Coastal Ecosystems (CIE IGCSE Geography)

• Coral reefs and atolls are formed through the build-up and compression of the skeletons of lime secreting, marine animals called polyps
• Living coral polyps are found in the upper and outer part of the coral reef only
• Their skeletons are hard, calcareous masses, which form when one generation dies and the next grows on top, creating an upward and outward reef
• There must be a solid surface to begin the growth of corals, this can be from a shipwreck or debris from elsewhere
• Coral reefs run parallel to the coast, with breaks where river mouths exit
• Coral reefs are very sensitive and cannot grow anywhere

• 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

	Global Features
Temperature	Corals cannot tolerate water temperatures below 18°C but grow best at 22°C – 25°C. Some can stand temperatures as high as 40° C for short periods. This is why coral reefs normally grow between the Tropic of Capricorn and the Tropic of Cancer -30° of the equator
Light	Corals need light for photosynthesis due to the algae, called zooxanthellae, that live in their tissue
Water	Corals are generally found at depths of less than 25m where sunlight can penetrate. The water must also be clear and clean to allow for optimum photosynthesis to occur
Salinity	Since corals are marine animals, they need salty water to survive, ranging from 32-42% salt water

• 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:

Type	Example	Features
Fringing	Coral Coast of Fiji	These are low, narrow bands of coral, running parallel to the coast and form around a land mass. They are covered by narrow, shallow lagoons at high tide. Their outer edges slope steeply down into the sea beyond. The landward side of the reef has a higher outer edge that rises to the high tide level.
Barrier	Great Barrier, Australia	They range from 500m to several kilometres from the coast and are separated by wide deep lagoons below the depth at which the polyps can live. The Great Barrier Reef has almost 3000 reefs, separated by channels stretching more than 2300km.
Atolls	Maldives Suvadiva Atoll	These are narrow, ring-shaped reefs, consisting of a coral rim that encircles a deep lagoon.  Sometimes, they may encircle and protect an island. Channels between islets connect a lagoon to the open ocean or sea.

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

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 halophyte
• 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

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