When seawater and fresh water combine, the result is water that has a salinity level lower than seawater but higher than fresh water. This is called brackish water or sometimes “briny water”.
As you would probably expect, brackish water usually occurs in areas where fresh water meets sea water. Thus, the most common brackish water spots are estuaries—areas of water between a river and sea. Another example is mangrove swamps, while there are also specific lakes, seas, and marshes that are brackish in nature.
Just like there are differences in organisms that live in seawater and freshwater, organisms that reside in brackish water can also be different. One primary reason for this is the adaptation to the chemical composition of brackish water. While this may differ on specific locations, brackish water contains the same major components as seawater but in different relative abundance.