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Coral reefs 3

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Coral reefs 3

Coral reefs are one of the oldest types of living systems on earth, and certainly one of the most spectacular. They are massive underwater structures formed by the limestone skeletons of tiny invertebrate animals. Reefs house a greater diversity of body forms, chemistry, and animal phyla (thirty-two compared to the eight that inhabit the most biodiverse ecosystems on land). Phyla comprise the second largest category of living things, after kingdoms.

Coral animals begin life as free-floating larvae, but settle on the sea floor in sedentary colonies. The term "coral" applies both to these animals and to their skeletons, particularly the skeletons of stone-like corals.

Many different organisms, including molluscs, sponges, and worms, help shape reefs, but hard corals and various algae are the major architects. In effect, the corals build limestone, because their skeletons are made of Calcium Carbonate. The skeletons deposited by these corals and other organisms accumulate, along with sand and other debris, to form the backbone of the reef. Over tens of thousands of years, chemical and mechanical changes turn the reef into true rock.

The body of a coral animal consists of a polyp, which is the living portion of the coral. A polyp is a hollow, cylindrical structure attached at one end to a surface, the other end is a mouth surrounded by tentacles which gather food and can sting prey to paralyse it. Polyps live in colonies, which grow from 1 to 7 inches, depending on the species. Coral polyps are classified as animals. Microscopic algae live within the animal tissues in a symbiotic relationship. The algae turn sunlight into carbon and sugars, which are then available to the polyp. In turn the polyp filters particles out of the water and excretes waste (nitrogen and phosphorus) that becomes available to the symbiotic algae. It's this very tight nutrient recycling within the coral itself that allows these corals to live in very low-nutrient waters.

There are three kinds of reefs: atolls, barrier reefs, and fringing reefs. Atolls are formed out in the middle of the ocean by volcanic subsidence, while fringing and barrier reefs form near continents. Florida contains both of these kinds of reefs, not as far offshore as Australia's Great Barrier Reef, but not terribly close either. Most are eight to eleven kilometres (five to seven miles) offshore. All three kinds of reefs can have associations, called patch reefs, which are small, shallow-water clusters or offshoots. In Florida, patch reefs can be as close as one hundred meters to the shore. Ancient limestone reefs have occupied the Florida peninsula intermittently over the past 150 million years. Florida's present coral reefs came into existence 5,000 to 7,000 years ago, when sea levels rose following the Wisconsin Ice Age. The reefs in the Florida Keys are part of the third largest barrier reef system (360 square kilometres from Miami to the Dry Tortugas).

Coral reefs are continuously being both built up and decomposed, so different parts of a reef are in varying stages of succession. Coral reefs are very fragile, because reef-building organisms cannot thrive if the surrounding water changes significantly. Coral reefs require very specific conditions in order to grow: a solid structure for the base; warm and consistent water temperatures (averaging between 20 and 30 degrees Celsius); stable salinity; moderate wave action; and clear water that is low in nutrients and plankton. The water on a healthy coral reef is clear because there are very few nutrients, so plankton that would cloud the water are few. In general corals grow slowly, but they are extremely efficient at living and reproducing in these conditions

Reefs matter in many ways:

' Links to other coastal ecosystems: such as mangroves and sea grasses.

' Sources of medicine: Because corals and most other reef-dwelling species move either very little or not at all, they rely on biochemical warfare for both offence and defence. They have developed strong and very diverse chemical compounds, and a number are proving to have significant anticancer, antibacterial, antiviral, and anti-inflammatory properties. For example, cytarabine, derived from the Caribbean sponge Tethya crypta, induces remission in certain forms of leukaemia and is also useful against the herpes virus; pseudopterosins from the Caribbean sea-whip Pseudopterogorgia elisbethae have powerful anti-inflammatory and analgesic properties; and prostaglandins, which have a wide range of clinical applications, can be found in large quantities in the Caribbean sea fan Plexaura homomalla. About six thousand unique chemical compounds have been isolated from reef organisms so far, and this potential pharmacopoeia has barely been tapped. And coral, which is porous and quickly reabsorbed, is used to repair human bone, with no risk of implant rejection or transmission of infection.

' Living breakwaters: Reefs protect coastal areas from storms, floods, and erosion. They're key to many a surfer's "perfect wave," and contribute sand to the growth of beaches.

' Evaporation basins: Reef flats and lagoons may play a key role in regulation of the sea's salt content, removing salt by acting as evaporation basins.

