Salinity measures the amount of dissolved salts in the water. The Chesapeake Bay has a wide range of salinity. Up north where the Bay is fed by freshwater rivers like the Susquehanna, the salinity is as low as 0.5. By the time you reach the mouth of the Bay down in Virginia, the salinity can be as high as 30. The middle of the bay is considered brackish which is used to describe waters that are a mixture of fresh and salt water.
Salinity not only varies by location, but by time of year and depth as well. The Bay has a higher salinity during drier months and a lower salinity during the wetter months, especially after the winter snow melts and the spring rains. Salinity also increases with depth because the less dense fresh water remains on the surface.
Many plant and animal species are limited by the different salinity ranges so depending where you are in the Bay, you will be able to see different flora and fauna.