The characteristics and distribution of coral reefs

Coral reefs are unique marine ecosystems constructed entirely from living organisms, primarily corals, which form vast deposits of calcium carbonate.

Factors influencing coral reef distribution

• Temperature requirements - Coral growth demands a minimum sea temperature of 18°C, with optimal growth occurring between 23°C and 25°C.
• Light availability - Corals rely on sunlight for growth, restricting their presence to shallow waters.
• Water depth constraints - Due to light needs, most reefs are found in seas less than 25 metres deep.
• Salinity dependence - As marine organisms, corals require saltwater to survive.

Local conditions affecting coral reef development

• Wave action necessity - Strong wave activity ensures well-oxygenated water, which is essential for coral health.
• Exposure to air risks - While corals need oxygenated water, prolonged exposure to air can be fatal.
• Sediment impact - Clear water is crucial; sediment can obstruct feeding mechanisms and reduce light penetration.

Global distribution

Coral reefs with the greatest biodiversity are predominantly located in South-East Asia and northern Australia. The Great Barrier Reef in Australia stands out for both its immense size and diverse species.

The features and global spread of mangroves

Mangroves are unique coastal ecosystems that straddle the boundary between land and sea, thriving in the intertidal zone.

Global distribution of mangroves

Mangroves are most prevalent in South-East Asia, where they are believed to have originated before spreading worldwide. They are generally found within 30° latitude of the equator, though some resilient species extend to temperate regions, such as the North Island of New Zealand.

Adaptations to challenging environments

• Intertidal survival - Mangroves endure regular flooding by seawater and occasional freshwater floods during high rainfall, adapting to rapid changes in salinity and temperature. They also cope with heat and mud.
• Salt filtration - Each mangrove species possesses a filtration system to exclude much of the salt from seawater.
• Root adaptations - Complex root systems, including snorkel-like roots for air intake and prop roots or buttresses for stability, enable survival in soft, muddy sediments.
• Reproductive strategy - Floating fruits and seedlings allow mangroves to colonise new areas as they drift with tides and lodge in mud to grow.

Biodiversity and ecological role

South-East Asia hosts mangroves with the highest biodiversity globally, with species ranging from small shrubs to towering trees exceeding 60 metres tall. Their root systems trap sediment, gradually building up land in the intertidal zone and extending coastal areas.

The formation and ecology of salt marshes

Salt marshes are dynamic ecosystems situated in the intertidal zone, positioned between permanently submerged mudflats and terrestrial vegetation above the high-tide mark.

Locations and conditions for salt marsh development

• Sheltered environments - Salt marshes form in areas protected from open sea waves, such as bay heads and estuaries.
• Water composition - In estuaries, water is brackish (a mix of salt and freshwater), while in bays, it is predominantly salty.
• Tidal flooding variation - Low marsh areas may flood twice daily, whereas high marsh zones flood only during storms or exceptional tides.

Ecological zoning and vegetation

The type of vegetation and wildlife in salt marshes depends on salinity and the frequency of tidal flooding. Different zones within a marsh support distinct plant communities:

Physical features and sediment deposition

Salt marshes are characterised by meandering creeks that facilitate the inflow and outflow of tidal water. These creeks, along with marsh vegetation, reduce tidal and wave energy, leading to continuous deposition of silt and mud. Over time, this process causes the marsh to extend seawards.

The development and vegetation of coastal sand dunes

Coastal sand dunes are wind-shaped accumulations of sand forming mounds and ridges along shorelines.

Conditions for sand dune formation

• Sand availability - Dunes require a wide beach with abundant sand to form.
• Wind direction - Onshore prevailing winds (blowing from sea to shore) are essential for transporting sand inland.
• Accumulation sites - Sand often gathers around obstacles like driftwood or seaweed at the back of the beach.

Stages of dune development

Sand dunes develop in a sequence of ridges parallel to the shoreline. When a beach dries at low tide, onshore winds blow sand inland, forming small embryo dunes around obstacles. As these grow, new dunes may form seaward, pushing older dunes further inland where they become sheltered and more stable.

Plant succession in sand dunes

Vegetation plays a crucial role in stabilising dunes, with plant communities changing as dunes age and move inland. Early colonisers face harsh conditions, while later species benefit from improved environments.

Initial challenges for pioneer plants:

• High salinity levels.
• Limited moisture due to quick-draining sand.
• Strong winds.
• Temporary burial by wind-blown sand.

Early species like sea twitch and sea couch grass must withstand these conditions. As conditions improve with plant establishment, more species appear, leading to dune heath in temperate regions. This sequence is known as plant succession.

Zonal vegetation in a typical dune system