Coral Bleaching – Why Do Coral Bleach?

Corals are colony-forming polyps, invertebrates (backboneless animals) that excrete a calcium carbonate skeleton, which forms the coral reefs. The polyps live in symbiosis with photosynthetically active algae (zooxanthellae). The algae supply the coral polyp with nutrients such as glucose, glycerol and amino acids. The algae are also responsible for the colors of the hard corals.

Coral reefs form an important habitat for many species of fish. Thousands of reef fish, invertebrates, and other species live in coral reefs. Nests are built here, fish hatch, and juvenile and adult fish find protection from predators or find their food here. Riffs form barriers along coasts and surrounding islands, protecting shorelines from storms. They also promote fishing, scuba diving, boating, and other recreational activities that earn billions of dollars worldwide each year.

Coral reefs are referred to as sea rainforests. The number of invertebrate species living in coral reefs is believed to be between 1 and 10 million, with many of them yet undiscovered and unknown. Despite covering less than 0.5 percent of the Earth’s surface, coral reefs are home to an estimated 25% of all marine species. Coral reefs are the major coastal ecosystem in the tropical Pacific, accounting for more than 25% of all reefs worldwide — over 66,000 km².

Many Pacific Island and Territories (PICTs), notably French Polynesia, Kiribati, and Palau, have at least double the amount of reef as land area (SPC data). Corals and coral reefs can be found all over the Pacific Island region, which includes the Hawaiian Islands (State of Hawaii), the Marianas Islands (Territory of Guam and Commonwealth of the Northern Mariana Islands), the eastern portion of the Samoan Islands (Territory of American Samoa), and several islands and atolls in the central Pacific referred to as (Howland, Palmyra Atoll, Johnston Atoll, Kingman Reef, Jarvis Island, Wake Island, and Baker Island).

healthy vs bleached coral

What causes coral bleaching?

Coral bleaching is mainly caused by warm seawater. This leads to the disruption of the symbiosis between polyps and algae, which in turn causes the corals to bleach and die.

The symbiosis between polyp and algae is highly sensitive and reacts to environmental pollution (sunscreen, oil, antifouling from ship hulls, agricultural runoff, herbicides, insecticides, etc), excessive solar radiation, changes in the salinity of the seawater, and especially heat stress.

When the temperature of the seawater is too high, the (algae) zooxanthellae are expelled by the corals, causing them to lose their color and “bleach”. Then the corals first look bright blue, green, yellow or pink to be white like lime at the end. When everything is dead, the coral skeleton will be overgrown by filamentous algae. Temperature-related bleaching events have been observed all around the world, including the Pacific Ocean.

How does global warming affect coral reefs?

Since the 1970s, there has been a great deal of research indicating that coral populations around the world including in the Pacific Ocean are deteriorating due to both climatic and non-climatic factors. Many ecologists and marine biologists have raised alarms about the global reduction in coral cover caused by global warming, coral bleaching, overfishing, and coastal pollution.

Coral bleaching has been produced by abnormally high Sea Surface Temperatures (SSTs), decreased cloud cover, higher-than-average air temperatures, and higher-than-average atmospheric pressures. Climate change is currently the single most powerful factor affecting the bleaching and death of coral reefs around the world. During cyclical El Nino periods, these coral bleaching events will intensify even more.

How does coral bleaching affect fish?

Global warming is causing the oceans to become warmer. Warmer water causes corals to bleach and die, depriving fish of their habitat, food sources and reducing the habitat for their offspring. This destruction of the fish habitats naturally has the consequence that the fish populations become smaller, or that certain species disappear completely.

In the Pacific Islands region, there are around 7,000 species of fish, they constitute an important component of fragile coastal ecosystems. The different species fill special niches in the reef ecosystems. Coral eaters feed on corals (e.g. butterflyfish, flagfish), herbivores feed on algae that grow on corals or on the bottom and various predators (e.g. groupers, snappers, and sharks) prey and feed on all sorts of fish that live in coral environments.

The importance of coral reefs in ensuring food security across the tropical Pacific is enormous — fish contribute 50–90% of the animal protein in the diets of coastal populations in the region, with the bulk of these fish acquired through subsistence fishing on coral reef ecosystems.

