A plume of sediment-laden river water is slowly spreading along the southern reaches of Lake Pontchartrain and working its way toward the Gulf of Mexico as the Bonnet Carre Spillway throws 186,000 cubic feet per second of fresh river water into the salty lake.

The potential effects of the continuing operation of the spillway, which relieves flooding pressure downriver, are a mixed bag.

In addition to sediment, the colder and fresher river water also carries nutrients from upriver agricultural practices, carries the possibility of invasive species and has at least the temporary effect of moving out certain species of fish that want more-brackish water.

At the same time, the additional nutrients can provide a base for better growth of other species, the cold water may mean the impact on oysters will be lessened and nutrient blooms apparent in previous spillway operations may be delayed, if they happen at all.

Only time will tell, because a January opening of the spillway hasn’t happened since 1937. Although not unprecedented, the early opening could mean different effects will be seen in the coming months.

Harry Blanchet, fisheries biologist with the state Department of Wildlife and Fisheries, said the influx of fresh water will move fish around and mostly out of the system as they look for saltier waters.

After the spillway opens, “Lake Pontchartrain gets a lot fresher and a lot muddier,” he said. The amount of fish movement depends on how long the spillway is opened, how much fresh water gets pumped into the system and if any major rains add even more fresh water to the lake.

On Wednesday, the U.S. Army Corps of Engineers held steady at 210 of the 350 bays opened on the spillway as the river reached its crest along the lower river.

Charter boat captain Dudley Vandenborre said it could take two or three months for the salinity to return to the lake.

“Being this early, I don’t think anyone really knows what’s going to happen,” he said. However, he said, on many levels, the lake hasn’t really returned to normal since the 2011 spillway opening.

In addition to possible reduction in crab catch this year, the additional fresh water could reduce the movement of young brown shrimp into the lake for the spring shrimp season, Blanchet said.

Although brown shrimp will continue moving into the lake and surrounding marsh through April, the true impact won’t be known until May, when Department of Wildlife and Fisheries staff check the numbers and size of the shrimp to set the season.

On the flip side, there could be benefits for the white shrimp season later in the year, Blanchet said, noting that those shrimp are fresh-water tolerant.

Although there’s not much oyster harvest in Lake Pontchartrain, there are oyster leases in Lake Borgne and Mississippi Sound, both of which could be affected. In the past, that impact has ranged from widespread deaths of oysters to one of little effect, Blanchet said.

The good news this year is that oysters can tolerate more fresh water if water temperatures are colder like they are now. The state is monitoring water conditions to determine if emergency actions are needed to relocate oysters, but so far, that doesn’t appear to be necessary, Blanchet said.

In fact, the fresh water may help, as parts of the Mississippi Sound are closed for oysters because of a red tide algae bloom. Red tide, he said, doesn’t do well in fresh water, so the extra flow from the Bonnet Carre could help mitigate that problem.

Continuing water-quality monitoring is being done by a number of agencies, including the Lake Pontchartrain Basin Foundation, the Department of Wildlife and Fisheries and the state Department of Environmental Quality.

Tom Killeen, administrator with the DEQ inspection division, said they are monitoring water characteristics every two weeks and taking water samples to analyze for nutrients and chemicals every month throughout the high-water event.

The first round of testing was done Jan. 8, before the spillway was opened, and another round was conducted Jan. 13 although the river water hadn’t yet reached the Causeway, where the water monitors are located.

The next tests will be conducted Jan. 27, Killeen said.

“I expect that we will see a slight elevation in the nutrient parameters,” Killeen said. “But it will be temporary.”

One concern of a Bonnet Carre opening is always that the extra nutrients carried by the Mississippi River could promote algae blooms in the lake. And that, in turn, could cause dead zones of low oxygen levels as the algae die and decompose.

“In 1997, that was the last time we really saw massive (algae) blooms in Lake Pontchartrain,” said John Lopez, coastal program coordinator with the Lake Pontchartrain Basin Foundation, adding that it included toxic blooms as well. “It really was terrible.”

In 2009, the Bonnet Carre opening was smaller in scale and there were only local algae blooms that occurred later in the year, typically in the northwest corner of the lake.

In 2011, there were no major blooms in Lake Pontchartrain, although dead zones of low oxygen and algae blooms did appear in the Mississippi Sound, Lopez said.

As monitoring continues this year, the foundation will be looking for how the nutrients flow through the system and watching for any algae blooms. However, with the cold water temperatures, it’s possible that any algae growth could be stunted, he said, unless warm weather continues and springlike conditions arrive early.

Follow Amy Wold on Twitter, @awold10.