Final answer:
Estuaries are characterized by a salinity gradient, which is influenced by the mixing of river water with ocean water. The varying salinity levels present a physiological challenge to local flora and fauna, which have adapted in various ways to cope with these conditions.
Step-by-step explanation:
A key feature of estuaries is a salinity gradient. An estuary is where a river meets the ocean, mixing fresh water with salt water, creating brackish water. This mixture results in areas with varying salinity levels, which are influenced by factors such as the rate of flow of freshwater, tide patterns, and seasonal rainfall. The diverse salinity in estuaries poses a significant physiological challenge to the plants and animals living there. Many species have adapted to these conditions; for instance, halophytic plants have specialized adaptations to manage salinity. Similarly, animals such as mussels and clams have developed behavioral adaptations to cope with rapid changes in salinity and oxygen levels.