The Impacts of Climate Change on the Coral Reefs of the Caribbean

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1 The Impacts of Climate Change on the Coral Reefs of the Caribbean Introduction Global climate change has emerged over the past decade as one of the major environmental issues facing countries worldwide. The Caribbean, with its small islands and low lying states, is facing serious challenges as they seek to grapple with the impacts of a changing climate. Global temperatures have increased by 0.6 o C over the last century and temperatures in the Caribbean have shown a similar trend (Taylor et al, 2007). Research indicates that this increase in temperature has contributed to over 500 coral bleaching incidents within the Wider Caribbean Region since 1980 and it has been predicted that by 2020 bleaching will become an annual event. Sea surface temperatures are presently at the upper threshold for coral survival and this, along with the increased occurrence of coral diseases and anthropogenic stressors, pose a serious threat to the future of Caribbean reefs (Burke & Maidens, 2004). What are corals and coral reefs? Corals comprise various marine organisms known as cnidarians which have hard or flexible skeletons. The body is made up of one or more polyps with tentacles used to catch food (Figure 1; Barnes, 1987). The corals that form reefs are hard corals with skeletons of calcium carbonate. These reef building corals are usually composed of many polyps (colonial) and have microscopic plants (zooxanthellae) in their tissues, which provide some additional food for the coral by means of photosynthesis. Because of the presence of the zooxanthellae, the reef building corals need light and so grow close to the surface of the sea. Massive reef structures are formed when each stony coral polyp secretes a skeleton of calcium carbonate. Over thousands of years dead coral skeletons build up giving rise to large structures known as coral reefs. This build up of coral skeleton, into a complex three dimensional framework provides habitat for numerous other species such as algae and sponges (Figure 2; Westmacott et al, 2000) Figure 1: Coral polyps with extended tentacles feeding on zooplankton ( 1

2 Figure 2: The three dimensional structure of the coral reef provides habitat for a number of other species. Climate Change Threats Coral reefs are now faced with a combination of threats arising from global climate change. These climate change factors include sea level rise, increased sea surface temperature, reduced calcification rates, altered circulation patterns and increased frequencies of severe weather events (Figure 3). Most unstressed coral reefs should be able to keep up with the predicted sea level rise. However, those weakened by temperature increases and other anthropogenic factors may not be able to grow and build their skeleton at a rate to keep up with rising sea levels. Increasing temperatures, translates into the increased likelihood of seasonal fluctuations that are likely to exceed the tolerance levels of most coral species. Figure 3: A number of climate change factors including sea level rise, increased sea surface temperature, reduced calcification rates, altered circulation patterns and increased frequencies of sever weather events pose a threat to coral reefs (Westmacott et al, 2000). Global emissions of greenhouse gases have raised the level of carbon dioxide in the atmosphere and in the oceans. Higher concentrations of carbon dioxide causes increased acidity of the seawater, which reduces calcification rates. Altered ocean circulation patterns could alter the dispersal and transport of larvae and affect the development and distribution of reefs worldwide. The increased frequency of severe weather events is likely to cause increased damage to coral reefs (Westmacott et al, 2000). If these trends continue as predicted by the IPCC (2007) coral bleaching will be a more regular feature of reefs worldwide. 2

3 Coral Bleaching Corals bleach as a result of a number of stresses both natural and manmade. These include prolonged elevated sea surface temperatures, high levels of UV light, low light conditions, high turbidity, sedimentation, disease, variable salinity and pollution. When water temperatures (or the level of any of the other stressors) increase above a critical threshold, usually over a wide area, corals begin to lose their zooxanthellae, eventually turning white. At this stage the corals are still living and if the stressful conditions subside they have the ability to regain their zooxanthellae, but should the temperature stress continue the corals are likely to die (Figure 4). Where mass bleaching causes high levels of coral mortality, these ecosystems typically take years to recover. Figure 4: Stages in mass coral bleaching (Adapted from Marshall & Schuttenberg, 2006) Mass bleaching is a recent phenomenon. Records of bleaching go back to as far as 1870 but since the 1980s bleaching events have become more frequent. The bleaching event in was the most geographically widespread with the highest level of coral deaths on record. Sea surface temperature stayed above the threshold for longer periods than previously recorded. It is estimated that up to 16% of the worlds reefs were destroyed. Mass bleaching events, although of a lesser severity, were recorded in 2002 in the southwest Pacific and in 2005 in the Caribbean and Western Atlantic. The extent and severity of mass bleaching events have increased worldwide over the last decade as illustrated in Figure 5. 3

