An estuary is a partially enclosed coastal body of water where freshwater from rivers and streams meets and mixes with saltwater from the sea. Estuarine habitats are characterised by brackish water (a mixture of fresh and salt water) and fluctuating salinity.
A lagoon is a classic example of an estuarine habitat. Lagoons are shallow bodies of water separated from the open sea by sandbars, barrier islands, or coral reefs, but they maintain a connection with the sea. In West Africa, well-known examples include the Lagos Lagoon and the Ebrié Lagoon. The water in lagoons is typically brackish because freshwater from rivers drains into them while seawater enters through inlets.
The other options are not estuarine habitats:
A river is a freshwater habitat. Although a river may flow into an estuary, the river itself is not estuarine.
Lakes are inland freshwater bodies with no direct connection to the sea.
The ocean is a fully marine (saltwater) habitat, not a zone of freshwater-saltwater mixing.
For examination purposes, remember that estuarine habitats are defined by the mixing of fresh and salt water. Lagoons, mangrove swamps, and river deltas are the most commonly cited examples.