How much of the world’s coral reefs are dead?

The mass coral bleaching events of the last 20 years have resulted in almost 20% of the corals in the tropics having died. About 50% of the world’s coral reefs are damaged or suffering from heat stress. However, the geographical differences are quite pronounced. There are areas where coral mortality from temperature is over 90%, other areas are less affected. Even the world’s largest reef system, the Australian Great Barrier Reef, is severely affected by coral bleaching and around 30% of the coral reef there has now died.

There is a great risk that this will continue because global warming cannot be turned off over night. We can only try to slow it down a bit. If we fail then the outlook is grim for the world’s coral reefs. Climate models predict that if the temperature rises up to 1.5 degrees by the year 2100, up to 80 percent of the coral reefs will disappear. It looks even worse if the global temperatures rise to two or more degrees, then the tropical coral reefs can be expected to disappear completely by 2100.

Global warming and its impact on Fiji and the south pacific

Various recent oceanographic studies in the South Pacific Ocean have shown that an increase in water temperature has a long sequence of effects. Coral bleaching has destroyed the habitat of a wide variety of marine species including fish, sponges, oysters, clams, sea urchins and starfish, etc.

As we have seen above, the coastal communities are very dependent on a functioning ecosystem in their food procurement and overall lifestyle. On the one hand, seafood is harvested here, which makes up an important part of the protein intake of the people living here. On the other hand, new dependencies have also arisen, in recent years tourism has become a driving factor in the islands’ economy. So tourists worldwide come to see the beauty of the coral reefs while snorkeling or diving. The disappearance of the coral reefs will inevitably make these destinations less attractive.

Why is coral bleaching bad?

The loss of coral reefs has catastrophic ecological and economic consequences. The coral reefs are home to around 25 percent of all marine species so they are among the richest ecosystems in the world. Millions of species of animals and plants live in the reefs, and new species are constantly being discovered.

Humans also benefit from these ecosystems. Fish from the coral reefs provide food, reefs provide protection to coastal communities from storms and the forces of the sea. Reef tourism is also an important economic source of income for many small island states.

The real economic and environmental damage that the loss of the coral reefs will cause cannot yet be foreseen. But the prospects are terrifying when you consider how important and how dependent humans are on coral reefs.

How can coral bleaching be stopped?

Crucial for the protection of the coral reefs is that all countries in the world work together to stop climate change and reduce environmental destruction so that coral reefs and nature have a chance to recover.

Measures to reduce the coral bleaching and destruction of coral reefs:

  • Sustainable management of marine resources and sustainable fishing (stop overfishing) and sensitive, environmentally friendly aquafarming methods must be developed and implemented consistently.
  • Local community-based adaptation and change in the use of marine resources is an important way to curb the negative effects of climate change and coral dying.
  • As a snorkeler or diver, you should make sure that the boat is moored to permanent moorings. Boycott tourist operators who are not environmentally friendly.
  • When snorkeling or diving, keep your distance and don’t touch fragile corals. Corals break off too easily. As a diver, body control and perfect buoyancy control are fundamental skills.
  • Do not buy souvenirs, jewelry, or decorative items made from corals. Not only does this promote the destruction of the reefs, but can also be heavily fined when crossing borders, since most of the species of corals are on the CITES list of protected species.
  • Uses environmentally friendly sunscreen. Certain ingredients in conventional sunscreens are powerful environmental pollutants and can damage coral reefs. Some island states are even banning the use of conventional sunscreens for this reason. As an alternative, there are environmentally friendly organic sunscreens.

The main cause of the decline in corals, however, is the rise in sea temperature – caused by climate change. It is up to each one of us to do the best to better protect the environment as well as politics to implement measures that protect the climate and curb global warming with all its consequences.

Sources and more to read:

Global warming and recurrent mass bleaching of corals

 Global Coral Reef Monitoring Network (GCRMN): Status of Coral Reefs of the World: 2020

Impacts of Climate Change on Corals Relevant to the Pacific Islands…Climate_Change_on_Corals…