corals = tiny marine animals (polyps). reefs formed from millions of them living
together in colonies. skeletons are calcium carbonate cups and they join to form a
stony, hard mass of limestone. next one grows on top of the generation that just
died, reef grows up and out as corals fight for food.
reefs run parallel to coast. has to be a base for reef to grow, base is seabed rock.
corals can also grow on shipwrecks. once established, reefs can grow up to 60cm a
year if conditions are right.
form huge structures which can extend for miles along a coastline. great barrier
reef, north-east coast of Queensland, Australia. length = 2600km area =
350000km. not one reef, 3000 separate reefs. taken 5 million years to grow.
extremely diverse ecosystem. over 800 types of coral, reefs provide habitat for
wide range of creatures. reefs occupy less than 1% of worlds seas but provide
habitat for 25% of marine species, high levels of biodiversity.
polyps are tiny animals that live in a symbiotic relationship with algae called
zooxanthellae. algae live in polyps’ bodies and use the polyp’s products of
respiration (CO2 and H2O) to carry out photosynthesis. produces sugar and
oxygen. polyps use those products to grow and respire. the product recycling
between polyps and algae is driving force behind the reef’s growth and
productivity
living coral = colourful, dead coral = white skeletons. dead reefs are called
‘bleached coral’. becoming more common showing that the ecosystem is under
threat.
Conditions for growth
Temperature: sea temps have to be above 18 degrees. grow best where sea
surface temps average between 22-25 degrees. only found within 30 degrees of
equator. unable to grow where there’s cold sea current.
Light: can’t grow in water deeper than 30m, shallow water up to 10m deep are
best. have a symbiotic relationship with algae which needs sunlight to
photosynthesis. coral feed on plankton which need sunlight.
wave action: outer edge of reef grows height as that is where oxygen and food
supplies are most plentiful – in zone where waves break
salinity: grow best where salt levels are high. prefer salinity levels of 32-42 psu
(practical salinity units)
exposure to air: polyps can’t survive for long above water. most of reef
is at or below low tide level
sediment: polyps need clean, clear, sunlit water so can’t live where
rivers bring muddy sediment to sea
sea bed: has to be solid surface where reef can grow. why there is
coastal features forming in shallow offshore waters.
Fringing reef
• low, narrow bands of coral next to coast at low tide level
• consists of a platform of coral, connected to the land and which has grown out from
it
• platform surface is slightly concave and filled by a shallow lagoon which occupies
the space between the outward edge of the reef and land.
• lagoon is less than 500m wide
• highest part is seaward edge of the reef as the breaking waves oxygenate the
water and make food available for polyps
• reef outer edges slope steeply down in sea beyond
• example: coral coast of Fiji and reefs along coast of Mauritius
Barrier reef
• similar structure to fringing reefs
• several km from land and over 0.5km from shore
• separated from land by deep lagoons which polyps can’t live in
• lagoon floor is made of old coral, so they may have originated from fringing reefs
• seaward edge of reef is highest part and outer edges of reef slope steeply down
into sea beyond (same as fringing)
• example: Great barrier reef, Aus. Made up of 3000 sections of reef operated by
channels
Atolls
• narrow circular reefs broken by channels
• surround deep circular lagoon
• example: Suadiva atoll and Gan atoll in Maldives. common in south Pacific Ocean
Formation of coral reefs
Fringing: coral grew in shallow waters where it’s found today
Barrier: fringing that continued to grow outwards from coastline
Atolls: more difficult to explain as appear from deep water, far away from coastline
with no connection to land
Charles Darwin developed a theory, 1842, also supported by Dana about formation
of coral reefs. It suggested that formation of 3 different reef types is linked. As
fringing reef grows upwards and outwards, could develop into barrier reef. Process
would be enhanced if an island was sinking (proved possible now by plate tectonic
theory). Barrier reef around a sinking island could turn into an atoll. As coral grows
upwards to maintain access to supplies as island beneath sinks. Coral growth most
strong on outer edge of reef so reef continues to grow outwards as islands sink,
creating even wider lagoon where island used to be. 200-year-old theory but many
people accept its explanation of atoll formation.
Why are coral reefs important?
Net primary productivity of reefs (2500g/m2/year) is higher than the average
rainforest (2200g/m2/year).
Seafood: in 1 year if properly managed reefs can yield up to 15 tons of fish and
other seafood per km2. provide food for up to a billion people in Asia alone
Other commercial products: corals and shells can be made into jewellery. live coral
and fish are harvested and sold for aquariums. sand and limestone mined for
building industry, destructive of ecosystem and unsustainable.
Tourism: coral reefs visited because of their attraction. snorkelling and scuba is
popular in red sea. tourism is rapidly growing and provide much needed income in
LIC’s.
