It is puzzling why the recent 2017 publication in Nature, Global
Warming And Recurrent Mass Bleaching Of Corals by Hughes et al. ignored
the most critical factor affecting the 2016 severe bleaching along the northern
Great Barrier Reef – the regional fall in sea level amplified by El Niño.
Instead Hughes 2017 suggested the extensive bleaching was due to increased
water temperatures induced by CO2 warming.
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Reef at Low Tide Around Lizard Island Great Barrier Reef |
In contrast in Coral Mortality Induced by
the 2015–2016 El-Niño in Indonesia: The Effect Of Rapid Sea Level Fall by
Ampou 2017, Indonesian biologists had reported that a drop in sea level had bleached the upper 15 cm of the reefs
before temperatures had reached
NOAA’s Coral Reef Watch bleaching thresholds. As discussed by Ampou 2017, the
drop in sea level had likely been experienced throughout much of the Coral
Triangle including the northern Great Barrier Reef (GBR), and then accelerated
during the El Niño. They speculated sea level fall also contributed to the
bleaching during the 1998 El Niño. Consistent with the effects of sea level
fall, other researchers reported bleaching in the GBR was greatest near the
surface then declined rapidly with depth. Indeed if falling sea level was the
main diver in 2016’s reef mortalities, and this can be tested, then most
catastrophic assertions made by Hughes 2017 would be invalid.
Indeed the Great Barrier Reef had also experienced falling
sea levels similar to those experienced by Indonesian reefs. Visitors to Lizard Island had reported
more extreme low tides and more exposed reefs as revealed in the photograph
above, which is consistent with the extremely high mortality in the Lizard Island
region during the 2016 El Niño. Of course reefs are often exposed to the air at
low tide, but manage to survive if the exposure is short or during the night.
However as seen in tide gauge data
from Cairns just south of Lizard Island, since 2010 the average low tide had
dropped by ~10 to 15 cm. After
previous decades of increasing sea level had permitted vertical coral growth
and colonization of newly submerged coastline, that new growth was now being
left high and dry during low tide. As a result shallow coral were increasingly
vulnerable to deadly desiccation during more extreme sea level drops when warm
waters slosh toward the Americas during an El Niño.
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Furthermore, an El Niño in the Coral Triangle not
only causes a sudden sea level fall, but it also generates a drier
high-pressure system with clear skies, so that this region is exposed to more
intense solar irradiance. In addition, El Niño conditions reduce regional winds
that drive reef-flushing currents and produce greater wave washing that could
minimize desiccation during extreme low tides. And as one would predict, these
conditions were exactly what were observed
during El Niño 2016 around Lizard Island and throughout the northern GBR.
Aerial surveys, on which Hughes 2017 based their analyses,
cannot discriminate between the various causes of bleaching. To determine the
cause of coral mortality, careful examination of bleached coral by divers is
required to distinguish whether bleached coral were the result of storms,
crown-of-thorns attacks, disease, aerial exposure during low tides, or
anomalously warmer ocean waters. Crown-of-thorns leave diagnostic gnawing
marks, while storms produce anomalous rubble. Furthermore aerial surveys only
measure the aerial extent of bleaching, but cannot determine the depth to which
most bleaching was restricted due to sea level fall. To distinguish bleaching and
mortality caused by low tide exposure, divers must measure the extent of tissue
mortality and compare it with changes in sea level. For example, the Indonesian
researchers found the extent of dead coral tissue was mostly relegated to the
upper 15 cm of coral, which correlated with the degree of increased aerial
exposure by recent low tides. Unfortunately Hughes et al never carried out, or
never reported, such critical measurements.
However a before-and-after photograph presented in Hughes
2017 suggested the severe GBR bleaching they attributed to global warming
primarily happened between February and late April. Their aerial surveys
occurred between March 22 and April 17, 2016. And consistent with low tide
bleaching, that is exactly the time frame that tide
tables reveal reefs experienced two bouts of extreme low tides coinciding
with the heat of the afternoon (March 7-11 & April 5-10). And such a
combination of sun and low tide are known to be deadly.
