An ecosystem that contains both salt water and fresh water is called an estuary.

Here's why:

* Estuaries are areas where freshwater rivers and streams meet the ocean. This creates a unique environment where salty ocean water mixes with fresh water, resulting in brackish water (water with salinity levels between freshwater and saltwater).

* Examples of estuaries include:

* Salt marshes

* Mangrove swamps

* River deltas

* Fjords

Estuaries are incredibly diverse and productive ecosystems, supporting a wide variety of plant and animal life. They play a crucial role in:

* Filtering pollutants from freshwater runoff

* Providing nursery habitats for fish and shellfish

* Protecting coastlines from erosion

Let me know if you'd like to learn more about any specific type of estuary!