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‘Dead zone’ identified off Sanibel

By Staff | Sep 12, 2018

A “dead zone,” an underwater patch with no plant, or animal life, has been found off the coast of Sanibel by Sanibel Captiva Conservation Foundation scientists.

Sanibel Captiva Conservation Foundation Scientist Dr. Rick Bartleson said about two weeks ago they started noticing the “dead zone” off the Sanibel coast. He said there is no oxygen in the water about 3 to 5 meters down to the surface, in some places all the way down to the bottom in about 8 meters of water.

“It’s a pretty large area. There is no life on the bottom, plant, or animal life,” Bartleson said Thursday afternoon. “We do not know the boundaries exactly, or know if the oxygen levels have gone up since last week’s storm.”

He said it is hard to determine where the dead zone is located, exactly.

“We don’t know if the oxygen levels are still low or not,” Bartleson said.

The dead zone is associated with freshwater runoff, which includes a lot of nutrients in the water. With algae blooms killing marine life, it will accelerate the oxygen demand.

“When fresh water is flowing over salt water there is stratification and little exchange of oxygen,” Bartleson said.

Storms could improve condition because the surface layer would move to offshore, which would take the high concentrations of red tide and push it out, he said. Upwelling replaces surface water, which can bring in oxygenated water from deeper out.

“Most of the marine life needs to recolonize, either come in from eggs, larvae or with fish,” he said. “They can swim back in when there is oxygen in the water. The recolonization of the fish from events like this takes a long time depending on the type of fish.”

Compared to recent events, they are not seeing as many dead fish and the beach levels of red tide are way down from where they were when the winds started blowing from the east, he said. Areas as Shell Point and Tarpon Bay are still seeing red tide in the area, but the levels are way down.

Bob Wasno, a professor with the Florida Gulf Coast University marine lab in Bonita Springs, said he placed cylinders out at Edison Reef about three weeks ago before the red tide started affecting the area. He and two volunteers went out and collected the cylinders last Tuesday.

“What the Edison Reef looked like a year ago to last Tuesday, complete zero life. We saw three pork fish and three snapper and that was it. It was pretty dramatic to see what the reef looked like,” he said. “It was a ghost town of anything alive. We took a water sample at the bottom of the reef and at the surface and every five miles. We received some pretty elevated cell counts, red tide cells per liter. It is out there.”

The Edison Reef typically contains hundreds of fish, many different species. The species span from flounders, bat fish, lizard fish, bait fish, as well as such larger fish as goliath grouper and Spanish mackerel.

“When we went down on Tuesday we spent a full hour going around the reef and only saw six fish total,” Wasno said.

He explained that there most definitely is some kind of nutrient overload taking place. His first impression would be that all these dead fish out there “gas up,” come to the surface, start to decompose, lose the gas and sink and roll across the bottom. Bacteria then acts on them and breaks them down. The bacteria exhausts through natural active feeding on dead carcasses, which sucks up oxygen in the water.

“Red tide fish die and then we have hypoxic zones where oxygen gets sucked up,” Wasno said. “We are hoping that the tropical storm came through and broke up the current red tide that is out there right now. We have our fingers crossed that we can get an early cool front that would come through. That seems to be what it takes to knock this stuff down a little bit.”

Wasno, who has been in Southwest Florida for 30 years, has seen a number of red tide instances in the past, but never of this extent. He said the red tide extends from Sarasota down to Marco Island, covering the island waters and is spreading out 30 miles from shore.

“The sheer volume of water that this red tide encompasses is mind blowing,” Wasno said.

When the red tide breaks up – which he said will take something pretty strong and mind blowing itself – he hopes the fish from offshore will fill the areas where there is no life now.

“Hopefully (we will) see a migration of offshore fish coming inshore filling that void, reproducing and bouncing back to where we can be,” Wasno said, adding, unfortunately will take a few years.