Striking a Deal with the Weed from Hell

The scene at Florida’s Crystal River National Wildlife Refuge in Kings Bay last October would have been familiar to anyone who has ever engaged in the battle to control the spread of invasive plants. Eager volunteers scurried about the shoreline of this manatee wintering ground, carting large plastic bins stuffed with water hyacinth, a notorious aquatic weed that’s caused headaches on five continents. Closer inspection, however, would have revealed the activity to be anything but business as usual: instead of hauling water hyacinth out of the bay, the conservationists were putting it back in—almost 4,300 gallons’ worth by day’s end.

Those volunteers were taking part in a bold pilot project that is the latest chapter in a half-century-long ecological story that reads like a fable. It starts with a well-intentioned campaign to rid Kings Bay of the water hyacinth, an aggressive nonnative species. Next come decades of additional control measures and a tragic downward spiral that transformed these crystal-clear waters into an unpleasant soup of slimy green algae. Then the story takes an unexpected turn, back to its original antagonist. Only this time, Bob Knight, the wetlands restoration ecologist leading this pioneering project, has recast water hyacinth as the unlikely hero. He believes this South American native, if controlled, could help solve the algae problem and return the bay’s ecosystem to a more desirable state. The irony in this approach is not lost on anyone involved.

Sometimes referred to as the “weed from hell,” water hyacinth is a free-floating plant with an explosive growth rate. After being introduced to Americans at the World’s Industrial and Cotton Centennial Exposition held in New Orleans in 1884, water hyacinth seduced gardeners with its beauty and hardiness. From the ponds and lakes where it was intentionally planted, it quickly spread—due in part to floods. But a bigger boost came from post–World War II development. As humans dammed rivers, drained wetlands, and filled water bodies with sewage and fertilizer, they created ideal water hyacinth habitat. At its worst, it can form impenetrable mats, boost mosquito numbers, and deplete the water of dissolved oxygen—effectively suffocating fish. Many ecologists have spent their careers trying to eradicate water hyacinth, and the idea of encouraging its presence, especially in an idyllic spot such as Kings Bay, leaves them almost speechless.

Supporters of the initiative, on the other hand, insist there’s nothing to fear. For one thing, the plant has persisted in the backwaters of Kings Bay—for the most part innocently—for decades. It’s almost impossible to eradicate. However, it is easy to contain and control, especially compared to algae. In addition to shading out algae, water hyacinths can provide habitat for fish and other wildlife. Their dangling root masses filter algae and promote denitrification. What’s more, they’re veritable salad bars for manatees. “If you ignore the label ‘exotic’ and focus on function,” says Jason Evans, a University of Georgia ecologist and advisor on the Kings Bay project, “this plant is almost exactly what you would think this ecosystem needs.”

The adversaries could hardly have chosen a more emotive battlefield. Covering more than 600 acres and feeding into the Gulf of Mexico through the six-mile Crystal River, Kings Bay is actually more lake than bay. Its constant inflow of underground water from more than 30 major springs keeps it at a steady 22 degrees Celsius. It provides an ideal habitat for fish and other wildlife and is a warm wintertime haven for sensitive manatees. Each year tourists come by the tens of thousands, many to experience what it’s like to swim amidst the bay’s passive sea mammals. By anyone’s criteria, it’s a magical place.

Or at least it was. Trouble began in the late 1950s when the initial spread of water hyacinth became a nuisance for boaters. Following chemical control measures, the hyacinth presence was reduced to acceptable levels, but a new problem soon arose in the form of hydrilla. Another fast-growing nonnative, hydrilla grows from the bottom, making it harder to manage. Initial control efforts in the 1960s included dousing parts of the bay with thousands of gallons of sulfuric acid, which was later recognized as harmful to other life forms. Copper-based herbicide treatments in the 1970s were also halted when high levels of copper were detected in sediments and in the organs of dead manatees. Managers then tried harvesting, mechanical shredding, and applying a new battery of herbicides that included diquat, endothall, and fluridone.

As the battle against hydrilla raged into the 1990s, more frequent explosions of algae, particularly Lyngbya wollei, arrived on the scene and filled the water column with fragments and long, slimy ribbons. Eventually, higher salinity and increased murkiness overcame the hydrilla and the native eelgrasses that once nourished the manatees, leaving mostly algae soup. Longtime area activist Helen Spivey recalls when the water was so clear that nighttime strollers would walk into it by accident. Now, says the 85-year-old former state representative, “the algae get belched off the bottom and float across the shore and into the small trees along the bank, where it hangs and dries out and ends up looking like toilet paper. We call them Lyngbya Monsters.”