A study of a September 2005 bleaching event on Pelorous and
Orpheus Islands in the central GBR by Anthony 2007, Coral
Mortality Following Extreme Low Tides And High Solar Radiation, had
reported extreme deadly effects when extreme low tides coincided with high
solar irradiance periods around midday. As in Indonesia, they also reported bleaching
and mortality had occurred despite water temperatures that were “significantly lower than the threshold
temperature for coral bleaching in this region (Berkelmans 2002), and
therefore unlikely to represent a significant stress factor.” Along the reef
crests and flats, “40 and 75% of colonies in the major coral taxa were either
bleached or suffered partial mortality. In contrast, corals at wave exposed
sites were largely unaffected (<1% of the corals were bleached), as periodic
washing of any exposed coral by waves prevented desiccation. Surveys along a
1–9 m depth gradient indicated that high
coral mortality was confined to the tidal zone.” [Emphasis mine]
The fortuitous timing of Ampou’s coral habitat mapping from
2014 to 2016 in Bunaken National Park (located at the northwest tip of
Sulawesi, Indonesia) allowed researchers to estimate the time of coral
mortality relative to sea level and temperature changes.
Ampou reported that in “September 2015, altimetry data show that sea
level was at its lowest in the past 12
years, affecting corals living in the bathymetric range exposed to unusual
emersion. By March 2016, Bunaken Island (North Sulawesi) displayed up to 85% mortality on reef flats” and that
almost “all reef flats showed evidence of mortality, representing 30% of
Bunaken reefs.” Based on the timing of reef deaths and changes in temperature
they concluded, “the wide mortality we observed can not be simply explained by
ocean warming due to El Niño.”
They concluded, “The clear link between mortality and sea level fall,
also calls for a refinement of the hierarchy of El Niño impacts and their
consequences on coral reefs.”
From the illustrations (below) of a generalized topography
of a fringing or barrier reef, we can predict the effects of low sea level by
examining where bleaching and mortality would occur within the whole reef
system. Coral occupying the reef crests
are most sensitive to drops in sea level and desiccation because they are first
to be exposed to dangerous periods of aerial exposure and last to re-submerge.
The inner reef flats are vulnerable
to lower sea levels, as those shallow waters are more readily exposed at low
tide because the reef crest prevents ocean waters from flooding the flats. If
reefs flats are not exposed, the shallow waters that remain can heat up
dangerously fast. Accordingly Anthony 2007 found 40 to 75%, and Ampou 2017
found 85% of the reef flats had bleached. In contrast coral in the fore reefs are the least vulnerable to
desiccation and higher temperatures due to direct contact with the ocean,
upwelling and wave washing. Accordingly Anthony 2007 reported <1% bleaching
in the fore reefs.
Coral mortality due to a drop in sea level leaves other
diagnostic telltale signs such as micro-atoll formation. As illustrated below
in Fig. 4 from Goodwin
2008, during neap low tides (MLWN) sea water can still pass over the reef
crest and flush the inner reef with relatively cooler outer ocean water.
However during the low spring tides (MLWS), the reef crest is exposed and ocean
water is prevented from reaching the reef flats. As mean sea level falls (MSL),
coral on the crest and flats are increasingly exposed to the air for longer
periods, and the upper layer of coral that had previously kept up with decades
of rising sea level, are now exposed to increasing periods of desiccation and
higher mortality.
There are over 43 species in the coral triangle that can be
characterized as “keep-up” coral whose growth rates are much greater than
average 20th century sea level rise. However their vertical growth
is limited by the average low water level
(HLC-Height of Living Coral in Fig. 4). Average low water level is
calculated as the mean water level between low neap tides and lower low spring
tides. (Due to the linear alignment of the sun, earth and moon and the
resulting stronger gravitational pull during a full and new moon, spring tides result in both the highest
high tides and lowest low tides. In contrast neap tides exert the least
gravitation pull. Spring tides typically
happen twice a month, but usually no more than once a month will spring low
tides coincide with the heat of the midday sun.)
