The Calm Water Behind a Reef: It's Called a Lagoon

Definition

A lagoon is a shallow body of water separated from a larger water body (usually the ocean) by a barrier—coral reef, barrier island, sandbar, or atoll rim.

Formation Types

• Coral reef lagoons: Fringing, barrier, or atoll reefs enclose shallow water (e.g., Great Barrier Reef lagoon, atoll lagoons)
• Coastal lagoons: Barrier islands/spits parallel shore (e.g., Venice Lagoon, Outer Banks sounds)
• Estuarine lagoons: River mouth partially blocked by sediment

Characteristics

• Calm water: Barrier dampens wave energy
• Shallow: Typically < 10 m, often < 2 m
• Variable salinity: Brackish to hypersaline depending on freshwater input/evaporation
• High productivity: Light penetration + nutrients = seagrass, algae, nurseries
• Sediment traps: Accumulate organic/inorganic matter

Ecological Importance

• Nursery grounds for fish, crustaceans
• Seagrass meadows, mangrove fringes
• Bird feeding/roosting
• Carbon sequestration (blue carbon)
• Storm buffer for coastlines

Human Connections

• Tourism: snorkeling, swimming, overwater bungalows (Maldives, Bora Bora)
• Fisheries: artisanal, aquaculture
• Transport: sheltered navigation
• Threats: pollution, dredging, sea level rise, reef degradation

Famous Examples

• Great Barrier Reef Lagoon (Australia)
• Chuuk Lagoon (Micronesia) — WWII wreck diving
• Venice Lagoon (Italy)
• Lagoa dos Patos (Brazil) — largest coastal lagoon
• Lake Worth Lagoon (Florida)