4 Since 1998 coral bleaching has become a common occurrence (Marshall & Schuttenberg, 2006). A special report compiled by the Global Coral Reef Monitoring Network (GCRMN) entitled Status of Caribbean Coral Reefs after Bleaching and Hurricanes in 2005 (Wilkinson & Souter, 2008) has been recently published and reveals the full extent of these bleaching events in the Caribbean. Figure 5: Global trends in the extent and severity of mass bleaching. (Adapted from Marshall & Schuttenberg, 2006). Thermal Stress in the Caribbean In early October 2005, NOAA issued a Coral Reef Watch Bleaching Alerts for both Puerto Rico and the U.S. Virgin Islands, where thermal stress was at record levels (NOAA, 2005). Thermal stress had reached Degree Heating Week (DHWs) values of over 15 at some locations (Figure 6). Each DHW represents one week of temperatures 1 degree Celsius above the maximum highest monthly average. Accumulated over three months, DHWs above four are virtually always accompanied by considerable bleaching, whereas levels above eight are believed to represent increased coral mortality and result in their inability to recover Bleaching Episode in Jamaica Between August and October 2005 Jamaica and other Caribbean nations experienced prolonged high sea surface temperatures. Jamaica experienced 5 6 weeks of exposure to higher than normal temperatures, which resulted in widespread bleaching. The Centre for Marine Sciences, with support from Reef Check International and NOAA, documented this event. 4

5 Figure 6: Degrees heating map for the Caribbean in October 2005 (NOAA) During the period November 2005 to May 2006, regular Reef Check surveys were conducted which included an additional bleaching assessment component. Sites located in Negril, Discovery Bay, Portland, Port Royal and the Portland Bight Protected Area were assessed on two occasions, first to determine the extent of the bleaching and secondly to determine the level of recovery. Twenty six assessments were conducted at 16 sites. Bleaching was first noted on the north coast of the island in late August to early September and later became evident on the south coast in late September to early October. The effects of the bleaching episodes were more pronounced on the south coast than on the north (Jones et al, 2008). The sites exhibited varying degrees of bleaching ranging from 10% to 95%. At Lime Cay and Drunkenmans Cay on the south coast and Dairy Bull on the north coast, 80% of the corals colonies were bleached. On the resurvey, up to 50% of the bleached corals had recovered. Figure 7: Bleached Montastrea annularis observed at Drunkenmans Cay, Jamaica in October 2005 (Jones, 2006). 5

