John D. Madsen

Whole Lake Fluridone Treatments For Selective Control of Eurasian Watermilfoil: II. Impacts on Submersed Plant Communities

ABSTRACT The aquatic herbicide 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 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 spicatum L.) 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 crispus L.) 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.

Whole Lake Fluridone Treatments For Selective Control of Eurasian Watermilfoil: I. Application Strategy and Herbicide Residues

ABSTRACT The herbicide fluridone is being used in northern lakes and reservoirs to control the exotic species Eurasian watermilfoil (Myriophyllumspicatum L.). 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−1 declining 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−1 fluridone 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−1 in 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−1 during the first 2 weeks posttreatment, and by main taininga concentration >2 μg·L−1 through 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.

ENVIRONMENTAL FACTORS AFFECTING BIOMASS AND DISTRIBUTION OF STUCKENIA PECTINATA IN THE HERON LAKE SYSTEM, MINNESOTA, USA

The Heron Lake System historically has been an important resource for waterfowl in Southern Minnesota, USA. In the early 1900s, the system was a major nesting, feeding, and staging area for breeding and migrating waterfowl mainly due to the extensive growth of sago pondweed, Stuckenia pectinata. In recent years, the abundance of S. pectinata has decreased dramatically. We conducted a study from 2002 through 2003 to identify factors limiting the success of S. pectinata. Distribution and biomass of S. pectinata differed significantly over the years of this study. Biomass was related to environmental factors, including water transparency and water temperature early in the season. Water transparency during May (time of early growth) had a negative relationship with maximum biomass at each site over all years. Water temperature had a positive relationship with increases in seasonal biomass yield of S. pectinata in the Heron Lake System.

Factors Limiting the Growth of Stuckenia pectinata (Sago Pondweed) in Heron Lake, Minnesota

ABSTRACT Point intercept surveys were conducted in Heron Lake, Minnesota to identify environmental factors limiting the growth of Stuckenia pectinata (L.) Börner (sago pondweed), formerly Potamogeton pectinatus L. Stuckenia pectinata was found at 50.2% of the sample sites. Mean tuber weight was 73 mg; tuber density was 31.8 N m−2 (2.32 g m−2). The distribution of S. pectinata was limited by water depth and wave action close to shore and light attenuation in deeper water. There was a negative correlation (r= −0.88) between the amount of sand in the sediment and the presence of S. pectinata shoots. There was a positive correlation (r= 0.89) between the amount of silt in the sediment and the presence of S. pectinata shoots.