Lake Pontchartrain's fresh water surge changes ecology

LACOMBE, La. — Lake Pontchartrain is getting the biggest flush of fresh water it has had in decades, and for some that’s a bad thing.

The water in America’s 11th largest lake has become increasingly brackish over the years, allowing bull sharks and other marine life to flourish while killing off cypress swamps along the lake’s southwestern sides.

That chemistry has been disrupted, though, by the U.S. Army Corps of Engineers’ opening of a spillway on the Mississippi River for nearly 100 days — a record — sending fresh water gushing into the lake and into bayous and estuaries that feed into the Gulf of Mexico.

The immediate effect is explosive blooms of cyanobacterium, the blue-green algae that has led officials to shut down Mississippi’s beaches and close popular fishing spots in Lacombe and elsewhere.

It also has packed a wallop on some fisheries and marine life, prompting a scramble by local officials seeking federal assistance.

“Each opening of this flood control structure causes significant ecological impacts,” top state officials wrote in a request to the Commerce Department. “These impacts disrupt the natural processes of many different species of aquatic life; in particular oysters, crab, shrimp, and certain inshore and nearshore finfish species.”

Yet many scientists say it’s not an ecological catastrophe.

“I think the tendency is to think all fresh water is bad, but that’s not necessarily true,” said Brian LeBlanc, a professor at Louisiana State University and water quality specialist who also consults with the National Oceanic and Atmospheric Administration.

“This could turn out to be a good thing,” Mr. LeBlanc said. “Once you remove the insult to the system, it doesn’t usually take long for Mother Nature to repair itself. If they close the spillway soon, usually in a month or so these things settle and the flush is often beneficial down the road to organisms in the lake.”

But it poses some problems at least in the near term. The intricate network of southeastern Louisiana’s lakes and bayous has become saturated with nitrates and phosphorus that has run off from farmlands upstream on the Mississippi, the river that serves as the continent’s large intestine, draining rivers and streams in parts of 31 states and two Canadian provinces.

Just how bad the situation has become is not clear.

NOAA has declared an “unusual mortality event” with bottlenose dolphins along the Gulf of Mexico’s northeastern rim.

From Feb. 1 to July 18, 291 bottlenose dolphins were stranded in coastal Gulf waters from Louisiana to the Florida Panhandle, said Teri Rowles, who runs NOAA’s Marine Mammal Health and Stranding Response Program.

“We do not know the causal factor with certainty right now,” Ms. Rowles said. “We do know it’s been one of the wettest years in the past 100 years, and so a lot of fresh water is pouring into the area.”

Dolphins tend to stay in an area — researchers call them ¨resident dolphins¨ — even when their environment becomes less hospitable.

Other species may do better.

“For a lot of animals, we expect they can adapt,” said Julie Anderson Lively, a fisheries specialist and assistant professor at LSU, who noted that most marine life will simply swim to places where the water is more to their suiting.

One organism that doesn’t have the luxury of choosing a new home is the oyster. Oysters typically take three years to grow to market size, and the beds are just recovering from freshwater inundations in 2010 designed to push off oil spilled in the Deepwater Horizon drilling disaster and to prevent flooding in 2011, Ms. Lively said.

The impact from Hurricane Barry last weekend is still being assessed. A storm surge would inject more salt water into the system, but the churn can break up the algae blooms. Just how successful that process was isn’t clear.

“These storm surges are something of a mixed blessing,” Ms. Rowles said. “It’s true they inject more salt water, but there’s also the addition of the rainfall and greater runoff.

All of this is happening because the Bonnet Carre Spillway has been open a record number of days. It also marks the first time in 88 years that the Army Corps of Engineers has had to open the release valve twice in one year and the first time it has done so in back-to-back years.

Heavy rains upstream are the culprit, scientists say, because they raise the Mississippi’s levels. When those hit certain points — 18 or 19 feet, say, on some gauges — the corps is required to open the spigot and blow off pressure.

The corps said it wants to begin turning off the faucet this month, but more heavy rain anywhere in the Mississippi River drainage basin could change those plans.

The fishing industry grumbles that the corps concentrated the release in the eastern sectors and didn’t equalize the situation by opening the Morganza Spillway, a valve roughly 100 miles northwest that pours fresh water into the Atchafalaya Basin southwest of the Mississippi River’s course.

Now analysts wonder whether the “dead zone” that develops in the Gulf every year will increase or shrink.

The fertilizer that pours into the Mississippi is spreading into areas beyond its usual limits, and the estimated amounts are more than two-thirds higher than the long-term average from 1980 to 2018, federal officials said.