In the wake of a fish kill that littered the shores of northern Biscayne Bay with floating fish carcasses, scientists are trying to parse exactly what triggered the lethal event.
Was the kill caused by the decades of pollution, the water temperatures that soared more than 4 degrees above the 10-year average, the stagnant winds, the low tide or the water gushing from the Little River at its highest rate in two decades? Or, was it all of the above?
Conditions conspired to create a perfect storm, scientists say, but not an unpreventable one.
“I guarantee you that my folks are going to huddle up after this is all done and say, ‘Holy smokes, what could we have done?’” said Todd Crowl, director of Florida International University’s Institute of the Environment. “What instruments could we have had? Where should we have been watching so that we could have said, ‘Oh folks, we've got an issue.’”
The fish kill was first spotted near Morningside Park on Aug. 10 by a neighborhood swim club and Miami Waterkeeper staff collecting regular water samples for swim advisories. Within a day, dead fish reports reached from Virginia Key to the Broad Causeway, with most along the western shores.
County environmental staff tested water and detected low oxygen levels. Samples collected by Miami Waterkeeper and provided to state regulators also confirmed low oxygen levels, leaving state and county officials to blame high water temperature, which can drive down oxygen levels.
When Miami Waterkeeper discovered unusually high flows coming out of the Little River, just north of Morningside, the South Florida Water Management District also blamed heavy rains.
But what set the stage is likely more complicated.
The bay covers 428-square miles from Key Largo to Bal Harbour, with three distinct regions, each with their own unique problems. In the southern bay, where run-off from agriculture and hot water from Turkey Point’s cooling canals seep into waters, 93 percent of seagrass beds have vanished. In the Central Bay, pollution from leaky septic tanks have fueled seaweed and clouded water.
And at the north end, along the dense urban coast, dirty canals and rivers now supply the majority of the bay’s freshwater, replacing the overland sheetflow that once crossed transverse glades up and down the bay. Those canals and other modern drainage efforts by the district’s regional flood control system also lowered the local groundwater table, which dried up freshwater springs that used to bubble from the bay bottom.
“There have been warning signs of declining water quality in Biscayne Bay since the first seagrass die off,” said Tiffany Troxler, an FIU wetlands ecologist who served on the county’s Biscayne Bay Task Force.
The task force, which wrapped up an 18-month-long review earlier this summer, concluded that the bay was not only in trouble, but in need of urgent attention.
“Despite its many layers of county, state, and federal protection for water quality, habitat, and wildlife, Biscayne Bay is at a tipping point,” the group wrote in a draft report being reviewed by Mayor Carlos Gimenez and expected to be presented at the next county commission meeting.
That conclusion echoed a 2019 report by the National Oceanic and Atmospheric Administration that warned the bay was undergoing a regime shift. Without serious effort, NOAA scientists warned, bay waters that draw anglers to hunt bonefish, tarpon and permit in shallow flats clear enough to spot fish would turn dark with nutrients and thick with algae.
What’s startling is that the north end, despite decades of development and heavy boat traffic, had been thriving and filled with manatee grass until about 2013. Unlike other seagrasses, manatee grass can thrive in water filled with nutrients. But then the grass started dying. By 2017, more than 75 percent had vanished.
Scientists now worry that die-off helped set the stage for the fish kill, removing a major producer of oxygen that also helped anchor the bottom and keep waters clear, which also allowed grasses to thrive.
Miami-Dade environmental regulators have been monitoring the seagrass beds and bay bottom since 1985. In their 2019 report, they reported that phosphorus and chlorophyl in waters that once had none have been steadily climbing now for 20 years. Seagrass in the Tuttle Basin likely got depleted by having too much algae, bacteria, seagrass and other matter competing for oxygen, the report said. So far, none of the seagrass has returned. By comparison, the central part of Florida Bay has mostly recovered from a massive seagrass die-off in 2015.
The amount of phosphorus, which can feed algae, flowing from the Miami River and Little River nearly doubled in 2015 and has remained higher than the previous decade. Nitrates and ammonia also started climbing in 2016 and 2017.
That’s leading some scientists to point to the need for closer monitoring of releases from the rivers.
“We've put so much pollution into the bay that when it gets too hot or a lot of fresh water gets flushed into the bay or we get really low wind, we are getting these apocalyptic conditions,” said Miami Waterkeeper executive director Rachel Silverstein, who is also coral biologist. “That's where the fish are literally suffocating. They can't get enough oxygen.”
In its comparison of river flows and rainfall in the Little River basin, the water management found flows spiked with increased rain, a logical result. After a deluge on May 26, rainfall in the basin for the month topped 18 inches, three times the average. The amount dropped below average in June, but climbed slightly above it in July. River flow remained high.
University of Miami coral researcher Chris Langdon, who specializes in ocean chemistry, said a simple comparison doesn’t always establish cause. He said data needs to be correlated to confirm rainfall as the increased flow, and not a change in canal operations.
“When one goes up, the other goes up in direct proportion to it. That would be the case if rainfall is the only thing that's causing discharge and the Little River to vary from time to time,” he said. “But if instead they're holding the water back and releasing it in episodes, then I would be able to see that.”
The district maintains long-term monitoring stations in the central and south end of the bay, but none in the north end. That’s left to about a dozen stations overseen by the county.
“I think we have to do a better job of having a sentinel sensor system out there to give us a much earlier warning,” said Crawl, whose $12 million state budget request for the institute, which would have helped expand monitoring, was rejected this year. “I basically laid out this was going to happen and this is the instrumentation we had to have in order to at least warn people.”
Crowl said he’s now working with NOAA and Waterkeeper to come up with recommendations for the district to help regulate the flows and clean gates in the river that routinely fill with trash.
FIU researcher Henry Briceno has also been working to document circulation in the basin, which has never been plotted.
“It was astounding,” Crowl said. “It's like, wow, we really don't know where this water is going.”
The Biscayne Bay Task Force is also calling for a new management board oversee the bay’s recovery, using Tampa Bay as a model for success. Tampa Bay had lost nearly all its seagrass by the 1980s. But since the management board took over, it’s restored more than 40,000 acres.
“Seagrass can come back,” Troxler said. “It's just that we need to get the water quality right.”
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