Currently, the third massive worldwide coral reef bleaching event is unfolding. Corals from Hawaii to the Indian Ocean are affected, and recently, the worst bleaching ever witnessed in the Great Barrier Reef was reported.
(From The Global Scientist / by Tamakiu Bieri)– Once rare, coral bleaching events have increased both in frequency and severity over the past decades. Similar to the event in 1998, the current bleaching is caused by a combination of global warming and a strong El Niño. Whereas the last two global bleaching events (in 1998 and 2010) lasted one year each, the current event is spanning two years.
Coral reefs are among the world’s most complex ecosystems and are of immense ecological, economical, and aesthetic value. They provide habitat for a unique community of organisms including fish, turtles, sharks, crabs, shrimps, urchins, sponges, and microbes. For coastal communities, they provide food, coastal protection, and recreational possibilities.
Despite their importance, the health and extent of coral reefs around the world has declined rapidly over the past several decades due to human influences. These include pollution, overfishing (fish remove macro-algae that compete with corals for living space), and increasing atmospheric CO2 concentration that causes the oceans to become warmer and more acidic. Reef-building corals live near their upper thermal tolerance limits, meaning that even small increases in seawater temperature can cause stress, and consequently, bleaching. Furthermore, acidic seawater harms corals because it dissolves their skeletons, which are made of calcium carbonate.
Unfortunately, this trend of warmer and more acidic oceans is likely to continue, as global average ocean surface temperature has increased by 0.7°C, and pH has decreased by 0.1 units to 8.0 during the 20th century. Average sea-surface temperature (top 700 m) is projected to increase by another ~2.6°C, and pH is projected to decrease to ~7.7 by the year 2100 if the worst-case IPCC (Intergovernmental Panel on Climate Change) scenario is applied.
Read the full article here: https://theglobalscientist.com/2016/05/20/how-algae-disappear-from-corals-during-a-bleaching-event/