Coral Bleaching

Warming ocean temperatures cause coral bleaching by disrupting the relationship between coral and zooxanthellae algae. Coral bleaching decreases coral growth, reduces fecundity and can kill coral. Global warming is making bleaching events more frequent. How fast the oceans warm will largely determine whether coral survive.

Global warming is the gradual increase of the Earth’s surface temperature, and is caused by increasing levels of greenhouse gases such as carbon dioxide (CO2). Warming temperatures have been increasing the size and magnitude of coral bleaching events over the last 30 years. Additionally, corals in a shallow Ofu Island lagoon in American Samoa experience highly variable hot water temperatures. Coral in this lagoon thrive in conditions that would kill coral that have not adapted to the heat (see the “Research” section for four microdocs that talk about the Ofu corals)

Coral can adapt to a changing climate. Some coral species have been around for thousands of years and adapted to many changes in climate since then. However, coral may not be able to adapt fast enough to the present rate of climate change.

Several factors influence coral’s ability to adapt to climate change including:

  • local adaptation
  • rate of temperature increase
  • zooxanthellae algae

Local adaptation
Coral from warmer waters can tolerate higher temperatures better than those from cooler areas. For example, the seawater in the Arabian Sea is warmer than most waters at this latitude. Arabian Sea coral bleach at higher temperatures than those from cooler waters.

Rate of temperature increase
The slower the rate of increase, the more chance coral has to adapt.

Type of zooxanthellae algae
Many different types of zooxanthellae algae live in symbiosis with coral, and some cope better with hot water than others. Coral with these temperature-resistant algae bleach at higher temperatures than coral with other types of zooxanthellae.

Coral are able to build reefs because of their symbiosis with single-celled zooxanthellae algae. Coral shelters the algae in return for food, and this extra food enables coral polyps to build reefs.

Zooxanthellae stop producing food when the water temperatures get too hot. Seawater tends to stay within a relatively narrow temperature range even with fluctuations caused by the seasons, sun exposure, and other factors. Algae and corals adapt to these local conditions. Global warming creates greater variability in seawater temperatures which stop zooxanthellae algae from producing food.

Zooxanthellae algae are ejected from the coral if they don’t produce food, but whether zooxanthellae choose to leave or the coral kick them out is unknown. Coral polyps are translucent and without algae you can see the coral’s white skeleton. This is why this process is called “coral bleaching”.

Reef-building coral can catch their own food and survive for a short time without zooxanthellae, but will eventually die unless it can get more.

Global warming adds to the level of disturbance experienced by coral reefs. This makes it harder for the coral to recover from other types of disturbance. Too much disturbance lowers ecosystem resilience and degrades the reefs diversity and productivity.

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Last modified Wed, 25 May, 2016 at 10:46