Coral Bleaching
www.ogp.noaa.gov
Why Bleaching?
Sun exposed areas bleach first
Photosynthesis
(normal conditions)
Photosynthesis under thermal stress
(Photoinhibition bleaching model)
Thermal thresholds
(Temperatures at which bleaching occurs)
Hoegh-Guldberg 1999
La Niña
El Niño
Tahiti Sea Surface temperature
Hoegh-Guldberg 1999
Number of reefs severely bleaching
Hoegh-Guldberg 1999
1998 Massive Bleaching
Question 1:
• Why are corals growing so close to
their thermal limit?
Predicted evolution SST
(Global coupled atmosphere-ocean-ice model)
Hoegh-Guldberg 1999
Question 2:
• Why are corals growing so close to
their thermal limit?
• Why are there few reports of coral
bleaching before 1979?
Predicted evolution SST
(Global coupled atmosphere-ocean-ice model)
Hoegh-Guldberg 1999
Question 3:
• Why are corals growing so close to
their thermal limit?
• Why are there few reports of coral
bleaching before 1979?
• Will coral bleaching increase in the
future?
Hoegh-Guldberg 1999
Possible scenarios of increasing SST
• Strategy shift:
– Hardy spp. replace sensitive spp.
• Tolerance: Corals acclimate + evolve
– spp. with highest genetic variability expected to
survive
• Phase shift: corals are replaced by algae
– Already occurring in many regions!
Simple Model
Model with interspecific
differences in thermal
thresholds
Model with thermal threshold
differences + acclimation & evolution
Hughes et al. 2003
Interspecific bleaching Susceptibility
Diverse Communities
Monospecific communities
Interspecific Bleaching Susceptibility
Raiatea, French Polynesia (May 2002)
Hughes et al. 2003
Coral species boundaries
(geographical differences)
1- Local Temperature differences
2- Genetic Variability differences
Low-Isolated endemic populations
High-Central and Mainland populations
Hughes et al. 2003
Facts on the future of Coral Reefs due
to Global warming
• Few indications that coral acclimation /
rapid evolution is occurring
• Oceans warming 2oC / 100 years
• Annual massive bleaching events by
2030-2070
• Phase shift away from coral dominated
communities by 2050
• Economical impact of Trillions of $,
affecting 100’s of million humans
Coral-Algae Phase Shift (Jamaica)
1984
199592%
Algae
3%
Coral (% cover)
53%
4%
Herbivory in Coral Reefs
Coral reef herbivores?
• Green Turtles
– Ecologically extinct
• Manatees & Dugongs
– Ecologically extinct
• Parrotfish (& surgeonfish)
– Generally overfished
• Sea Urchins
– Variable abundance (diseases & predation)
Jamaican History 101
•
•
•
•
•
1492: 16 million Green Turtles (Caribbean)
1688-1730: 13000 turtles/year (slave food)
1730: 6.5 million Turtles (Caribbean)
1800: Turtle fishery crashes, Fish fishery develops
1881: Jamaica imports 85% of its fish
(local overfishing)
• 1962: Historical high fishery catches (15% local origin)
(local overfishing still)
No Turtles, No manatees, Very few parrotfish
ONLY SEA URCHINS LEFT
(Diadema antillarum)
Jamaican History 102
• 1980: Hurricane Allen
• 1983: Diadema die-off across Caribbean (99% mortal.)
NO HERBIVORES LEFT!
• Late 1980’s: Shift to Algal Domination
• 1991: Hurricane Gilbert
• Today:
– Algae dominate reefs
– Extensive overfishing of herbivore fish species
– Slow and patchy recovery of Sea Urchins populations
% reef sites
Historical coral reef
community changes
P = Prehuman
H = Hunter Gatherer
A = Agricultural
CO+CD = Colonial
M1 = Modern
M2 = Present
Historical coral reef degradation
Increase Coral Diseases
Massive Bleaching
OVERFISHING
Principle 15 of the Rio
Declaration
• “Where there are threats of serious or
irreversible damage, lack of full scientific
certainty shall not be used as a reason for
postponing cost-effective measures to
prevent environmental degradation.”