Excessive rain is reducing salinity and affecting seafood

Published 7:00 am Saturday, June 1, 2019

By Skip Rigney

Freshwater is pouring into the coastal waters of Mississippi in volumes that have rarely been seen in our lifetimes. The result is an ecological upheaval that will likely hammer the Coast’s seafood industry.

The primary source of the freshwater is a manmade diversion of water from the Mississippi River.  For much of the spring a portion of the River’s waters have been diverted north of New Orleans through the Bonnet Carre Spillway into Lake Pontchartrain. A large portion of that freshwater then exits through the Rigolets Pass into Mississippi Sound.   

Sign up for our daily email newsletter

Get the latest news sent to your inbox

Mississippi Sound is the body of water between the Mississippi coast and the barrier islands that lie 5 to 15 miles to its south. The Sound is home to shrimp, oysters, and a variety of commercial and sports fisheries.

One key to the Sound’s biological richness is the brackish water usually found there. Saltier than freshwater, but less salty than deep ocean water, salinity in the Sound supports a Goldilocks-like habitat for many marine species at various stages of their life-cycles.

Salinity is a measure of how much salt is dissolved in water. In the deep ocean, salinities of 35 parts of salt per thousand parts of water are common. In Mississippi Sound salinities usually vary between 10 and 25 parts per thousand. According to data from the U.S. Geological Survey (maps.waterdata.usgs.gov) that was the case throughout almost all of 2018. Much of the normal variation is due to dry and rainy spells in south Mississippi and the northshore of Lake Pontchartrain in Louisiana and the resulting changes in local river runoff.

But in February a much bigger source of freshwater began to come into play. Salinities in the Sound have now dropped to less than 5 parts per thousand. The Mississippi Sound has gone from brackish to practically fresh.

The massive influx of freshwater is because the Army Corps of Engineers opened the Bonnet Carre Spillway from February 27 to April 10. And, then, for the first time in history, they opened the Spillway for a second time in one year on May 10. And, it’s still open. As of Thursday, 145,000 cubic feet of river water was pouring through the Bonnet Carre every second. That’s right. Every second.

This week the Corps announced that, for only the third time in history, they plan to open the Morganza Spillway on June 6, sending millions of tons of water into Louisiana’s Atchafalaya River Basin every hour. Most of that freshwater will affect the central and western Louisiana coast. Why the need to divert all this water from the Mississippi River? In the words of an Army Corps’ press release this past Tuesday: “The current flood fight is historic and unprecedented. Today marks the 214th day of the flood fight, and it is expected to surpass the 1973 event (225 days) as the longest flood fight.”

Heavy winter and spring precipitation in the northern and central reaches of the Mississippi River basin has raised the River, which was already high at the end of 2018, to dangerous levels. New Orleans must be protected, and the tools that are available are these spillways. Expect to be hearing a lot more about impacts to the Mississippi and Louisiana seafood industries in the coming months.