodes of mass coral bleaching have led to catastrophic coral mortality globally. Remotely monitoring and forecasting such biotic responses to heat stress is key for effective marine ecosystem management. The Degree Heating Week (DHW) metric, designed to monitor coral bleaching risk, reflects the duration and intensity of heat stress events and is computed by accumulating SST anomalies (HotSpot) relative to a stress threshold over a 12-week moving window. Despite significant improvements in the underlying SST datasets, corresponding revisions of the HotSpot threshold and accumulation window are still lacking. Here, we fine-tune the operational DHW algorithm to optimise coral bleaching predictions using the 5 km satellite-based SSTs (CoralTemp v3.1) and a global coral bleaching dataset (37,871 observations, National Oceanic and Atmospheric Administration). After developing 234 test DHW algorithms with different combinations of the HotSpot threshold and accumulation window, we compared their bleaching prediction ability using spatiotemporal Bayesian hierarchical models and sensitivity–specificity analyses. Peak DHW performance was reached using HotSpot thresholds less than or equal to the maximum of monthly means SST climatology (MMM) and accumulation windows of 4–8 weeks. This new configuration correctly predicted up to an additional 310 bleaching observations globally compared to the operational DHW algorithm, an improved hit rate of 7.9%. Given the detrimental impacts of marine heatwaves across ecosystems, heat stress algorithms could also be fine-tuned for other biological systems, improving scientific accuracy, and enabling ecosystem governance.
... The need for non-destructive assessments of living corals in a laboratory setting has led others to develop similar protocols including X-ray computed tomography (CT) and 3D modeling (Laforsch et al., 2008), but these techniques have drawbacks, including high instrument cost and long outof-water exposure times. Others have measured live corals using structured-light 3D scanning to assess live corals that are larger and more complex than microfragments, but noted that Glynn and D'Croz, 1990Stimson and Kinzie, 1991Chancerelle, 2000Vytopil and Willis, 2001Hoegh-Guldberg et al., 2005Holmes et al., 2008Naumann et al., 2009Veal et al., 2010aLatex Meyers and Schultz, 1985Dye-dipping Hoegh-Guldberg, 1988 Surface Index (SI) calculation ...
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Rapid and widespread declines in coral health and abundance have driven increased investments in coral reef restoration interventions to jumpstart population recovery. Microfragmentation, an asexual propagation technique, is used to produce large numbers of corals for research and restoration. As part of resilience-based restoration, coral microfragments of different genotypes and species are exposed to various stressors to identify candidates for propagation. Growth rate is one of several important fitness-related traits commonly used in candidate selection, and being able to rapidly and accurately quantify growth rates of different genotypes is ideal for high-throughput stress tests. Additionally, it is crucial, as coral restoration becomes more commonplace, to establish practical guidelines and standardized methods of data collection that can be used across independent groups. Herein, we developed a streamlined workflow for growth rate quantification of live microfragmented corals using a structured-light 3D scanner to assess surface area (SA) measurements of live tissue over time. We then compared novel 3D and traditional 2D approaches to quantifying microfragment growth rates and assessed factors such as accuracy and speed. Compared to a more conventional 2D approach based on photography and ImageJ analysis, the 3D approach had comparable reliability, greater accuracy regarding absolute SA quantification, high repeatability, and low variability between scans. However, the 2D approach accurately measured growth and proved to be faster and cheaper, factors not trivial when attempting to upscale for restoration efforts. Nevertheless, the 3D approach has greater capacity for standardization across dissimilar studies, making it a better tool for restoration practitioners striving for consistent and comparable data across users, as well as for those conducting networked experiments, meta-analyses, and syntheses. Furthermore, 3D scanning has the capacity to provide more accurate surface area (SA) measurements for rugose, mounding, or complex colony shapes. This is the first protocol developed for using structured-light 3D scanning as a tool to measure growth rates of live microfragments. While each method has its advantages and disadvantages, Frontiers in Marine Science | 1 April 2021 | Volume 8 | Article 623645 Koch et al. 3D-Scanning and Coral Microfragment Growth disadvantages to a 3D approach based on speed and cost may diminish with time as interest and usage increase. As a resource for coral restoration practitioners and researchers, we provide a detailed 3D scanning protocol herein and discuss its potential limitations, applications, and future directions.
