The Great Barrier Reef is the largest coral reef
system in the world. It covers an area of 350,000 square kilometres, and
extends about 2,300 kilometres (Frith and Frith 1997) down the East Coast
of Queensland, Australia.
Although coral polyps are tiny, some of the reefs
they have created are so large they can be seen unaided from space.
Mathew Flinders, the first European to circumnavigate most of the coast
of Australia, named it the "Great Barrier Reef'. It is, however, not a
single barrier of coral, but a series of almost 3000 separate reefs (Frith
and Frith 1997).
The current living reef is also not as old as commonly thought. Parts
of it were formed as far back as 18 million years ago, but due to differing
sea levels, the actual living coral we see today has only been able to
develop in the last 5 to 8 thousand years.
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THE GREAT BARRIER REEF MARINE
PARK
Much of the Great Barrier Reef is contained and protected within
a national park, which is administrated by the 'Great Barrier Reef Marine
Park Authority'. This is acronym to GBRMPA, often pronounced as 'ga-broump-pa-'.
It is divided into four different sections; the Far Northern Section, the
Cairns section, the Central section and the Mackay Capricorn section
Containing as it does tropical coral reef, sea grass, mangrove and
island ecosystems, it is of course incredibly diverse. There are over 400
species of corals, 4000 molluscs, 1500 fish and 200 birds (McCoy 1999). |
CORALS
Many people still think of corals as some sort
of plant when in fact they are members of the animal kingdom. However,
they do have plant like features such as a relatively sedentary lifestyle
and the ability to gain energy from the sun. |
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This is accomplished through photosynthesis by symbiotic
algae called zooxanthellae living within the coral polyps tissues. While
the tentacles of the individual polyp animal captures food in the water
(usually by night), studies have shown that coral actually gets more than
ninety percent of its energy from this photosynthetic algae (Slater 1997).
While the algae provide food to the animal, in return, the algae get a
safe place to live within in a stinger cell protected animal. |
This need for sunlight by the algae means that corals
have to live towards the surface of the ocean, where the light penetrates
The fastest growing corals feed by night on microscopic animals with their
tentacles, and capture sunlight by day with their in-built algae. Those
corals that do not have zooxanthellae in their skin are those that grow
in deeper unlit waters (Slater 1997).
Most of the tiny individual corals actually live together in colonies,
which form what we humans see as an individual coral head. Additionally,
the so-called 'hard corals' excrete a limestone skeleton, and as the previous
animals of the colony die, provides the base upon which the next coral
polyps grow. Thus, as Davidson (1998) points out, corals can be thought
of as 'animal, vegetable and a mineral'.
CORAL REEF ISLANDS
There are three main types of islands associated
with coral reefs; 'Coral Atolls', 'Continental Islands' and 'Coral Cays'.
Coral atolls form around previous recent volcanic activity, and so it is
unlikely there are any true coral atolls on the Great Barrier Reef. 'Continental
islands' are essentially parts of a former coast; the lower valleys have
been flooded by a rising ocean and the islands are merely former mountains
that have been 'drowned'. 'Coral Cays' are essentially accumulations of
sand and broken coral.
CORAL CAYS
A Coral Cay is essentially an accumulation of sand
that begins to collect in an area protected by a reef. It can only fully
develop once the sea level has stabilised, and thus the smaller cays found
along the Great Barrier Reef must be less than 5000 or 6000 years old.
The 'sand' is a mixture of coral, shells, coralline algae and forams that
have been smashed and pounded into tiny bits by actions of the ocean (Martin
1995). If this accumulation manages to survive cyclones and storms, it
may become further stabilised by low vegetation and bird droppings (Martin
1995). |
| Further stability can occur
if 'beach rock' develops. This occurs when the calcium carbonate provided
by shells and coral is dissolved by rain and runs into the sand where it
eventually cements together into beach rock (Martin 1995). Another hard
substance called 'cay sandstone' can develop from bird droppings undergoing
a similar process and being washed into and cementing with the sand (Martin
1995). |
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| Eventually a cay can support trees and resorts.
At the present time, there are estimated to be around 240 coral cays on
the Great Barrier Reef (Martin 1995). Some of the larger examples of these
cays include 'Green Island' off Cairns and 'Heron Island' at the southern
end of the Great Barrier Reef. |
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