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Bleaching and Local Ecological Effects

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The Bleaching Working Group (BWG) was founded by the UNESCO /Intergovernmental Oceanographic Commission(IOC) in April of 2001. The group’s initial efforts included the development of indicators specifically for coral bleaching. Subsequently, it expanded its mandate to examine specific physiological mechanisms for coral bleaching as well as the local ecological factors that cause bleaching and its after-effects, and differences between direct human stresses and those related to climate change.

The Working Group has prioritized hypotheses at various levels of interaction related to stress tolerance and the basis for vulnerability and resilience of corals reefs to bleaching.

Example hypotheses include:

Molecular-level Hypotheses:

1. The basis of heat stress tolerance in corals rests in the molecular mechanisms that reduce photoinhibition.
2. Failure of the primary steps of photosynthesis leads to a build-up of oxygen radicals, which then cause cellular damage.
3. Both coral host and zooxanthellae have a series of coral bleaching specific markers that may be useful as bio-markers.

Cellular and Physiological Hypotheses:

1. Coral bleaching and mortality is driven by the primary variable elevated temperature, but is influenced by light, flow and other factors.
2. Thermal stress will reduce growth rates, coral metabolism, and regenerative capacity.
3. Seasonal fluctuations in the density and quality of zooxanthellae are important to understand coral bleaching.

Within-Reef Ecological-level Hypotheses:

1. Climate change will reduce reef resilience by:
   • increasing whole colony mortality on coral reefs,
   • changing differential mortality patterns (species, size) reducing recruitment (loss during larval phase failure of settlement)
   • having a greater effect on larval survival compared to the adult phase
   • causing a change in relative abundance of populations,size frequency distributions, and causing a functional shift.
2. Other stressors (natural and/or anthropogenic) will have a compounding effect on the tolerance of corals and zooxanthellae to thermal stress.