Tom Vierus, Marine Biologist | Amanda Ford, Coral Reef Ecologist
It’s April 2016, just off the coast of Beqa – a small island to the south of Fiji’s main island Viti Levu.
Amanda and myself are currently living on the beautiful Fiji Islands to conduct our scientific fieldwork. While I am investigating a potential multi-species nursery of hammerhead and blacktip sharks in a remote part in the north of Fiji, Amanda and her team are focusing on the coral reefs. For her PhD at the Leibniz Centre for Tropical Marine Research in Germany, she investigates the impact of human communities on coral reef health around Pacific islands.
Her team has spent the last half a year here studying how agricultural land-use changes, sewage input and fishing are affecting reefs. To gain a better understanding of the impacts of a remote island community, the team has been working in a small village on Beqa Island. By surveying various parameters of reefs at increasing distances from shore, she can compare how their structure and function changes. We have traveled back here one last time on this trip to follow-up on the ‘white corals’ of which local fishermen are telling tales…
70% of all corals bleached
Tropical corals live in symbiosis with small single-celled algae, called zooxanthellae. Up to one million of them can be found per cm2 of coral tissue, giving corals the vibrant colors which we so strongly associate with tropical reefs. The algae synthesize sugars out of sunlight and carbon dioxide (photosynthesis) and feed them to their host, providing the coral animal with enough energy to build its energetically-costly calcium carbonate skeleton. The coral animal in turn supplies a sheltered home alongside a constant supply of carbon dioxide and nutrient-rich waste products. This symbiotic relationship breaks down if surrounding water temperatures become stressful, causing the coral to digest or expel the zooxanthellae, leaving behind the clear animal tissue and the visible white coral skeleton, hence the term ‘coral bleaching’.
Looking down from the boat in Beqa, the unusual bright whiteness of the reef gives the impression that it is glowing from the depths. As soon as we get below the surface to measure the extent and severity of the bleaching, the devastating scene takes our breath away. Though there is admittedly a ghostly beauty and elegance to a coral reef that is bleached, it is an incredibly unnerving to see the large-scale effect of ocean warming on corals that can be up to hundreds of years old. Up close to bleached colonies you can start to differentiate distinct parts that have already died and are being colonized already by small algae. As we return to the surface, our eyes convey what we both feel. “At least 70% of all soft and hard corals are showing at least some bleaching. It doesn’t look good,” Amanda summarizes to me with a sigh. Even though bleached corals may still recover, extended periods of elevated sea surface temperatures and any additional stress from competing organisms or disease can lead to eventual death of the corals which are so depleted in energy without their symbiotic algae.
‘This is the same situation as most of the reefs I have surveyed during the last weeks’, Amanda says to the fishermen as she explains to them what they are seeing around their coast. Some shallow areas along Fiji’s coast experienced unusually high water temperatures for several weeks. Besides the coral bleaching there have also been local mass fish die-offs which were likely caused by anoxic conditions triggered by the unusually high temperatures during low tide.
El Niño and the third global bleaching event
El Niño events represent one of Earth’s most important natural weather-producing phenomena by influencing pressure systems, winds and precipitation. They occur generally every two to seven years following a weakening of the trade winds that push water from east to west in the Pacific Ocean. These winds usually allow cool and productive waters from the depths to replace the surface water in the east as the winds blow the water west. When the trade winds weaken, the warm surface water stays in the east, restricting the upwelling of the deeper water. Without the currents being driven from east to west, water across the Pacific starts to warm up.
Though the direct effect of climate change on El Niño events is debated, these events are now occurring in combination with global ocean warming, thus intensifying the effects. 2015 was the hottest year since record keeping began according to the United Nations; only 8-10% of the warming was attributed to the El Niño. The combination of warm temperatures and the extended and intense ‘super El Niño’ seemed to be the worst yet for coral reefs worldwide. After 1998 and 2010, this was the third time an El Niño event has led to global bleaching events in various parts of the world. Since the end of 2014 the predictably returning El Niño weather event had been warming surface temperatures in wide parts of the Pacific region, with additional catastrophic effects in the eastern Pacific. By suppressing the cold and nutrient rich waters currents flowing upwards along Peru’s coastline, millions of fish failed to arrive leading to the collapse of the otherwise flourishing Peruvian fishing industry. While fishers returned with empty nets, heavy rains all along South America’s west coast caused flooding and in the western Pacific, dry, hot air led to miserable agricultural yields.
Coral reefs among the most diverse habitats on earth
Healthy coral reefs belong to the most productive and diverse habitats on earth, harboring around 25% of marine species despite only covering less than 1% of ocean floor. In turn they are vital for tropical coastal communities in terms of food and coastal protection, with hundreds of millions of people depending on them directly and indirectly. Furthermore, coral reefs generate billions of dollars every year in the tourism industry. In fact, it is estimated that the generate financial revenue in the region of US $30 billion– every year!
Coral bleaching and associated mortality can be catastrophic for coastal human communities in many ways. Once hard corals die, they are overgrown quickly by other organisms such as algae. Overtime, their remaining hard skeleton is eroded by a mixture of biological, chemical and physical processes, reducing the originally complex reef structure to flattened rubble. When this happens, the hugely diverse array of organisms that rely on the reef’s three-dimensional structure for shelter and protection are essentially left homeless. Furthermore, as reefs are flattened, their original capacity to dissipate up to 90% of wave energy is lost, leaving coastlines very susceptible to storm surges. Many of the people living close to tropical coral reefs are restricted in their ability to move or to switch to alternative livelihoods or food sources, rendering them highly vulnerable to coral reef loss.
The future is concerning regarding thermal stress on reefs. Already over 25% of tropical coral reefs are already lost and without large-scale action to minimize carbon dioxide emissions, most reefs are predicted to have disappeared by the middle of this century.
Better management can help improve the situation
All is not lost. There are things that can be done to improve the situation. Maintaining healthy fish populations and good water quality can keep fast-growing algae under control. Similar to how a stressed person isn’t going to feel better with a poor diet and no sleep, local conditions can be changed to allow reef a maximum chance to recover from bleaching. If local factors are better maintained, bleached coral have much more capacity to survive, re-establish their zooxanthellae and continue to grow. Many management tools can achieve this. Marine protected areas can provide a refuge from fishing for fish communities, as can catch limits and size-limits that protect important life-stages. Wastewater management, reduced fertilizer usage near to coastlines and rivers and mangrove reforestation can improve water quality around coastlines. Now more than ever, it is critical that we implement local management tools to maximize the future survival of this ecosystem. Otherwise, this is not only an ecosystem in peril, but its loss will affect millions of our neighbors around the world.
We need to act united
As the afternoon sun starts to descend we make our way back to the village. Exhausted from the long day surveying the reefs, our eyes stare indistinctly towards the green lush coast line. Although we have seen bleaching all around, it doesn’t necessarily mean it is all doomed. The data collected by research teams like Amanda’s will help to further understand combined impacts of local stressors and climate change on reefs, and with solid data the likelihood of implementing better local management increases. We have certainly seen that the fisherman’s tales of white corals were true and it makes one thought very clear: as so often, many of the poorest people around the world will suffer the most. Climate change is real and we need to speed up our efforts to combat and reduce its impacts. Though educating decision makers and spreading the word to as many people as possible are important, these efforts should start with ourselves and reducing our own carbon footprint as much as we can.
