Final answer:
The influx of seawater at depth in an estuary creates a dynamic environment of varying salinity, necessitating adaptations by local plant and animal species. Halophytic plants and behavioral adaptations in animals like mussels and clams help them survive the changing conditions.
Step-by-step explanation:
The influx of seawater at depth within an estuary brings in saltwater during high tides, which mixes with freshwater from sources such as rivers, creating a brackish water environment. This mixing action results in the short-term and rapid variation in salinity, posing physiological challenges for the local flora and fauna.
To deal with these changes, many estuarine plant species have become halophytes, equipped to tolerate high salt conditions, while animals, such as mussels and clams, have developed behavioral adaptations like switching between aerobic and anaerobic respiration depending on the salinity and oxygen levels.
During periods of low salinity, these animals conserve energy by closing their shells and ceasing to feed. However, when the tide brings in higher salinity waters, they open their shells and resume feeding using aerobic respiration. The introduction of saltwater thus plays a crucial role in both the biological and physical characteristics of estuaries, impacting nutrient cycles and the very adaptations of species that inhabit these transitional zones between the riverine and marine environments.