Coral Reef Ecosystems (AQA A Level Geography)

Categories of coral reefs

• There are two major categories of coral reef:
  • Warm - found within warm tropical waters 
    • Typically found between 30° north and south of the equator
    • Water remains above 16°C
    • Examples include:
      • Red Sea Reef - narrow body of water between Saudi Arabia and northern Africa
      • Great Barrier Reef - world's largest coral reef off the coast of north-eastern Australia
      • Mesoamerican Reef - located in the Caribbean Sea, east of the Yucatan Peninsula
      • New Caledonia Barrier Reef - consists of two reefs parallel to each other in south-west Pacific Ocean
  • Cold - found in deep, nutrient-rich, strong current sea waters; where temperatures and salinity levels are more stable, compared to coastal waters
    • Can develop in waters as cold as 4°C and at depths of 40 - 2000m 
    • Coral larvae attach to an initial hard surface and feed through catching particles in the passing currents
    • These strong currents remove excess sediments, preventing build-up on the coral
    • Cold water corals are most successful on seamount slops (underwater mountains), due to the accelerated water flow down the slope
    • The largest cold water reef to be discovered (2002) is Røst Reef, off the coast of the Lofoten islands approx. 100km west of mainland Norway. It extends over a length of about 43km (27 mi), and has a width of up to 6.9km (4.3 mi) 
    • Most cold water reefs (to date) have been found in the North Atlantic, including off the west coasts of Scotland and Ireland

Distribution

• Corals are scattered throughout the tropical and subtropical Western Atlantic and Indo-Pacific oceans, generally within 30°N and 30°S latitudes
• Western Atlantic reefs include these areas: Bermuda, the Bahamas, the Caribbean Islands, Belize, Florida, and the Gulf of Mexico
• The Indo-Pacific ocean region extends from the Red Sea and the Persian Gulf through the Indian and Pacific oceans to the western coast of Panama
• Corals grow on rocky outcrops in some areas of the Gulf of California
• The Great Barrier Reef in northern Australia is renowned for its great biodiversity and size and can be seen from space
• Their distribution is controlled by four factors: 
  • Temperature
  • Light
  • Water depth
  • Salinity

Map showing generalised global distribution of warm and cold water coral reefs

Anatomy

• A coral reef is made up of three main components:
  
  • A hard surface
    
    • Submerged rocks and hard surfaces give a place for corals to attach to
    • They also serve as a habitat for other coral reef animals
  • Coral polyps
    
    • Coral polyps are tiny little animals that are related to anemones and jellyfish
    • They can live individually, or in large colonies that make up the coral reef
  • Reef animals
    
    • There can be between 1 and 8 million species within the coral reef ecosystems
    • Many play a vital role in maintaining a healthy, functioning, and balanced reef
• There are two main types of corals:
  • Hard corals - reef building corals that create skeletons out of calcium carbonate, that harden eventually into rock
  • Soft corals - soft and bendable and resemble plants or trees, with wood-like cores and fleshy outer-skin for protection

A cross section of a typical coral polyp

• Coral reef formation starts with free-swimming coral larvae attaching themselves to any hard submerged surface
• They begin to secrete a hard, calcium carbonate, skeleton for protection against predation
• As they grow, the polyps rely on a symbiotic relationship with tiny algae called zooxanthellae
  • The polyps provide the algae with shelter
  • The zooxanthellae algae provide the corals with essential nutrients in return
• Over time, the build-up of calcium carbonate provides a new surface for other polyp larvae to attach themselves to
• The allows the reef to continue growing and develop a self-sustaining process
• The build-up of calcium carbonate skeletons into reefs takes hundreds to thousands of years and consist of millions of polyps that grow on top of the limestone remains of former colonies, that will eventually form massive reefs
• Growth of large reefs is usually slow - 0.5 to 2cm a year
• But given the right conditions (good light, temperature, and wave action), some polyp species can grow as much as 4.5cm a year

Types of coral reefs

• Fringing Reefs
  • These are reefs that form around a land mass
  • Found very close to the shore, in shallow waters
  • They do not have a lagoon separating them from the land
• Barrier Reefs
  • Run parallel to the shore but are separated by a lagoon
  • The Great Barrier Reef in the Coral Sea, off the coast of Queensland, Australia is a good example of a barrier reef.
  • It is the world's largest coral reef system with over 2,900 individual reefs and 600 islands that stretches for over 2,300 kilometres and can be seen from space
• Atolls
  • Horseshoe-shaped rings away from the shore
  • Consists of a coral rim encircling a lagoon

Darwin's theory of coral reef formation from volcanic island uplift to subsidence

Importance of coral reefs

• Coral reefs account for just 18% of total marine environment, but are home to 25% of all marine species
• They form one of the richest ecosystems on Earth
• Coral reefs support more than 450 million people with food and income (Caribbean reefs generate approx.US$10bn a year)
• Reefs form a natural barrier against storms and wave surges
• Coral reefs are a carbon store (calcium carbonate skeletons) and contribute to the carbon cycle
• Algae and sponges have value with their medicinal properties

• Coral reefs need specific conditions to grow in tropical warm waters

Environmental Conditions Required for Warm Water Coral Reef Growth

• At a local level, other factors will affect development:
  • Wave action - corals need well oxygenated, clean water and wave action provides this
  • Exposure to air - although corals need oxygenated water, they cannot be exposed to air for too long or they will die
  • Sediment - all corals need clear, clean water
    • Any sediment in the water will block normal feeding patterns by reducing the availability of light affecting the photosynthesis of the microscopic algae living in the polyp tissue

Nutrient cycling in coral reefs

• The waters of the coral reefs are nutrient-poor, necessitating efficient nutrient recycling to maintain ecosystem health
• It is the symbiotic relationship between the coral polyp and zooxanthellae algae that is central
• The algae capture nitrogen and phosphorous from the polyps waste, which fertilises the algae
• The polyps are able to digest zooplankton, bacteria and edible detritus from ocean floor upwelling
• The algae provide food and nutrients by photosynthesis

Simplified diagram of coral reef nutrient recycling