Although corals are found in temperate and tropical waters, shallow-water reefs are formed only in a zone extending at most from 30°N to 30°S of the equator. (This zone is very important to whales because many types of plankton live there). Tropical corals do not grow at depths of over 50 m (165 ft). Temperature has less of an effect on the distribution of tropical coral, but it is generally accepted that they do not exist in waters below 18 °C.[
“Corals are certainly threatened by environmental change, but this research has really sparked the notion that corals may be tougher than we thought,” say researchers at Stanford’s Woods Institute for the Environment in the USA.
Corals in danger?

Coral locations

Coral reefs form the basis for thriving, healthy ecosystems throughout the tropics. They provide homes and nourishment for thousands of species, including massive schools of fish, which in turn feed millions of people across the globe. Corals rely on partnerships with tiny, single-celled algae called zooxanthellae. The corals provide the algae a home, and, in turn, the algae provide nourishment, forming a symbiotic relationship. But when rising temperatures stress the algae, they stop producing food, and the corals spit them out. Without their algae symbionts, the reefs die and turn stark white, an event referred to as coral bleaching.
During particularly warm years, bleaching has accounted for the deaths of large numbers of corals. In the Caribbean in 2005, a heat surge caused more than 50 percent of corals to bleach, and many still have not recovered. In recent years however, scientists discovered that some corals resist bleaching by hosting types of algae that can handle the heat, while others swap out the heat-stressed algae for tougher, heat-resistant strains.
In 2006, researchers at Stanford, travelled to Ofu Island in American Samoa. Ofu, a tropical coral reef marine reserve, has remained healthy despite gradually warming waters with numerous corals hosting the most common heat-sensitive and heat-resistant algae symbionts. Ofu also has pools of varying temperatures that allowed the research team to test under what conditions the symbionts formed associations with corals.
In cooler lagoons, Oliver found only a handful of corals that host heat-resistant algae exclusively. But in hotter pools, he observed a direct increase in the proportion of heat-resistant symbionts, suggesting that some corals had swapped out the heat-sensitive algae for more robust types. These results, combined with regional data from other sites in the tropical Pacific, were published in the journal Marine Ecology Progress Series in March 2009.
Global pattern
To see if this pattern exists on a global scale, the researchers gathered worldwide oceanographic data on a variety of environmental variables, including ocean acidity, the frequency of weather events and sea-surface temperature. They then compiled dozens of coral reef studies from across the tropics and compared them to environmental data. The results revealed the same pattern: In regions where annual maximum ocean temperatures were above 84 to 88 degrees Fahrenheit (29 to 31 degrees Celsius), corals were avoiding bleaching by hosting higher proportions of the heat-resistant symbionts. Most corals bleach when temperatures rise 1.8 F (1 C) above the long-term normal highs. But heat-tolerant symbionts might allow a reef to handle temperatures up to 2.6 F (1.5 C) beyond the bleaching threshold. The scientists believe that this might be enough to help get them through the end of the century, Oliver said, depending on the severity of global warming.
A 2007 report by the United Nations International Panel on Climate Change concluded that the average surface temperature of the Earth is likely to increase 3.6 to 8.1 F (2 to 4.5 C) by 2100. In this scenario, the symbiont switch alone may not be enough to help corals survive through the end of the century. But with the help of other adaptive mechanisms, including natural selection for heat-tolerant corals, there is still hope, scientists believe.
It comes down to a calculation of the rates of environmental change versus the rates of adaptation. Heat-resistant corals also turn out to be more tolerant of increases in ocean acidity, which occurs when the ocean absorbs excess carbon dioxide from the atmosphere—another potential threat to coral reefs. This finding suggests that corals worldwide are adapting to increases in acidity as well as heat, and that across the tropics, corals with the ability to switch symbionts will do so to survive.
Future protection
Researchers from the Institute say that it’s hard to imagine that these corals, which have existed for a quarter of a billion years, only have 50 years left. Part of their job might be to figure out where the tougher ones live and protect those places.

Journal reference:
Oliver TA, Palumbi SR. Distributions of stress-resistant coral symbionts match environmental patterns at local but not regional scales. Marine Ecology Progress Series, 2009; 37893 DOI: 10.3354/meps07871

This article was adapted from materials provided by Stanford University.

He writes that human activities are cumulatively driving the health of the world’s oceans down a rapid spiral, and only prompt and wholesale changes will slow or perhaps ultimately reverse the catastrophic problems they are facing.

Jackson cites the synergistic effects of habitat destruction, overfishing, ocean warming, increased acidification and massive nutrient runoff as culprits in a grand transformation of once complex ocean ecosystems. Areas that had featured intricate marine food webs with large animals are being converted into simplistic ecosystems dominated by microbes, toxic algal blooms, jellyfish and disease.
Professor Jackson is the director of the Scripps Center for Marine Biodiversity and Conservation and has tagged the ongoing transformation of the oceans as “the rise of slime.” His new paper, “Ecological extinction and evolution in the brave new ocean,” is a result of Jackson’s presentation last December (2007) at a biodiversity and extinction colloquium convened by the National Academy of Sciences. The purpose of his talk was to make clear just how dire the situation is and how rapidly things are getting worse. He said, “It’s a lot like the issue of climate change that we had ignored for so long. If anything, the situation in the oceans could be worse because we are so close to the precipice in many ways.”

In this new assessment, Jackson reviews and synthesizes a range of research studies on marine ecosystem health, and in particular key studies conducted since a seminal 2001 study he led analysing the impacts of historical overfishing. The new study includes overfishing, but expands to include threats from areas such as nutrient runoff that lead to so-called “dead zones” of low oxygen. He also incorporates increases in ocean warming and acidification resulting from greenhouse gas emissions.
Jackson outlines in detail the powerful, destructive effects that come about when forces combine to degrade ocean health. For example, climate change can exacerbate stresses on the marine environment already brought by overfishing and pollution. He writes that ‘all of the different kinds of data and methods of analysis point in the same direction of drastic and increasingly rapid degradation of marine ecosystems.’

He goes further in his analysis by constructing a chart of marine ecosystems and their “endangered” status. Coral reefs, which are his primary area of research, are “critically endangered” and among the most threatened ecosystems on earth; also critically endangered are estuaries and coastal seas, threatened by overfishing and runoff; continental shelves are “endangered” due to, among other things, losses of fishes and sharks; and the open ocean ecosystem is listed as “threatened” mainly through losses at the hands of overfishing.

“Just as we say that leatherback turtles are critically endangered, I looked at entire ecosystems as if they were a species,” said Jackson. “The reality is that if we want to have coral reefs in the future, we’re going to have to behave that way and recognize the magnitude of the response that’s necessary to achieve it.” To stop the degradation of the oceans, he identifies overexploitation, pollution and climate change as the three main “drivers” that must be addressed.

“The challenges of bringing these threats under control are enormously complex and will require fundamental changes in fisheries, agricultural practices and the ways we obtain energy for everything we do,” he writes. “So it’s not a happy picture and the only way to deal with it is in segments; the only way to keep one’s sanity and try to achieve real success is to carve out sectors of the problem that can be addressed in effective terms and get on it as quickly as possible.”

Journal reference:
Jackson et al. Colloquium Paper: Ecological extinction and evolution in the brave new ocean. Proceedings of the National Academy of Sciences, 2008; DOI: 10.1073/pnas.0802812105
This article used material from the University of California – San Diego via Eurekalert.