How +2°c can kill a coral reef

How +2°c can kill a coral reef
This time last year in the Seychelles, the lethal heat wave associated to the 2015/6 El Niño was weakening after several months of deadly impacts on the reefs. P4O was in the field, monitoring coral-bleaching impacts on a restored reef engineered by the NGO Nature Seychelles.

Juvenile damselfish taking shelter in a bleached Acropora coral in the Seychelles during the 2016 mass coral-bleaching event. ©people4ocean
Spreading from the western Pacific, the lethal heat wave associated to the 2015/17 El Niño hit the western Indian Ocean around February 2016. Here in the Seychelles, the water temperature averaged 30°C for four consecutive months, peaking over 31°C on some days!

Corals are peaky when it comes to temperature: not too hot, not too cold. Reef building corals thrive in temperatures ranging from 23°C to 29°C. Exceed that limit – even by 1°C - for too long and you may witness coral bleaching...
Under normal conditions, each polyp of the colony live in symbiosis with millions of small algae called zooxanthellae. Using the energy of the sun (you may know a little about photosynthesis :-), the zooxanthellae make food for the coral, and provide up to 90% of its daily energy requirements! Last but not least, it is also the zooxanthellae that give their colours to corals.

Millions of symbiotic algae (zooxanthellae) - visible as bright green dots (picture above) - live within the coral tissue and provide the coral colony with up to 90% of its daily energy requirements.
If the water goes too hot for too long, those little food-makers start releasing free radicals and become toxic for their host. When this happens, the polyps will start expelling them in the water column. In a matter of days, the coral gradually looses its color and eventually turns white and starts to starve. Without its main source of energy, the colony’s immune system weakens, making the colony more susceptible to attacks from sponges and seaweeds. If the heat stress ceases, the polyps of the colony will slowly re-absorb zooxanthellae from the water column and the colony will recover. However, if the heat stress persists for too long, the colony will eventually starve to death.
We witnessed a mass coral-bleaching event in the Seychelles from February to August 2016. A particularly hot summer coupled with an intense El Niño caused water temperatures to rise and exceed season averages by 1 to 2°C for several weeks, triggering the phenomenon of coral-bleaching on the reef. As we received bleaching alerts from the US National Oceanic and Atmospheric Administration (NOAA) as well as bleaching reports from across world, we new what was coming...but what could we do to prevent it? From the start of March, we monitored over 20,000 vibrant colorful corals turning snowy white and remaining bleached for over 3 months. Mortality across the Seychelles averaged 80-90%. Despite the lethal effect of this bleaching event, many coral recruits (juvenile corals) settled on the reef in the following months, thus initiating the recovery of the reef.

Reef managers find themselves powerless against climate-related events because they happen on such a large-scale and because water temperature is a factor out of our control. Watching corals die-off in 2016 was completely out of our control...however, as we observed some corals resisting and surviving the event, we realized something could be done to help the reef recover and resist future heat waves. One month after the end of the bleaching event, we collected naturally detached fragments and coral colonies that were still alive after the heat wave. We re-fragmented them to small pieces (to increase stock size) and seeded them on a nursery. After 10 months of growth, we had generated a stock of about 3,000 heat-resistant corals that we transplanted back onto the reef. These “super-corals” are now living their life on the reef and generating more super-corals during each spawning seasons.
This bleaching event was an opportunity to build a new strategy for reef management in the context of climate-change. People4ocean now applies that strategy to restore coral reefs at large-scale.
Pocillopora corals turning fully white due to heat stress in 2016, Seychelles. photo: people4ocean

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