by Rebecca Chillrud
March 20, 2020
Have you ever noticed that the air smells salty in some places on the Chesapeake Bay, while in others it does not? The salt you smell has to do with the salinity, or amount of dissolved salt, in the water.
The Chesapeake Bay is an estuary—a partially enclosed body of water where fresh water from rivers and streams mixes with salt water from the ocean. About half of the water in the Bay comes from salt water from the Atlantic Ocean. The other half drains into the Bay from its enormous 64,000-square-mile watershed.
The Chesapeake Bay's salinity is highest at its mouth, where sea water from the Atlantic Ocean enters. On the upper Bay in Maryland, where the Bay's salinity is lowest, you probably won't smell that “ocean” smell. But near the mouth of the Bay in Virginia, where salinity is highest, the air may smell very salty.
Of course, this “salty air” test isn't exactly scientific. Salinity is usually measured in parts per thousand (ppt), the number of grams of dissolved salts present in 1,000 grams of water. Fresh water, like you would find at the head of the Bay and its tidal rivers, contains few salts (less than 0.5 ppt) and is less dense than the ocean-strength salt water at the mouth of the Bay, which averages 25 to 30 ppt.
In addition to the difference in salinity from north to south, the water on the Bay’s Eastern Shore tends to be saltier than water on the western side, due to two factors:
- Most fresh water enters the Bay from its northern and western tributaries, including the Susquehanna and Potomac rivers.
- The Coriolis Force, a phenomenon caused by the earth’s rotation, pushes flowing water in the Northern Hemisphere to the right, so saltier water moving up the Bay veers toward the Eastern Shore.
Most of the water in the Bay, including in the middle portion of the Bay and its tidal rivers, is brackish. Brackish water is a mixture of salty and fresh, with a salinity level of greater than 0.5 ppt but less than 25 ppt.
Salinity varies widely from season to season and from year to year, depending on the amount of fresh water flowing from the Bay’s rivers. The Bay tends to be fresher in spring, when snow melts and heavy rainstorms are frequent. During drier months, the Bay is usually saltier.
Drastic changes in salinity can have severe consequences for the Bay’s underwater life. For example, oysters require salty waters and they can’t move if conditions change. They can close their shells, but only for a short period before they begin to suffer. Underwater grasses can be affected as well, as different species require different salinity ranges. These grasses are also unable to migrate if salinity changes.
Blue crabs and finfish are able to move to different places in the Bay to find the right conditions for survival, but this can impact commercial and recreational fisheries. When record rainfall led to fresher conditions in the Bay in 2018 and 2019, blue crabs stayed in the saltier southern regions far longer than they would in a typical year. This meant that watermen in the upper Bay experienced a poor crabbing season in 2019, despite the 60% increase in the blue crab population that year.
While much of the Bay’s native life struggles to adapt to heightened freshwater flows, the Bay’s invasive species can take advantage of the low-salinity conditions. One such species is the blue catfish, an invasive fish that used the fresh conditions of 2018 and 2019 to spread and establish populations in freshwater tributaries throughout the Bay. “Today, blue catfish are more widely distributed than perhaps ever,” says Peter Tango, monitoring coordinator for the Chesapeake Bay Program.
So, the next time you're by the Bay—whether at the beach, on a boat or in your neighborhood—take a deep breath and enjoy the salty (or not-so-salty) smell of the air. While it may simply be a breeze for you, the amount of salt you smell means a lot to the critters living below the water's surface.
This post is adapted from one written by Alicia Pimental for the Chesapeake Bay Program blog in October 2007.