While the oil spill in the Gulf of Mexico dominated the news during summer 2010, it was a somewhat unusual summer for the rest of the world as well. Extreme heat and fires in Russia caused millions of dollars worth of damage. Flooding in Pakistan has resulted in millions losing their homes. And droughts in most parts of sub-Saharan Africa and Southeast Asia have led to crop failures. One climate-related event that we haven’t heard much about, however, is the widespread bleaching of coral reefs all across the globe, largely due to this year’s severe heat stress. Scientists fear widespread die-offs of coral reefs, endangering not only the richest ecosystems in the ocean, but also fisheries that feed millions of people, and tourist industries worth billions of dollars.
Rainforests of the Sea
Known as the “rainforests of the sea”, coral reefs occupy a small part of the ocean, but they harbor nearly a quarter of all fish species. Reefs are made up of teeming communities of animals, a few millimeters in size, called polyps . The polyps secrete calcium carbonate to form skeletal structures, and it is this combined skeletal structure of the polyp communities that forms what we call reefs. In order to survive, these coral polyps form crucial symbiotic relationships—interactions that are mutually beneficial for both partners—with algae. Algae need the shelter and nutrients that coral polyps provide, and in turn the photosynthetic algae supply food in the form of sugar to the coral polyps. Without the supply of food from algae, the coral polyps cannot live. These captive algae in the coral polyps are what give reefs their brilliant colors. Many fish living in these reefs sport similarly brilliant colors that allow them to be camouflaged.
A coral is ‘bleached’ when it loses its color and appears ghostly white. Bleaching occurs when higher than average ocean temperatures (even an increase of just ~ 0.5-1°C)  cause the algae to have increased metabolic rates that induce them to secrete toxins. These toxins cause the coral polyps to recoil and literally spit out the algae. With most of the algae gone, the coral is stripped of its color and looks white, and it will have a much more limited supply of food. In such bleached and malnourished conditions corals are more vulnerable to diseases, and should the heat stress continue, they essentially starve to death. However, if the temperature drops, the few remaining algae can reproduce and help the coral polyps recover. Even when reefs have been dead for sometime, new coral polyps can still take hold, though the overall ecology and diversity of the reef will not be the same as before the bleaching event. In most cases these altered reefs are not as resilient as their former incarnations and are more vulnerable to environmental stresses. In the worst-case scenario, the reef dies from the heating and bleaching and never recovers.
Turning up the heat
This year’s extreme heat has led to a die-off of coral colonies that scientists fear may just be the starting point of a major environmental catastrophe. This is not the first time such die-offs have been seen — major die-offs were also seen in 1984, 1988, and 1998 . Coincidentally, all of these years were severe El Niño years. ‘El Niño’ is the name given to a semi-periodic climate phenomenon, occurring every 3-7 years, due to warming of sea surface temperatures off the coast of Peru. The warming causes the ocean-atmosphere system to reorganize in ways that cause major, albeit temporary, environmental changes all across the world, some of which were mentioned at the beginning of this article. However, the El Niño phenomenon has been occurring for at least the last five to six thousand years, and fossil records of corals have been around for an equal or even longer period of time . Yet, only over the last 60-70 years have we observed severe bleaching events like the ones this year that are increasingly causing coral die-offs. This suggests that factors other than the El Niño effect maybe responsible for these severe bleaching events in the recent past.
While global warming could be one cause, other factors, such as coastal development and overfishing, need to be considered. Coastal development leads to water pollution, and associated deforestation causes increased sedimentation rates; if trees and other plants are removed, soil washes off the land and into the rivers and oceans. Both pollution and sedimentation degrade water quality and negatively affect reefs, which need clear water to thrive. Additionally, coastal development also contributes indirectly to the declining health of reefs by promoting large-scale fishing industries. Species diversity of all organisms living in a coral reef, including fish and the coral polyps themselves, are important in maintaining the genetic diversity of reefs that makes them more resilient to environmental stresses. Overfishing and malpractices in fishing reduce species diversity in the coral colonies, making the reefs more vulnerable to, and causing delayed recovery from, environmental stresses like El Niño events.
An early warning?
Coral bleaching has not yet reached catastrophic levels. But such a phenomenon brings to light the fragility and importance of rich, diverse ecosystems like the coral reefs. While it is true that coral reefs have rebounded from crises due to natural phenomena in the past, it is less clear how they will fare in response to human actions. Coastal development, overfishing and greenhouse gas-related warming are all relatively recent occurrences, and we still don’t fully understand how coral reefs will react to artificial stresses like these. Close to 60% of the world’s coral reefs are threatened directly by human activity . In addition to its effects on natural ecosystems, the dying of coral reefs can also have a significant impact on our society as well. The populations of dozens of small island nations and the coastal regions of countries like Indonesia and the Philippines rely heavily on reef fish for food. If the coral polyps do not recover from a stress event, the reef ecosystem could eventually collapse, eliminating the livelihood and food source for a multitude of people.
Waters off the coast of Peru have cooled since this summer. This year, the worst heating appears to have passed and there seems to be a slowdown in coral bleaching, although scientists cannot be sure if it is indeed a sign of recovery or simply a temporary stalling in an ongoing series of bleaching events. The total magnitude of this calamity, however, remains unknown. There are still regions in the world where ocean temperatures peak quite a while after the El Niño months. Questions remain: are these die-offs permanent, or will there be new polyp re-growth on the dead reefs? And how do we best manage such rich ecosystems under stresses that seem unlikely to ease in the near future? The United Nations just agreed on a ten-year plan to slow biodiversity loss, setting the goal of ensuring protection of the 10% of the world’s oceans by 2020. The plan includes specific goals including making “special efforts to reduce the pressures faced by coral reefs.”  Will this be enough? The jury is still out on that. In the meantime, scientists will continue to study these critical living networks and the factors that affect them; hopefully this knowledge will help in the conservation of the world’s rainforests of the sea.
With special thanks to Jessica Carilli (scientist at the Australian Nuclear Science and Technology Organization and an expert on coral reef ecology) for critical input on the article.
1. San Diego State University/Scripps Oceanography Project: http://coralreefsystems.org/
2. National Oceanic and Atmospheric Administration (http://www.coral.noaa.gov/cleo/coral_bleaching.shtml)
3. Tager et al. (2010) Ecology 91 1:191-2000
4. 1998 report by World Resource Institute (http://www.wri.org/publication/reefs-at-risk)
5. Pres Release about the Nagoya Biodiversity Summit http://www.cbd.int/doc/press/2010/pr-2010-10-29-cop-10-en.pdf
Links of Interest
New York Times slideshow of the coral reefs of the Line Islands
NOAA Ocean Service Education site on Corals
Publications by John Pandolfi, a leading scientist in ecology of coral reefs