Final answer:
A key feature of estuaries is the mixing of fresh and saltwater which creates brackish water and affects the organisms within. Adaptations of estuarine organisms are specialized to deal with varying salinity, leading to a diverse ecosystem.
Step-by-step explanation:
Estuaries are dynamic environments where the mixing of fresh and saltwater creates unique ecological conditions. This mixing leads to brackish water with varying levels of salinity, making estuaries distinct from other aquatic biomes. A key feature of estuaries is not high salinity nor low biodiversity, nor specifically sandy soil; rather, it is the characteristic mixing of freshwater from rivers and saltwater from the ocean, influencing the adaptations of the organisms that thrive in these environments.
Organisms in estuaries, such as halophytes and mollusks, must adapt to the changing salinity levels. These plants and animals have specialized adaptations that allow them to survive in conditions that can rapidly fluctuate due to tidal movements. Plants may have salt-filtering roots, while animals like mussels and clams adjust their feeding and respiration strategies based on the current salinity and oxygen levels.