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Ocean warming is leading to more frequent coral bleaching events. However, cold stress can also induce bleaching in corals. Here, we report observations of a boreal winter bleaching event in January 2020 in the central Red Sea, mainly within a popu- lation of the branching coral Stylophora pistillata on an offshore reef flat. Sea surface temperatures (SSTs) rarely fall below 24°C in this region, but data loggers deployed on several nearby reef flats recorded overnight seawater temperatures as low as 18°C just 3 days before the observations. The low temperatures coincided with an extremely low tide and cool air temperatures, likely resulting in the aerial exposure of the corals during the night time low-tide event. The risk of aerial exposure is rare in winter months, as the Red Sea exhibits seasonal fluctuations in sea level with winter values typically 0.3–0.4 m higher than in summer. These observations are notable for a region typically characterized as a high-temperature sea, and highlight the need for long-term monitoring programs as this rare event may have gone unnoticed.
Scleractinian (hard layered) corals live for several centuries or longer in the tropical surface waters that comprise about half of the total surface area of the world’s oceans. Coral reefs in the tropical surface waters are the largest biologically produced natural features over the Earth’s surface. About 20% of modern carbonate accumulation takes place in coral reefs. The relatively thick annual growth bands of coral skeleton (usually around 10 mm a year) have provided a wealth of information on the climate and environmental changes that occurred in the past. These environmental archives are becoming essential to forecast the future climate and environmental changes in their local habitats in the tropical regions including the Indo-Pacific Warm Pool region that plays a significant role in the world ocean and atmospheric circulation, hence in the entire globe. Deep-sea scleractinian corals often living more than a millennium have been found in most oceans, and these slow-growing corals (a few micron meters a year) have also been found to faithfully record climate and environmental changes that occurred in the ocean. This chapter introduces the status of the scientific investigation on a coral skeleton climate and environmental proxies to the audience who are interested in coral reef with respect to climate and environmental change. It will briefly cover the biomineralization process, methods of sampling coral cores and subsequent cleaning for further chemical analysis, skeleton age determinations, and the utilities of selected chemical elements and selected isotope proxies (Li, B, C, N, O, F, Na, Ca, V, Cr, Mn, Fe, Ni, Cu, Zn, Y, Mo, Cd, I, Ba, REEs, Nd, Pb, U, Pu). This chapter is largely dealt with surface-dwelling tropical corals, but it also includes some proxy studies on deep corals.KeywordsCoral skeletonChemical element and isotope compositionsClimate and environmental proxies/tracersDating
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Most major climate models project that rapid increases to ocean temperature (1-3 o C per century) will continue if atmospheric greenhouse gases continue to accumulate. This scenario, together with the explicit link between coral bleaching, mortality and sea temperature, has led to the bleak prediction of annual mass coral bleaching event by the end of the current century. This paper explores this projection for 12 sites in the Pacific Ocean, and improves the projections by including intensity as well as frequency information. The behaviour of coral reefs over the past twenty years indicates a strong predictive association between the size and length of sea surface temperature anomalies and the intensity of mass coral bleaching (Degree Heating Months, DHM). The DHMs associated with severe bleaching events seen in 1998 in Palau, Okinawa, Seychelles and Scott Reef (Australia) were 3.9, 3.0, 3.1 and 2.6 respectively. By contrast, DHMs for the less affected reefs of Moorea, Cook Islands, and the southern and central sectors of the Great Barrier Reef were 0.9, 0.4, 1.7 and 1.4 respectively. Using the Global Circulation Model, ECHAM4/OPYC3 (forced by the moderate IS92a scenario) sea surface temperatures were projected for the next 100 years at the 12 Pacific sites. In all cases, thermal stress increases so rapidly such that DHM values of greater than 10 (triple current extreme DHM values) are projected to be commonplace by 2080. Data showing that corals are rapidly adapting to these changes remains extremely scant, as is evidence that corals can rapidly swap their symbiotic algal for more thermally tolerant ones. The inescapable conclusion from this study is that the projected changes in thermal stress are likely to result in major negative changes in the distribution and abundance of corals and related organisms as this century progresses.
