Explain the conditions for coral reef and mangrove development, the coastal services they provide, and the threats they face

What this dot point is asking

SEAB wants you to explain the conditions that coral reefs and mangroves need, the services they provide to tropical coasts (especially coastal protection and biodiversity), and the threats they face. The central insight is that these are biogenic coasts, built and held together by living organisms, so they are extremely valuable but acutely sensitive to changes in temperature, water quality and sediment.

The answer

Conditions for coral reef growth

Reef-building (hermatypic) corals live in symbiosis with zooxanthellae, algae in their tissues that photosynthesise and supply most of the coral's energy. This symbiosis sets the conditions:

• Warm water, about 23 to 29 degrees Celsius, confining reefs to the tropics and subtropics.
• Clear water, low in sediment, so light reaches the algae.
• Shallow water, generally less than about 25 metres, within the sunlit photic zone.
• Normal marine salinity, so reefs avoid river mouths with freshwater dilution.
• Clean, agitated, well-oxygenated water delivering nutrients and clearing sediment.

The temperature requirement is the dominant control on their tropical distribution.

Conditions for mangrove growth

Mangroves are salt-tolerant trees of sheltered tropical and subtropical intertidal zones. They thrive where:

• The coast is sheltered and low-energy (lagoons, estuaries, behind reefs), so seedlings can establish;
• There is a muddy, fine, anaerobic substrate rich in sediment;
• The water is warm and brackish to saline, in the intertidal range.

Their aerial roots cope with waterlogged, low-oxygen mud and trap sediment.

Coastal services

• Coastal protection. Reefs act as natural breakwaters, dissipating wave energy offshore; mangrove roots absorb wave energy and storm surge and bind sediment, reducing erosion and flooding.
• Biodiversity and fisheries. Both are hotspots of marine biodiversity and act as nurseries for fish, underpinning local fisheries.
• Carbon and water quality. Mangroves store large amounts of blue carbon and filter pollutants and sediment from runoff.
• Economy. Both support tourism and livelihoods.

Threats

• Coral reefs: warming seas cause bleaching (corals expel their zooxanthellae and may die); ocean acidification weakens skeletons; sedimentation and pollution from land runoff smother corals; destructive fishing and coastal development add direct damage.
• Mangroves: clearance for aquaculture (shrimp ponds), reclamation and development; pollution; and altered sediment supply.

Examples in context

Example 1. Singapore's reefs and mangroves under pressure. Despite heavy reclamation and sedimentation, Singapore retains coral around the southern islands such as Pulau Hantu and mangrove at Sungei Buloh and Pulau Ubin's Chek Jawa. Conservation includes coral relocation during reclamation and mangrove protection in nature reserves, showing both the threat from coastal development and active efforts to retain biogenic coasts in a highly urbanised setting.

Example 2. The Great Barrier Reef and mass bleaching. Australia's Great Barrier Reef has suffered repeated mass-bleaching events in recent years as marine heatwaves raised sea temperatures beyond corals' tolerance, killing large areas of coral. It is the clearest large-scale demonstration of how warming seas threaten reefs globally, and of the consequent loss of coastal protection and biodiversity.

Try this

Q1. Explain why coral reefs are confined to shallow tropical seas. [3 marks]

• Cue. Reef corals depend on symbiotic zooxanthellae that photosynthesise, so they need warm water (about 23 to 29 degrees Celsius) and clear, shallow, sunlit water within the photic zone, conditions found mainly in the tropics.

Q2. Explain how mangroves reduce coastal erosion. [2 marks]

• Cue. Their dense aerial roots increase friction and absorb wave and tidal energy, while trapping sediment; this dissipates energy and promotes accretion, protecting the shore behind.

Q3. Explain why coral bleaching threatens the survival of a reef. [3 marks]

• Cue. Heat stress makes corals expel their zooxanthellae, losing their main food source; prolonged bleaching starves and kills the corals, so the reef stops growing and can collapse, taking its protective and ecological functions with it.