Final answer:
Brackish water is created in estuaries, where freshwater from rivers mixes with ocean saltwater. The salinity of these waters varies, leading to unique adaptations in resident plants and animals, such as halophytic plants and mussels that can tolerate salinity changes.
Step-by-step explanation:
Brackish water is formed at the interface where freshwater, such as from a river, meets the saltwater of the ocean. This mixing of waters occurs in environments known as estuaries, which are biomes characterized by diluted saltwater or brackish water. The salinity in these areas varies depending on factors such as the rate of freshwater flow and tidal movements. During high tide, saltwater enters the estuary, and during low tide, the flow reverses, which leads to the fluctuating salinity levels that can pose physiological challenges for the resident plants and animals. For instance, halophytic plants in estuaries have developed adaptations, such as filters in their roots, to deal with these saline conditions. Similarly, animals like mussels and clams exhibit behavioral adaptations to cope with the changing salinity, such as switching between aerobic and anaerobic respiration.