The rising threat of coral bleaching

Context

A recent study conducted on coral reefs in the Indian and Pacific Oceans has found that heat was not the only driver of coral bleaching but it is also influenced by location and several other factors.

Background

• The stunning colours in corals come from marine algae called zooxanthellae, which live inside their tissues. This algae provides the corals with an easy food supply thanks to photosynthesis, which gives the corals energy, allowing them to grow and reproduce.
• When corals get stressed, from things such as heat or pollution, they react by expelling this algae, leaving a ghostly, transparent skeleton behind. This is known as ‘coral bleaching’. Some corals can feed themselves, but without the zooxanthellae most corals starve.

Significance of coral reefs

• A Coastal Protection - By their massive formation between the surface and the first few tens of meters deep, coral reefs are a very effective for absorbing elements coming from the ocean. They absorb wave’s energy and contribute to environmental protection through the reduction of coastal erosion in case of storms, hurricanes etc.
• Habitat - Estimates suggest that these living limestone scientists over one million plant and animal species are involved and they host receive more than 25% of all species of marine life. It is one of the most important biodiversity hotspot on earth.
• Food Resource - Reef animals are an important source of protein, contributing about a quarter of the fish catch average in these countries. “Well managed” reef can provide between 5 and 15 tons of fish, crustaceans, molluscs and other invertebrates per square kilometre.
• Economic Importance - Millions of people around the world depend on coral reefs for food, protection and employment.
• Tourism Wealth - The reefs are often essential element in the economy of tropical regions they inhabit. They attract divers in effect, freedivers, recreational fishermen and lovers of white sand beaches. Local economies benefit from billions of dollars from visitors to reefs with their companies exploiting reef ecosystems. Through tourist services, billions of dollars are collected.
• Medical Future - Reef organisms are used in the treatment of diseases such as certain cancers including leukaemia, HIV, cardiovascular diseases, ulcers. In addition, long coral skeleton, because of its very close similarity to our bones nature, served as material for bone grafts. Coral reefs also contribute to the advancement of research.

Factors responsible for Coral Bleaching

• Increased water temperature (most commonly due to global warming), or reduced water temperatures,
• Oxygen starvation caused by an increase in zooplankton levels, Increased solar irradiance (photosynthetic active radiation and ultraviolet light)
• Increased sedimentation (due to silt runoff)
• Bacterial infections, Herbicides
• Changes in salinity and Extreme low tide and exposure
• Pollutants such as oxybenzone, butylparaben, octyl methoxycinnamate, or enzacamene: four common sunscreen ingredients that are nonbiodegradable and can wash off of skin
• Ocean acidification due to elevated levels of CO2 caused by air pollution
• Being exposed to Oil or other chemical spills
• It may result from increases in seawater temperature, particularly when associated with elevated levels of solar irradiance (e.g., ultraviolet radiation)
• Earlier studies had established that bleaching occurred when corals underwent stress from water temperatures rising significantly above normal. It was believed that reefs closer to the warmer equator had bleached more in the past, and were expected to degrade further.

New research findings

• Researchers have found that coral bleaching events across the world are influenced not only by rising water temperatures but also by a wide range of other factors like the longitudinal location at sea where the reefs are present
• It has suggested that that bleached corals are highly variable in terms of warm water temperatures and location, with some going through bleaching levels of up to 60 percent, and others surviving with no impact.
• The bleaching depended heavily on the location of reefs along the longitudinal gradient from East Africa to Fiji.
• Different stress-tolerances - The researchers also found that since different coral reefs were composed of different species of coral and were exposed to varying levels of heat and stress, they had developed different tolerances to these factors, leading them to react differently to bleaching events.

Consequences

• Coral can survive short-term disturbances, but if the conditions that lead to the expulsion of the zooxanthellae persist, the coral's chances of survival diminish.
• If the coral polyps die of starvation after bleaching, they will decay and the hard coral species will then leave behind their calcium carbonate skeletons, which will be taken over by algae, effectively blocking coral re-growth and eventually, the coral skeletons will erode, causing the reef structure to collapse.

Way forward

• There is much that we can do locally to protect coral reefs, by making sure there is a healthy fish community and that the water surrounding the reefs is clean.
• Well-protected reefs today typically have much healthier coral populations, and are more resilient (better able to recover from natural disasters such as typhoons and hurricanes).
• Fish play important roles on coral reefs, particularly the fish that eat seaweeds and keep them from smothering corals, which grow more slowly than the seaweeds. Fish also eat the predators of corals, such as crown of thorns starfish.
• Marine protected areas (MPAs) are an important tool for keeping reefs healthy. Large MPAs protect the Great Barrier Reef and the Northwestern Hawaiian Islands, for example, and in June 2012, Australia created the largest marine reserve network in the world. Smaller ones, managed by local communities, have been very successful in developing countries.
• In the long run, however, the future of coral reefs will depend on reducing carbon dioxide in the atmosphere, which is increasing rapidly due to burning of fossil fuels. Carbon dioxide is both warming the ocean, resulting in coral bleaching, and changing the chemistry of the ocean, causing ocean acidification. Both making it harder for corals to build their skeletons.