It’s not clear why the ecology of the bay flipped. One theory is that growing pressure on underground aquifers from the surrounding population reduced inflow from the springs, raising salinity and lowering the bay’s ability to flush away floating algae. Another idea is that decades of invasive plant control inadvertently gave algae a competitive advantage. Such unintended consequences have been observed elsewhere. During an eight-year study at a reservoir in São Paulo, Brazil, researchers documented periodic single-species algal blooms while water hyacinths were present. After hyacinth eradication, there was total domination by multiple algal species, some potentially toxic.

Either way, the situation in Kings Bay is now a cause of widespread concern. For some, it’s merely unsightly. For others, there is fear for the area’s lucrative tourism industry. And even though the bay’s manatee numbers have increased in recent decades—a trend that may be due partly to conservation measures and partly to the loss of wintering habitat elsewhere—others are concerned for the long-term health of the bay’s charismatic residents. For one thing, manatees are not overly fond of grazing on algae. More worrisome is the threat of toxicity. There were 829 reported manatee deaths in Florida in 2013, the highest total ever, and nearly one-third of them have been attributed to contact with algae toxins. While most were marine algae, analysis of a manatee corpse from the Crystal River area found toxins associated with L. wollei.

Managers have run out of options. Attempts in recent years to replant eelgrass have failed when manatee grazing outpaced planting. Desperate volunteers now periodically extract loads of algae using hand rakes, a backbreaking and costly approach that’s as permanent as a haircut.

Which brings the story full circle—back to water hyacinth. Research since the 1970s has shown it to be something of a wonder plant when it comes to ridding water of everything from excess nutrients to heavy metals and other contaminants. In China, where water hyacinth has been a nuisance for decades, authorities recently enlisted the plant in their long-running struggle to resuscitate algae-choked lakes that serve as water sources for millions. In Lake Dianchi near Kunming, major nitrogen reductions were recorded in 2012 after water hyacinth was grown for a year in floating pens spanning more than 1,000 acres, a project that included harvesting the plants for fertilizer and for use in biogas production. (1) And then there’s the Tominé Reservoir, part of the Bogotá River in Colombia. After years of declining health precipitated by the usual suspects, including excessive nutrient input, water hyacinth invaded this major source of regional drinking water in 2003. Recognizing the opportunity to test the plant’s water purification capabilities, researchers corralled the invader at one end of the reservoir representing 7.5 percent of its surface area. Then they took ongoing measurements of various water quality parameters at different locations and compared them with pre-invasion data. They found that water quality had improved so much by 2006 that it had reached standards originally set for 2020. What’s more, water quality increased with proximity to the hyacinth coverage zone. Manuel Rodríguez Susa, an environmental engineer at the University of the Andes in Bogotá, confirmed in a recent email that things continue to go well: “Today Tominé Reservoir is still in good health thanks to water hyacinth.”


At Kings Bay, where some residents have been advocating for a return of water hyacinth for decades, Knight thinks the plant’s powers can be utilized in a more direct fight against algal dominance. For starters, water hyacinth mats attract a rich diversity of organisms that includes snails and other important algae grazers. But more importantly, they provide shade. Having worked with aquatic plants for 30 years, Knight thinks that blocking growth of free-floating algae over a portion of the bay could reduce overall turbidity and help eelgrasses and other submerged plants regain a competitive edge. He aims to find out just how much hyacinth coverage is needed to achieve good water clarity.

Matt Cohen, an aquatic ecologist at the University of Florida, is skeptical that acceptable levels of water hyacinth would have useful impact on the entire bay. So Cohen is taking a wait-and-see stance regarding the Kings Bay experiment. “We don’t know what prompts this conversion to an algae state,” he says, “and that’s part of why I think doing this experiment in a careful way is totally warranted.”


It took Knight a year to acquire permits from six different government agencies in order to set up a small experiment with a treatment area less than a quarter-acre in size. Once the first floating pens were in place in March 2012—stocked with hyacinths as well as with native frog’s-bit and water lettuce—Knight discovered just how much manatees love water
hyacinth: The big sea mammals quickly emptied the corrals each time they were filled. As a result, he designed and built cage-like pens that restricted access without endangering the tenacious grazers.

With the new cages stocked, Knight is set to collect water quality data in the upcoming growing season. So far he’s been able to show that water hyacinth is a capable candidate. The plants are growing in the bay’s current salinity conditions, something critics had doubted and something that cannot be said for the two native floating plants in the experiment.

Ultimately, project supporters hope that yesterday’s enemy could be tomorrow’s friend. They believe the hyacinths can play an important role in a conservation strategy that also includes reducing nutrient runoff and restoring spring flows. “Since these invasive plants are here and we can’t get rid of them,” says the University of Georgia’s Evans, “I think it’s counterproductive to be killing them and not taking advantage of their functions. These are important tools. We should be using them.”

—Garry Hamilton

1. Wang, Z. et al. 2013. Chemosphere doi:10.1016/j.chemosphere.2013.03.014.

Top photo ©Peter Chadwick/DK Images; hyacinth experiment photos courtesy of Kings Bay Springs Alliance