Status of Caribbean coral reefs after bleaching and hurricanes in 2005

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Coral Reefs and Climate Change: Susceptibility and Consequencesgases such as CO 2. Current predictions of future coral reef bleaching events indicate that coralswill not adapt to warmer water without either a stabilization of greenhouse gas emissions oreven a decrease. If low emission technologies result in global temperatures stabilizing at 2 o Cabove the present, coral populations will initially decrease with the loss of temperature sensitivespecies, until they are replaced by more temperature resistant species. That will take decadesif not centuries. However, if greenhouse gases do not stabilize, the most likely scenario is thatcoral populations will decrease, with growing rates of extinction of corals and the thousands ofother species that depend on coral reefs. This will mean an end to the all-important ecologicalservices provided by coral reef ecosystems.2. In c r e a s i n g Co n c e n t r at i o n s o f CO 2in SeawaterThe cascading effects on ocean chemistry of rising atmospheric CO 2levels are referred toas ‘ocean acidification’. This does not generally invoke the same sense of urgency as coralbleaching, probably because:zx it is a creeping environmental problem;zx ocean acidification has only recently been accepted as a reality (partly becauseseawater carbonate chemistry is not intuitive); andzx the process is relatively invisible and does not appear to physiologically harm adultcorals.How ocean acidification will affect all life stages of organisms, reef communities, and reefstructures, however, is largely unstudied. While the nature and rate of ocean acidification iswell-known and predictable, the potential ecosystem effects of ocean acidification constitute aproblem of high uncertainty, but high risk.The uptake of atmospheric CO 2by the oceans is a double-edged sword. So far, the oceans haveabsorbed about a third of the excess CO 2released into the atmosphere from burning fossil fuelsand other human activities. Another third has been taken up by activities on the land, and theremaining third has remained in the atmosphere such that the concentration of CO 2has gonefrom 280 parts per million by volume (ppmv) before the industrial revolution to about 380 ppmvtoday. Ocean uptake of CO 2from the atmosphere reduces the severity of the greenhouse effectand climate change (and indeed the conditions that cause coral bleaching). Unfortunately, italso alters the chemistry of seawater resulting in lower pH (‘ocean acidification’) and decreasedcarbonate ion concentrations. Low pH values represent high hydrogen ion concentrationsand more acid conditions, and high pH values represent low hydrogen ion concentrationand alkaline conditions. pH is reported on a logarithmic scale; such that a 1.0 change in pHrepresents a 10-fold change in hydrogen ion concentration. The pH of tropical seawater hasremained around 8.2–8.3 for about a half million years, but will decrease to around 7.9–8.0 whenatmospheric CO 2 concentrations are double the pre-industrial levels; that represents about a30% increase in hydrogen ion concentration. This change in ocean acidity will also cause ashift in the relative proportions of the inorganic forms of carbon: dissolved CO 2; bicarbonate;and carbonate. A lowering of seawater pH also means that the carbonate ion concentration willdecrease by more than 30%. This represents a substantial change in the chemical conditionssupporting calcification, because the carbonate ion is a major skeletal building block for thecalcium carbonate (CaCO 3) skeletons of corals and other reef-building organisms.23
Status of Coral Reefs in the Caribbean after Bleaching and Hurricanes of 2005Atmospheric carbon dioxideCOCO + H O 2 2– +CO H CO HCO + H CO 2–++ H2 (aq) 2 333Dissolvedcarbon dioxideCarbonic acid Bicarbonate CarbonatePreindustrial versus doubled preindustrial CO2 concentrationsCOppmv280 8560CO + H COμmol kg162 (g)–2 (g) 2 (aq) 2 3–1 –1 –1HCO3μmol kg16351867CO 2 –3μmol KG272177pH8.117.93This diagram illustrates what will happen to ocean chemistry as more CO 2dissolves in seawater.When CO 2concentrations in the atmosphere effectively double from the pre-industrial levels, therewill be an increase in dissolved bicarbonate and a decrease in the available carbonate in seawater.Thus it will become more difficult and energy consuming for coral reef animals and plants to makeskeletons.DEEP-SEA CORALS AND OCEAN ACIDIFICATIONMost people are familiar with tropical coral reefs, but many are unaware of the widedistribution of deep-sea corals that live in cold waters below the photic zone, in depthsof 50-1000 m. The scientific world was unaware of the vast expanses of ‘deep-sea coralbioherms’ that occur along the edges of continental shelves until a few decades ago, whenunderwater mapping by remote cameras came into common use. These coral ecosystemsare known to support many fisheries, particularly in the North Atlantic Ocean and offthe coast of Alaska. A major threat to these communities has been bottom trawling,which physically destroys these slow-growing reefs. Now the global problem of oceanacidification is threatening their existence.Deep-sea corals are considered to be particularly sensitive to ocean acidification becausethey often grow just above depths where waters become under-saturated with calciumcarbonate. As CO 2concentrations rise and ocean acidification proceeds, that depth is‘migrating’ upwards, and many deep-sea coral ecosystems will soon be immersed inunder-saturated waters. Since the distribution of cold water corals today appears to belimited to depths above the saturation depth, it is likely that the deeper coral ecosystemswill disappear.Guinotte JM, Orr J, Cairns S, Freiwald A, Morgan L, George R (2006). Will human-inducedchanges in seawater chemistry alter the distribution of deep-sea scleractinian corals?Frontiers in Ecology and the Environment 4:141-146.The significance of these changes: Experiments with corals and coral communities culturedin predicted future seawater chemistry conditions show that calcification rates will decrease by20–50% of pre-industrial levels by 2050 (the predicted date for a doubling of atmospheric CO 2concentrations). However, there will be variations in calcification rates between species, and24
Page 1 and 2: Stat u s o fCa r i b b e a nCo r a
Page 3 and 4: Dedication: This book is dedicated
Page 5 and 6: Status of Caribbean Coral Reefs aft
Page 25 and 26: Status of Coral Reefs in the Caribb
Page 27: Status of Coral Reefs in the Caribb
Page 35 and 36: 3. Hu r r i ca n e s a n d t h e i
Page 37 and 38: Hurricanes and their Effect on Cora
Page 39 and 40: Hurricanes and their Effect on Cora
Page 41 and 42: 4. Th e 2005 Bl e a c h i n g Ev e
Page 43 and 44: The 2005 Bleaching Event: Coral-Lis
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Page 67 and 68: Coral Reefs of the U.S. Caribbean19
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The Effects of Coral Bleaching in t
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Managing for Mass Coral Bleaching:
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Status of Caribbean coral reefs after bleaching and hurricanes in 2005

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