Freshwater Plants: Tenacious Tendrils
South Carolina’s quarter-century battle against freshwater invasive plants has taken place on many fronts and against a variety of foes, ranging from beautiful floating water hyacinth to a ten-foot-tall common reed, particularly along the coastal plain, and in some of the state’s big reservoirs.
The S.C. Department of Natural Resources’ response to the hydrilla invasion, meanwhile, has received national recognition as an example of using smart science and cooperative action to reverse an ecological disaster.
Unlike the frightening red-bellied pacu, hydrilla is pretty. The long, swaying tendrils of this submersed species make it an ideal aquarium plant, and it was imported from India and Asia to line North American fish tanks. Unfortunately, hydrilla produces dense surface mats that displace native species, especially in freshwater lakes.
“From a wildlife perspective, hydrilla draws fish and waterfowl,” says Chris Thomason, a freshwater fisheries biologist at the DNR. “But at certain levels, it impacts everything from boating to industrial power intakes. It quickly escalates, and we have to spend enormous amounts of money to clean it up.”
With tendrils that grow as long as 25 feet, each hydrilla plant can span the diameter of an average swimming pool. Its rapid reproduction almost always leads to major municipal and industrial water intake clogs, decreased oxygen levels, impaired boating activities, degraded water quality and, for good measure, a sizeable increase in mosquito breeding sites.
As a result, hydrilla holds the dubious distinction as South Carolina’s most expensive invasive freshwater plant, costing the state $14.7 million since 1982 to force back a presence that had expanded to cover as much as 58,000 acres statewide, easily enough to cover all of Lake Murray and more.
“Hydrilla is recognized by every state and federal government as one of the most problematic invasive species,” says Chris Page, coordinator of the Aquatic Nuisance Species Program at the DNR.
By the time hydrilla appeared on Lake Murray in 1993, the DNR and the Aquatic Plant Management Council had already battled a hydrilla infestation in lakes Marion and Moultrie that covered a large chunk of the massive pair of lakes and temporarily shut down the hydroelectric plant at St. Stephen.
The DNR immediately started treating the Lake Murray invasion with herbicides and later requested a lake drawdown.
“Lake homeowners don’t care for drawdowns, so that was not a popular thing,” says Page. While herbicides and drawdowns provided good short-term control, hydrilla continued to spread to more than 6,600 acres. At that point the DNR decided to unleash an exotic invader of its own: a large herbivorous minnow called triploid grass carp.
“We’re fortunate that hydrilla is like shrimp and grits to grass carp,” says Page. “They love it, and it’s what they eat first.”
The DNR first added grass carp to its biological arsenal while controlling hydrilla in the Santee-Cooper lake system in the early 1990s. By stocking only sterile fish, biologists were able to guard against permanent ecological disturbances. Scientific opinion was divided at the time, but the DNR decided to pursue the largest stocking of triploid grass carp in the world.
The gamble worked, and the state stocked 2,620 more grass carp in the Santee-Cooper lakes in 2007 to help keep hydrilla under control. “Small maintenance stockings will ensure that hydrilla doesn’t get out of control yet allow our native plant species to survive,” says Page.
Stocking triploid grass carp in the Santee-Cooper lakes has been very cost-effective, at less than $10 per acre per year, according to the U.S. Army Corps of Engineers.
An invasive plant that frequently shares the billing with hydrilla is water hyacinth, which hails from Brazil. As South Carolina’s second most problematic invasive aquatic plant, it has generated a bill of $1.3 million to treat 16,000 affected acres since 1985. As a result, water hyacinth was specifically named alongside hydrilla in section § 50-13-1415 of the South Carolina Code, which prohibits importation, possession or placement of the plants in state waters. Federal law also specifically forbids the interstate transportation of water hyacinth.
The renegade, free-floating water hyacinth was probably introduced to state waters by homeowners discarding decorative water-garden plants near Charleston in the 1960s and 1970s. Water hyacinth plantlets can double in less than twelve days, completely cloaking entire bodies of water and staking out waterfront real estate.
These dense mats of growth choke out native species and prevent sunlight from penetrating the water, reducing oxygen levels and causing widespread decay. Virulent infestations can even cause flooding by impeding the natural water flow during heavy rains. Water hyacinth is kept in check through low-level herbicide applications early in the season, while the plants are still small.
Another free-floating freeloader is water lettuce, which is known for its cabbage-like appearance. South Carolina was spared its first outbreak in 1991, when cold winter temperatures killed the initial infestation on the Waccamaw River. Water lettuce then reappeared in a private subdivision lake in Berkeley County and migrated to the Goose Creek Reservoir. Its tendency to tag along with humans, coupled with a virulent ability to reproduce from a single plant, have made it a prolific intruder.
A somewhat unexpected invading plant is the common reed, or Phragmites australis. While native to North America, the dense clusters invading South Carolina originated from Europe in the 1970s. This giant grass tends to overpower native plants that provide food and habitat for waterfowl and has cost South Carolina $1 million since 1985.
The most ferociously named freshwater invader is probably alligatorweed, but its bite comes in the form of mosquito breeding grounds. This aggressive South American perennial spreads rapidly through fragmentation and can be found throughout South Carolina. The S.C. Public Service Authority (Santee Cooper) made initial strides toward controlling alligatorweed in the 1940s, and the U.S. Rivers and Harbors Act of 1958 established a cost-splitting precedent among the state and federal governments to control nuisance aquatic plants.
Prior to the hydrilla invasion, Brazilian elodea was South Carolina’s most problematic submersed aquatic plant. Introduced to the United States more than a century ago, it has colonized the Saluda River below Lake Murray, the Savannah River near Augusta and small ponds in the Upstate, among others. Because it spreads through fragmentation, the elodea defies most mechanical controls like cutting and harvesting.
Looking forward, the DNR is most concerned about a free-floating fern called giant salvinia. Its eight-year tenure in South Carolina began in a private pond near Colleton County, where it was introduced from a contaminated shipment of water-garden plants from California. Thanks to a concerted effort among the DNR, the Clemson Extension Service and the USDA, giant salvinia was successfully eradicated—only to be reintroduced in the same manner in 2004 in a Jasper County plantation pond.
“I think salvinia is the big one knocking on the door, trying to get in,” says de Kozlowski. “It’s like a super duckweed, and its reproductive rate is bound to be a problem for us.”
Giant salvinia can double its biomass every week, wrecking irrigation systems, fisheries, transportation routes, power production and municipal and industrial water intakes. Smaller lakes and streams can be completely covered over by salvinia infestations.
South Carolina’s moderate climate and multitude of shallow, nutrient-rich water bodies make it particularly susceptible to freshwater plant invasions from new threats like giant salvinia. As a result, the DNR is enlisting public cooperation to minimize hitchhikers and cross-contamination among bodies of water.
Text for this supplement by Marc Rapport, free-lance writer and senior writer for the S.C. Department of Parks, Recreation & Tourism.
© 2007 South Carolina Wildlife Magazine - www.scwildlife.com