November 27, 2016
Here's why Lake Okeechobee discharges didn't wipe out Stuart's oysters
STUART — Vincent Encomio expected the worst last week as he waded out to oyster beds in the St. Lucie River.
After all, the 2013 Lake Okeechobee discharges, which dumped 136.1 billion gallons of polluted water into the river over 166 days, wiped out the oysters along Stuart's Riverwalk. And this year's discharges stretched over 279 days and totaled 220 billion gallons.
Encomio, director of scientific research at the Florida Oceanographic Society in Stuart, figured the oysters that had repopulated the beds since 2013 didn't stand a chance.
"It was amazing," Encomio said. "I picked up a cluster of shells and found live oysters; quite a few of them in fact."
Based on what he saw, Encomio estimated at least half the oysters along Stuart's waterfront survived the onslaught from Lake O.
Oysters naturally thrive in the St. Lucie's somewhat-salty water. But extended discharges that wipe out the river's salinity can wipe out the oysters, too. The rule of thumb is that oysters can survive about a month of salt-free water.
Making matters worse, the sediment carried by the discharged water — about 54.7 million pounds of it from Lake O this year — clogs the filters oysters use to feed. Oysters close up to keep the sediment out and starve to death.
So why did oysters fare so well in this year's onslaught?
During much of this year's discharges, the Army Corps of Engineers released Lake O water in "pulses" designed to mimic the natural flow of water through the river after heavy rains. The flow rate is high during the first part of the week and dwindles to two days of no discharges at all.
Over the 279-day span of this year's discharges, there were 24 no-flow days.
"Those stoppages, even though they were just a couple of days each time, really helped," Encomio said. "They might have been enough to keep oysters alive."
The breaks allowed incoming tides to wash oyster beds in the downtown Stuart area of the river with salty water, much like the oysters got closer to the inlet.
Also, although 2016's discharges lasted longer, 2013's were more intense.
"In 2013, the discharge rate was really high," Encomio said. "They shot up in mid-July and stayed high until September."
The high flows coming down the river in 2013 kept tides from coming up the river; lower flow rates in 2016 allowed some salt water to reach upstream.
Freshwater is lighter than saltwater, and the light freshwater flowed downstream on top of heavier saltwater, which pushed upstream on the bottom of the river — down where the oysters are.
Encomio's findings could lead scientists to encourage the corps to use pulse releases and lower flow rates in future discharges.
"Our stance is still that there should be no discharges at all, that excess Lake O water needs to go south," Encomio said. "But if there are going to be discharges, there's room for making sure they do the least amount of environmental damage."