B4.1B Coral reefs, biomes and biome adaptations

Reef Corals Build Only Where Several Abiotic Conditions Line Up At Once

1. Reef-building corals are called hermatypic corals, and they lay down hard skeletons of calcium carbonate.
2. Their polyps live in a mutualism with zooxanthellae, photosynthetic algae that live inside the coral tissue.
   1. The algae supply sugars from photosynthesis, and the coral supplies shelter and nutrients in return.
3. This partnership is why reef corals only build where light, temperature, water clarity, salinity, pH, and oxygen all fall inside a narrow range at the same time.

Shallow, Sunlit Water Powers The Algae That Feed The Coral

1. The zooxanthellae need bright sunlight to photosynthesise.
2. Without enough light the algae cannot supply energy, so the coral cannot grow or build its skeleton.
3. Light fades quickly with depth, which limits reefs to shallow water.
   1. Reefs usually form in the upper 50 metres of water.
   2. Below this depth too little light reaches the algae for useful photosynthesis.

Clear, Low-Sediment Water Lets Light Reach The Reef

1. Low turbidity keeps water clear so sunlight can reach the zooxanthellae.
2. Low sedimentation stops particles settling on and smothering the coral.
3. Coastal development and deforestation raise sediment runoff into reef waters.