' Shapers of landmass: Reefs play a part in the formation of tropical islands through deposition and accumulation of Calcium Carbonate rock (limestone) and sand.

' Mediators of global climate: Corals remove large amounts of carbon from the atmosphere as they grow, actually "fixing" 700 billion kilograms a year. Carbon dioxide is a greenhouse gas that contributes to global warming.

Coral Reefs are also very important to the Florida economy:

' Recreation and tourism: tourists spend about $1.2 billion annually in the Florida Keys.

' Food fisheries: the value of reef fisheries (including shellfish and other invertebrates) off the Keys is estimated at $48.4 million.

' Other industries: reefs support a large trade in aquarium fish and are a habitat for sport fish.

Most present-day reefs have probably been growing for 5,000 to10, 000 years. But their continued survival is now threatened. Reefs around the world are now declining at an unprecedented rate--one that far outstrips our understanding of the problem. Few long-term studies of coral reefs have been conducted, and there is considerable debate about the overall health of reef ecosystems. Still, most scientists agree that reefs worldwide are in crisis.

Caribbean reefs appear to be in worse condition than Pacific reefs. They naturally have lower levels of biodiversity, which makes them more vulnerable to structural change. They house significantly fewer species of fast-growing and reef-building corals, and diseases have affected the entire basin while the far greater size of the Pacific has tended to keep outbreaks reef-specific or regional. The third largest barrier reef system in the world is located off the Florida Keys. Its recent decline has been attributed to multiple causes, almost all of which involve human activity. Most scientists agree that the greatest threat to Florida's reefs is degraded water quality. When land is cleared for development or agriculture, fertilisers, pesticides, and eroded soil wash out to sea when it rains. Pesticides can weaken the corals and make them more susceptible to disease. Sediments can smother or scour the reef, impairing coral growth. The sea is also where most of the sewage and wastewater from Florida Key residents and their one million annual visitors ultimately ends up, and this pollution also degrades the water quality. In particular, fertilisers and sewage have significantly increased the level of nutrients in the water of Florida Bay. As a result, phytoplankton utilise these nutrients and grow exponentially. This causes various kinds of algal blooms: phytoplankton turning the water green, toxic blooms including red tides, and macro-algae overgrowing and smothering the reef. As phytoplankton increases, so does the turbidity of the water. This cuts down the amount of light reaching the zooxanthellae (tiny one-celled algae that live inside coral polyps), so photosynthesis within the coral is adversely affected.

Poor water quality affects other parts of the coastal ecosystem. Coastal ecosystems act as buffers between land and sea, reducing negative impacts in both directions. When stressed, they are less effective. There are three key environments in Florida that are intimately related. First of all, the mangroves along the shore, secondly, the grass beds in shallow water, and finally, the coral reefs at the edge of the shelf. Water flows through this system, and the health of each system determines the health of the next. Every time somebody cuts down mangroves, it affects the sea grass beds somewhat further off shore. Every time a sea grass bed is destroyed, it affects the coral reef even further off shore. So all these interconnected habitats need to be preserved as a whole. Increased nutrients in the water were blamed for a major sea grass die-off in the bay in 1987, further stressing the reefs.

Silt normally held down by the sea grass flowed out of the bay and ended up on the coral reefs, clouding the water. This sediment hurts the reef in several ways: it impairs photosynthesis; it forces corals to expend energy cleansing themselves; and it can even bury them entirely. Studies showed corals only 4 kilometres from each other grew at dramatically different rates: the corals closer to Florida Bay grew only half as much as offshore corals.

The location of the Florida Keys makes them particularly vulnerable. They are close to the heavily populated North American coast, and ocean currents place them downstream of the Caribbean basin. The Loop Current, which travels clockwise in the Gulf of Mexico from the Yucatan Peninsula, to the Mississippi River, by Tampa Bay, and ultimately to the Florida Keys, carries stormwater and agricultural runoff containing pesticides, heavy metals, oil, and other toxic waste from more than half of the United States. Even sediment from the Amazon finds its way through the Florida straits. Compounding the problem is the fact that most of the bedrock underlying the islands of the Florida Keys is highly porous limestone, the remains of ancient reefs, through which contaminated waters easily flow in and out. Preliminary studies indicate that ground waters beneath the reefs do contain nutrients, principally ammonia, at levels many times higher than that of normal seawater. These nutrient-rich ground waters can seep ...

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