Coral Bleaching: An Overview

Corals or more specifically called coral reefs (coral polyps) are basically invertebrate animals that belong to a vast group of bright and colorful class of animals called Cnidarians.

These organisms that are identically genetical, possess microscopic algae called zooxanthellae which stay inside their tissues. the corals and the algae possess a mutual relationship where the corals provide the zooxanthellae, the components that are essential for photosynthesis. in return, the zooxanthellae supply organic compounds of photosynthesis to the corals for example carbohydrates which the coral polyps utilize for synthesizing their calcium carbonate skeletons. Additionally, the zooxanthellae also provide the corals with their unique, beautiful and vibrant colors. 

Corals can either be stony-shallow-water corals, the ones that build reefs or they may be soft corals /deep water corals that reside in dark cold waters.

Some of the most conspicuous, sudden and destructive outcomes of global climate change on the coral reef ecosystems reflect by a physiological stress response of polyp corals, a phenomenon referred to as coral bleaching. When corals face abiotic stress by alteration in conditions such as temperature, light, or nutrients, they expel out the symbiotic zooxanthellae living in their tissues causing them to turn completely white. The pale white colors of the corals show the translucent tissues of calcium carbonate that are visible because of the loss of zooxanthellae that provide the pigments to the corals. 

Coral bleaching should not be confused with the corals being dead. Corals can survive a bleaching event if the stress is not severe, however, if they are under more and continuous stress for a length of period, they may be subject to mortality, this means if the temperature continues to be high the corals would not let the algae back and thus it would die. 

In the year 2005, Unites States lost about fifty per cent of its coral polyps because of massive leaching. The corals present in the Caribbean, Indian and Pacific Oceans are undergoing bleaching on a regular basis. According to a published report by the National Ocean and Atmospheric Association, the period around 2014 to 2017 witnessed heat stress that were severe enough to trigger a bleaching action in near about 75% of the world’s tropical coral reefs and for approximately 30% of the world’s coral reefs that stress was sufficient enough to kill them.

Causes

• Increase in sea/ocean temperature: The majority of the coral polyps are seen floating or attached to their substrate where the water has the warmest temperature, they could tolerate which means even the slightest increase in the water temperature could cause harm to the corals.
• Ocean acidification: Due to the increase in the level of carbon dioxide in the atmosphere, ocean water absorbs more carbon dioxide. This causes an increase in the acidity of ocean water which inhibits the coral's ability to form calcareous skeletons, essential for their survival.
• Human induced threats: Overfishing, contamination caused due to agriculture and industrial runoff, excessive coral mining and establishment of industrial areas in the coral ecosystem’s proximity adversely impact corals.
• Chemical pollution: Increased in nutrient concentration promotes phytoplankton’s growth which consequently increases the number of other organisms' growth. This adversely affects the coral ecosystems since the other water organisms compete with corals for space.

Major consequences of coral bleaching 

• A variation in the coral community can affect the species that can depend on them. Fishes and other invertebrates, and sea turtles that rely on live corals for their food and shelter can be disturbed due to coral bleaching which could ultimately hamper the entire food chain. Corals also serve as a spawning ground for many sea creatures. Thus as reef ecosystems collapse, the species that are already at risk may face extinction.
• Coral bleaching could cause a heavy shift in the fish community. This could translate into a reduction in catches for fish which could impact food supply and associated economic activities.
• Coral reefs bleaching could heavily impact peoples’ livelihood, food security as well as safety. Corals acts as natural barriers and protectors of coastlines by constantly absorbing the force of waves and storm surges thus protecting the population of people residing along the coast from increased storm damage, erosion or flooding. Without them, we would need to rely on manmade seawalls, which would not only be costly and less effective but would also damage the environment while their construction process.
• Reef tourism greatly contributes to a nation’s economy by bringing in millions of dollars each year and supporting thousands of jobs. Bleached coral polyps, deprived of magnificent marine creatures, jeopardize it all.
• Genetic and species diversity could also be negatively affected and may decline when corals die as a result of bleaching.

How can we help?

Coral survival and recovery prospects can be promoted through appropriate marine protected areas (MPA) and design.

• Reducing storm water runoff might help in combating water pollution, reduction in floods and protecting our water resources in turn protecting corals.
• Solutions for protecting the future of corals must transcend social, economic and cultural boundaries.
• Halting unplanned coastal development could significantly play a beneficial role in the reversal of declining reefs in some locations.
• By practicing sustainable fishing and establishing opportunities for ecotourism could contribute to the conservation of coral ecosystems.
• Another way could be by mitigating water pollution wherever possible by preventing oil spills or dumping chemicals into water bodies.
• Minimizing the use of non-degradable chemicals based fertilizers, insecticides or herbicides which harm corals.
• Last but not the least taking all possible measures to prevent or reverse actions that worsen global warming, since climate change is the greatest threat to coral reef ecosystems.

Together we can contribute on our parts to save the world’s coral reefs from bleaching before it gets too late.