Final answer:
The salinity of water in estuaries varies both vertically and horizontally.
Step-by-step explanation:
The salinity of water in estuaries can vary both vertically and horizontally.
Estuaries are biomes where fresh water from rivers meets the ocean, resulting in a diluted saltwater. The rate of flow of freshwater sources determines the salinity of the estuary. High tides bring salt water into the estuary, while low tides reverse the current.
Organisms living in estuaries, such as crustaceans, molluscs, and fish, must be able to tolerate a wide variation of salinity due to the short-term and rapid changes in salinity caused by the mixing of fresh water and salt water.