Bay Water Salinity…Or Lack Thereof


The salinity where the Mobile Bay water meets the Gulf of Mexico has plummeted in the past month. 

Salinity in estuaries can change from day to day depending on a number of environmental factors such as tide and weather. 

Photo courtesy of the Mobile Bay National Estuary Program. 

What is an estuary? 

According to the NOAA Ocean Service Education, an estuary is “a body of water where saltwater from the ocean mixes with fresh water from rivers or streams. In freshwater, the concentration of salts, or salinity, is nearly zero. The salinity of water in the ocean averages about 35 parts per thousand (ppt). The mixture of seawater and fresh water in estuaries is called brackish water and its salinity can range from 0.5 to 35 ppt.” 

Here’s the Dauphin Island salinity graph provided by the Mobile Bay National Estuary Program. 

So what’s caused this major drop of salinity in the estuary?

All the rain we have been getting, both locally and far upstream. Since fresh water does not contain salt and we have gotten quite a bit of rain in the last two weeks, more fresh water is being mixed into the Gulf than sea water. You can really see the effect that the large amount fresh water has had on the amount of salt in the water with the graph above.

Although the rain we have received here locally contributed to the initial drop in salinity, it would not have been enough to have kept the levels as low for as long without the help of the rainfall flowing from upstream.

There’s a direct correlation between the two graphs shown above. Notice when the rainfall began to spike, the salinity began to fall. 

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