Why are coral lagoons limited mainly to tropical seas?

Coral lagoons are restricted to tropical seas because they are formed by coral reefs, which require specific environmental conditions to grow and thrive.12

These conditions include:

• Temperature: Reef-building corals are best developed in surface waters with mean annual temperatures of approximately 23–25 °C (73–77 °F). Significant reef development generally does not occur where temperatures fall below 18 °C (64 °F).3
• Light: Tropical seas provide the necessary light intensity and radiant energy required for the growth of the zooxanthellae, the symbiotic organisms essential to reef corals.4
• Salinity: Corals are restricted to seawater with normal oceanic salinity, typically between 30 and 40 parts per thousand, which ensures the water remains supersaturated in calcium carbonate for skeleton formation.3

Because these biological and geochemical requirements are primarily met in tropical or near-tropical waters, coral lagoons—which are defined by the presence of these organic structures—are geographically limited to these regions, typically within 25° latitude of the Equator.41