11-14-2016 Council Packet - Laserfiche WebLink

Home Browse Search

N Triclopyr for selective whole -bay miifoil management canopy formation is a focus of ecologists (Grace and Wetzel 1978, Nichols and Shaw 1986, Madsen 1998), it also drives citizen complaints. In response to ongoing EWM problems, the Lake Mm- netoWm Conservation District initiated a multiyear demon- stration project in 2007 to determine if targeting EWM at a whole -bay scale could reduce management inputs over time. Reliance on presencelabsence point -intercept method- ology (Madsen 1999, Mikulyuk et al. 2010) to assess effi- cacy of treatments led to stakeholder concembecause EWM frequency was often not congruent with perceptions of de- creased abundance following treatment Likewise, concern was expressed that point -intercept results were masking her- bicide impacts to native plant abundance and density. Use of point -intercept sampling as a primary assessment technique mustbe considered in terms of viable alternatives. Extensive labor requirements associated with biomass sam- pling and the need for costly equipment and dataprocessing capabilities associated with hydroacoustics has discouraged incorporating these efforts into routine plant assessments. Johnson and Newman (2011) describe a biomass sampling technique that reduces efforts associated with sample col- lection While hydroacoustic data can show temporal and spatial changes in plant coverage and abundance following management efforts (Valley and Drake 2005, 2007, Valley et al. 2006, Zhu et al. 2007, Sabol et al. 2009), issues with costs and data processing requirements have discouraged adoption by resource managers. Recent availability of low- cost recording sonar units has led to renewed consideration of incorporating hydroacoustic technology into submersed plant assessments (Netherland and Jones 2012). Our objective was to evaluate invasive and native plant frequency and abundance over multiple seasons following whole -bay management with triclopyr We assessed (1) bay - wide herbicide concentrations; (2) changes in percent area covered and biovolume of submersed vegetation via hydroa- coustic transects; and (3) multiseason changes in frequency and biomass of EWM and nativeplants following whole bay herbicide applications. Methods Study sites St. Albans Bay (64 ha; 63% littoral); Grays Bay (64 ha; 84% littoral), and Gideon Bay (133 ha; 54% haoral) are located in Lake Minnetonka, Mmnesota (Fig. 1). Both St. Albans and Grays bays have narrow inlets that limit hy- draulic exchange with the main body of water, whereas Gideons Bay is more open and thought to be subject to increased water exchange with the main lake. Grays Bay is the outlet for Lake Minnetonka, and despite the nar- row inlet, this bay can be subject to increased exchange in years when water flows are high. In 2011, the treatment of Grays Bay was postponed due to high flow rates and concerns of treated water being introduced to Minnehaha creek. In contrast, water flow at the outlet was mmrmal in May 2012 at the time of the whole -bay herbicide applica- tion. All 3 bays support dense beds of EWM that impact various stakeholder uses of the water. Although water qual- ity sampling was not a significant component of this study, data from the Minnehaha Creek Watershed District (MCWD 2013) are available forbays throughoutLake Minnetonkafor 2011 through 2013 (www.minnehahacreek.orgfdata). These data indicate that Secchi disk transparency for the bays ranged from 2.6 to 5.2 in, chlorophyll a ranged from 2.0 to 5.5 µglL, and total phosphorus (P) ranged from 15.3 to 19.7 µg/L during the 3 -year study period. The water clarity indicates that macrophytes were not light -limited during the course of this evaluation. All 3 bays were recognized by the MCWD for superior water quality during the course of these trials. Herbicide treatments and water sampling Herbicide treatments on the 3 bays were conducted to demonstrate a bay -wide approach to EWM control. These treatments required significant planning, stakeholder ap- proval and funding, and permitting by the Minnesota De- partment of Natural Resources (MDNR). Our objective was to monitor the treatment outcomes on Grays Bay that had been part of the large-scale demonstration since an original triclopyrlendodmU treatment m 2008, and on 2 bays that had notpreviously been managed at abay-wide scale (St. Albans and Gideons). Although the label allows triclopyr treatments at concentrations up to 2500 uglL, bay -wide strategies se- lectively targeting EWM resulted in the use of much lower treatment concentrations. The treatment rates described be- low were based on prior mesocosm research, the desire to achieve selective EWM control, and operational budgets. St. Albans Bay was treated on 26May 2011 to achieve a bay - wide concentration of 310 µglL triclopyr via the application of Renovate OTF granular herbicide (Renovate OTF, SePRO Corp., Carmel, IN). The treatment was calculated based on the entire volume of the bay. The treated area (ha), mass of product applied, target concentration in the treatment zone, bay -wide concentration, and water sample intervals are shown in Table 1. On 14 June 2013, St. Albans Bay received an application of Renovate OTF granular herbicide to achieve a bay -wide concentration of 300 AglL triclopyr above the thermocline (Table 1). Water samples were collected from 6 sites in the bay at mid - depth (above the thermochne) by personnel from Fresh- water Scientific Services LLC and the Three Rivers Park District (Fig. 2). To monitor vertical distribution of the 307