Edexcel IGCSE Geography 复习笔记 2.2.1 Coastal Ecosystems of the World

Distribution of Coral Reefs

• Coral reefs are large deposits of calcium carbonate built entirely of living organisms called coral polyps
• 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

Features of Coral Reefs

• 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 'zooxanthellae' living in polyp tissue. The corals provide algae with home and compounds for photosynthesis. In return, the algae produce food, oxygen and help remove wastes

Types of Coral Reefs

• Fringing Reefs - these are reefs that form around a land mass
• Barrier Reefs - these are found parallel to the shore but are separated by a channel of water:
  • 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,  consisting of a coral rim that encircles a lagoon

Distribution of Mangroves

• Both mangroves and coral reefs are found in warm tropical waters, however, unlike the sensitive coral reefs, mangroves are highly adapted to changing conditions
• This has made them the most successful ecosystems on Earth

Global Distribution of Mangroves

• Originate from south-east Asia and spread across the globe
• Mainly found in warm tropical waters and coastal swamps within 30° N and S of the equator
• Some have adapted to more temperate conditions and have colonized as far south as New Zealand's North Island
• They grow in the intertidal zone of the coast
• South-East Asia has mangroves with the highest biodiversity in the world

Characteristics of Mangroves

• Mangroves are trees that live on the coastline
• They sit in water between 0.5 to 2.5 metres high
• They range in size from small shrubs to trees over 60m high
• They have numerous tangled roots that grow above ground and form dense thickets
• They need high levels of humidity (75 - 80%) and rainfall per annum (1500 - 3000 mm)
• Ideal temperature is around 27° C but are adapting to more temperate climates
• Mangrove root system is complex, with a filtration system to keep salt out
• Some have snorkel like roots that stick out of the mud to help them take in air
• Others use 'prop' roots or 'buttresses' to keep their trunks upright in the soft sediment at the tidal edge

 Prop Roots      Mangrove Root Systems             Snorkel Roots

• It is the roots that trap mud, sand and silt which eventually builds up the intertidal zone into the new land
• At the same time, the mangrove is colonizing new intertidal areas
• The fruits and seedlings of mangroves can float and can travel many kilometres on ocean currents
• As they drift with the incoming tide, they become lodged in the mud and begin to grow, colonizing new areas

Distribution of Sand Dunes

• Coastal sand dunes are found all over the world
• They are the accumulation of sand, shaped into mounds and ridges by the wind
• Found at the back of a beach, above the maximum reach of the tide

Global Distribution of Coastal Sand Dune Systems

• Coastal sand dunes develop best when:
  • There is a wide beach and large quantities of sand
  • The prevailing wind is onshore
  • There is a large tidal range to allow time for the sand to dry
  • There are suitable locations for the sand to accumulate

Characteristics of Sand Dunes

• Sand dunes can be small ridges or large hills usually found at the back of a beach
• They can extend backwards for many miles as well as along the beach
• They are an important ecosystem supporting unique flora and fauna that have adapted to live within the dune system
• Dunes are vulnerable to erosion by natural processes and human activity:
  • It is common to see vulnerable sections of dunes fenced off to prevent public access, or for paths to be laid to prevent people from eroding the dunes further

Formation of a Sand Dune

• Wind blown sand is deposited against an obstruction - pebble or driftwood
• As more sand particles are caught, the dunes grow in size, forming rows at right angles to the prevailing wind
• Over time, the ridges of the dunes will be colonized and fixed by vegetation in a process called succession
• The first plants (pioneer species) have to deal with:
  • Salinity
  • Lack of moisture as sand drains quickly (highly permeable)
  • Wind
  • Temporary submergence by wind-blown sand
  • Rising sea levels

Coastal Sand Dune Succession

• Embryo Dunes
  • Wind blown dried sand is trapped by debris and deposition begins
  • Pioneer species such as Lyme Grass and Sea Couch Grass begin to colonise
  • There is little soil content and high pH levels (alkaline)
  • Embryo dunes  are very fragile and reach a maximum height of 1 metre
• Fore Dunes
  • The embryo dunes bring some protection against the prevailing wind
  • This allows other species of plant to grow such as Marram Grass
  • Marram grass begins to stabilise the dune with its root system
  • These plants add organic matter to the dunes making the dunes more hospitable for plants that later grow
  • A microclimate forms in the dune slack
  • Maximum height is 5 metres
• Yellow Dunes
  • These are initially yellow but darken as organic material adds humus to the soil
  • Marram grass still dominates the vegetation, but more delicate flowering plants and insects are found in the dune slacks
  • 20% of the dune is exposed, down from 80%
  • Height does not exceed 8 metres
• Grey Dunes 
  • Grey dunes are more stable, with less than 10% of exposed sand and have a good range of biodiversity
  • Soil acidity and water content increase as more humus is added
  • Shrubs and bushes begin to appear
  • Height is between 8 - 10 metres
• Mature Dunes
  • As the name suggests, these are the oldest and most stable of the dunes
  • They are found several hundred metres or more from the shoreline
  • The soil can support a variety of flora and fauna such as oak trees and alders (climax vegetation)
  • This is the final stage in succession which is known as the climax community stage

Distribution of Salt Marshes

• Salt marshes are found all over the world and are not temperature dependant
• Like mangroves, they are an ecosystem of the intertidal zone
• They are typically very flat, with numerous channels running through them
• They form in:
  • Coastal areas that are well sheltered, such as inlets and estuaries where fine sediments can be deposited
  • Areas behind spits and artificial sea defences where tidal waters can flow gently and deposit fine sediments
  • They form in brackish water

Global Distribution of Salt Marshes

Features of Salt Marshes

• Salt marshes are communities of nonwoody, salt-tolerant plants
• They begin as tidal mud flats, gaining height as more sediment is deposited
• This builds up to and above the level, and frequency of tidal flooding ensuring that the soil never dries out and remains muddy and sticky
• Pioneer species of halophyte plants begin to colonise
• As these plants die and add nutrients to the soil, sediment builds up. This makes the conditions more favourable and other species start to develop.
• The process of the development of vegetation, over time is known as succession. In a salt marsh, this is known as a halosere
• The lower marshes are flooded daily by the rising tide.
• They are good coastal defences in some areas, acting as a natural buffer against coastal erosion and flooding
• However, in many areas they have been reclaimed for agriculture or development, and are threatened by human activities