6 Capacity for Corals to Adapt to Climate Change Observations have shown that there is variation in the resistance and survival rate of corals to bleaching within species and between species. A genetic shift in population composition through the selection of more resistant genotypes may aide species in adapting to climate change. Corals may also have the ability to acclimatize to changing conditions by making physiological adjustments at the biochemical or cellular level. These adjustments are usually short term (days) and usually come at a cost by diverting energy from other physiological processes such as growth and reproduction. Incorporation of more heat resistant zooxanthellae and altering the density or position of pigments in the tissue are two of the mechanisms likely to be used in acclimatization. Coral may also adapt by shifting their range in response to increasing sea temperature. This would be accomplished by larvae from heat resistant corals being transported to reefs once dominated by less resistant genotypes (Marshall & Schuttenberg, 2006). Coral reefs are the foundation for many tropical marine ecosystems. The physically complex reef structure facilitates the presence of numerous other marine species therefore reduction in the abundance of coral species can cause significant decreases in biodiversity (Bruno, 2007). The increasing frequencies and intensity of climate change factors may outpace the adaptive mechanisms of many corals hence the long term impact of climate change on coral reefs will depend on the capacity of corals to adapt to changing conditions (Marshall & Schuttenberg, 2006). All the adaptation mechanisms must have the ability to keep up with sea surface temperature increases, sea level rise and reduced calcification rates. With the diversity that exists among coral species and the adaptive potential of many of these species the composition of coral reefs of the future in the Caribbean is likely to be much different from what presently exists today (Buddemeier, et al, 2004). Marcia Chevannes Creary Environmental Data Manager Caribbean Coastal Data Centre Centre for Marine Sciences University of the West Indies Mona, Kingston 7 Jamaica W.I. Tel: (876) (876) /8238 Fax: (876)

7 References Publications Barnes, R Invertebrate Zoology. Fifth Edition. Orlando, FL: Harcourt Brace Jovanovich, Inc. pp Bruno JF, Selig ER, Casey KS, Page CA, Willis BL, Harvell CD, Sweatman H, Melendy AM Thermal stress and coral cover as drivers of coral disease outbreaks. PLoS Biology Vol. 5, No. 6, e124 doi: /journal.pbio Buddemeier, R. W., J. A. Kleypas, R. B. Aronson Coral reefs & global climate change Potential Contributions of Climate Change to Stresses on Coral Reef Ecosystems. Prepared for the Pew Center on Global Climate Change. 42pp Burke, L. and J. Maidens Reefs at Risk in the Caribbean. World Resources Institute, Washington. 81 p Jones, L Caribbean seas too hot for corals? 2005 Caribbean Bleaching Event: Response and Assessment Workshop. International Tropical Marine Ecosystems Management Symposium 3. Global Problems, Local Solutions. Cozumel, Mexico, October 2006 Jones L., P.M. Alcolado, Y. Cala, D. Bobian, V. Coelha, A. Hernandez, R. Jones, J. Mellela and C. Manfrino The Effects of Coral Bleaching in the Northern Caribbean and Western Atlantic. In Status of Caribbean coral reefs after bleaching and hurricanes in Wilkinson, C., Souter, D. (eds). Global Coral Reef Monitoring Network, and Reef and Rainforest Research Centre, Townsville, pp IPCC Climate Change 2007: Synthesis Report. Summary for Policy Makers. Taylor, M.A., Centalla, A., Chalery, J., Borrajero, I., Benzanilla, A., Campbell, J., Rivero, R., Stephenson, T.S., Whyte, F. Watson, R Glimpses of the Future: A Briefing from the PRECIS Caribbean Climate Change Project. Caribbean Community Climate Change Centre, Belmopan, Belize. 24 pp Westmacott, S. Teleli, K, Wells, S., and West, J. M. (2000) Management of bleached and severely damaged corals reefs. IUCN. Gland, Switzerland and Cambridge. UK vii + 36 pp 7

8 Wilkinson, C., Souter, D. (eds) (2008). Status of Caribbean coral reefs after bleaching and hurricanes in Global Coral Reef Monitoring Network, and Reef and Rainforest Research Centre, Townsville, 152 p. Websites Marshall, P., Schuttenberg, H A reef manager s guide to coral bleaching. Great Barrier Reef Marine Park Authority, Townsville, Australia. ers_guide.pdf NOAA News Release Major Coral Bleaching Event Expands Across Caribbean, Severe In Puerto Rico And U.S. Virgin Islands. NOAA s New Coral Reef Watch Satellite Bleaching Alerts Aiding Managers. NOAA05 R FOR IMMEDIATE RELEASE Contact: Ben Sherman. 10/24/05 r html 8