Reefs are sensitive, extremely biodiverse ecosystems that are strongly affected by climate change and play a vital role in ocean health. As global temperatures rise, the fragility and importance of the world's reefs simultaneously makes them a source of serious concern for environmentalists and provides early indications of the severity of climate change's future effects.

Coral reefs occupy thousands of square kilometers of shallow inshore waters in the tropics over a region of millions of square kilometers and are often used to define the tropical marine environment. Unlike other marine communities, reefs are built entirely by biological activity. They are composed of calcium carbonate produced by corals (phylum Cnidaria, class Anthozoa), with some contributions from calcareous algae and other organisms. Corals and the organisms that live with them are extremely sensitive to climate and are important indicators of ocean health.

Reefs have a long history--reef-building organisms were some of the first marine life to develop hard skeletons. Reefs reached their periods of maximum development during the Middle Cambrian (513- 501 million years ago), Devonian (416-359 million years ago), and Carboniferous (354-290 million years ago) periods, when they were dominated by a now extinct order, the rugose corals. During the Late Cretaceous (100-65 million years ago) and Neogene (23 million years ago to present) periods they have been dominated by scleractinian corals. The fossil record shows that some reefs were formed by other organisms in the past. Calcareous algae and archaeocyathids (small, cone-shaped animals probably related to sponges) formed reefs in the Early Cambrian period (542-513 million years ago), as did rudist bivalves in the Late Cretaceous period (100-65 million years ago). However, corals have generally been the dominant reef formers through geologic history.

Although corals live throughout the world, even in polar and temperate regions, only tropical (hermatypic) corals form reefs. Most hermatypic corals have symbiotic unicellular organisms called zooxanthellae living in their tissues. Zooxanthellae are vital to coral reef health, providing up to 90 percent of a coral's energy requirements by photosynthesis. Reef distribution is also limited by temperature, water depth, light, salinity, sedimentation, and emergence into air.

Coral reefs are limited to shallow waters, typically less than 25 meters deep, which is why they are restricted to island and continent margins (with the exception of atolls). This limitation dervies from the symbiotic zooxanthellae, which require light for photosynthesis. Below 25 meters, there is insufficient sunlight to support photosynthesis (Spalding, 2001). When the photosynthesis rate is reduced, corals produce calcium carbonate at a slower rate and are unable to form reefs. Hermatypic corals also require water with salinity close to normal seawater, or, in some cases, higher. Excessive sediment from runoff can smother corals and make it difficult for zooxanthellae to photosynthesize. Coral reefs thrive in areas with strong wave action, but are limited by low tide levels, as exposure to air can kill them relatively rapidly.

Reefs are typically divided into three categories: atolls, barrier reefs, and fringing reefs. Atolls are ring-shaped reefs that rise from very deep offshore waters and enclose a lagoon. They are found primarily in the Indo-Pacific area. Barrier and fringing reefs occur near a landmass, with a barrier reef being separated from land by a greater distance and deeper water. These two categories grade into each other and are sometimes considered one. Although hard corals are the dominant organisms of reefs today, they are one of the most diverse, species-rich marine ecosystems as a result of efficient nutrient recycling. Some of the other important groups of organisms found in reefs are gorgonians or sea fans (order Gorgonacea), soft corals (order Alcyonacea), the fire coral Millepora (order Hydrocorallina), coralline algae such as Halimeda, mollusks, echinoderms, crustaceans, polychaete worms, sponges, and fish. Coral reefs may have hundreds or thousands of fish species, making them one of the richest environments for fish, which in turn contribute to the biological structure of the reefs. Finally, bacteria play a key role in decomposition and recycling of organic matter, as well as the biological productivity of reefs. Tropical marine waters are nutrient poor, while coral reefs are nutrient rich and extremely biodiverse. They provide crucial environments for many marine species. Severe tropical storms, which may be increasing in frequency and severity due to global warming, are a major cause of reef mortality. Although hard corals are extremely sturdy and stand up to strong wave action, hurricanes and typhoons can destroy large areas of reef by uprooting coral colonies and carrying them off the reef. Storm deposits of broken coral are common in Caribbean fossil reefs. The El Nino weather phenomenon, which sends an influx of nutrient-poor warm water into the normally cold, nutrient-rich surface waters off the coast of Ecuador and Peru, is also a periodic cause of catastrophic reef mortality. These changes in ocean patterns and climate can raise surface water temperatures for long periods of time, resulting in increased coral mortality rates, ranging from 50-98 percent overall.

The most puzzling cause of coral die-off is coral bleaching, which occurs when corals expel their zooxanthellae. If zooxanthellae do not recolonize quickly, the corals can die. Coral bleaching has been reported since the 1980's, sometimes associated with El Nino events. The causes are unclear, but it is likely associated with stress, particularly in warm water. Coral bleaching events have become more common, which some attribute to global warming. Reefs develop best in areas where the mean annual temperatures are between 23œ and 25œ Celsius, and only some can tolerate temperatures above 36œ to 40œ Celsius (Tackett, 2002).

Coral reefs weakened by bleaching and storms are less resistant to diseases such as black band and white band disease, the latter of which has decimated Caribbean corals on a regional scale. Reefs can take over a hundred years to recover from some types of catastrophic mortality events. Coral reefs are in increasing danger from the effects of global warming, from warmer waters to increasingly severe storms. Throughout history, reefs have completely disappeared for millions of years following mass extinction events, and there is evidence that reefs may soon disappear for the remainder of human history. Reefs act as an indicator of global warming and are crucial to marine biodiversity. It is also important to protect the health of coral reefs because they provide an important habitat for many food species, as well as protective barriers against storms for tropical islands.

References

1. Spalding, Mark D., Edmund P. Green, and Corinna Ravilious. World Atlas of Coral Reefs. Berkeley: University of California Press, 2001.

2. Tackett, Denise Neilsen, and Larry Tackett. Reef Life: Natural History and Behaviors of Marine Fishes and Invertebrates. Neptune, N.J.: TFH, 2002.

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