Chemical Control of Aquatic Vegetation

Aquatic vegetation affects everyone who uses the lake. Anglers adore it for its use in providing refuge and food for sportfish. Recreational boaters abhor it because it gets tangled in their boat propellers and water skis. However you feel about it, one thing is clear: aquatic vegetation must be controlled. If it is allowed to grow unrestrained, it does no one any good and can cause irreparable damage to the quality of a lake as a water resource. Hydrilla is one form of aquatic vegetation praised by fishermen and cursed by boaters. It is an exotic plant brought over from Sri Lanka in the 1950s for use as a decorative aquarium plant. It found its way into area lakes by compassionate owners of goldfish who wanted to set their pets free. This plant provides great fishing wherever it’s growing when it’s not overgrown. But when it is allowed to thrive unregulated, it creates a dense mat on the water’s surface and does not allow other underwater life access to sunlight or oxygen. When the underwater plants die out, fish cannot survive and the lake’s water quality becomes poor, according to scientists at the Lewisville Aquatic Ecosystem Research Facility (LAERF) located near Dallas. One sign of a lake of poor water quality, according to Doug Hannon, “the Bass Professor” on ESPN, is dirty, brown water. Another sign is the buildup of dead plankton in coves resembling soap suds. These are both beginning to be familiar sights on Lake Conroe. The problems created by Hydrilla have prompted state and federal governments to impose laws to maintain control of the weed. “Possession of this plant is a second degree misdemeanor, and punishable by a $500 fine and/or 60 days in jail.” The scientists at LAERF outlined several reasons why Hydrilla must be managed. It can quickly fill waterways, affecting boating, wildlife use and flood control. It outcompetes native species of plants, reducing plant diversity, and it reduces dissolved oxygen levels, increasing the potential for fish kills. It also causes an increase in organic sedimentation, accelerating the lake’s aging process. Hydrilla is most commonly controlled through the use of herbicides. These chemicals have proven to be the most effective and cost-efficient methods, but alternatives are continually being researched. Fluridone is presently the most commonly-used herbicide for the control of Hydrilla. This chemical is sold commercially as Sonar. According to Remmie Scarborough, the Manager of the Lake Conroe Division of the San Jacinto River Authority and a member of the Aquatic Management Society of Texas, Sonar was used early on in the eradication of Hydrilla from Lake Conroe. He is unaware of the effects of this particular chemical because shortly afterwards the Lake Conroe Marine Association and the Texas Parks & Wildlife Department decided to import a large number of white amur, or grass carp, to Lake Conroe to eat the Hydrilla. Sonar and Hydrothol 191 were both used to control Hydrilla on the lake until the white amur were introduced, Scarborough says. Hydrothol 191 is a chremical that burns Hydrilla down to a certain level, he says. This chemical is used as a temporary measure, much like mowing a lawn. The effects of both Sonar and Hydrothol 191 on Lake Conroe’s Hydrilla problem could not be accurately determined, but Scarborough says he’s read of Sonar having positive effects where it has been used to control Hydrilla on lakes in Florida. Waterhyacinth is another problem exotic plant. It was introduced to the United States from Brazil in the early 1900s as a decorative pond plant. It tends to grow in coves of a lake and can multiply at an accelerated rate to eventually choke off the cove from the rest of the lake. Today waterhyacinth is the #1 aquatic weed problem in the world. The State of Florida spends $5 million a year solely for the control of waterhyacinth. One of the ways waterhyacinth is controlled is with the herbicide Garlon. This chemical is manufactured by Dow Chemical and is only sold under research conditions. John Madsen, a research biologist at LAERF, says Garlon is about five years away from being released on the consumer market as an aquatic-use herbicide. Another exotic problem plant, Eurasian watermilfoil, is being studied at LAERF. It is typically thought to have been brought over the Atlantic from Europe or western Asia as a decorative aquarium plant, like Hydrilla. It ranked #1 as the biggest problem aquatic plant below the Mason-Dixon line until Hydrilla pushed its way into first place. Today, it ranks #3 nationwide after Hydrilla and waterhyacinth as an exotic weed. One interesting note about biological control: grass carp will not eat Eurasian watermilfoil, says Dr. Smart. They have no effect on the waterhyacinth or pond lilies of Lake Conroe either, says Blake Kellum, the Water Quality Supervisor of the San Jacinto River Authority. A chemical known simply as “2,4-D”, or 2, 4 dichlorophenoxyl, is commonly used for the control of waterhyacinth and positive results have been seen for the control of Eurasian watermilfoil as well, says John Madsen