What are the key features of estuarine ecosystems?

The correct choice emphasizes that estuarine ecosystems are formed by the mixing of freshwater from rivers and saltwater from the ocean. This unique combination creates a nutrient-rich environment that supports diverse biological communities. Estuaries are critical habitats for various marine species, including fish, birds, and invertebrates, providing essential breeding, feeding, and nursery grounds.

The mixing of freshwater and saltwater leads to varying salinity levels, which can support a wide range of organisms adapted to those conditions. This dynamic environment fosters high productivity and biodiversity, making estuaries vital for both marine and terrestrial life.

The other options are not accurate representations of estuarine ecosystems. For example, while certain areas of an estuary can be impacted by pollution, they are generally not devoid of life; rather, they can be rich in biodiversity. Additionally, estuaries are not characterized by stagnant water; they experience tidal flows that mix the water, facilitating ecological processes. Finally, it's incorrect to say that only saltwater species inhabit these regions, as estuaries support both freshwater and saltwater organisms due to their mixed conditions.