CORAL REEF:DEFINITION . CONDITION AND TYPE

 Coral reefs are one of the most spectacular and valuable ecosystems on the planet — and one of the most threatened. Climate change, coastal development and tourism, destructive fishing, and other human activities are endangering their very existence. At the current rate of destruction, more than one-third of the world’s coral reefs will be destroyed within our lifetime.

Coral reefs provide an important ecosystem for marine life, offering food and shelter among their crevices and branches for animals including fishes, molluscs, sea urchins and sponges.

Corals are found in all of Earth's oceans, from tropical to freezing temperatures, however they only build coral reefs in warm, shallow seas in the tropics. Among the biggest and best-known are the reef systems of the Great Barrier Reef of Australia, which is around 2,300 kilometres long. The most biologically diverse reefs in the world can be found in a region known as the Coral Triangle in Southeast Asia.

What Are Coral Reefs? 

Coral reefs are massive limestone structures that provide food and shelter for marine life. Hard corals are responsible for much of the solid, limestone (calcium carbonate) framework of the reef. Built over hundreds, if not thousands of years, some coral reefs are so large they can be seen from outer space. In and around the limestone framework of coral reefs live a complex array of plants and animals. Some of them — like calcareous algae (algae with limestone in its tissues) — help to build the reef up. Others — such as worms, grazing fish, urchins and boring sponges — break it down. Many other animals, including more than 4,000 different species of fish, find shelter on the reef and make it their home. This multitude of life is what we know as the coral reef. 

Environmental Conditions Necessary for a Healthy Coral Reef

 Corals are very sensitive organisms and environmental conditions need to be just right Corals need:..

 Abundant Sunlight 

Reef-building corals cannot survive without sunlight, since zooxanthellae, their symbiotic algae, require sunlight for photosynthesis. The majority of zooxanthellae reef-building corals do not grow below 45m (150ft).

Warm Temperatures Corals can only live within a narrow temperature range from around 16°C - 29°C (62°F - 85°F). This explains why corals thrive in the warmer waters of the tropics. 

 Low Nutrient Levels Corals are adapted to live in ocean water, which contains very low levels of nutrients. Nutrients, which are needed by all living organisms, are found in food and dissolved in water. Too many nutrients can upset the natural balance of life on the reef, creating conditions that favor other fast growing organisms such as marine plants and sponges. 

Clear Sediment-Free Water Corals prefer clear water with low levels of sediments (small particles of earth, rock and sand). Sediments can bury corals, blocking out needed sunlight and killing them. Corals tend to live in areas with some wave action as this helps supply them with food and oxygen as well as keeping corals free of sediment.

 Salty Water Corals are marine animals adapted to live in seawater with a salinity of around 35 parts per thousand. In areas that are too salty, or not salty enough, corals cannot survive. 

Hard Substrate Corals need a hard substrate or surface to attach to and cannot successfully colonize loose substrates such as rubble or sand. If the substrate is unstable, young coral colonies will be crushed and killed by wave action and storms. 

How Old Are Coral Reefs?

 Coral reefs are one of the oldest ecosystems on earth. 

• The ancestors of modern coral reef ecosystems were formed around 570 million years ago. During this time, blue green algae and later sponges (not corals), were the major reef builders. 

• Modern coral reefs as we know today, have existed for a staggering 240 million years.

 • The tropical seas were divided from 24 to 5 million years ago and coral reefs were distributed into the regions that we know of today.

 • Most existing coral reefs are between 5,000 and 10,000 years old. 

Over geological time there have been several mass extinctions associated with coral reefs. Reefs have survived changing sea levels, uplifting of landmasses, periods of widespread warming and repeated ice ages as well as recurrent short-term natural disasters such as cyclones and hurricanes. Over geological time coral reefs have shown a remarkable ability to adapt and survive. 

Where Are Coral Reefs Found?

Worldwide, coral reefs cover an estimated 284,300 square kilometers. This area represents less than 0.1 percent of the world’s oceans and less than 1.2 percent of the continental shelf area. Coral reefs are found all around the world within the tropics (between 30 degrees north and 30 degrees south of the equator). Some are also found farther from the equator in places where warm currents flow out of the tropics, such as Florida and southern Japan. 

Coral reefs can be found in 3 broad regions: 

• Caribbean and Atlantic

 • Indian Ocean and Red Sea

 • Pacific and Southeast Asia

 Most coral reefs are found in the Indo-Pacific, an area that stretches from the Red Sea to the Central Pacific. This is also the area of highest species diversity among reefs overall. Less than eight percent of the world’s coral reefs are found in the Caribbean and Atlantic, and species diversity is much lower in these regions.

Distribution of coral reefs: • 284,300 square kilometers of coral reefs worldwide. • Coral reefs live between 30°N and 30°S of the equator. • Most coral reefs are found in the Indo- Pacific.

Indonesia has more coral reefs and greater species diversity than any other country in the world.

Types of Coral Reefs

Fringing reefs lie around islands and continents, and are separated from the shore by narrow, shallow lagoons. They usually parallel the coastline and at their shallowest point can reach the water’s surface. 

Barrier reefs also grow parallel to the coastline, but are separated by deep, wide lagoons. At their shallowest point, they can reach the water’s surface, forming a “barrier” to navigation. The Great Barrier Reef in Australia is the most famous example, and is the largest barrier reef in the world. Atolls are rings of coral that create protected lagoons and are often located in the middle of the sea. 

Atolls usually form when islands surrounded by fringing reefs sink into the sea or the sea level rises around them (they are often the tops of underwater volcanoes). The fringing reefs continue to grow and eventually form circles with lagoons inside.