When growing in deeper waters, a keep-up species like
mounding Porites spp. grow at rates of 5 to 25 mm per year and form dome shaped
colonies. However due to increased aerial exposure when growth reaches the
surface, or due to exposure from sea level fall, the upper most surface dies
from high air temperatures, higher UV damage and desiccation. This results in a
flat-topped colony leading to the classic “micro-atoll” shape, with dead coral
in the center surrounded by a ring of live coral, as exemplified by a Kiribati
micro-atoll in the photograph below.
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Microatolls |
Micro-atoll patterns have been crucial for reconstructing
past fluctuations in sea level on decadal to millennial timeframes. As Ampou
2017 observed in Bunaken NP, mortality due to a drop in sea level was mostly
restricted to the upper 15 cm of coral, which leads to the formation of
micro-atolls. So before simply assuming
climate-change-warming has induced mortality, micro-atoll formation and other
associated patterns indicative of sea level change must be examined. A short
discussion on how sea level changes can shape micro-atolls can be read here.
Due to its regional sensitivity to the sea level change that
accompanies an El Niño, the northern Great Barrier Reef has an abundance of
fossil micro-atolls that have allowed researchers
to estimate El Niño activity and fluctuating sea levels over the past 4000
years. They estimated 4000 years ago low water neap tides were at least 0.7
meters higher than they are at present. Studies
of micro-atolls in the Cook Islands further to the east in the southern
Pacific, suggest that by 1000 AD during the Medieval Warm Period, average sea
level had fallen, but remained about 0.45 meters higher than today. During the Little Ice Age sea level fell to 0.2
meters below current levels during
the late 1700s and early 1800s, before recovering throughout the 1900s.
Hughes 2017 wanted to emphasize GBR bleaching as a
“global-scale event” in keeping with his greenhouse gas/global warming
attribution, but bleaching and mortality was patchy on both local and regional
scales. And although Hughes presented their analyses as “a fundamental shift
away from viewing bleaching events as individual disturbances to reefs,” the
unusually high mortality around Lizard Island demands a closer examination of
individual reef disturbances. The lack of mortality in 2016 across the southern
and Central GBR, was explained as a result of the cooling effects of tropical
storm Winston, but that does not explain why individual reefs in those regions
have not bleached at all, while others bleached only once, and still others
bleached twice or three times since 1998. Hughes’ shift away from examining
what factors affected individual reefs will most likely obscure the most
critical factors and yield false attributions.
Hughes reported the various proportions of areal bleaching
as degrees of severity. But that frightened many in the public who confused
bleaching with mortality, leading some misguided souls to blog the
GBR was dead. However
bleaching without mortality is not a worrisome event no matter how extensive.
Rates of mortality and recovery are more important indices of reef health. As
discussed in the article The
Coral Bleaching Debate: Is Bleaching the Legacy of a Marvelous Adaptation
Mechanism or A Prelude to Extirpation?, all coral retain greater
densities of symbiotic algae (symbionts) in the winter but reduce that density
in the summer, which often leads to minor seasonal bleaching episodes that are
usually temporary. Under those circumstances coral typically return to normal
within weeks or months. Furthermore by ejecting their current symbionts, coral
can acquire new symbionts that can promote greater resilience to changing
environmental conditions. Although symbiont shifting and shuffling promotes
adaptation to shifting ocean temperatures, symbiont shuffling cannot protect
against extreme low tide desiccation, and dead desiccated coral can no longer
adapt. Humans have little control over El Niños or low tides.
Hughes also contradicted past studies to mistakenly suggest
that recurring bleaching in a given reef is evidence that corals are not
adapting or acclimating. However bleaching happens for many reasons. Symbiont
shuffling to better adapt to warmer waters does not guarantee adaptation to
lower sea levels, cyclones or changes in salinity. Coral reefs deal with
changing sea levels with rapid growth to keep-up as sea level rises, and then
dying back when sea level falls. Decadal swings in regional sea level will
likely cause decadal swings in bleaching and are not evidence of coral
fragility.