During January 1977 three consecutive cold fronts crossed S Florida and the northern Bahamas which depressed shallow-water temperatures below the lethal limit for most reef corals. Digital thermal infrared data acquired by the NOAA-5 meteorological satellite, in situ water temperatures, and meteorological data were used to study the thermal evolution of Florida Bay and Bahama Bank waters. Coral mortality at Dry Tortugas was up to 91% during the 1977 event. Coral and fish kills were also reported from other parts of the Florida Reef Tract and northern Bahamas. Study results show that cold-water stress conditions can exist over vast shallow- water areas and have residence times of several days.-from Authors.
Knowledge of the critical levels for key environmental variables that are likely to cause bleaching in reef corals is of fundamental importance in conducting risk assessments of potential climate-change effects on coral reefs. Such knowledge can also be used to provide early warning of mass bleaching events. A number of factors have contributed to the difficulty in determining critical levels for coral bleaching. These factors include the fact that multiple stressors may be involved in bleaching, the duration of stress required to elicit a bleaching response varies with temperature, and bleaching triggers are known to be variable in space, time and by species. In this study, I identify sea surface temperature (SST) as the most important parameter for predicting coral bleaching from 4 possible environmental variables collected over 10 to 12 yr from weather stations at 2 locations on the Great Barrier Reef (GBR): temperature, wind speed, solar radiation and barometric pressure. Predicted bleaching-response curves are constructed from high-resolution in situ temperature records and historical observations of coral bleaching for 13 locations. These curves approximate reef-wide stress-response thresholds for bleaching of thermally sensitive (and often dominant) coral species. Distinct spatial trends exist in the thermal sensitivity of coral populations that correspond with position across the shelf and latitude in the case of mid- and outer-shelf reefs. This suggests that considerable thermal adaptation has taken place over small (10s of km) and large (100s to 1000s of km) spatial scales. Bleaching curves for inshore reefs do not correspond with latitude and are more variable, reflecting greater local-scale variability in temperature regimes.
The presence of zooxanthellae in tissues of the cold-temperate water coral Plesiastrea urvillei (Milne Edwards and Haime) has been confirmed histologically. Numbers of zooxanthellae per unit surface area and increases in submerged wet weight as a measure of calcification have been followed for 150 days under four different conditions: light-fed, light-starved, dark-fed, and dark-starved. No significant difference was found in density of zooxanthellae or calcification rates between light-fed and light-starved, and between dark-fed and dark-starved. After Day 48 the calcification rate in the dark dropped, however, by a factor of ≈4 to a constant lower rate and was correlated with a decrease in density of zooxanthellae. Zooxanthellae thus enhance calcification about 4 times during photosynthesis. Measurements of oxygen consumption and production indicated that even at the low light intensities experienced on a cloudy winter day by the coral in its natural environment more energy was fixed during photosynthesis than was required by the host. The retention of zooxanthellae and continued calcification in the dark for upwards of 48 days is considered to be an adaptation to the lower light levels experienced by P. urvillei compared with tropical corals.
Polar satellite-derived observations of sea surface temperatures (SSTs) have been used routinely since 1982 to provide a complete monitoring of our planet, covering all comers of the oceans (unless covered by clouds) twice each day. In 1992, an initial glimpse was published (Strong, 1992) of some tendencies that had been observed during the 1980s. Now that seven additional years of NOAA satellite SST data have become available, the earlier time-series (Strong, 1992) has been up-dated, hi this analysis of the global nighttime SSTs, care was taken to avoid the anomalous conditions found during the 1982-83 El Chichón aerosols, 1991-92 Mt. Pinatubo aerosols, and the strong El Niño of 1997-98. Evidence of warming is found to be present throughout much of the Tropics and in the mid-latitude Northern Hemisphere. Estimates from the Southern Hemisphere, while strongly indicative of compensatory cooling in the region, are found to be not as reliable.