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ABSTRACTSmall-plot treatments of triclopyr were conducted on Lake Minnetonka and Lake Minnewashta, MN, during June 1998 to investigate the herbicide's potential to selectively control Eurasian watermilfoil (Myriophyllum spicatumL.) at low doses. Applications were made on 1-ha plots with rates based on plot type: references (0 mg acid equivalent (ae)·L−1), protected plots (0.5 mg ae·L−1), semi-protected plots(1.0 mgae·L−1), and unprotected plots (1.5 mgae·L−1). Plot protection was a function of potential mixing in the water column. Herbicide residues were monitored to determine dissipation 1 through 72 h post treatment. Samples were analyzed with both a high performance liquid chromatography technique and an enzyme-linked immunosorbent assay method. Results from these two analytical techniques were compared, and found equivalent (R2= 0.96). Triclopyr had a relatively short half-life for each treatment (3.5 hr at 0.5 mg ae·L−1, 2.9 hr at 1.0 mg ae·L−1, and 4.2 hr at 1.5 mg ae·L−1). At 8 weeks post treatment, there was a 30 to 45% reduction in Eurasian watermilfoil distribution. Greatest Eurasian watermilfoil control was achieved in plots using higher triclopyr rates. Frequency of native plants decreased by 24% in the untreated reference plot, 20% in the 0.5 mg ae·L−1plot and 6% in the 1.0 and 1.5 mg ae·L−1plots. Mean species per point, however, either increased or remained unchanged in seven of the nine treated plots. Decline of native plants may be partially attributed to the onset of fall senescence. Larger contiguous areas, higher triclopyr rates, and sequential applications may be required to enhance Eurasian watermilfoil control in small-plot, partial lake treatments.

ABSTRACTThe aquatic herbicide fluridone is being used in northern tier states to selectively control the submersed exotic species Eurasian watermilfoil (Myriophyllum spicatumL.) 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 effect of low-dose fluridone treatments on the submersed plant communities in four lakes in Michigan. The overall study objective was to determine whether submersed plant species diversity and frequency were impacted by low-dose fluridone applications in the year of treatment, when targeting a whole lake for Eurasian watermilfoil control. The primary objectives of this portion (part II) of the overall study was to determine fluridone effectiveness on the exotic submersed species Eurasian watermilfoil (Myriophyllum spicatumL.) and to evaluate shifts in plant species diversity at one year posttreatment. Secondary objectives included determining fluridone effectiveness on the exotic submersed species curlyleaf pondweed (Potamogeton crispusL.) and verifying laboratory-derived results of fluridone concentration and exposure time relationships with respect to efficacy against Eurasian watermilfoil. 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 post treatment). The fluridone concentration and exposure time (CET) relationship resulted in good to excellent control of Eurasian watermilfoil through 15 months posttreatment on three of the treated lakes (Big Crooked, Camp, and Lobdell). On a fourth lake, Wolverine, the required CET relationship was not maintained and poor control of Eurasian water milfoil 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 posttreatment, in any of the fluridone-treated lakes. Native plant cover was maintained at levels >70% in the year of treatment and at one year posttreatment; a level above the range (20 to 40%) 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.

ABSTRACTThe herbicide fluridone is being used in northern lakes and reservoirs to control the exotic species Eurasian watermilfoil (MyriophyllumspicatumL.). Since quantitative information linking changes in plant communities following fluridone applications is limited, particularly with respect to water residue records, a study was conducted to investigate the effect of low-dose treatments on the submersed plant communities in four Michigan lakes. The overall study objective was to determine whether plant species diversity and frequency of occurrence were affected by low-dose fluridone applications in the year of treatment. The primary objective of this portion of the overall study was to provide an application strategy that would maintain a threshold dose of fluridone, 5 μg·L−1declining to 2 μg·L−1, in the treated lakes to selectively control Eurasian watermilfoil. Study lakes were 55 to 220 ha in size and contained an average of nine species of submersed plants. Big Crooked, Camp, Lobdell, and Wolverine lakes were treated in mid-May 1997 with the formulation Sonar® AS, to yield an initial concentration of 5 μg·L−1fluridone in the upper 3.05 m of each lake. Asequential application of Sonar® AS was conducted on each lake at 16 to 21 days after initial treatment (DAIT), intended to reestablish a fluridone concentration of 5 μg·L−1in the upper 3.05 m of each lake. Bass, Big Seven, Clear, and Heron lakes received no fluridone applications and served as untreated reference sites. Water residue samples were collected at prescribed intervals on each fluridone-treated lake from pretreatment up to 81 DAIT. Samples were collected from six littoral stations and from two deep locations throughout each lake, and temperature profiles were measured at the deep stations. Fluridone residues were analyzed using two separate techniques, the newly developed enzyme-linked immunosorbent assay and the standard high performance liquid chromatography method. 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 μg·L−1during the first 2 weeks posttreatment, and by main taininga concentration >2 μg·L−1through 60 DAIT. Residues became well mixed in the water column under isothermal conditions, and thermal stratification prevented mixing of fluridone into deeper and colder waters. Residue data indicated that thermal stratification, or the lack thereof, at the time of herbicide application can affect target herbicide concentrations. Using the volume of a pre-selected depth zones 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.

ABSTRACTAquashade™, a colorant and dye registered by the US Environmental Protection Agency (USEPA) for use in aquatic environments to manage plant growth, does not reduce the visibility of water that otherwise would meet safe swimmming standards. This product was examined for its potential impact on underwater visibility in surface waters used for recreational swimming. Aquashade™ treatments of 0, 1, and 5 mg L−1were evaluated in 3-m deep outdoor tanks containing reservoir water. Transparency and light attenuation were examined using a Secchi disk and an underwater photo synthetically active radiation (PAR) meter. In the first experiment, filtered lake water was used, while in the second, bentonite clay was added to provide a light transparency of approximately 1 m. In filtered lake water, Aquashade™ treatments (1 mg L−1and 5 mg L−1) reduced light transparency by approximately 10% and 30%, respectively, but the transparency (272 and 194 cm, respectively) was well above typical international (100 cm) and state (33 cm) standards for safe swimming. In the trial with clay turbidity, the 1.0 mg L−1Aquashade™ treatment was not significantly different in transparency when compared with the untreated reference, but the 5 mg L−1treatment reduced transparency to 67 cm (31%). Aquashade™ absorbed light from 550 nm to 650 nm, with peak absorbance in the 630 nm range. Field measurements of Aquashade spectral attenuation matched that observed in the laboratory. While treatments at the allowable level of 1 mg L−1will reduce transparency, the effect is small compared to inorganic or algal-induced turbidity that can occur in surface waters.