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Geowissenschaften

University of London

10/10, Mr Jones, 2010

Malte L. ©
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Coral reefs

Coral reefs are a unique marine ecosystem. They are built up entirely of living organisms. Reefs are huge deposits of calcium carbonate made up mainly of corals. It is mainly controlled by four factors:

● temperature – coral growth needs a minimum water temperature of 18°C. They grow best between 23°C and 25°C

● light – is needed for the coral to grow; because of this need, corals grow only in shallow water

● water depth – because of the need for light, most reefs grow where the sea is less than 25 metres deep

● salinity – since corals are marine creatures, they can only survive in salt water.

At a local level, there are other factors affecting where coral reefs develop:

● wave action – corals need well oxygenated salt water; this occurs in areas of strong wave action


Coral reefs are often called the rainforests of the sea, both due to the vast amount of species they harbour, and to the high productivity they yield. Aside from the hundreds of species of coral, reefs support extraordinary biodiversity and are home to a multitude of different types of fish, invertebrates and sea mammals.

Covering less than one percent of the ocean floor, reefs support an estimated twenty-five percent of all marine life, with over 4,000 species of fish alone. Reefs provide spawning, nursery, refuge and feeding areas for a large variety of organisms, including sponges, cnidarians, worms, crustaceans (including shrimp, spiny lobsters and crabs), molluscs (including cephalopods), echinoderms (including starfish, sea urchins and sea cucumbers), sea squirts, sea turtles and sea snakes.


Reef structures play an important role as natural breakwaters, which minimize wave impacts from storms such as cyclones, hurricanes or typhoons.


Also, their beauty makes coral reefs a powerful attraction for tourism, and well managed tourism provides a sustainable means of earning foreign currency and employment for people around the world, even in remote areas of developing countries.


Several attempts have been made to estimate the value of coral reefs in terms of dollars. Benefits from coral reefs can be categorized into 2 types: "direct use values" (fisheries and tourism industry), and "indirect use values" (benefit derived from coastline protection).

According to a United Nations estimate, the total economic value of coral reefs range from US$ 100,000 to 600,000 per square kilometre per year (Source: UNEP-WCMC, 2006)


The value of coral reefs lies in:

● their biodiversity – within the Great Barrier Reef there are 700 species of coral; 1500 species of fish and 4000 species of mollusc

● the protection they give to low-lying coasts from the impact of tropical storms

● their rich fish stocks – they supply the basic food requirements of many LICs

● their appeal to tourists and the recreational opportunities they offer such as snorkelling and scuba diving. Over 150 million people each year take holidays in areas with coral reefs.

Coral reefs are easily stressed by human actions. Any contact with the human body is likely to kill the coral immediately around the point of contact. Reefs are also threatened by pollution, over fishing and the quarrying of coral for building stone.

If the stress persists, the death of the reef soon follows. Figure 2.25 above shows the sequence of coral reef decline that follows from the development of the coastal area nearby. The development involves the spread of urban areas, the growth of a fishing industry, the coming of commercial farming and the rise of tourism.

A recent survey of the world’s coral reefs showed that 27% of them were highly threatened by human activities. Another 31% were classified as being under ‘medium threat’.


In summary, healthy coral reefs provide:


Habitat: Home to over 1 million diverse aquatic species, including thousands of fish species

Income: Billions of dollars and millions of jobs in over 100 countries around the world

Food: For people living near coral reefs, especially on small islands

Protection: A natural barrier protecting coastal cities, communities and beaches

Medicine: The potential for treatments for many of the world's most prevalent and dangerous illnesses and diseases.


Mangroves

Mangroves are most common in South-East Asia (Figure 2.19). It is thought that they originated here and subsequently spread around the globe. Today, most mangroves are found within 30 degrees latitude of the Equator, but a few hardy types have adapted to temperate climates. They reach as far as the North Island of New Zealand.

This quickly alters the salt levels, as well as temperatures. Mangroves are not only able to survive these changing water conditions, they can cope with great heat and choking mud.


It is these roots that trap mud and sand, and eventually build up the intertidal zone into land. At the same time, the mangrove is colonising new intertidal areas. The fruits and seedlings of all mangrove plants can float. As they drift in the tide away from the parent trees, they become lodged in mud where they begin to grow.

So a new area of mangrove takes root.


Mangroves are valuable nurseries for fish and crustaceans, and are rich in wildlife. Mangrove roots, which are exposed at low tide, trap silt and help to create new land. Mangrove timber provides fuel and building material. However, perhaps the greatest value of mangroves in this age of rising sea levels is the protection from storm surges they give to low-lying coastal areas.


It is widely believed that mangrove swamps are disease-ridden. For this reason, they are being cleared at a fast rate. Just over half of the mangrove swamps are being cleared to make way for aquaculture – the farming of fish and shrimps. A quarter is being cleared (deforestation) to provide timber for fuel and building purposes.

The land reclamation is being undertaken to provide sites for the building of tourist hotels and other amenities. The diversion of freshwater is intended to meet the needs of tourists and expanding agriculture. The farming requires the application of herbicides to prepare cleared areas for cultivation.

Salt marshes

Salt marshes occupy a midway location between mud flats that are permanently submerged by water and terrestrial (land) vegetation lying above the high-tide mark. Like mangroves, they are an ecosystem of the intertidal zone.

Salinity (how salty the water is), and the frequency and extent of flooding of the marsh determine the types of plants and animals found there. In some cases, the low marsh zone floods twice daily while the high marsh floods only during storms and unusually high tides.

These different environmental conditions result in differences in the types of plants and animals found in different parts of the same marsh area.

Salt marshes are criss-crossed by meandering creeks, which allow tidal water to drain in and out. The creeks slow down tidal energy and the marsh plants slow down wave energy. As a result, there is an almost continuous deposition of silt and mud. Over time, this means that the salt marsh gradually extends seawards.

On the face of it, areas of salt marshes appear to be of little obvious value. However, the reality is that many salt marshes are among the most used and therefore the most threatened ecosystems in today’s world. Specific threats include:

  • Industrial pollution – particularly of water, as many marshes occur in estuaries which are favoured as sites for ports, power stations and oil refineries

  • Agricultural pollution – heavy applications of fertilisers and pesticides on adjacent farmland lead to eutrophication (an increase in the concentration of chemical fertilizers in an ecosystem) of marshland waters

  • Pressure from developments such as marinas and other recreational facilities.

  • Salt marshes are also threatened by changes associated with global warming such as more storms and higher water levels.



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