Case study of Coral Bleaching

 Coral bleaching happens when the algae on the coral die and the coral turn white. These algae and coral live in a symbiotic relationship. The algae are responsible for about 90% of coral's energy generation. Bleached coral cannot survive for a longer time. Eventually, they will die due to starvation. The main reason for bleaching is the increasing temperature of the ocean. Many other factors like ocean acidification and heavy metal pollution are also responsible for bleaching. Corals in almost every part of the world are affected due to increasing temperature. Many bleaching events have occurred in the past few decades. But the bleaching event of El Nino from 2014 to 2017 was the deadliest bleaching event ever.

Between 1980 and 1990 sixty major coral bleaching events have occurred. The longest bleaching event was recorded in 2016. Over 70% of the world's corals were affected during that time. According to a study, the oldest bleaching event was that of Frasnian time of the Late Devonian era which was triggered due to a rise in sea level and ocean temperature. 

The reason behind Bleaching:

Zooxanthellae are colourful marine algae that live inside the corals. These algae are responsible for the colour and entry production of the corals. However, these algae cannot survive in high-temperature water. If the temperature of the ocean rises then they die. The loss of these algae is known as bleaching. Without zooxanthellae, the coral tissue becomes transparent, revealing its white skeleton. The corals cannot survive without the algae. without the algae, the corals become more vulnerable to diseases, their growth and reproduction decreases and sometimes they face starvation. 

The Great Barrier Reef Bleaching:

The GBR, spanning more than 340,000 sq km has the highest number of coral species. In the last 4 decades, the Great Barrier Reef has experienced more than nine major bleaching events. In some locations, almost 90 to 95% of corals have died during the bleaching event. From 1985 to 2002 the population of corals is dropped by almost 10% and it is still decreasing. After 2014, the ocean is experiencing a severe increase in temperature. Hence the death rate of corals is also increasing in the GBR. The shallow water corals of Queensland has the highest mortality rate due to bleaching. During the summer season, the bleaching event becomes even more frequent. 

Today 40% of the corals still have no effect of global warming, another 25% are under the severe effects of bleaching and 35% are moderately bleached. In 2020, corals of coastal cities like Townsville and Mackay has shown the most severe bleaching. According to Morgan Pratchett, a researcher at ARC Centre of Excellence for Coral Reef Studies, "Bleaching is not always fatal, it affects some species more than others.". A pale or lightly bleached coral can regain its colour under favourable conditions.

The fatality of bleaching depends on the type of corals. For some corals, bleaching weakens their immune system or calcium content but is not fatal. While for some corals, bleaching leads to starvation and eventually death. There is also a chance of recovery after bleaching. The metabolism and colour can be restored within few days of bleaching for some species. While some take decades to recover their original colour. Back to back mass bleaching events makes it very difficult for corals to recover speedily. It was f\proved from one study that most corals need more time to recover. 

According to Professor Terry Hughes, director of ARC Centre of Excellence for Coral Reef Studies, greenhouse gases emission in Australia must be stopped. Instead of controlling the pollution, the government is promoting new coal mines and fossil fuels. 

Effects of bleaching on fishes:

A team of researchers spent more than 600 hours underwater to understand the behaviour of butterflyfish after the bleaching event of 2016. The experiment lasted for nearly 2 years. Researchers found very unusual behaviour of these fishes. In this experiment, they studied nearly 5,560 butterflyfishes of 38 different species. They studied these fishes living near the corals at different locations of GRB, Japan, the Philippines and Thailand.

The butterflyfishes living near the coral reefs have shown less aggression in their behaviour. This change in behaviour is higher at places where corals in large number were dead after bleaching. The reason is, corals are an important diet of butterflyfish. As a large number of corals were dead, the fishes have to face a major change in their diet. However, the increasing temperature and acidification could also be the reason for their unusual behaviour.

So today much more work is needed to be done in order to save the corals. The oceans are getting warmer and warmer, more and more glaciers and ice caps are melting. All these factors are very harmful to the marine ecosystem and any changes in the marine ecosystem directly affect the inland creature including humans.