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The seas surrounding corals reefs are normally blue. Why is this reef green?

The Sea is Green

Many coral reef organisms eat algae, and prevent it from taking over the reef. If one type of herbivore is reduced in numbers other species can often take over and control algae. However, coral reefs that have lost diversity may not have herbivores waiting to take over. These reefs will quickly be smothered by algae.

 Why is the sea green?

The waters surrounding a coral reef are normally blue and very clear. Coral surfaces are tan or brown. But the coral reef turns green if algae take over, the reef turns green because floating particles of algae color the water.

Algae can grow much faster than coral and if not controlled will overgrow the reef.  Many coral reef organisms eat algae and prevent it from taking over. But what happens when if these herbivores are lost?

 Where are the herbivores?

Sea urchins such as the black-spined sea urchin (Diadema antillarum) eat and, in large numbers, control algae. Likewise, some reef fish are mostly vegetarian, surgeon fish and others such as parrot fish graze the coral surface. This gives coral polyps the space to settle and grow.

In 1983 and 1984, a disease struck the Caribbean killing over 93% long-spined sea urchin, leading to high amounts of algae, and coral death.

Algae immediately began to flourish.

This increase was minimal on reefs with many herbivorous fish, because they took over as the main control of algae. Coral still had space to develop and grow on these reefs.

Unfortunately, these fish are good eating and were overfished on many reefs. Overfished coral reefs were smothered by algae because there was nothing to control them. These reefs turned green.

 Can we turn the sea back to blue?

The easiest way to control algae is by preserving the organisms that eat it. Setting aside areas where these herbivores are free from interference gives them time and space to grow and reproduce. These protected areas provide a reservoir of herbivores (and algae control) for the protected area and the surrounding region.

Marine parks have been created in the Caribbean islands of Bonaire, St. Lucia and many others. These protected reefs are recovering from the loss of the black-spined sea urchin.  Fish have increased in size and numbers and help control the algae.

They also add to the local economy as an additional benefit. These marine parks attract thousands of tourists, and fishing in the waters surrounding these parks has improved dramatically.

 Sustainability and green reefs

Diversity influences ecosystem resilience because high diversity ecosystems resist or recover from disturbance faster. Ecosystems often become more vulnerable to further diversity loss once diversity has initially been lost.

Weedy algae were able to dominate overfished reefs after the loss of the long-spined urchin because diversity and resilience had already been reduced. These overfished reefs then lost even more diversity because coral was overgrown and the species that rely on it were lost.

In contrast, more diverse reefs had fish to take over as algae control and the loss of the urchins had much less impact. However, these reefs are now more vulnerable to disturbance unless urchin numbers build up and diversity is restored.


Lessios, H. (1988). Mass mortality of Diadema antillarum in the Caribbean: What have we learned? Annual Review of Ecology and Systematics 19:371-93.

Lessios, H. et al. (1984, 19 October). Spread of Diadema mass mortality through the Caribbean. Science. PDF retrieved 22 August 2008 from http://www.sciencemag.org/cgi/reprint/226/4672/335.pdf?ijkey=0a485cd939fe5b4ec3df4e19a9d5e8a88bce4997.

McGinley, M. (2007, October 18). Threats to coral reefs. Encyclopedia of Earth. Retrieved 31 July 2008 from http://www.eoearth.org/article/Threats_to_coral_reefs.

Mumby, P. et al. (2006, January 6). Fishing, trophic cascades, and the process of grazing on coral reefs. Science. Retrieved 22 August 2008 from http://www.sciencemag.org/cgi/content/full/311/5757/98

Palumbi, S., McLeod, K., & Grünbaum, D. (2008). Ecosystems in action: lessons from marine ecology about recovery, resistance, and reversibility. Bioscience 58(1): 33-42.

Palumbi, S. et al. (2009). Managing for ocean biodiversity: Creating a national biodiversity conservation agenda to sustain marine ecosystem services. Frontiers in ecology and the environment.

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