By: Eva Gruber
With the recent tragic news of the Great Barrier Reef experiencing the largest mass coral bleaching event ever, we can no longer ignore the fact that coral reefs the world over are in sheer danger.
Coral reef ecosystems take centuries to grow and develop, and it is no small matter to be losing them due to anthropogenic activities in a matter of years.
At least 19% of the world’s coral reefs – including 50% of those in the Caribbean – are already gone, and within 20 years, if current trends continue, we could lose another 15%, according to the National Oceanographic and Atmospheric Administration.
If climate change continues unabated, all the coral reefs on the planet could be gone within one human generation. That means all the global coral reef system – with all of its biodiversity and fisheries supporting millions of poor people around the world – will be wiped out. There will be cascading effects on the rest of the ocean’s marine habitats as well, and as a result there will be widespread hunger, poverty, and political instability.
Coral bleaching occurs when the zooxanthellae (which provide color to the corals) are expelled due to stress – this is most often due on a large scale to climate change, as increased water temperatures are the main stressors to corals. Lots of other harmful human activities also stress corals – acidification, pollution, coastal development, heavy tourism, overfishing, and harmful fishing methods all hammer the health of coral reefs.
Sometimes bleaching can be reversed – bleaching does not immediately kill the coral, and if the zooxanthellae are able to return then the coral can recover. However, if the coral polyps go for too long without zooxanthellae, then they can die. Death will occur if the stressor does not reverse – and unfortunately the trend of warming ocean waters means that rarely is there an instance where the water then cools down enough for the zooxanthellae to return.
When bleaching is combined with overfishing – which often is the case, then the result can be catastrophic. In the 1970s, Jamaica painted a stark picture of what happens when a coral reef ecosystem is compromised. Due to overfishing, the resident fish of the reef (many of which eat algae) were decimated. In their absence, algae could grow unabated, blocking crucial sunlight to the photosynthetic organisms.
With the fish gone, urchin populations exploded as they gorged on the seaweed. Then, the urchins were struck with a disease and their population crashed – upon which the seaweed returned and choked out the rest of the corals. A marine biologist and his team studying Jamaica found that when algae-eating fish were prevented from returning to the reef and eating seaweed, coral growth was slowed by 700 percent.
Acidification is also a serious stressor – as corals can only form polyps when the temperature and acidity of the water is within a small range. With the increasing absorption of CO2 from the atmosphere, the ocean is becoming more and more acidic, and this trend is severely impacting corals. Research from the University of Queensland shows that at current rates of acidification, corals will be permanently pushed out of their ideal polyp-growing range within 20 to 30 years, if international action on emissions is not enacted.
Without corals and the fish species that rely on them, the entire ecosystem crashes, and seaweed forests take over. What we will be left with is dead coral and algal-dominated ecosystems from which the benefits of coral reefs – hotspots of biodiversity, productive fisheries, and as-of-yet-undiscovered medicines – will vanish.
“A world without coral reefs is unimaginable,” said Jane Lubchenco, a marine biologist who heads NOAA. “Reefs are precious sources of food, medicine and livelihoods for hundreds of thousands around the world. They are also special places of renewal and recreation for thousands more. Their exotic beauty and diverse bounty are global treasures.”
Thermal heating stress map: http://coralreefwatch.noaa.