of LAERF. The herbicide 2,4-D is being used by the TP&WD to control waterhyacinth in the state. It kills plants by disrupting the growth cycle. The use of chemical poisons as a method of killing weeds is a relatively new development. A study of Carol Van Strum in the 1983 book, A Bitter Fog: Herbicides and Human Rights, says phenoxy herbicides, the most effective modern herbicides, were developed to stimulate growth of plants. Chemicals used for this purpose had the opposite effect when used in excess. In fact, the chemicals could kill selectively when used in overdose amounts. This attracted the interest of scientists in 1943 for their use in chemical warfare research. Agent Orange, a chemical used in Viet Nam, is composed of 50% 2,4-D, according to Van Strum. This has spawned a great deal of research into the effects of 2,4-D on humans and wildlife and the amount of 2,4-D that could be used safely. A study conducted by Bionetics Research Laboratories released in 1969 shows the effects of phenoxy herbicides on human health. Specifically, it shows 2,4-D “to be capable of causing birth defects at doses lower than lethal amounts.” It further lists 2,4-D along with six other pesticides under the heading “these should be immediately restricted.” Dr. Ruth Shearer, a molecular geneticist and consultant in genetic toxicology from Washington state has been studying the genetic effects of 2,4-D and its relationship to cancer since 1979. She asserts, “2,4-D induces mutations in both animals and human cells in culture, and damages DNA….It causes developmental toxicity when given to the pregnant female animal. It causes fetal hemorrhage at a low dose in rats….2,4-D caused malformations and fetal death in the animals only at a high dose, but it caused various kinds of malfunctions and growth retardation at very, very low doses.” According to an Environmental Protection Agency Pesticide Fact Sheet, “certain formulations of 2,4-D are highly toxic to fish and/or aquatic invertebrates.” In 1989, an EPA report stated that there was some potential of 2,4-D contaminating the groundwater. A label from a distributor of 2,4-D lists safe handling procedures for the herbicide. Among several warnings, it reads, “1) Do not contaminate water used for irrigation, domestic or spray purposes 2) Do not spray directly in water 3)Do not contaminate water by cleanup of equipment or disposal of waste” In the areas that 2,4-D has been used, there have been many bad effects on humans and animals attributed to the herbicide and confirmed by scientific studies: swollen eyes, mouth and lips, rashes, urinary disturbances, renal damage, fatigue, nausea, anorexia, diarrhea, swelling and pain in the extremities, paralysis, serious irregularities of the heartbeat, neuritis and numbness in the legs, fingers and toes. 2,4-D is felt to be linked to an outbreak of muscular disorders reported in the Lake Livingston area where the TP&WD is using it to control the spread of waterhyacinth. However, 2,4-D is still a very common ingredient used in lawn care chemicals. It is an active ingredient in the consumer herbicides Rodeo and Roundup. A study released by the National Cancer Institute in 1988 shows farmers who were exposed to 2,4-D for 20 or more days per year are six times more likely to develop non-Hodgins lymphonia (a type of cancer) than non-farmers. Farmers who mixed or sprayed the herbicide stood a greater chance of developing the disease. 2,4-D has never been used in Lake Conroe, according to Blake Kellum, the Water Quality Supervisor of the SJRA. Today, Lake Conroe’s aquatic vegetation consists of pond lilies and some waterhyacinth, according to Kellum. The pond lilies tend to colonize the same area each year. They sprout purple and yellow blooms and are very attractive, Kellum says, and fishermen love to fish in them. Waterhyacinth is not a problem on Lake Conroe right now, Kellum says, but it does have the potential of becoming a problem. “No one on Lake Conroe has reported a problem with waterhyacinth. If it does become a problem, we don’t want property owners running out with a gallon of herbicide,” says Kellum. The public is not allowed to use any chemicals without the approval of the SJRA. The scientists at LAERF are studying the largest number of plants available to any one of the research facilities under the U.S. Army Corps of Engineers’ Waterways Experiment Station located in Vicksburg, Mississippi. They said that in a lake devoid of vegetation, like Lake Conroe, a colonizing plant is the only type that will grow.
This can be good or bad. Hydrilla is a colonizing plant, but so are several native, preferable plants, such as muskgrass, horned pondweed and American pondweed. Once the native colonizers get going, other native stable plants can be introduced. All these plants need is clean water, time to grow and a chance to grow without being suffocated by Hydrilla or another exotic weed. Lake Conroe will only benefit from this strategy of plant introduction, and boaters and anglers alike will gain a new appreciation for their lake.

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