The Washington PostDemocracy Dies in Darkness

Opinion Manatees are starving in Florida. Pollution is the likely culprit.

Manatees swim with their calves at Blue Spring State Park in Orange City, Fla., in 2017. (Red Huber/AP)

Stephanie Seneff, a senior scientist at the Massachusetts Institute of Technology, is the author of the forthcoming book “Toxic Legacy.” Jennifer Margulis is a science journalist.

Manatees are an iconic mammal species in the Florida Everglades and surrounding rivers and estuaries. Their large, round bodies with protruding flippers and huge heads give them an adorable cartoonish appearance that has wide appeal. Thanks to multiple conservation laws, manatee populations have expanded, from 1,300 in the 1990s to more than 6,000 today.

But here’s the bad news: Manatees in South Florida’s rivers, estuaries and lagoons are not doing well this year. Since the beginning of 2021, 432 have already died. This is nearly three times the average number of deaths in the same time interval (Jan. 1 to March 5) over the past five years. That puts 2021 on pace to be one of the deadliest years for manatees in the past decade. And while the reasons for this decline are not entirely understood, we do have a lead on what’s driving the devastation: herbicide pollution in Florida’s waterways.

Scientists attribute the increase in deaths mainly to the declining availability of sea grass, which is manatees’ primary source of nutrition. The manatee population looks emaciated, and it seems obvious that a primary factor in their decline is inadequate food supply.

Sea grass depends on sunlight for photosynthesis. But sunlight can’t easily filter through dense layers of algae that build up in the waters during algae blooms, when huge numbers of single-cell organisms explode under conditions that promote their exuberant growth.

One species common in Florida’s freshwater algae blooms is the so-called blue-green algae, which is actually a type of bacteria called cyanobacteria. Their numbers mushroom out of control when there are excessive levels of micronutrients — particularly nitrogen and phosphorus — from fertilizer runoff.

Therefore, to understand why manatees are starving, we must understand the reason for the algal blooms that lead to the loss of sea grass. A primary factor could be the herbicide glyphosate. Glyphosate, the main ingredient in Roundup, is commonly used as a ripener on sugar-cane fields surrounding the waterways. Because the chemical can dissolve in water, it can easily wash away into waterways after heavy rain. It is also applied directly into the waterways to control invasive species such as the West Indian marsh grass, which competes with native species of grass.

A team of Florida scientists recently looked at the prevalence of glyphosate in the blood plasma of manatees and in their waterways. Of the 105 manatees tested, 56 percent tested positive for glyphosate or a derivative of the chemical. Equally troubling, the researchers found that the concentration of glyphosate in manatees’ blood has been increasing over the past decade. They also discovered that glyphosate and its derivatives are ubiquitous in the Florida waterways the manatees frequent.

Why does this matter? Because cyanobacteria are among the few species that can feed on glyphosate. In fact, glyphosate stimulates cyanobacteria growth, while it suppresses the growth of many plants that are sensitive to it. In other words, cyanobacteria thrive in the presence of glyphosate, whereas sea grass is directly harmed by it — and then dies from lack of sunlight when there are extensive algal blooms.

In the central watershed of the Indian River Lagoon system, there has been increasing urbanization in recent years, and this results in excess nutrient supply to the river system from overfertilized lawns. But glyphosate is also used extensively in urban areas to control weeds in lawns.

There might be other factors contributing to the problem. Sewage pollution from septic tanks is another factor that has figured into the excessive supply of nitrogen to fuel rampant algae blooms, and this, too, has increased in volume due to urbanization. Scientists also say that cold weather has also put extra stress on Florida’s manatees this year.

Nevertheless, science shows that glyphosate works synergistically with excess nutrients and other toxicants to pollute the waterways and harm aquatic life. If we want to stop manatees from starving, we have to stop using this harmful chemical on our crops, on our lawns and in our waterways.

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