What defines an estuary ecosystem?

An estuary ecosystem is defined as a transitional area where saltwater from the ocean meets and mixes with freshwater from rivers or streams. This mixing creates unique environmental conditions that are distinct from both marine and freshwater systems. In an estuary, the salinity varies due to the inflow of freshwater and tidal actions, leading to a diverse range of habitats that support various plant and animal species.

The presence of both saltwater and freshwater makes estuaries incredibly rich in nutrients, which support high biological productivity. Estuaries often serve as vital nurseries for many marine species and provide important ecosystem services such as water filtration, flood protection, and habitat for wildlife.

The other options describe different types of ecosystems that do not embody the characteristics of an estuary. For instance, a freshwater lake surrounded by wetlands does not involve the mixing of saltwater and freshwater, while a coastal area with no freshwater input or a location with only saltwater would not be classified as an estuary either. These details help to clarify why the correct answer highlights the unique blending of saltwater and freshwater in defining an estuary ecosystem.