Coral Bleaching

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 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 2 o C / 100 years Annual massive bleaching events by 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) AlgaeCoral (% 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 : 16 million Green Turtles (Caribbean) : 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 : 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 % reef sites 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.”