They contain saltwater and freshwater
They contain elements of both freshwater and saltwater ecosystems.
Only animals that can live in freshwater or saltwater live in an estuary because estuaries are places where freshwater and saltwater meet.
In all of the world's oceans. Some can even live in estuaries (where the freshwater from rivers or streams connect with the saltwater from sea).
Estuaries
saltwater freshwater saltwater freshwater
they are both saltwater and freshwater
An area where fresh river water mingles with tidal sea water is termed 'brackish'. Examples are: estuaries and mangrove swamps.
The Persian Gulf is saltwater, not freshwater.
Estuaries.
Most lakes contain freshwater. From this fact, I'm assuming it is made out of freshwater, rather than saltwater.
freshwater
Freshwater