Reefs in peril
Reefs in Peril

map showing world locations where coral reefs are in greatest peril
Coral reefs in many parts of the world are in declining health.  At greatest risk are reefs in the Philippines, Indonesia, East Africa, Okinawa, and many areas in the Caribbean (see map). Loss of reefs in the island archipelagos of Indonesia/ Philippines and West Indies would be devastating, in no small part relating to their value to the study of evolution of coral biodiversity in the two ocean systems.  Wilkinson 1992 Proc 7th Int Coral Reef Sympos 1: 11. 

status of the world's remaining reefs



Left: status of world reefs in the 1990s




Below: two healthy reefs in the 1990s: Caribbean (Left) and Fiji (Right

  photographs of crinoids in Caribbean and Indo-Pacific regions
The ultimate cause of decline in health of coral reefs is population growth

Just a few decades ago poor health of reefs appeared to stem mainly from visible manifestations of world-population growth, such as overfishing, pollutants, enrichment from sewage discharge and terrestrial-fertiliser run-off, and so on. Some of these, at least, were thought to be manageable. In present times, however, a far more insidious and potentially fatal risk to health of coral reefs stems from exponential increase in atmospheric carbon-dioxide levels. This invisible factor has two main effects on seawater, warming temperature and increasing acidification. Unless controlled, either factor could prove lethal to reefs through effects on coral reproduction, and growth and survival of larvae with calcareous skeletons. Buddemeier 2001 Bull Mar Sci 69: 317.  Graph from Wilkinson 1992 Proc 7th Intern Coral Reef Sympos 1: 11.


graph correlating reef status with world population numbers
The proximal causes of decline in health of coral reefs are many hot buttons for peril part of BIOLOGY OF CARIBBEAN CORAL REEFS overfishing/reef collecting on Caribbean coral reefs disease on Caribbean coral reefs SCUBA/snorkeling recreation on Caribbean coral reefs future of Caribbean coral reefs pollution on Caribbean coral reefs eutrophication of Caribbean coral reefs hot button for bleaching part of Biology of Caribbean Coral Reefs
They include overfishing/reef collection, disease, SCUBA/snorkelling/ recreation, pollution, and eutrophication, all dealt with in their own sections along with the topic of future of reefs

graph showing rise in carbon dioxide levels from 1960 to present courtesy NOAA and Scripps Institution of Oceanography
Global warming and ocean acidification are associated with increasing level of carbon dioxide (CO2) in the atmosphere. CO2, along with methane, water vapour, nitrous oxide, and ozone, is a "greenhouse" gas, so named because it forms an insulating blanket over the earth's surface. It originates from combustion of wood and fossil fuels including coal, oil, gas, and other substances previously stored in the ground. Historical levels fluctuated around 225-275ppm (data from deep Vostok Lake and ice cores in Antarctica; see graph on Right). Prior to the Industrial Revolution in the mid-1800s, levels were relatively stable at about 280ppm, but from then on and correlative with increase in world population numbers, it has risen to over 380ppm (see both graphs) and will, within a few years, reach 400ppm. At present rates of increase, it is estimated that by the end of the century levels could reach 600ppm or even higher, accompanied by general atmospheric global warming of up to 4.5oC. Such warming will result in sea-level rise through melting of ice packs and glaciers, and lead to warmer ocean temperatures. Published data on air/ocean temperatures indicate a past rise of 1oC from 1910 to 2000. Present thinking suggests that a 2oC rise will be critical for health of coral reefs through deleterious effects on reproduction and exacerbation of bleaching events. graph showing historical levels of atmospheric carbon dioxide courtesy NASA As atmospheric CO2 levels rise, so more dissolves in ocean waters resulting in increased acidity. Even a small increase in acidity of ocean waters can have serious effects on organisms that utilise calcium carbonate in their skeletal parts. Such organisms range from microscopic foraminiferans and coccolithophores to large invertebrates such as corals, gastropods, sea urchins, and cuttlefishes. Just as buildings and monuments constructed of marble and limestone slowly dissolve and become pitted from acid rain, so invertebrate skeletons can be deleteriously affected by increasing ocean acidity. Many invertebrates including snails, sea urchins, and feather stars have delicate larvae with skeletons of calcium carbonate and these, owing to large surface area to volume relationships in the larvae, are particularly vulnerable to increased acidity. Ocean acidity has risen only a relatively small amount on the pH scale, but effects are already being seen. Keep in mind that a pH scale is on a logarithmic scale, so a rise of one full unit is equivalent to a 10-fold increase in acidity.

