Threats to Coral Reef Ecosystems (AQA A Level Geography)

Human Activity & Coral Reefs

• Anthropogenic (human-caused) activities that threaten coral reefs are:

  • Pollution

  • Over and destructive fishing practices using dynamite or cyanide

  • Collecting live corals for the aquarium market

  • Mining coral for building materials

  • Plastic waste

  • Global warming -  lead to increased acidification and coral bleaching

• Pollution is the most significant threat

  • Land-based runoff and pollutant discharge from:

    • Dredging

    • Coastal developments

    • Agricultural and deforestation activities

    • Sewage treatments

    • Chemical and heavy metal pollution

    • Shipping produces oil and human waste, toxic paint is used on the hulls of ships to prevent barnacle growth and reduce oxidation

• Any of these runoffs may contain sediments, nutrients, chemicals, insecticides, oil, and/or debris

• Unsustainable tourism and coastal development cause lasting damage to a coral reef

• Touching or standing on a coral reef or construction close to the shoreline, can instantly damage a reef that is thousands of years old

• Unsustainable boating practices, like anchoring on a reef, also causes destruction

• Unsustainable fishing reduces fish populations that feed on algae

  • The algae grows unchecked, eventually smothering corals

  • Blast fishing destroys 5.9m2 of reef with a single blast, destroying entire sections of coral reefs

• Plastic waste and sediments (from bags and packaging) choke coral reefs systems by suffocating them

  • It is expected to worsen as development in LDE's and EME's continues to grow

• Desalination

  • Many coral reefs are found in areas that face water scarcity and depend on seawater desalination for freshwater supply

  • Along with freshwater production, desalination plants discharge brine (salt) and chemical waste into the sea, changing the levels of salinity, water chemistry and damage the sensitive ecosystems of the reef

• Drainage basin schemes

  • Changes to drainage basins affect runoff into coral reefs

  • In Queensland, Australia, 200,000ha of forest was cleared (2012-14)

  • Sediment, freshwater runoff and land based pollutants all increased into the Great Barrier Reef which resulted in parts of the reef dying

  • The Burdekin Falls Dam in Queensland has resulted in changes in the amount and timing of freshwater flow into the reef system, disrupting those ecosystems that rely on small amounts of freshwater for their life-cycle

• Onshore development

  • Any form of coastal development affects marine life either directly through dredging the sea floor to deepen channels for large ships to pass through

  • Indirectly through clearance of mangroves forest, which increases the amount of sediment and nutrients added to the reef ecosystem

    • Too much sediment limits available light for photosynthesis 

    • Increased nutrients can be toxic to coral reefs 

Natural Impacts on Coral Reefs

• Coral reef ecosystems can be disturbed through:

  • Extreme rain events

    • Freshwater dilutes the seawater, unbalancing the water chemistry of the coral reef

    • Storm waves associated with tropical cyclones break apart or flatten large coral heads, scattering their fragments

  • Volcanic activity and earthquakes

    • Volcanic ash can smother and block coral polyp feeding tubes

    • Subglacial volcanic eruptions produces large volumes of melt water, resulting in catastrophic freshwater flooding

    • Hot, volcanic magma smother vast areas of living coral reefs rapidly

    • Underwater earthquakes and volcanic eruptions can break apart the hard coral reef support and damage the ecosystem

    • It is now thought that volcanic activity actually supports the growth and spread of coral reefs

    • Volcanic activity is responsible for atolls

  • Thermal stress from unusually warm water (such as El Niño events)

    • Causes coral bleaching as the polyps become stressed and expel their symbiotic algae

  • Changes in tidal patterns - unusually long periods of low or high tides

    • Low tide exposes the coral heads to ultraviolet light, which overheats and dries out the coral tissue

    • This over stresses the polyps and they begin to expel their symbiotic zooxanthellae, leading to bleaching and death of the reef

    • Exceptionally long high tides or sea-level rise, restricts light to the reef, preventing photosynthesis by the symbiotic algae, leading to the death of the polyps

  • Marine predation - coral are vulnerable to predation

    • Fish, marine worms, barnacles, crabs, snails and starfish etc. prey on the polyp's soft, inner tissues 

    • Entire reefs can be damaged, if the number of predators become too high

    • The crown-of-thorns starfish is a particular problem on the Great Barrier Reef where almost a quarter of the reef was destroyed in the 1980s

• The coral reef ecosystem usually recovers over time as natural impacts rarely destroy entire reefs

  • But, if the coral reef is exposed to continuous stresses, natural or human, the strain can become overwhelming and the polyps will die

Future Prospects for Coral Reefs

• Impact of climate change

  • Climate change can be due to natural effects such as tectonic shift, meteorite strikes, volcanic eruptions, increased sunspot activity and changes in the orbit of the Earth

  • These natural impacts have brought periods of cold and warm climates, all of which have impacted on the growth of coral reefs

  • However, it is anthropogenic activity that is having the greatest impact on the survival of coral reefs

  • This is leading to a warming of the oceans, causing thermal stress that is contributing to coral bleaching and infectious diseases

  • Rising sea levels increases sedimentation which can smother coral polyp and block feeding tubes 

  • Storm patterns change - they are becoming stronger and more frequent

  • Warming climate increases precipitation rates (warm air can hold more water vapour), this leads to increased freshwater, sediment and land-based pollutants entering the water, increasing algal blooms and reducing available light for photosynthesis

  • Ocean currents are altered, which changes the interconnectivity of ecosystems and temperature regimes, leading to lack of food and dispersal of coral larvae

  • Increased levels of CO2 is causing ocean acidification and changing the pH levels of the coral waters, which restricts growth rates and structural integrity of the coral reef

Future of coral reefs 

• The World Resources Institute reached the following conclusions in 2021:

• Human activities have already caused damage to coral reef systems and climate change is enhancing these impacts

• Mass coral bleaching is becoming more frequent, giving less time for the ecosystem to recover between bleaching events

  • In 2016, the northern section of the Great Barrier Reef lost 67% of its corals due to coral bleaching

• Damage to any coral reef puts thousands of other species at risk 

• Management of coral reefs needs to be effective to minimise the damage and include:

  • Restriction of fishing - number of fish caught is limited, especially protected species

  • Commercial fishing pay a fee that funds research, education and management of the reef

  • Tourist industry indirectly benefits from the conservation efforts - ecotours to monitor the reef and report on the extent of coral bleaching or numbers of specie diversity

  • Limiting of water pollution through land based activity - governments, farmers and industry working together to limit the type and amount of pollutants running off the land into the reef

  • Setting up of a reef trust to improve water quality and habitats in and around a reef

  • Education of the local people to monitor and protect their reef, and report any changes whilst still be able to support their way of living off the reef

• These local changes are effective but do not tackle the global issue of climate change and sea level rise