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smother the coral because the coral is attached and cannot move upward to get above the surface of the sediment. As a result of these processes, coral reefs are not found near the mouths of rivers that release huge volumes of suspended fine sediment into coastal waters. In Papua reefs are found around small islands, and along the western mainland coast. However there are no reefs in the eastern part of the province, where the landmass is large and larger rivers such as the Mamberamo, Pulau, and Digul empty into the ocean. Because the island of New Guinea is a geologically young, very high island in an area of very high rainfall (over 3,000 mm/yr in Papua; Tomascik et al. 1997), runoff of fresh water and sediment is very high. If the temptation to reap large quick profits by cutting the rainforests of Papua is not resisted, the amount of sediment runoff will grow many-fold, and coral reefs will be rapidly killed.
Reef building corals need light to live and grow. Suspended sediment in the water scatters and absorbs light, reducing its availability to coral. Although there are limits to how much sedimentation corals can tolerate, some are able to thrive in areas with moderate levels of sedimentation. A coral reef named Middle Reef about a kilometer offshore of Townsville, Queensland, Australia, survives with high coral cover in water that has a visibility of only about one to two meters. The coral species on this reef are quite different from those found on the nearby outer edge of the Great Barrier Reef, in clear oceanic waters. Further, sediment input to the nearshore waters of Queensland increased dramatically when sheep and cattle were introduced to the area about 100 years ago (McCulloch et al. 2003). This increased sedimentation stresses the surviving corals and renders them less resilient, perhaps to the point of being unable to recover from other types of disturbance.
TEMPERATURE
Coral reefs are restricted to warm waters where the minimum temperature is above about 18 C. The world’s most northern coral reefs are at Kure Atoll in the northwest Hawai’ian Islands, and in Japan, and the most southern are at Lord Howe Island, off southeastern Australia. Some coral communities can be found at even higher latitudes, but they do not accumulate calcium carbonate, and therefore do not form coral reefs. An example is in the Solitary Islands off New South Wales, Australia, where occasionally storms with waves up to ten meters tall sweep most corals off into deep water. Newly settled coral recruits then grow on the non-carbonate rocks, rebuilding the coral community, but their skeletons do not accumulate (Harriott, Smith, and Harrison 1994). Such coral communities can also be found in the tropics near the equator in marginal environments, such as sandy, high sediment, or low circulation areas. In addition, at the extremes of latitude, coral reefs tend to be small, and to have low diversity (Yamano et al. 2001, discussed below). Papua is situated in an area close to the equator, where temperatures are warm year-round (27.5–28.5 C in the Java Sea; Tomascik et al.1997) and nearly ideal for coral reefs.
Most coral reefs are found in warm, clear tropical water. The water is clear because it contains few of the tiny drifting plants and animals, which together are known as plankton. Temperate and polar waters are often opaque with a green or brown color, due to masses of plankton. The tiny drifting plants are called phytoplankton, and they require nutrients such as nitrogen and phosphorous, just as do other plants. In the tropics, the hot sun heats only the surface water but does not penetrate deeper water. Hot water rises above cold water because it expands slightly when heated. The boundary between the warm surface water and colder deep water begins at a depth of about 50 m in Indonesia, and extends down to about 300 m depth (Tomascik et al. 1997). The fact that warm surface waters float on top of the cold deeper water means that these two bodies of water do not mix. Phytoplankton absorb nutrients from the surface water as they perform photosynthesis and grow. The phytoplankton are fed on by zooplankton (tiny drifting animals), which are in turn fed on by larger animals in a food chain or food web. When any of these organisms die, they sink slowly down into the deep cold water and to the bottom, taking nutrients with them. While currents called upwelling bring nutrients from the bottom back up to the surface fueling blooms of plankton in temperate and polar waters, upwelling is rare in tropical waters. The result is low nutrient levels in warm tropical surface waters, low densities of plankton, and hence clear water.
ZOOXANTHELLAE
The low nutrients in warm, clear, shallow tropical waters pose a paradox for coral reefs. Coral reefs have abundant organisms, and high rates of photosynthesis and growth. How can this occur in waters that are low in nutrients? How can this oasis flourish in such a biological desert? A variety of mechanisms probably contribute, but perhaps the most important is the algae living in corals. Corals are animals related to sea anemones and jellyfish. Corals have small polyps that are nearly identical to sea anemones. However, they contain within them the seeds of their success. These are the tiny single-celled algae known as zooxanthellae. The algae are members of a group called dinoflagellates. The algae and the coral animals live in a mutualistic symbiosis (i.e., mutually beneficial coexistence). The waste products of the animal contain the nitrogen and phosphorus that the algae need. The algae perform photosynthesis in the sunlight, and leak much of what they produce into the surrounding animal cells (around 80%). So the algae benefit from the nutrients the coral animal produces, and the coral benefits from the food that the algae produce. In effect, this is a tight recycling arrangement, with nutrients passed from one partner to the other, and then back to the other partner. As a result, the combination of these two partners needs a smaller input of nutrients from the outside, and can survive in nutrient-poor, warm, clear, tropical waters.
Plants on Coral Reefs
On most coral reefs, animals are obvious and appear to be common, while plants are less obvious and appear to be less common. But only plants can produce food through photosynthesis. When animals eat plants, most of the food is used to produce energy to run the animal’s bodily functions, while only a small part is added to the material of the growing animal. As a rule of thumb, only about 10% of what an animal eats is incorporated into its body in growth. As a result, there must be about ten times as much biomass of plants as herbivores that eat them. And there must be about ten times as much herbivore mass as carnivore mass that eats them, and so on up the food chain. Thus there must be about 100 times as much plant biomass as carnivore biomass for carnivores that eat herbivores; 1,000 times as much to support carnivores that eat other carnivores. And yet on coral reefs animals are obvious and appear abundant, while plants are usually less obvious and appear less abundant. How can this be? First, some plants are hidden. Zooxanthellae are found in hard corals, soft corals, giant clams, and a few other animals on coral reefs. They provide a large part of the food production on a coral reef, and yet are not obvious. They actually provide much of the color in corals and giant clams, yet we normally don’t recognize them as plants. Second, the algae which can be seen on coral reefs include some species which are small and hard to see but are highly productive. Large fleshy algae grow slowly and put most of their growth into defenses such as woody cellulose that is hard to digest, calcium, and chemicals that are bad tasting or toxic. Defenses are necessary for a plant to grow large on a coral reef, since there are many hungry mouths of herbivores, such as fish, sea urchins, and snails. Fish alone bite algae about 40,000 to 156,000 times per square meter of reef per day! So herbivory is intense on coral reefs. A second group of algae is the filamentous algae. These algae are made of tiny strings of cells with little or no defense. Their main defense is their ability to grow very rapidly. Herbivores bite most of their growth off daily or hourly, but the base of each filament attached to rock rapidly grows the string back. So filamentous algae are highly productive fast growing algae, but have a very low standing biomass and are hard to see.
Species Diversity
Coral reefs are not only geological structures, but also biological communities. Coral reefs are amazingly diverse and complex ecosystems. They are the most diverse marine ecosystem known (i.e., they are the most species-rich). Sometimes they are said to be the most complex ecosystem on the planet, but they actually have fewer species than tropical rainforests. Rainforests have large numbers of insect species, and insects are by far the most species-rich group of organisms on the planet. There are more insect species known than all other organisms combined, and there are more insects in tropical rainforests than anywhere else on earth. There are very few marine insects, and none known on coral reefs. The total number of species is not known for either coral reefs or rainforests. Around 1.8 million species have been described on earth, with a majority of those being insects. Estimates for the total number of species