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The Seagrass Died. That May Have Triggered A Widespread Fish Kill In Biscayne Bay

mikesell fish kill 0811.jpg
Kathryn Mikesell
Swimmer Kathryn Mikesell encountered mats of floating dead fish during her morning swim Monday in northern Biscayne Bay.

A seagrass die-off that began in 2013 and eventually wiped out three-quarters of the meadows in Biscayne Bay’s Tuttle Basin may have helped set the stage for a widespread fish kill this week.

The seagrass die-off left much of the bay bottom barren and without the thick grasses that helped produce oxygen in shallow waters that can reach steamy temperatures in the summer heat, said Piero Gardinali, a professor at Florida International University who specializes in chemical oceanography. When waters reach high temperatures like they did this week, that can leave fish even more vulnerable.

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“If the grasses were there and they were healthy, things would be different,” he said.

Christopher Boykin
Pelican Harbor Seabird Station
Stingrays struggling for oxygen crowded the shoreline near the Pelican Harbor Seabird Station.

Scientists monitoring the bay first began noticing the die-off in 2013 in the busy Tuttle Basin. By 2017, three-quarters were gone. Without the grasses to photosynthesize during the day and produce oxygen, that could leave the basin more vulnerable when temperatures soar and oxygen levels drop in the hot water, Gardinali said. Across the bay, more than 20 square miles of seagrass have vanished.

Combined with rising pollution feeding algae and increasing nutrients in a bay that thrived on low levels, environmental scientists warn the bay is at at tipping point.

“It's a series of unfortunate events that we just keep adding. And then eventually we are here,” he said. “We're going to start dealing with an increase in frequency of some of these events. I know people like to use the word resilience. You know, the grasses and the habitat needs to be resilient.”

The poor conditions can also become chronically unhealthy for marine life, he said.

It's the same as me eating unhealthy,” he said. “We all understand if we're out of shape, you suffer on the treadmill. Organisms in the wild are no different. They are on an environmental treadmill.

State and county environmental regulators are still investigating what may have triggered the fish kill first reported Monday by swimmers and Miami Waterkeeper staff collecting water samples near Morningside Park. Initial sampling showed water temperatures nearing 90 degrees and low oxygen levels.

The Florida Fish and Wildlife Conservation Commission is testing water for harmful algae blooms that may have contributed. Miami-Dade’s Division of Environmental Resources Management, which inspected the site of the kills Monday, says no odor or discoloration in water was detected by staff.

Gardinali has deployed monitors to map the extent of the low oxygen conditions and determine whether they are spreading.

Because most of the dead fish appeared on the bay’s western edge, he worries winds and currents may have carried the fish away from the site where the kill possibly started.

While the University of Miami’s Rosenstiel School has conducted drifter tests to track currents in the Central Bay, not much is known about currents in the enclosed Tuttle Basin, Gardinali said. Dead fish have been spotted as far north as the 79th Street Causeway and south to Virginia Key.

“What I'm afraid of is that what we’re seeing on the west end is probably wind driven and not necessarily where things are happening,” he said.