NOTE on dissolution CO2 converts to bicarbonate ion, which combines with a molecule of H2O to form bicarbonate with release of H+. More H+ means greater acidity reflected by a lower pH value. During the 2.5 centuries from when the first pH values were measured, pH has decreased from about 8.25 to 8.14 representing an increase in H+ concentration in world's oceans of about 30%. At predicted rates of increase in atmospheric CO2 concentrations, ocean acidity will have increased by about 70% in 2050, and by about 125% by 2100, over pre-industrial levels. Anyone with high-school knowledge of chemistry will be scratching their heads on reading this, because they know that any pH above 7 is actually basic. So, what is really meant by "ocean acidification" is "oceanic decreasing basicity". It is unlikely, even at extreme levels of CO2 predicted for the future, that the world's oceans will become actually acidic (i.e., <7 pH)

NOTE marble is a type of limestone (calcium carbonate) converted by heat and pressure into hard, crystalline form. Limestone is a soft, sedimentary rock consisting primarily of calcium carbonate formed from skeletal parts of corals and of single-celled plants, such as coccolithophores and foraminiferans. Depending upon conditions of formation, limestones can be hard like concrete or soft like chalk. The well-know chalky White Cliffs of Dover are formed from deposits of coccollthophoric plants


photograph of mound coral Montastrea sp. showing bleachingSome 3-4 decades ago scientists began seriously to implicate global warming as a cause of reef decline, most particularly in cases of episodic bleaching of corals.  Now there is little argument that higher temperatures are involved in bleaching, but in those early days scientists still cautioned that attempts to link bleaching events to global warming may be premature in view of lack of long-term records to correlate with changing health of coral-reef ecosystems. Atwood et al. 1992 Bull Mar Sci 51: 118. 

NOTE loss of symbiotic photosynthetic cells from the coral tissues. For more information on bleaching go to NUTRITION: CORALS: CORAL BLEACHING



Bleached and partly diseased boulder
coral Montastrea sp., 0.3X 


Current thinking by 19 world coral-reef scientists on the relationship between CO2 levels (with associated global warming), and bleaching events presents a bleak view.graph showing

FAR LEFT: the graph shows historical increase in global bleaching events relative relative to increase in atmospheric CO2 content

PHOTO: extensively bleached
corals on a shallow reef in the
Maldives (Indian Ocean)


BELOW: reefs, or some semblance of what we know today as a reef, may come back eventually, but not in our lifetimes


graph showing
For coral reefs a high-temperature anomaly or "hotspot" can be as little as 2oC. If "business as usual" relative to CO2release into the atomosphere continues into mid-century then reef survival is doubtful. For a useful suggestion from these scientists as to what can be done to potentially mitigate the effects of global warming on coral reefs go to "So what's next?" in another section of BCCR. Beyer et al. 2018 Conservation Letters e12587; photograph courtesy R. Vevers, 2015.


While most attention with regard to global warming has, understandably, concerned stress of higher temperatures, cold stress can happen in subtropical areas bordering continents such as along the coast of Florida. In February 2010, for example, a 12d period of unusually cold weather in the Florida Keys killed corals (90% of mound corals Montastraea annularis killed or partially damaged; other corals suffer 39% loss in cover), gorgonians (48% loss in cover), sponges (39%), and macroalgae (91%) living at depths of 5-8m. Colella et al. 2012 Coral Reefs 31: 621. Photographs courtesy the authors.

photographs showing cold-temperature damage to brain coral Colpophyllia natans and mound corals Montastraea annularis in the Florida Keys in 2010NOTE seawater temperatures in some areas dropped from 20-24oC to about 12oC over a 32h period

BEFORE (summer 2009) and AFTER (winter 2010)
views of a cold-shocked colony of brain coral Colpophyllia natans and 2 smaller colonies of
mound coral Montastraea annularis. Note that
both colonies of the latter species appear to have
been destroyed by the cold temperature. Note
also that the camera angle changed between shots


graph comparing loss of corals in shallow and deep reefs in Bonaire and Curacao over a 30yr periodNot surprisingly, reef degradation is more apparent in shallow water than deep. This is evident in a 30yr study in Bonaire and Curacao that shows through photograph comparisons an approximate 56% decline in number of coral colonies in reefs at 10-20m depth, but only a 14% decline at 30-40m depth. Principal degradative agents in shallow waters are thought to include bleaching, disease, and such anthropogenic activities as artisanal fisheries and shoreline development. Bak et al. 2005 Coral Reefs 24: 475.


Thirty-year change in abundance of
coral colonies in Bonaire and Curacao


Does global warming imply that coral-reef species may gradually expand their ranges poleward? This is not known for the Caribbean basin, perhaps because the land area of the islands is so discontinuous, but reports are coming in of such range extensions for species on the Great Barrier Reef, which has a north-south length of about 2300km to about 25o S latitude. Such changes are documented by the establishment of photograph of staghorn coral Acropora intermedia recently discovered in the Solitary Islands, Australiapopulations of tropical reef-fish species in temperate regions along the eastern Australian coast, and by range extensions polewards of staghorn corals Acropora spp. in both Japan and Australia. Baird et al. 2012 Coral Reefs 31: 1063; photograph courtesy the authors.

NOTE however, the second largest barrier reef in the world, the Mesoamerican Barrier Reef System, extends about 1000km from the Yucatan Peninsula in the north to the Bay Islands of Honduras in the south, and might be expected to show such range extensions

A colony of staghorn coral Acropora intermedia
recently discovered in the Solitary Islands, Australia
(30o S latitude, 2011) may be evidence of such
poleward expansion. Average winter temperature
in these islands has increased by 0.4oC since 1975

overfishing/reef collecting on Caribbean coral reefs SCUBA/snorkeling recreation on Caribbean coral reefs future of Caribbean coral reefs pollution on Caribbean coral reefs eutrophication of Caribbean coral reefs disease on Caribbean coral reefs