Hughes 2017 modeled the 2016 GBR bleaching event as a
function of surface ocean temperatures that surpass bleaching thresholds,
although reefs will bleach below that threshold and will fail to bleach despite
temperatures above that threshold. Despite the fact El Niños are well known to
cause rapid sea level fall along the GBR, Hughes’ model never accounted for
falling sea level. Nor did they account for past observations that falling sea
levels induced bleaching when temperatures were below bleaching thresholds.
More disturbing because sea level fall caused bleaching in various reefs, with
some experiencing good water quality and others poor quality, Hughes asserted there
was “no support for the hypothesis that good water quality confers resistance
to bleaching.” However this contradicts an abundance of regional studies
attributing increased coral disease and bleaching to high nutrient loading.
Woolridge
2013 have argued that coral eject their algal symbionts and bleach when
temperature, light and nutrients increase to a level that accelerates the
symbionts growth. Increased growth consequently reduces the amount of energy
transferred to the coral, resulting in ejection of the slacking symbiont.
Because increased nutrient loads can promote increased symbiont growth at relatively
lower temperatures, higher nutrient loads can promote bleaching at lower
temperatures.
Furthermore while coral’s symbiotic relationships allow them
to recycle limited nutrients and out compete seaweeds, higher nutrient loads
enable greater seaweed growth, which reduces corals’ competitive advantage.
Furthermore seaweeds have been shown to harbor allelopathic chemicals that
inhibit coral growth, as well as serving as reservoirs for bacteria that cause
coral diseases. Higher nutrient loads induce more dissolved organic carbon that
bacteria feed upon, allowing disease-causing bacteria to rapidly multiply.
Higher nutrient loads also increase the survival of crown-of-thorns larvae,
which then increases coral depredation and bleaching.
In a 2013 experimental study, Chronic
Nutrient Enrichment Increases Prevalence And Severity Of Coral Disease And
Bleaching, Vega-Thurber reported that higher nutrient loads caused a
“twofold increase in both the prevalence and severity of disease compared with
corals in unenriched control plots” as well as a “3.5-fold increase in bleaching
frequency relative to control corals.”
Although Hughes 2017 suggests the pattern of recurring
bleaching is simply a function of temperature and global warming, as
illustrated in Hughes’ Figure “e” below, recurring bleaching is not a global
phenomenon. (Black dots represent reefs that bleached during all 3 surveys:
1998, 2002, 2016; light gray represents reefs that bleached only once, and dark
gray reefs bleached twice.) . In most cases the degree of recurring bleaching
does not predict the recurrence of bleaching in nearby reefs despite similar
ocean temperatures. Although an El Niño generates widespread bleaching,
bleaching is still a regional issue affecting individual reefs differently.
During an El Niño sea level rises in the eastern Pacific and falls in the
western Pacific. Recurring bleaching in the Far North and Southern regions of
the GBR are uncommon, while recurring GBR bleaching has been frequent between
Cookstown and Townsville where temperatures have been quite variable. And in
accord with prior research, the region between Cookstown and Townsville has
suffered from lower water quality and higher nutrients loads, causing more
frequent bleaching and greater crown-of-thorns attacks.
After perusing Hughes 2017, it was clear they had been led
to incorrectly embrace the prevailing bias of CO2-induced catastrophic bleaching because they failed to
address the fall in sea level before and during the 2016 El Niño, and likewise
they failed to address how weather created by El Niños promotes clear skies and
increased solar heating. To add insult to injury, because sea level drops
bleached reefs in both good water quality and bad, and bleaches reefs in both
protected preserves and unprotected, Hughes 2017 presented a statistical
argument that disparaged any significant value of ongoing conservation efforts
to minimize bleaching by reducing nutrient loading and by protecting reefs from
overfishing. By belittling or ignoring most critical factors affecting coral
bleaching other than temperature, Hughes suggested our only recourse to protect
reefs “ultimately requires urgent and rapid action to reduce global warming.”
And because such an apocryphal analysis was published in
Nature and will undoubtedly mislead coral conservation policies,
I wept.
