3.1.3 Coral Reefs Flashcards
What are coral reefs?
Build of thousands of tiny polyps which are related to anemones and jellyfish. Can live individually or in large colonies comprising an entire structure
- Use calcium and carbonate ions from seawater to build white skeleton made from calcium carbonate/limestone to protect the soft body
- Polyps nocturnal and at night extend tentacles to feed on minerals in the water.
- Colour comes from zooxanthellae living in the tissues - several million live, producing pigments in just one square inch of coral
- When a generation of coral die, the next grows on top of it, growing the reef upwards and outwards to compete for food
- Usually run parallel to the coast and create huge structures
- Often extremely diverse and provide habitats for thousands of creatures, covering 1% of the world seas yet providing habitats for over 25% of all marine species
- Coral gets CO2, phosphorous and nitrogen for photosynthesis and zooxanthellae give oxygen, sugars and fats to the coral.
What conditions are required for coral to grow?
- Must be shallow for sunlight to reach as coral need zooxanthellae for oxygen so light needed to photosynthesize
- Water rarely deeper than 30m
- Feed on plankton who also need sunlight - 19m usually best
- Low tidal range needed to avoid long periods of exposure
- Warm water temperatures, normally 20-32
- Clear and clean water - let sun in
- Too much sediment or plankton can stop light
- Need saltwater with balance ratio of fresh and saltwater
- Gentle wave movements to keep water oxygenated
- Rocky sea surface for reef to row on
What are fringind reefs?
Fringing reef: colonises fringes of coast of a land mass, often growing along the shores protected by barrier reefs and contain organisms best adapted to low wave energy conditions.
Characterised by an outer reef capped by algal ridge. Broad, flat reefs with no lagoons between the shoreline and the reef - susceptible to human impacts. This creates a platform of coral connected to the land and grown out from it
What are barrier reefs?
Similar in structure but usually several km offshore, typically along the outer edges of the continental shelf. They run parallel to the shoreline, commonly separated from the shore by a wide, deep lagoon.
They are broader, older and more continuous than fringing reefs.
They may have began as fringing reefs but slowly grew out from the coastline
What are atolls?
Narrow circular reefs broken by channels surrounding a deep circular lagoon. They often rise from submerged volcanic foundations.
They are similar to barriers reefs as they are confined to the flanks of submerged oceanic islands. Contains a coral rim, surrounding a lagoon. If the rim is only partial water may flow in and out through channels
What is Darwin’s theory?
-Coral reefs build upwards as land sinks to access light, oxygen and food as the island sinks. The coral grows fastest on the outer edge, so it continues to grow outwards as the island sinks, creating a wider lagoon where the island used to be. As a result, fringing reefs become barrier reefs and barrier reefs become atolls.
-Coral polyps flock along submarine platform on edge of an island and colonise.
-They grow upwards and reach sea level, forming a fringing reef
-Land subject to subsidence due to tectonics, so coral polyps end up deeper where they will die, so forced to grow upwards by depositing calcium carbonate and building up on rock to get food and light
Growth slowed near the shores but faster on edges, creating a lagoon between coats and fringing reef, forming a barrier reef.
-Island continues to sink, a ring of coral is formed known as an atoll. The growth is slow in the middle as conditions are warmer and saltier due to evaporation but better for growth on the outer edges
What is Murray’s theory?
Suggest coral grow on suitable platforms and start to grow out to form fringing reefs. Continuous growth forms barrier reefs. Atolls form due to outward growth of coral in al directions at the top of a submarine platform. On the lagoon side there is dead corals and seaward side has living corals which continuously grow outwards
What is Daly’s theory?
As sea level rise underwater wave cut platforms made due to erosion when sea levels were lower, creating barriers for reefs to form. Fringing reefs form on narrow ones, barrier reefs on long ones and atolls formed around isolated island peaks.
Why are coral reefs important?
- Provide habitats to thousands of species
- High productivity
- Valuable seafood
- Commercial products like jewelry and aquariums
- Source of limestone
- Tourism
- Medical products
- Coastal protection
- Estimated $100bn benefit to global economy
What is coral bleaching?
When coral is stressed it causes bleaching, as they expel zooxanthellae and die.
- Z release charged oxygen molecules toxic to the polyps so they are expelled and no longer feed polyps through photosynthesis.
- They can become bleaches when high temperatures persist for a week or more so polyps reject. Normally happens in summer but if happens for too long they will die.
- Algae may then grow on dead coral, making it impossible for new coral to form.
- When Z is released it loses colour and becomes white.
What is the effect of global warming?
- 1 to 2 degree rise causes bleaching
- Most reefs naturally bleach in summer, however large heatwaves cause mass bleaching.
- If this is frequent can threaten survival
- Marine heatwaves more common due to global warming
- In 2016/17 mass bleaching of GBR killed 50% of corals. In 1998, 16% of corals were affected, many permanently bleached.
- Some experienced temps over 32 degrees.
- Sea level rises cause reefs to drown if it rises faster than coral growth, causing overexposure to UV
What is ocean acidification?
Global warming is causing carbon dioxide to be absorbed by the sea, reducing calcification in corals and making them have weak skeletons and more prone to erosion
- Causes dissolution as water dissolved the coral
- Slows the growth of new corals and corrodes existing ones
- Carbonates blocked by hydrogen ions, so coral weaker and more vulnerable
- 50% of global CO2 absorbed by oceans - carbonic acid removes calcium carbonate from water.
- Some polyps can survive without a skeleton or use the bicarbonate instead of carbonate
How are reefs physically damaged?
- increased storm erosion through wave activity, high winds, hurricanes etc - may link to climate change
- physical damage and overfishing
- blast fishing
- cyanide fishing
- fish fences
- coral mining
- coastal development
- tourism - trampling
- turbulence and sedimentation
How does pollution affect coral reefs?
- Economic growth means industrial and agricultural pollutants get into rivers and sea
- Chemicals grow plankton which swarm the reefs and eat the coral
- Oil pollution smothers the coral
- Raw sewage crows plankton and kills coral
- Plastic waste kills coral due to bacteria
- Sewage carries nutrients - eutrophication and blooms
- This can impact food chains, oxygen levels and invasive species
- Oil, chemicals biocides, sewage, waste and anchors can smother, damage and destroy corals.
How is coral managed?
- Combating global warming emissions to reduce temperature and acidity
- Reduce local threat and damages
- Protective legislation
- Locals with jobs that do not harm coral
- Natural protected reserves
- Reduce sedimentation, fishing, blasting etc.
- Local assistance
- Coastal management practices
- Monitoring, manage and enforcement
- More coral reefs designated as marine protected areas to act as reservoirs of biodiversity
- Coral mesh fences to stabilise and hold together
- Marine Park acts
- Zoning and management plans
- Pesticide managements
- Management of coastal development
- Tourism management
- Fishing management
- Cleaning water
- Restoration of estuary habitats
- Paris Agreement on climate change
- Local pollution and destructive fishing
- Research of biology - genetic selection of heat resistant corals.
- Regular monitoring of reefs, water quality and other environmental factors
