The extra nutrients from the Mississippi River that poured into Lake Pontchartrain while the Bonnet Carre spillway was open haven’t created the expected large algae blooms, at least not yet.
The U.S. Army Corps of Engineers started opening the Bonnet Carre spillway May 9 to divert part of the flow of the Mississippi River during this spring’s high water. The corps began closing the spillway structure June 20. During that month and a half, fresh water, sediment and nutrients from the river were carried into the lake.
With the closing of the spillway, the lake is slowly returning to normal, but water monitoring continues.
“There really weren’t too many surprises,” said Chris Piehler, administrator with the state Department of Environmental Quality’s Inspection Division. As expected, the salinity in the lake dropped while nutrient levels increased, he said.
“We saw water quality in the lake look like the Mississippi River,” he said.
The only surprise was what didn’t happen: There were no algae blooms that usually grow with the addition of nutrients.
Those algae blooms occurred in the lake after the Bonnet Carre spillway had been opened in 1997 and 2008, he said. “We really expected that to be very significant this time,” Piehler said.
Although samples have shown algae levels are higher than years when the Bonnet Carre structure remains closed, the levels weren’t higher than those in 1997 and 2008.
Piehler said there’s been some speculation as to why. One hypothesis is that wave energy on the lake kept the water stirred up so much that algae couldn’t access the sunlight it needs to grow, he said.
Another hypothesis is that in previous years the algae — which prefers freshwater environments — was flushed out of streams that run into the lake, essentially providing a seed source for the algae blooms. This year, the drought kept the streams and bayous from being flushed out, so the source algae seed wasn’t put into the lake, Piehler said.
It may also be that not enough time has passed for the blooms to have occurred.
John Lopez, director of the coastal sustainability program with the Lake Pontchartrain Basin Foundation, said in previous years when the Bonnet Carre was opened, the algae blooms didn’t appear until two months after the closure of the spillway.
“So we still think the blooms are coming,” Lopez said. “But it’s possible we may not see it.”
If the salinity levels in the lake increase before conditions improve for algae growth, it’s possible the salinity could hamper the algae blooms, he said.
The foundation is continuing water monitoring in the lake along the Causeway bridge and recently started going by boat to the west and northwest areas of the lake — where algae blooms have been located in the past.