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Coral Bleaching

Coral bleaching occurs when corals expel the symbiotic algae living in their tissues due to thermal stress, leading to loss of color and vital functions. Factors such as increased sea surface temperatures, La Niña, and El Niño events exacerbate the phenomenon, pushing corals close to their thermal limits. Historical data shows a rise in bleaching events since 1979, with predictions suggesting more frequent occurrences by 2030-2070. The decline in herbivorous fish populations also contributes to coral ecosystem degradation, emphasizing the need for effective conservation strategies.

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Coral Bleaching

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  1. Coral Bleaching www.ogp.noaa.gov

  2. Why Bleaching?

  3. Sun exposed areas bleach first

  4. Photosynthesis (normal conditions)

  5. Photosynthesis under thermal stress(Photoinhibition bleaching model)

  6. Thermal thresholds(Temperatures at which bleaching occurs) Hoegh-Guldberg 1999

  7. La Niña El Niño

  8. Tahiti Sea Surface temperature Hoegh-Guldberg 1999

  9. Number of reefs severely bleaching Hoegh-Guldberg 1999

  10. 1998 Massive Bleaching

  11. Question 1: • Why are corals growing so close to their thermal limit?

  12. Predicted evolution SST(Global coupled atmosphere-ocean-ice model) Hoegh-Guldberg 1999

  13. Question 2: • Why are corals growing so close to their thermal limit? • Why are there few reports of coral bleaching before 1979?

  14. Predicted evolution SST(Global coupled atmosphere-ocean-ice model) Hoegh-Guldberg 1999

  15. 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?

  16. Hoegh-Guldberg 1999

  17. 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!

  18. Simple Model Model with interspecific differences in thermal thresholds Model with thermal threshold differences + acclimation & evolution Hughes et al. 2003

  19. Interspecific bleaching Susceptibility Diverse Communities Monospecific communities

  20. Interspecific Bleaching Susceptibility Raiatea, French Polynesia (May 2002) Hughes et al. 2003

  21. 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

  22. 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

  23. Coral-Algae Phase Shift (Jamaica) Algae Coral (% cover) • 3% 53% 1995 92% 4%

  24. Herbivory in Coral Reefs

  25. Coral reef herbivores? • Green Turtles • Ecologically extinct • Manatees & Dugongs • Ecologically extinct • Parrotfish (& surgeonfish) • Generally overfished • Sea Urchins • Variable abundance (diseases & predation)

  26. 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:Historicalhigh fishery catches (15% local origin) (local overfishing still) No Turtles, No manatees, Very few parrotfish ONLY SEA URCHINS LEFT (Diadema antillarum)

  27. 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

  28. Historical coral reef community changes % reef sites P = Prehuman H = Hunter Gatherer A = Agricultural CO+CD = Colonial M1 = Modern M2 = Present

  29. Increase Coral Diseases Massive Bleaching OVERFISHING Historical coral reef degradation

  30. 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.”

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