Coral bleaching

Assembled from 43 blocks · 3 evidence · 22 related

1. Story
2. The breakdown of the coral-algal symbiosis under stress, in which the host loses its symbionts, or the symbionts lose their pigments, and the white skeleton becomes visible through the transparent tissue. The dominant trigger on modern reefs is heat: sea temperature held roughly one degree Celsius above the local maximum monthly mean for a period of weeks, with high light making it worse and cloud or turbidity making it better. The mechanism is understood in outline as damage to photosystem II in the symbiont under combined heat and light stress, producing reactive oxygen species that the host expels the symbiont to escape. The crucial point for a reader is that bleaching is not death. A bleached colony is starving, not destroyed; if the heat abates within weeks it can re-acquire symbionts and survive, though often with reduced growth and no reproduction that year. If the heat persists, it starves. Cold shock, freshwater, sedimentation and disease can bleach coral too, which is why an individual bleached colony is not by itself evidence of a marine heatwave.
3. Knowledge
4. Reef-building corals (Scleractinia)
5. Coral bleaching
6. Marine heatwave
7. Connections
8. Reef-building corals (Scleractinia)
9. Marine heatwave
10. Degree Heating Weeks (DHW)
11. Symbiont shuffling and thermal tolerance
12. The first global mass bleaching event
13. A published projection that reefs would bleach faster than they could recover
14. The second global bleaching event
15. The third global bleaching event
16. Mass bleaching on the Great Barrier Reef
17. A second consecutive year of mass bleaching on the Great Barrier Reef
18. The interval between severe bleaching events is measured, and it has collapsed
19. Mass bleaching on the Great Barrier Reef during a La Nina
20. The fourth global bleaching event is confirmed
21. The fifth mass bleaching on the Great Barrier Reef since 2016
22. The current state of the fourth global bleaching event is not established in this package
23. Marine heatwave
24. Zooxanthellae (Symbiodiniaceae)
25. Zooxanthellae (Symbiodiniaceae)
26. Coral bleaching
27. Crown-of-thorns starfish (Acanthaster)
28. Elkhorn coral (Acropora palmata)
29. Darwin publishes The Structure and Distribution of Coral Reefs
30. The first global mass bleaching event
31. A published projection that reefs would bleach faster than they could recover
32. Mass bleaching on the Great Barrier Reef
33. The interval between severe bleaching events is measured, and it has collapsed
34. Reef-building corals (Scleractinia)
35. Coral bleaching
36. Thermal refugia
37. Coral restoration methods
38. Mass bleaching on the Great Barrier Reef during a La Nina
39. Extreme water temperatures in the Florida Keys kill nursery and wild coral
40. Evidence
41. On projected warming, the interval between bleaching events would fall below the interval corals need to recover, making event frequency rather than event severity the binding constraint on reefs. V55 verification basis: not consulted in this session; identified and summarised from the producing model's knowledge of the paper and its subsequent reception.
42. Reef thermal stress is measured as HotSpot (sea surface temperature above the local maximum monthly mean) accumulated over a rolling twelve-week window into Degree Heating Weeks, with roughly four DHW indicating likely significant bleaching and roughly eight indicating likely widespread bleaching and mortality. V55 verification basis: the product documentation was NOT retrieved — coralreefwatch.noaa.gov was refused by the egress proxy — and these definitions and thresholds are stated from the producing model's working knowledge of the products. Any alert levels added above Level 2 are not described here.
43. In the 2016 Great Barrier Reef bleaching event only a small minority of the more than eleven hundred reefs surveyed escaped bleaching, with the most severe bleaching in the northern third of the system, and water quality and past exposure did not protect reefs from it. V55 verification basis: not consulted; the survey design and headline result are stated from the producing model's knowledge, and the precise escaped-reef percentage is given as "around nine per cent" rather than to a decimal place.