What Are They?
Coral reefs are structures found underwater,
constructed of calcium carbonate secreted by limestone
objects known widely as corals. Most coral reefs are
composed from stony corals, which consists of tiny
animals belonging to the family Cnidaria called polyps.
Polyps excrete a hard exoskeleton to protect their
bodies. They are one the most diverse ecosystems in
the world and the vibrant colours of coral reefs
astound people internationally.
An oblique photograph of a
Blue Linckia Starﬁsh
Three Main Types
Of all coral reefs in the world, only three types
distinguish itself from the others. These include...
A barrier reef is a coral reef
parallel to the shore but is
separated by a channel of water.
An atoll is a ring-shaped coral
reef, consisting of a coral rim
that encircles a lagoon.
A fringing reef is a reef that
forms around a land mass.
Apart from an atoll, fringing reef and a barrier reef, other coral reefs remain within our oceans today including...
Patch Reef - a common, isolated reef, similar to an atoll with grass or plants growing around it.
Apron Reef - similar to a fringing reef but shorter and steeper, beginning at a point or peninsula.
Bank Reef - linear or semicircular shaped-outline larger than a patch reef.
Ribbon Reef - long, narrow, similar to a ribbon featuring a lagoon.
Table Reef - an isolated reef with all the characteristics of an atoll excluding the lagoon.
An oblique photograph of a
school of vibrant orange ﬁsh
in a ribbon reef.
Coral reefs are estimated to cover 284,300 square kilometres, just under 0.1% of the oceans’ surface area which is
about half the size of France but still support over 25% of the world’s marine population. The majority of the location of
coral reefs are in the Indo-Paciﬁc region (consisting of the Red Sea, Indian Ocean and the Paciﬁc Ocean), covering
91.9% of all coral reefs, followed by Australia with 40.8% and Southeast Asia with 32.3%.
Although coral reefs exist within both temperate and tropical waters, shallow water reefs form only within 30 degrees
north and south of the Equator. Coral reefs are rare along the western African and American coasts. This is primarily
due to upwelling, which is the process when the current is pushed back by the wind, and cold currents that reduce the
temperature required for reefs to grow. Coral reefs are seldom found in South Asia and South America for similar
Coral reefs are varied around the world and most well-renowned reefs are excellent tourist destinations including...
Great Barrier Reef
The world’s largest reef, comprising 2,900
individual reefs and 900 islands stretching for
Mesoamerican Barrier Reef
The world’s second largest reef stretching for
1,000 km, connected by various islands.
The deepest photosynthetic reef, found
just off the coast of Florida.
ABOVE: This map shows the location of coral reefs around the world. BELOW: This map depicts the areas in which coral reefs grow in shallow water.
The majority of the coral reefs found today were formed prior to the last glacial period when melting ice caused the sea level to increase and ﬂood the
continental shelves which are the extended perimeters of each continent and associated coastal plain. This proved that modern coral reefs are less than 10,000
years old. As ‘communities’ were established on the shelves, the coral reefs rose against the rising sea level. Coral reefs that were too slow became ‘drowned
reefs’ as the water surrounding them provided lack of essential light. Coral reefs are formed in the deep sea away from these shelves, often around oceanic
islands or as atolls (ring-shaped coral reef, consisting of a coral rim that encircles a lagoon). The majority of the islands were previously volcanic and others
are formed by tectonic origins with the movement of the plates causing the ocean ﬂoor to rise above surface.
The ﬁrst man to endeavour to interpret how coral reefs became fringing, barrier and as atolls was Charles Darwin. In 1842, Charles Darwin set out his theory
of the formation of coral reefs, in his monograph ‘The Structure and Distribution of Coral Reefs’. Darwin theorised that it was due to uplift and subsidence of
the Earth’s crust underwater that was forming coral reefs. Darwin’s theory is as follows...
begins with an
extinct volcano or
As the island and
growth forms a
including a shallow
the land and the
As the subsidence
becomes a larger
between itself and
the island is
becomes an atoll
According to Charles Darwin, the bottom of the lagoon should be bed rock, remnants of the
volcano. A team of scientiﬁc drillers proved this correct and the rest of Darwin’s theory. Darwin’s
theory succeeded his work on coral reefs, proving that the polyps of coral reefs thrive in the clean
seas of the tropics, where the water is agitated but not at a substantial depth. He discovered that
coral reefs must start of formation in a shallow condition to survive, hence the theory. When the
bottom of the sea is rising , fringing reefs grow around the coast but if they leave the water they die
and become white limestone. To stay alive, the coral reef rises with the sea level, forming a lagoon
within. The encircling of a lagoon can be broken by storms or extremely strong winds. Similar to a
rising sea level, a subsiding bottom can also overwhelm the reef, killing the polyps and causing the
to drown. Coral reefs that rely on algae, such as zooxanthellae, to survive are vulnerable to
Formation of coral reefs vary between fringing reefs, that uses nutrients to stay with the sea
elevation; barrier reefs, that form when a subsiding island is still not submerged, separating the reef
and the land mass; and an atoll, which forms when a subsiding island is submerged.
