Technical Report TR-01-7 US Army Corps of Engineers, Waterways Experiment Station

Aquatic Plant Control Research Program

Kurt D. Getsinger, John D. Madsen, Tyler J. Koschnick, Michael D. Netherland, R. Michael Stewart, David R. Honnell, Alicia G. Staddon, and Chetta S. Owens

ABSTRACT

The aquatic herbicide Sonar AS (fluridone) is being used in Northern tier states to selectively control the submersed exotic species Eurasian watermilfoil (Myriophyllum spicatum L.) growing in lakes and reservoirs. Reliable quantitative information linking changes in the submersed plant community following fluridone applications is limited, particularly with respect to water residue records; therefore, a study was conducted to investigate the impact of low-dose fluridone treatments on the aquatic plant communities in eight lakes of southern Michigan. The main objective of the study was to determine whether submersed plant species diversity and frequency were impacted by low-dose fluridone applications in the year of treatment, when targeting for Eurasian watermilfoil control. Secondary objectives included (a) determining fluridone effectiveness on the exotic submersed species curlyleaf pondweed (Potamogeton crispus L.), (b) evaluating shifts in plant species diversity at 1-year posttreatment, (c) measuring the effect of thermal stratification on water column distribution of fluridone residues, and (d) verifying laboratory-derived results of fluridone concentration and exposure time relationships with respect to efficacy against Eurasian watermilfoil. Study lakes were 55 to 220 ha in size and contained an average of nine species of submersed plants, including Eurasian watermilfoil and curlyleaf pondweed. Four lakes (Big Crooked, Camp, Lobdell, and Wolverine) were treated in mid-May 1997 with Sonar AS to yield a target concentration of 5 g " L -1 (ppb) fluridone in the upper 3.05 m (10 ft) of each lake. A sequential (booster) application of Sonar AS was conducted on each lake at 16 to 21 days after initial treatment (DAIT). This whole-lake booster application was intended to reestablish the target concentration of fluridone (5 g " L -1 ) in the upper 3.05 m of each lake. Four water bodies (Bass, Big Seven, Clear, and Heron) received no fluridone applications and served as untreated reference lakes. Water residue samples were collected on prescribed intervals on each of the fluridone-treated lakes from pretreatment up to 81 DAIT. Samples were collected from six littoral zone stations and from two deep locations throughout the lakes. Water temperature profiles were measured at the deep stations at each water-sampling event. Fluridone residues were analyzed using two separate techniques; (1) the newly developed enzyme-linked immunosorbent assay (ELISA), and (2) the standard high-performance liquid chromatography (HPLC) method. Quantitative sampling of vegetation was performed using point-based frequency of species occurrence to evaluate whole-lake distribution and diversity of the submersed plant community of all eight study lakes. The technique was implemented using global positioning and geographic information systems, with a minimum grid resolution of 50 m by 50 m. Plant surveys were conducted in early to mid-May and in mid-August in 1997 (year of treatment) and 1998 (12 and 15 months posttreatment). Aqueous fluridone levels on three of the treated lakes met the laboratory-derived criteria for achieving good control of Eurasian watermilfoil by providing a peak concentration of approximately 5 g " L -1 during the first 2 weeks of posttreatment, and by maintaining a concentration > 2 g " L -1 through 60 DAIT. This fluridone concentration and exposure time (CET) relationship resulted in good to excellent control of Eurasian watermilfoil through 15 months of posttreatment on these lakes. On a fourth lake, however, the required CET relationship was not maintained and poor control of Eurasian watermilfoil was observed. There was no strong evidence of long-term curlyleaf pondweed control in any of the fluridone-treated lakes. The herbicide application strategy used in this study did not significantly impact the native plant species diversity or cover in the year of treatment, or through 15 months of posttreatment, in any of the fluridone-treated lakes. Native plant cover was maintained at levels > 70 percent in the year of treatment and at 1 year of posttreatment; a level above the range (20 to 40 percent) recommended for healthy fish and wildlife habitat. The selective control of Eurasian watermilfoil achieved in this study verified results from previously conducted laboratory and outdoor mesocosm evaluations. Fluridone residues became well mixed in the water column under isothermal conditions, and thermal stratification prevented mixing of fluridone into deeper and colder waters. Thermal stratification, or the lack thereof, at the time of herbicide application can impact target concentrations of fluridone. Using the volume of a preselected depth zone to calculate the amount of fluridone needed to achieve a particular target concentration can result in an over- or under-dosing of a water body, leading to poor or nonselective control of Eurasian watermilfoil. Higher initial and booster rates of fluridone (e.g. 6 g " L -1 ) could mitigate the loss of product to mixing processes associated with deep thermoclines likely to occur in early spring treatments, if 3.05-m-depth contour is preselected for volume calculations. These slightly higher treatment rates should provide for more consistent control of Eurasian watermilfoil, while still maintaining selective control properties per nontarget native plants.

Complete report available in PDF format at: http://libweb.wes.army.mil/uhtbin/hyperion/EL-TR-01-7.pdf