coral bleaching . why bleaching?

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

El Niño

La Niña

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 differences2- Genetic Variability differences

Low-Isolated endemic populationsHigh-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)

Algae Coral (% cover)

1984 3% 53%

1995 92% 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 parrotfishONLY 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

Historical coral reef community changes

% r

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site

s

P = PrehumanH = Hunter GathererA = AgriculturalCO+CD = ColonialM1 = ModernM2 = Present

Historical coral reef degradation

Increase Coral DiseasesMassive 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.”