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Bay Blog: salinity

Oct
01
2007

Fresh or Salty? Bay's Salinity Makes a Big Difference to Underwater Life

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 salt, in the water you're near.

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 scientifically defined as the number of grams of dissolved salts present in 1,000 grams of water. It is usually expressed in parts per thousand (ppt).

Fresh water contains few salts (less than 0.5 ppt) and is less dense than full ocean-strength sea water, which averages 25 to 30 ppt.

The Chesapeake Bay's salinity is highest at its mouth, where sea water from the Atlantic Ocean enters. As you head north in the Bay, salinity gradually decreases.

Water with salinity greater than 0.5 ppt but less than 25 ppt is called brackish, meaning a combination of salt water and fresh water. Most of the water in the Chesapeake Bay is brackish.

On a map, salinity contours called isohalines mark the salt content of surface waters. Isohalines tend to show a southwest-to-northeast tilt for two reasons:

The greatest volume of fresh water enters the Bay from its northern and western tributaries, such as the Susquehanna and Potomac rivers.

The earth's rotation causes a phenomenon called the Coriolis Force. Flowing water in the Northern Hemisphere deflects to the right, which means that saltier water moving up the Bay veers towards the Eastern Shore. Therefore, water near the Eastern Shore is saltier than water on the western side of the Bay.

Salinity in the Chesapeake Bay fluctuates depending on precipitation and the season. In drought years, such as 2007, salinity increases because less fresh water flows from the Bay's rivers. This can have both mixed effects on Bay species like oysters, blue crabs and underwater bay grasses.

Oysters have higher spat sets in years when salinity is high. However, MSX and Dermo, the two diseases that have ravaged Bay oyster populations, also flourish in saltier waters.

When more sea water enters the lower Bay and creeps northward, crabs migrate to the upper reaches of the Bay and the headwaters of rivers to escape the high salinity. This northerly migration can benefit crab reproduction, as females lay their egg masses further up the Bay rather than near the ocean. Their location, coupled with decreased fresh water flow, allows crab larvae to stay closer to the Bay, where they are more likely to survive than if they were swept out into the ocean.

Dramatic changes in Bay salinity can have adverse effects on bay grasses because most species specifically require either salty, brackish or fresh waters to grow. Since bay grasses cannot migrate with changing salinity, they ultimately die.

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.



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