Download The biology of coral reefs

B io Factsheet
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Number 197
Biology of coral reef ecosystems
This Factsheet:
• Describes the structure and functioning of coral reefs
• Explains why their conservation is considered vital
• Reviews the kind of exam questions you can expect on this
synoptic aspect
Typical Exam Question
Suggest why the relationship between coral and the
xooxanthellae can be considered symbiotic.
Answer
The algae carry put photosynthesis producing sugars and
oxygen which the coral can use. The algae also assist the coral
in calcification
Coral reefs are one of the most diverse, productive and complex
communities on Earth. Found mainly in shallow, clear, and warm
tropical marine oceans, coral reefs are the largest and most
spectacular structures made by living organisms; reefs such as the
Great Barrier Reef in Australia can be seen from space.
In return the algae have a relatively safe place to live and are
provided with a source of carbon dioxide from coral respiration
The dominant species in a reef ecosystem is the hard (stony) coral,
which secretes the exoskeleton of calcium carbonate (CaCO3) .
However, there are over one million species of organism associated
with coral reefs, thus food chains and webs are complex. However,
all of this complexity is dependent upon the primary producers:
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•
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Table 1 summarises the benefits of symbiosis.
Table 1. Coral and zooxanthellae algae –mutual benefits
reef building corals (which contain photosynthetic
zooxanthellae algae)
microscopic cyano-bacteria (blue-green algae) which fix nitrogen
and photosynthesise
(multicellular) seaweeds
phytoplankton floating in the waters around the reef.
Benefits of coral to algae
Benefits of algae to coral
Allows sunlight to reach
algae for photosynthesis
Provides nutrients to coral
Protects algae from harsh
environment
Speeds up coral growth
Provide CO2 from
respiration for algal
photosynthesis
Removes waste products from coral
Fig 1. Coral reef food web
Reef forming coral is not found at depths below 50 metres as the
photosynthetic algae require sunlight. Photosynthesis also
demands a water temperature range of 21-290C, whilst water clarity,
salinity and water movement need to remain within narrow limits.
Colonies of zooxanthellae live endosymbiotically within the tissues
of other coral reef organisms such as soft coral, as well as
invertebrates such as giant clams, jellyfish, seasquirts, sponges,
flatworms and sea slugs.
Coral Reef structure and function
A coral reef is being continuously formed in two stages: (a) Growth of the framework of the reef and
(b) Erosion and cementation.
(a) Growth of the framework
The reef substrate is mainly composed of the calcium carbonate
from living and dead polyps. Millions of polyps grow on top of the
limestone remains of former colonies to create massive reefs. Singlecelled calcareous algae called Foraminifera also add calcareous
compounds to the reef when they decompose.
(b) Cementation
The reef structure can be physically eroded by wave action and
biologically broken down by grazing animals such as parrot fish,
sea urchins and sponges. Small fragments of coral settle into the
small spaces in the reef structure. Further cement is supplied by
algae and the nitrogenous excretory products of fish.
Reef-building corals have evolved a symbiotic relationship with a
type of unicellular brown algae called zooxanthellae. Millions of
these live within the tissues of the polyps.
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Bio Factsheet
197 Biology of coral reef ecosystems
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Recently, a species of brown seaweed, Dictyota menstrualis has
been suffocating the fragile coral reefs in Florida, the Bahamas and
the Caribbean. In some areas of the reefs, the seaweed has covered
between 20% - 70% of the coral reefs.
Threats to Coral Reefs
Coral reefs are under threat from human activities (anthropogenic
disturbances), as well as natural phenomena.
Excess numbers of predators can also upset the sensitive balance
of the reef ecosystem. The crown-of-thorns starfish (Acanthaster
planci) have been observed spreading over the Great Barrier Reef
since the 1970’s, killing the coral polyps at a speed of 6 square
metres per year per starfish.
This increase in A. planci may be due to eutrophication. The Giant
clam C. Tritonis which is a predator of the Crown-of-Thorns, has a
very decorative shell and is collected to be sold as souvenirs.
The seaweed is fast growing and tangles itself around the coral. By
smothering the coral, it stops the photosynthetic reactions in the
zooxanthellae, and suffocates the coral polyps.
Natural disturbances such as storms or hurricanes bring extreme
wave conditions to the reefs which break the coral into small
fragments. Heavy rain from storms increases run-off and
sedimentation.
Table 2. Main Threats to Coral Reefs
Pollution from marine commercial shipping
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Dumping waste at sea
Emptying fuel, oil and other chemicals
Use of toxic chemicals as anti-fouling
Pollution from the land
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Poor water treatment or sanitation systems
Raw sewage dumping
Release of nitrogen and phosphorus which causes eutrophication
Sedimentation
•
Removal of mangrove or deforestation of areas of land close to the sea
creates excess sediment flows into the reef.
Overfishing
•
Alters ecological balance by changing the natural predator/prey
relationships. Fishing with explosives destroy habitats
Aquarium Trade
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Nearly 1,500 fish species are traded throughout the world.
It is difficult to breed fish or coral in captivity and most fish do not
survive the journeys around the world.
Souvenirs
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Coral is popular for use in jewellery.
Invertebrate species are traded as marine ‘ornamentals’.
Coral mining
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Coral can be used as a building material
Coastal development
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39% of the world’s population lives within 100km of a coral reef.
These populations are expected to double by the year 2050
Commercial aquaculture
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Use of antibiotics increases resistant strains of bacteria.
Overcrowding of fish creates excess decay bacteria which deplete the
oxygen supply.
Anchor damage and divers
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Dive boats anchor on the reefs.
Poorly trained divers, physically damage the reef
Toxic waste released into rivers
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Toxins such as cyanide pollute water supplies and kill aquatic life.