This line drawing summaries the formation
of a fringing reef, barrier reef and atoll.
A coral reef ecosystem has different zones with distinct areas for numerous habitats. There
are three predominant zones consisting of a fore reef, reef crest and back reef (frequently
referred to as a reef lagoon). These components interconnect both physically and
ecologically. Reef life and oceanic processes assist in exchanging of nutrients, seawater,
sediments and marine life between themselves. Therefore, the three zones provide an
essential role to the nurturing of the coral reef ecosystem and the abundant marine
Although the fore reef, reef crest and back reef are the major zones, other zones
feature in various coral reefs over the world. These consist of...
Reef Surface - the shallowest part of a coral reef, making
itself exposed to sunlight for photosynthesis and digestion
of zooxanthellae and plankton . It is subject to shoal due
to the incoming waves (diagram on right).
Off-Reef Floor - the shallow sea ﬂoor of a coral reef that
is often found on a continental shelf. It is a home to
plants, sea grass and ﬁsh.
Reef Drop-Off -similar to a cliff but home to many marine
life and plankton.
Reef Face -found between the reef ﬂoor and reef drop
off, this zone is the most diverse, providing hiding places
such as crevices and is a dominant ground for algae and
Reef Flat - a sandy ﬂoor, where chunks of coral are
found. Possibly bordering a lagoon, it is a preferred
place of habitat for ﬁsh.
Reef Lagoon - an entirely enclosed section, festooned
with various coral reefs. This area is not affected by
This diagram signiﬁes the shoaling of water. Water in
the reef surface is often agitated and when it
encounters shallow water, it shoals,which is when the
wave height increases but the movement is slowed.
Geographical processes determine the lives of coral reefs. They are a combination of
human and physical forces that form and transform parts of the world.They shape the
way coral reefs grow and interact and plays an essential role is investigation of coral
reefs. The geographical processes consist of atmospheric, biotic, geomorphic and
Atmospheric Processes - physical or chemical processes operating simultaneously in complex and
interdependent ways. They cover predominantly on climate, location and conditions of the area.
Biotic Processes - any process associated with life and living things including bacteria.
Geomorphic Processes - processes corresponding with topography and appearance of the
landscape. They focus on erosion, weathering and mass movement.
Hydrologic Processes - processes that convene on water and marine life. It is the basis of
understanding the life of a landscape.
The atmospheric processes involved with coral reefs identiﬁes key questions about a coral reef. Why
do coral reefs grow where they are? What conditions do coral reefs require to thrive? The
atmospheric processes of coral reefs determine these factors and ultimately the survival of a coral
Coral reefs thrive in shallow, warm waters most commonly near land. Predominantly found in the
tropics, they require a temperature of 21-30 degrees Celsius. The majority of coral reefs is found in
waters up to 50m deep, as they do not rise into tide level to decrease the risk of exiting the water.
They are usually found in waters of latitude 30 degrees north and 30 degrees south as these are
ideal temperatures for growth. For coral reefs to survive they must also rely on saltwater and
zooxanthellae to assist their growth. Consequently, coral reefs become dependent and independent
biomes, creating a vast contrast of atmospheric processes, varying for each type.
This ground-level image shows
the ﬂourishing of coral reefs.
Biotic processes have a signiﬁcant inﬂuence on coral reefs. The vibrant and diverse ecosystem can be
distinguished by the large limestone structures known as coral. Coral provides a shelter, a hiding
place and as a reliable resource of food for the marine life residing in a coral reef. As a coral reef is
a securely interconnected ecosystem, damage to a part of a coral reef will consequently affect the
Although this biome is only found in 1% of the oceans internationally, coral reefs host 25 % of the marine
life population. The vast variety of marine animals and plants is a representation of the biodiversity of
coral reefs thriving within our oceans. Symbiotic relationships exist within this ecosystem, an example of a
mutually beneﬁcial interaction between objects of coral reefs is the clownﬁsh and the anemone or between
different zones of the coral reef.
The biodiversity of the
ecosystem is depicted using an
Geomorphic processes determine the physical shape and structure of coral reefs and also the
formation of this ecosystem. Formation of coral reefs consist of...
Fringing Reef - forms along a coastline, growing on a continental shelf in shallow water (1).
Barrier Reef - parallel to the shore, these reefs are separated from land by a channel of water.
Atolls - a ring-shaped reef enclosing a partially or entirely closed lagoon (3).
The topography of a coral reef is varies as contemplates
Joseph H. Connell, in his book ‘Diversity in Rainforests
& Coral Reefs’. ‘"topography of coral reefs is constantly
changing. Each reef is made up of irregular patches of
algae, sessile invertebrates, and bare rock and sand.
