Coral: Lost at sea?

Coral reefs support a quarter of the world’s marine life, but rising ocean temperatures are killing them. The impact of their decline could be huge, says marine biologist Olivia Durkin.

A a result of rising sea temperatures, we are seeing the degradation and eventual destruction of one of the most beautiful ecosystems on Earth. Corals around the world are succumbing to yet another mass “bleaching event”; reefs that were once a rich mosaic of colours are now shockingly white as corals fade and die.

Corals are in fact a combination of animal, algae and “rock”. Colonies are made up of many identical individuals called polyps that secrete a stony skeleton. Polyps contain microscopic algae called zooxanthellae living within the coral animal tissue; the relationship is mutually beneficial, or symbiotic. Zooxanthellae use sunlight to provide energy and nutrients for the coral through photosynthesis, in return they are provided with shelter. In a reef, each colony acts as a building block, pieced together to form intricate structures that provide habitats for an abundance of reef fish and many other creatures.

Bleaching is the ultimate stress reaction, when environmental conditions decline to a point where they cannot sustain the coral-algae relationship. Zooxanthellae, which are responsible for the magnificent colour of corals, are expelled from the coral, leaving the transparent tissue on a stark white skeleton. Although still alive, bleached corals must work harder without zooxanthellae to obtain the energy they need for growth and survival. If periods of stress are prolonged, the corals will die. Other reef residents such as giant clams, sea anemones and soft corals, which also contain zooxanthellae, are sharing the same fate.

Widespread bleaching tends to be attributed to abnormally high sea temperatures in addition to high levels of light. This happens in times of prolonged calm weather and crystal-clear water. While this may sound like the picture-postcard perception of a typical coral reef environment, it is in fact turbulent waters that keep coral healthy and scientists worry that the current event has the potential to be the worst ever.

Sea temperatures are at an all-time high. Major bleaching incidents are increasingly prevalent. The US National Oceanographic and Atmospheric Administration (NOAA) revealed that 2010 has been the hottest year in recorded history. Prior to this, 1998 was the hottest for 130 years, leading to unprecedented intense bleaching and coral mortality worldwide which wiped out more than 90 per cent of shallow water corals in the Indian Ocean. NOAA’s Coral Reef Watch monitors and predicts bleaching events using HotSpots, a measure which highlights areas where sea surface temperatures rise above levels that can lead to bleaching.

As predicted by NOAA, bleaching began this February in Mauritius and it has progressed throughout the Indian Ocean and South East Asia, including reefs off Thailand, Cambodia, Malaysia and Philippines. Florida and the Caribbean are next, with strong warming of the surrounding sea and severe bleaching expected for the coming months.