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

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 low­lying states, is facing serious challenges as they seek to grapple
with the impacts of a changing climate. Global temperatures have increased by
0.6oC 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 “reef­building”
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 reef­building 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
build­up of coral skeleton, into a complex three­dimensional 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 (www.coris.noaa.gov).
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Figure 2: The three­dimensional 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.
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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 1997 –1998 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.
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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.
2005 Bleaching Episode in Jamaica
Between August and October 2005 Jamaica and other Caribbean nations
experienced prolonged high sea surface temperatures. Jamaica experienced 5­6
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.
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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. Twenty­six
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).
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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 long­term 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) 927 1609
(876) 935 8276/8238
Fax: (876) 977 1033
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References
Publications
Barnes, R. 1987. Invertebrate Zoology. Fifth Edition. Orlando, FL: Harcourt Brace
Jovanovich, Inc. pp. 149­163.
Bruno JF, Selig ER, Casey KS, Page CA, Willis BL, Harvell CD, Sweatman H,
Melendy AM. 2007. Thermal stress and coral cover as drivers of coral disease
outbreaks. PLoS Biology Vol. 5, No. 6, e124 doi:10.1371/journal.pbio.0050124
Buddemeier, R. W., J. A. Kleypas, R. B. Aronson. 2004. 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. 2004. Reefs at Risk in the Caribbean. World Resources
Institute, Washington. 81 p
Jones, L. 2006. Caribbean seas too hot for corals? 2005 Caribbean Bleaching
Event: Response and Assessment Workshop. International Tropical Marine
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Mellela and C. Manfrino. 2008. The Effects of Coral Bleaching in the Northern
Caribbean and Western Atlantic. In Status of Caribbean coral reefs after
bleaching and hurricanes in 2005. Wilkinson, C., Souter, D. (eds). Global Coral
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IPCC. 2007. Climate Change 2007: Synthesis Report. Summary for Policy
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Wilkinson, C., Souter, D. (eds) (2008). Status of Caribbean coral reefs after
bleaching and hurricanes in 2005. Global Coral Reef Monitoring Network, and
Reef and Rainforest Research Centre, Townsville, 152 p.
Websites
Marshall, P., Schuttenberg, H. 2006. A reef manager’s guide to coral bleaching.
Great Barrier Reef Marine Park Authority, Townsville, Australia.
http://www.coris.noaa.gov/activities/reef_managers_guide/pdfs/reef_manag
ers_guide.pdf
NOAA News Release 2005. 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. NOAA05­R499­04. FOR
IMMEDIATE RELEASE Contact: Ben Sherman. 10/24/05
http://www.publicaffairs.noaa.gov/releases2005/oct05/noaa05­r499­04.html
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