The size, shape and relative abundance of these
patches changes from year to year in response to the
various factors that favour one type of patch over
another. Growing coral, for example, produces constant
change in the ﬁne structure of reefs. On a larger scale,
tropical storms may knock out large sections of reef and
cause boulders on sandy areas to move."
Hydrologic processes are a prominent feature in coral reefs. Coral reefs are composed
of numerous individual creatures known as coral polyps. Coral polyps are the
foundation of the structure of coral reefs. The calcium carbonate gives the coral its
shape and due to the inaccurate quantity of calcium carbonate, the topography is
varied. The coral polyps feed on zooplankton, tiny animals present in the watery
presence of the hydrosphere. The range of colour of coral is due to the amount of
zooxanthellae, an algae produced in the polyps.
Coral reefs have an extremely productive biodiversity, hosting numerous plants and animals in
complex and varied habitats that sustain life for a wide variety of creatures. They are a home to an
abundant variety of living creatures such as algae, sponges, ﬁsh, invertebrates and other animals.
Algae - coral reefs are chronically threatened by algae and it’s consequences to the ecosystem. Inhabiting a large
percentage of area of surveyed locations, algae consists of turf algae, coralline algae and macro algae.
Sponges - sponges are an essential role in the ensuring the ecosystem maintains function. It ﬁlters the organic produce
secreted by corals and algae that are absorbed by corals in turn.
Fish - the reasons for the vast diversity of 4000 species of ﬁsh remain controversial. Around 35 tonnes of ﬁsh are found
every square kilometre of a coral reef unless damaged that will reduce the quantity.
Invertebrates - numerous invertebrates, such as sea urchins and sea slugs, play a pivotal part in preventing algae from
Other - although the main animals found in the proliﬁc biodiversity of a coral reef are eminent, the other animals such
as seabirds such as herons, monitor lizards and sea snakes are all another aspect of coral reefs that must be implied.
Here are some examples of diverse animals that inhabit coral reefs.
Interactions between human beings and coral reefs have been evident from the discovery of the diverse ecosystem. Organisations such as UNESCO
and Greenpeace assist and nurture coral reefs to ensure their growth is sufﬁcient. To prevent numerous hazards, they carefully monitor each coral
reef and observe any changes or improvements to the area. Some Aboriginal Dreamtime stories also use the presence of coral reefs to represent
exuberance and a vibrant perspective of our world. Although interactions between humans and coral reefs are at substantial levels, humans pose
threats that might cease the life of this ecosystem on the face of this planet. Some practices consist of...
Cyanide and Dynamite Fishing - coral reefs are in close proximity due to such activities that have depleted the population of coral reefs substantially,
overturning the ecosystem and gravely impacting life for marine life.
Pollution - human pollutants are causing a lot of harm to coral reefs. Fertilisers, herbicides and pesticides all runoff from farmland on the shore that
results in less clarity in the water. This leads to lack of sun exposure that may kill the coral reef.
Climate Change - the consequences of human-induced climate change are colossal. The impacts of increased ultraviolet radiation, anomalies in ocean
temperature and increased ocean acidiﬁcation result in tissue damage, exposure to disease, bleaching frequency and change of skeletal formation
including the secretion of calcium carbonate that will ultimately destroy coral reefs.
Sea Life Depletion - as over 25% of ﬁsh species reside in coral reefs, when marine life is caught, this not only affects coral reefs but the entire species
of that speciﬁc ﬁsh. This not only affects coral reefs by gradually decreasing its population but also humans as well, especially those that are
dependent on food for sustenance.
Diving - when divers touch coral reefs, they damage the delicate structure of the ecosystem. Photographs, residue left from boats such as petroleum
and waste material, collisions between humans and the ecosystems shape the entire coral reef.
Minimum - Marine.
Range - 15%
Rank - 27%
Average - 33.33%
Location of Coral Reefs
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Presence of Coral Reefs
A physical map is a interpretation of natural location, depicting where a feature is on the Earth’s surface. This physical
map displays the location of coral reefs, the red marks indicating the presence of the ecosystem. As observable, SouthEast Asia hosts the most amount of coral reefs including the Great Barrier Reef (18.261°S,147.7°E)
Topography Of The Formation Of Coral Reefs
A topographic map is a detailed, large-scale map of part of the Earth’s surface, illustrating selected features of the
physical environment. As the topography of coral reefs is inconsistent, the hand-drawn map shows the formation of
Climatic Map of Coral Reefs
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A thematic map is used to illustrate a theme such as climate or rainfall. This map shows the temperature of the water in
coral reefs around the world. As you can see, the Mesoamerican Barrier Reef (14.1°N, 87.22°W) thrives in both warm
and cold water.