Coral Bleaching
Coral bleaching is the whitening and the subsequent death of the coral caused by stress in the coral polyps. The effect of the stress
causes the coral to release its symbiotic zooxanthellae algae. The nutrients supplied by the zooxanthellae are then reduced, which
leads to the death of the coral polyps.
Table 3.
The stresses which can cause bleaching:
Stress
Cause of Stress
Increased water temperatures. Corals tolerate a
temperature range of 250C-290 C. Prolonged exposure to
higher temperatures causes bleaching.
Global climate change
Solar irradiance
This is a natural phenomenon as UV light increases in calm waters
Sediment run-off
Caused by construction of marinas, seawalls and land clearing.
Pathogenic diseases
Disease-carrying bacteria originating from sewage, can infect coral.
Pollution
Engine exhausts, oil, agricultural and land runoff. Waste disposal
and toxic waste dumping.
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Bio Factsheet
197 Biology of coral reef ecosystems
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Typical Exam Question
Fig.2 shows the decline in coral cover on the Great Barrier Reef.
Fig.3 shows the number of reefs on the Great Barrier Reef damaged by episodes of coral bleaching. Bleaching results when changed
abiotic conditions cause the coral polyps to expel algae or when the algae lose chlorophyll. The coral then becomes totally white.
Fig.2 Decline of coral cover
Fig.3 Coral bleaching
60
500
Cumulative number of reefs
Coral cover (%)
50
40
30
20
10
1960
1970
1980
1990
2000
400
300
200
100
0
1960
Year
1970
1980
1990
2000
Year
Each point = mean cover of up to 241 reefs sampled in each year
(a) Suggest two abiotic factors that could influence coral bleaching. (2)
(b) Explain why bleaching may lead to the death of the coral reef
(2)
(c) Fig.4 shows the number of reefs on the Great Barrier Reef damaged by outbreaks of crown-of-thorns starfish (COTS). Fig.5 shows
part of the food web of the reef.
Fig 4 Outbreaks of crown-of-thorns
Fig 5 Coral reef food web
Octopus
Giant clam
Cumulative number of reefs
500
Crabs
400
Crown of Thorns Starfish
Sea urchins
300
Coral
200
Sea stars
Sea slugs
100
Sponges
Zooplankton
0
1960
1970
1980
1990
2000
Year
Phytoplankton
Explain how overfishing may have contributed to the decline of the reef (3)
Markscheme
(a) temperature;
salinity;
sedimentation;
heavy metals;
(b) loss of photosynthesis/key producer;
reduced food supply/oxygen;
(c) Removal of predators of e.g. COTS;
Results in population explosion;
Increasing predation of reef;
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Bio Factsheet
197 Biology of coral reef ecosystems
www.curriculum-press.co.uk
Practice Questions
Answers
1. With reference to a coral reef, explain each of the following
terms:
(a) habitat (2 marks)
(b) niche (2 marks)
1. (a) habitat is the place in which specific organisms live (e.g.
invertebrate sea urchins live on coral, or octopus live inside
caves, reef crevices or tube coral during the day, coming
out at night to hunt);
2. When undertaking conservation work on a coral reef ecosystem,
transects were used to obtain quantitative data.
(a) What is meant by quantitative data?
(2)
(b) niche is a term which describes how organisms fit into an
ecosystem/ their role;
e.g. different fish species have different prey/feed at different
times.
(b) Data was collected by using transects from two areas of a
coral reef. An index of diversity may be calculated using the
formula
N(N-1)
d=
Σ n (n-1)
2. (a) Numerical information/ numbers of organisms;
e.g. sessile (non-moving organisms), such as coral species/
slow-moving invertebrates.
Where N = total number of organisms of all species
and Σ = total number of organisms of each species.
(b) (i) Transect A;
d=
Species in
transect A
Number of
organisms
in sample
Species in
transect B
Number of
organisms
in sample
Pore coral
126
Pore coral
4
Star coral
227
Star coral
2
Goblet sponge
4
Goblet sponge
0
Pin cushion
sea urchin
3
Pin cushion
sea urchin
0
Giant clams
12
Giant clams
0
Crown of Thorns
Starfish
5
Crown of
Thorns Starfish
0
= 2.05
Transect B
(11 × 10)
d=
= 3.2
(4 × 3)+(2 × 1)+(5 × 4)
(ii) There is more diversity in the area of Transect B. The
ecosystem is more likely to be stable.
(iii) The predator Crown of Thorns Starfish is more prevalent
in Transect Area B, as there are less Giant clams to eat
the Crown of Thorns Starfish; The Giant clam has a very
decorative shell and is collected for sale as a marine
souvenir.
3. (a) Marine organisms are collected for souvenirs; diver damage
in popular reef diving areas; boat damage by tourist boats/
anchoring;
(i) Calculate an index of diversity for each area. (2)
(ii) Which part of the reef A or B has the greatest diversity? (1)
The diagram below shows part of the food web for the reef.
Octopus
Crabs
(372 × 371)
(126 × 125) + (227 × 226) + (4 × 3)+(3 × 2)+(12 × 11)
Giant clam
(b) Fishing industry/using explosives for fishing/overfishing;
mining coral; aquarium trade; commercial aquaculture;
(any 3)
Crown of Thorns Starfish
Sea urchins
Coral
Sea stars
Sea slugs
Sponges
Zooplankton
Phytoplankton
(iii) Suggest an explanation for the reduction in coral and sea
urchins in Area B. (2)
3. Outline how each of the following human activities have
disrupted the health of coral reef ecosystems.
(a) Tourism (3)
(b) Industry (3)
Acknowledgments: This Factsheet was researched and written by Gloria Barnet. Curriculum Press, Bank House, 105 King Street,
Wellington, Shropshire, TF1 1NU
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ISSN 1351-5136
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