Record winter rain on the heels of a severe summer drought that withered acres of seagrass may not be enough to stem the fever ailing Florida Bay.
The seagrass die-off, which spread from about 25 square miles to more than 62 square miles through the winter, blanketed the central bay in a plume of yellow sulfide. While scientists say the die-off appears to have stopped for now, they worry that rising water temperatures over the summer could trigger a more lethal blow: algae blooms. Record highs have already been topped three times in the bay in recent months, they say.
“It’s kind of like the fuse to the bomb was lit last summer,” said Stephen Davis, a wetlands ecologist with the Everglades Foundation. “It either snuffs itself out, or the bomb is a large-scale algal bloom.”
Scientists fear higher summer temperatures will essentially cook what has become a soup of dead seagrass, where rotten plants soak up oxygen and produce even more grass-killing sulfide. So far, they have not seen any blooms, in part because the high salinity that lingered during a massive 1987 die-off has not occurred. Salinity is back to near normal, helped by an increase in water flowing through creeks emptying from marshes. But parts of the bay are weak, putting everyone on edge.
“Right now, it’s a wait-and-see situation,” said David Rudnick, science coordinator for Everglades National Park’s South Florida Natural Resources Center.
And unfortunately, with Everglades restoration work still incomplete, the tools don’t exist to fix it.
“We’re definitely making positive strides,” Rudnick said. “But at this point, there’s not much we can do about the damage that’s already been done to the seagrass community and the fact the bay is at risk of algal blooms.”
Unlike Florida’s other troubled waters — the St. Lucie and Caloosahatchee estuaries muddied by water releases from Lake Okeechobee over the winter — the bay and its network of two dozen basins present a far more intractable problem. Once conditions worsen, it can take decades for things to get right again.
“When you start getting nutrients in the bay, it just takes a lot longer to recover from that because it can’t get rid of them as quick,” said Margaret “Penny” Hall, a Florida Fish and Wildlife Conservation Commission seagrass expert leading an investigation of the die-off.
Next month, Hall’s team will return to the bay to expand their survey and take a look at hard-to-reach basins by kayak, where mud banks make it impassable for boats. The U.S. Army Corps of Engineers also plans to send up a drone in late July or early August for aerial mapping, a key tool in studying the vast, 850-square-mile bay, said spokesman John Campbell.
The last time a similar die-off hit the bay in 1987, it occurred in nearly the same locations but only included about 15 square miles. For two years after the die-off, the region got little rain. That helped fuel an algae bloom that coated some areas with a stinky pea-green slime that left a sport-fishing industry valued at $722 million a year reeling. The 94-square-mile collapse, which helped propel demands for Everglades restoration, took 20 years to recover from.
This time around, record rain hit the mainland, raising hope that freshwater flowing south would save the bay. In addition to water from creeks, the South Florida Water Management District moved about 69.2 billion gallons into Shark River Slough by June 1 to relieve flooding in vast conservation areas north of Everglades National Park.
But scientists say that while the water helped marshes in the park, it has done little for the bay.
“Because we had so much rain, it should have been plenty of water to provide to the bay, but it just points again to the fact that the infrastructure is not there,” said Audubon Florida biologist Jerry Lorenz. “It points to the inability of the Water Management District infrastructure to supply Florida Bay with its needed water supply.”
A century ago, water from the lake flowed south into the bay. Now the bay gets most of its fresh water from rainfall, leaving isolated pockets with little of the tidal flushing that can help freshen water when temperatures rise, evaporation increases and salinity spikes.
Last July, when rainfall at Garfield Bight was about 10 inches below average, salinity jumped to more than double the levels typically found in bay water, according to monitoring by Everglades National Park. Water temperatures hovered over 93 degrees in the bight for more than 77 days.
Normally, oxygen in the water increases during the day to keep plants breathing overnight. But with so much heat and salt trapped in the water, oxygen plummeted, essentially suffocating grass at night. Turtle grass, which had rebounded with a fury in the central bay, needs lots of oxygen. As it died and began releasing sulfide, it started a lethal reaction.
“That’s fuel basically for all these bacteria that decompose that organic matter, just like you’d have on a compost pile,” Rudnick said.
Ninety percent of the turtle grass in Rankin Lake and Rankin Bight — prime hunting grounds for redfish — died, leaving vast rafts of dead, floating grass. But what may be more worrisome is what’s trapped in the bay’s muddy bottom when the grass dies.
Sturdy turtle grass has deep roots to help it survive low light. If it grew in sand, the dead plant matter might get easily washed away. But this is Florida Bay, where the bottom can feel like quicksand.
“It’s like a wad of moist clay compared to a handful of sand,” Davis said. “Water can run through the sand, but it’s not going to pass through the interior of a ball of clay.”
On top of that, Rudnick said scientists believe the hot water stratified, preventing oxygen in the air from reaching the bottom. Because the seagrass is dying in nearly the same location, Hall said scientists are starting to look at what makes the basins so vulnerable. Along with the drought, they now suspect basins with a thicker blanket of grass and little flushing got hit harder.
“They increase their respiration rate just like you would breathe more on a hot day,” Hall said.
Which points to the problem of moving more water into the bay. Under historic conditions, the bay was like a lawn with a variety of grasses: shoal and widgeon grass grew in shallow, fresher water, while turtle grass grew in saltier conditions. Manatee grass filled patches in between. But with water from the north damned, parts of the bay became almost entirely turtle grass.
The park is now trying to move more water into Taylor Slough, Rudnick said. It’s not clear whether more water would ever entirely prevent a die-off, Hall said, but it would likely prevent a single species from dominating areas and keep a die-off from growing so large.
“Places like Rankin Lake are always vulnerable. But if there was more fresh water, it would take a longer period of drought,” she said.
The good news is that on their last trip out in May, Hall’s team was beginning to see signs of faster-growing shoal grass filling turtle grass meadows, which could help stabilize the sandy bottom, keeping water clear for more grass to grow.
What happens next has scientists split.
Lorenz worries that years of flood-control and climate change-projections for rising temperatures have created a “perfect soup” for years of poor conditions. Paul Carlson, an ecosystem biologist with the state’s Fish and Wildlife Research Institute and one of the first scientists to document the toxicity of sulfide in sediment in the 1987 die-off, is more hopeful. The quicker return to more normal salinity may prevent a large-scale bloom that can kill fish and devastate marine life, he said.
“In the previous episode, it took several years for the salinity to drop back to normal, and we’re seeing that happen within a year,” he said.
That’s the good news. The bad news?
The bay “might take another 20 years to recover,” he said. “Or it could be as little as 10 or 15.”