BECKYHITCHIN
In hot water
Becky Hitchin considers the impact of rising water temperatures on coral reefs across the world
Coral reefs are one of our most precious ecosystems , critical in providing safety , food security and livelihoods to more than 1 billion people worldwide . They contribute to local economies , not least through tourism , with an estimated 70 million visitors to coral reefs annually .
However , approximately half of the world ’ s live coral cover on coral reefs has been lost since the 1870s , with accelerating losses in recent decades , particularly associated with bleaching events . The first mass coral bleaching event we know about occurred in 1998 , and it killed approximately 8 % of the world ’ s coral .
Since then , as bleaching events have increased in frequency and intensity on the world ’ s coral reefs , coral has found it ever more difficult to recover to some extent between events , and by 2018 , 14 % of coral is known to have been lost .
Coral reefs are particularly vulnerable to climate change and are projected to decline further to 10 to 30 % of former cover if we undergo a 1.5 ° C warming , and to less than 1 % of former cover at 2 ° C warming . The future of our reefs varies greatly among and within countries , depending on forecasts of water temperature and weather patterns . Climate change affects coral reefs in many ways . The main one , as mentioned above , is through bleaching , though ocean acidification also plays a role .
Let ’ s start with coral bleaching . Corals live in a mutually beneficial relationship with microscopic algae that live in their tissues , known as zooxanthellae . These tiny algae gain a protected place to live , and provide the coral with up to 95 % of its needed nutrients , such as amino acids and glucose . These are needed by the coral to grow their calcified skeleton and their food products .
When corals are stressed , they can expel the algae , which we see when corals turn from their normal bright and beautiful colours to a stark white . If the temperature stays high , the coral will not let the algae return after being expelled , and the coral will die . All it needs is a change in water temperature of 1-2 ° C over several weeks to cause coral to drive out their algae .
Also affecting corals is a phenomenon known as ocean acidification , the ongoing decrease in pH through the global ocean . It ’ s caused by the increasing uptake of carbon dioxide from the atmosphere into seawater . The problem for corals reefs is that the more acidic seawater is , the less calcium carbonate it can hold , and corals need calcium carbonate for their exoskeletons . With more acidic waters , corals become weak , and the skeletons easier to break , leading to faster erosion . The impact of ocean acidification seems to be greater when the temperature is higher .
“ When corals are stressed , they can expel the algae "
However , corals can adapt to a higher water temperature – at least to some extent . Some types of zooxanthellae seem to have greater thermal resistance , and corals can shuffle the types of zooxanthellae they keep inside them . If they can bring in species with a better thermal resistance , they may have an inbuilt advantage . Some corals seem to be less susceptible to thermal stress , including Acropora , and while this may give them added protection , it may lead to changes in coral communities .
It used to be thought that deeper coral reefs could act as refuges for bleaching events , but recent research has suggested that they too are impacted – if not as severely . In the 2016 Great Barrier Reef bleaching events , impacts on the deep reefs were still severe ( 40 % bleached and 6 % dead colonies at 40m ) but significantly lower than at shallower depths ( 60 – 69 % bleached and 8 – 12 % dead at 5-25 m ).
As you can see , understanding how any particular coral reef will respond to warming temperatures will be a huge challenge . The impact will depend on the corals involved , the shape and size of the reef , whether it continues into deeper and cooler waters , and how temperature will continue to change . What is a refuge today may not be a refuge tomorrow .
It will also be affected by how already stressed corals can withstand overfishing , local nutrient loading , sedimentation , invasive species and disease . It ’ s a learning process for scientists to understand these changes , and another learning process to work out how to restore the damaged reefs . Time will tell what the long term impact will be on our wonderful coral reefs . �
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