Previous Year Question

Coral bleaching mainly occurs when corals remain under thermal stress for a prolonged period because seawater temperature rises above its normal range.

The tissues of corals contain microscopic symbiotic algae commonly known as Zooxanthellae. Through photosynthesis, these algae provide energy to corals and also play an important role in giving coral reefs their characteristic colours.

When seawater temperature becomes excessively high, the symbiotic relationship between the coral and Zooxanthellae becomes disturbed. Under stress, the coral may expel these algae from its tissues.

After the loss of Zooxanthellae, the transparent coral tissue allows the white calcium carbonate skeleton underneath to become visible. The coral therefore appears white, and this process is known as Coral Bleaching.

Bleaching does not mean that the coral has died immediately. If thermal stress ends quickly, surviving corals may regain Zooxanthellae and recover. However, if high temperature persists, starvation, disease susceptibility and physiological stress increase and may result in mass coral mortality.

Major mass-bleaching episodes in the Andaman and Nicobar coral reefs have been associated with elevated Sea Surface Temperature (SST). During the severe bleaching event of 2010, SST in the Andaman region was recorded at approximately 1–2°C above normal.

An important point is that tropical corals already live close to their upper thermal tolerance. Therefore, even a sustained rise of only about 1–2°C above the normal seasonal maximum can trigger extensive bleaching.

Among the other options:

Coastal industrial activities and marine pollution can damage coral reefs, but the principal cause of the catastrophic mass-bleaching event referred to in the question was abnormally high sea temperature.

A 4°C decrease in temperature was not responsible for this event; the documented bleaching event was associated with warming and thermal stress.