Medical products: some organisms living in the reef produce chemicals that can
cure diseases
Coastal protection: protect coastline from storm waves and can break tsunami
wave power. important where tropical storms are likely. protect coastline from
erosion, flooding and can shelter ships
reefs continue $100 billion to global economy. there’s good economic reasons why
they should be used sustainably. coral reefs are under threat. when environmental
conditions become stressful coral bleaching can happen.
Why are they under threat?
Threat 1: enhanced global warming…
• if seawater temps increase by 1/2 degrees coral bleaching can be the result. temp
increases = temporary, effects = permanent. summer 1998 temps rose so much
that bleaching affected 16% of the world’s coral reefs. some recovered but majority
was permeant.
• global warming leads to rising sea levels which means reefs will drown if they can’t
grow fast enough
• global warming is increasing acidity of ocean waters. more CO2 in atmosphere
means that more of it dissolves in oceans. this stresses the polyps making it more
difficult to build and maintain their limestone shells
Threat 2: pollution…
land-based pollution: economic growth in many LIC’s means industrial and
agricultural pollutants are more likely to get into rivers which then bring pollutants
to the sea. chemicals in agricultural fertilisers have encouraged growth of
phytoplankton which in turn feed increased number of the crown of thorns starfish
which swarm onto the reefs and eat the coral.
shipping: Persian Gulf have high levels of oil pollution from oil tankers. areas
around ports have large amounts of shipping that pollutes the sea. plastic waste
can smother and kill coral.
sewage: raw sewage from growing coastal settlements and from tourist hotels can
pollute seawaters. treated sewage is a problem as it contains phosphates from
detergents when encourage the growth of phytoplankton and the crown of thorns
starfish
Threat 3: physical damage…
population: as pop. grows the food demand increases. overfishing takes more fish
from the sea than can be replaced which reduces the reefs biodiversity. some
fishing methods can physically damage the reefs. blast fishing can be especially
damaging to reef
tourism: more and more people visiting reefs. walking on reef can kill coral,
swimmers stir up sediment and divers break off reef pieces. locals gather shells
and coral as sediment. construction of beachside hotels so sand and limestone are
quarried from reefs
shallow water: propellers churn up sea bed and add sediment to the water
sediment: sediment draining into the sea from degraded and eroded river basins
can damage the reef. polyps can’t live in muddy water
natural hazards: tropical seas are susceptible to tropical storms. large/storm waves
can damage reefs.
in 2011, it was estimated that 10% of worlds reefs had been completely damaged
and 75% are under threat. the degradation of reefs is a major problem that has
economic, environmental and social consequences.
Management strategies
combating global warming by reducing emissions. this should stop sea temps rising
which will stop coral bleaching and will slow down the increase of ocean water
acidity
reduce local threats such as trampling, mining, over-fishing, sedimentation and
coral blasting
help LIC’s introduce protective legislation and provide funds for new laws to be
enforce
provide locals with jobs that don’t involve damaging the coral
designating coral reefs as nature reserves, especially the ones that haven’t
experienced too much damage.
Case study: coral reef conservation in Fiji
All the Fijian islands are surrounded by coral reefs, 2/3’s thought to be at risk.
Reefs are important for environmental and economic reasons. Very rich
ecosystems with great biodiversity. Fiji’s 300 coral species are habitats for 2000
fish species (sharks, tuna, shellfish and endangered turtle species). Reef
ecosystem has links with bigger ecosystems through food chain: plankton —> coral
—> parrot fish —> larger fish —> humans.
Important barriers to storm waves, reducing their impacts on the land. Plants and
animals in reef ecosystem are being researched as possible future medicine
sources.
Economic benefits: subsistence and commercial fishing. selling broken off pieces of
coral for aquariums. reefs and marine life attract tourists, stopover point for flights
Why are reef conservation measures needed in Fiji?
human causes…
sensitive organisms that can die if touched
been affected by over-fishing
Fiji was one of the world’s largest exporters of coral and fish for aquariums
tourists walking and diving
Fiji population is rising, and most people live on coats
development of settlements has led to harmful sediment sewage and other
pollutants being washed out into reefs
natural causes…
every 3/4 years, Fiji battered by cyclones which generate destructive waves that
break coral
storms lead to larger amounts of nitrates and phosphates reaching reefs from runoff from agricultural land, causes number of corn of thorns starfish to rise
increase of sea temps causes reef bleaching, polyps also die.
el Niño event, reversal of equatorial ocean currents leads to warms water from
Indonesia moving east and warming the sea.
lower salinity is another bleaching cause
Efforts by a Fijian resort to conserve the reef
many hotels and communities are trying to conserve, Fiji’s coral coats is fringed
with the world’s second-largest reef. e.g. hide-away resort.
visitors can snorkel at high tide but no putting feet down. guided walks organises
along specially provided reef path. high tide, guests can view marine life on reef
from resorts glass bottom boat. in lagoon there’s a coral nursery where coral is
planted and protected.