Two weeks of Lake Okeechobee discharges haven't caused much damage to the St. Lucie River estuary, but an environmental expert says extending the releases much longer could be devastating.

The Army Corps of Engineers, which started discharging lake water to the river estuary Feb. 25 at an average daily rate of about 323 million gallons, plans for the releases to continue at least until March 16.

More: Lake Okeechobee discharges to St. Lucie River to last 3 weeks

They'd better not last much longer, said Mark Perry, executive director of the Florida Oceanographic Society in Stuart.

On Thursday, the environmental education, research and advocacy nonprofit gave the river's overall water quality a "B" or "good" grade, but gave an "F" to the river's South Fork, where discharged Lake O water enters the estuary.

Low salinity and poor visibility were the main reasons.

More:Lake O discharges a public health disaster, Treasure Coast residents say

Continuing the discharges beyond March 16 could affect the estuary's annual oyster spawning season, which typically begins in late March or early April.

That would threaten a year's worth of new oysters, Perry said.

Feel the pulse?

Oysters are important to the estuary's ecosystem. Oyster beds support more than 300 species of aquatic animals, and the bivalves help make the estuary healthier by filtering impurities out of the water.

Many of the estuary's oysters survived the extended discharges in 2016 because the water was released in pulses: small releases that ramp up and drop down over the course of each week of the discharges.

The pulses are meant to mimic natural heavy rainfall events that add a lot of stormwater runoff into the estuary for a couple of days and slack off before the next storm hits.

The Army Corps says it's pulsing the current round of discharges, but Perry argued it's not a true pulse release.

"During pulse releases in the past, the flow would go up and then come down so that we'd have a day or two with absolutely no water discharged at all," he said. "We're not getting those no-flow days this year, and we really need them. Those are the days when tides are able to reach all the way up into the South Fork and the North Fork, to bring in salty water."

Reasons for releases

The discharges were started in the dry season to reduce the probability of needing to release much higher volumes of water during the summer, when algae blooms are more likely, said Col. Andrew Kelly, the Army Corps commander for Florida.

More: Will discharges now prevent another lost summer?

Under its current operations guidelines, the Army Corps tries to maintain the lake at elevations between 12 feet, 6 inches each June 1, the beginning of the summer rainy season, and 15 feet, 6 inches in late October, the beginning of the dry season.

Several environmental groups have been urging the Army Corps to take the lake lower in the dry season. U.S. Rep. Brian Mast, a Palm City Republican, has suggested going as low as 10 feet, 6 inches.

More: How high, how low should Lake Okeechobee go?

People living south of the lake, including the Glades Lives Matter group, say taking the lake that low threatens water supply for farm irrigation and municipal use.

Likely to drop

The Army Corps doesn't have a target level for the lake to reach before the rainy season, Kelly said.

"We want to take the lake lower in a responsible manner," he said. "Our goal is to release what water we can without causing any significant impacts to the estuaries."

It's likely the lake will drop below 12 feet, 6 inches. It was only half an inch higher than that Friday morning; and with or without more discharges, there are 2½ more months of dry season ahead.

Perry doesn't buy the argument that low discharges now will prevent high discharges in the summer.

"It's like a bully saying, 'Let me beat you up a little now and I won't beat you up hard in the future — unless I get really mad, and them I'm gonna wail on you," Perry said. "Even if the lake starts the summer a foot lower than normal, if it jumps up a couple feet because of heavy rains or a tropical storm or hurricane, the Corps is going to get nervous and we're going to get discharges."

See how discharges ramp up and down — but without no-flow days — in the graphic below:

By the numbers

Since the discharges started Feb. 23:

4.5 billion gallons: Total Lake O water sent to the St. Lucie River

3 ⅜ inches: Amount Lake Okeechobee has dropped

18 to 5 parts per thousand: Drop in salinity in the South Fork of the St. Lucie River

By the end of the 3 weeks of discharges:

6.8 billion gallons: Total amount of Lake O water to the St. Lucie at rate of 323 million gallons a day

Half an inch: Approximate amount discharges to the St. Lucie will contribute to lowering the lake

Sources: Army Corps of Engineers, Florida Oceanographic Society