Cloudy waters are driving Florida’s massive manatee die-off

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The cause of death: starvation because of the loss of seagrass in increasingly polluted waters,

Florida’s most recent winter dealt a blow to its West Indian manatees, iconic marine mammals that are a big tourist attraction. In the first 5 months of this year, 761 manatees wintering in one Florida lagoon died, according to the state’s Fish and Wildlife Conservation Commission (FWC). The number represents about 10% of Florida’s population of Trichechus manatus latirostris, the subspecies found there, and is more than the total number of the manatees that died across the whole state in 2020.

The cause of death: starvation because of the loss of seagrass in increasingly polluted waters, a problem not easily fixed. “I would not be surprised if this happens again next year,” says Daniel Slone, a research ecologist with the U.S. Geological Survey (USGS).

The only vegetarian marine mammal, manatees—often called sea cows—thrive in subtropical waters, where they feed on seafloor grasses, algae, and floating plants. In the Atlantic Ocean north of the equator, two subspecies exist. One lives in the southern part of the Caribbean. The other meanders through Florida’s rivers, springs, and coastal waters, sometimes straying north to Massachusetts in the summer. Manatees cannot tolerate water colder than 20°C, so in Florida during colder months they gather in warm springs, or in the water discharged at coastal power plants from cooling generators. 

One key spot is the Indian River Lagoon midway down Florida’s east coast, which hosts about 2000 manatees each winter thanks to water releases by power plants. There, agricultural runoff, sewage discharges, and other human-driven activities have been increasing. Since 2011, the excess nitrogen and phosphorus that flow into waterways have fueled lengthy “superblooms” of brown or green algae that make the lagoon’s water “look like pea soup,” says Martine deWit, an FWC veterinarian. With sunlight unable to penetrate the lagoon, seagrasses and other photosynthesizing organisms disappear. “Each year, the seagrass gets a little worse,” deWit says.

This year, “the hammer just fell,” Slone says. The 2020–21 winter started with a cold snap, requiring manatees to increase their calorie intake to stay warm. Then, even though there wasn’t enough grass to feed all the manatees, the animals stayed put. “They chose warmth over being hungry,” deWit says. Some of the starving animals that made it through the winter and started to spread out across Florida’s coast are now showing up dead elsewhere because they could not recover.

About 10 years ago, USGS researchers determined the Indian River Lagoon could sustain the number of manatees that were learning from their mothers to go there. “That is clearly not the case now,” Slone says.

For the moment, the options for restoring seagrass are limited. “We don’t want to replant seagrass until we have [better] water quality,” says Charles Jacoby, an environmental scientist with the St. Johns River Water Management District, based in Jacksonville, Florida. His agency is just 5 years into a 15-year plan to lower the nutrient load in the lagoon. The plan includes removing “muck”—muddy, nutrient rich sediment that’s been accumulating in the lagoon, as well as in canals and tributaries, because there’s not enough water flow to flush it out. It also calls for improving sewage treatment plants and hooking up households that now use septic tanks to the sewer system. Because these solutions will take time, he and others are considering short-term fixes for the manatees, such as feeding the sea cows or trying to coax them to winter elsewhere.

But other wintering spots are getting rarer, says Jaclyn Lopez, an attorney and manatee expert with the Center for Biological Diversity, a conservation organization. “We have not done enough to preserve warm water habitats.”

Until now, Florida’s manatees, which were one of the first species to be placed on the U.S. endangered species list in the 1970s, seemed to be doing better. In 1991, the first aerial census counted about 1200 individuals. Now, there’s an estimated 7000 to 8000. In 2017, that progress prompted then-President Donald Trump’s administration to upgrade the manatee’s legal status to “threatened” under the federal Endangered Species Act, despite objections from conservation groups.

Manatees also fall under the protection of the federal Marine Mammal Protection Act.

On 26 May, a working group established under that act declared the die-off an “unusual mortality event,” which triggers a federally supported investigation of the cause.

Meanwhile, manatee rehabilitation centers have been overloaded with emaciated animals, and officials are struggling to find new places to care for sick animals. “We don’t have the ability to satisfy that rescue need,” Lopez says.

But Sloan and Jacoby are optimistic about the species’ future in Florida. Populations on the state’s west coast are doing fine. And the long-term prospects for manatees on Florida’s east coast, Jacoby says, “are probably all right.”

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