Coventry Lake – Hydrilla Management Program
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Coventry Lake – Hydrilla Management Program 2018 Final Report Draft - February 2019 Prepared For: Prepared By: SŌLitude Lake Management 590 Lake Street Shrewsbury, MA 01545 Northeast Aquatic Research George W. Knoecklein, Ph.D. 74 Higgins Highway Northeast Aquatic Research Mansfield Center, CT 06250 Table of Contents Introduction ............................................................................................................................................ 1 2018 Management Approach .................................................................................................................. 1 Coventry Lake Aquatic Plant Survey Results ............................................................................................. 2 Recommendations .................................................................................................................................. 7 Introduction Hydrilla (Hydrilla verticillata) in Coventry Lake was treated with the systemic herbicide Fluridone for the first time in 2018. Hydrilla, first found at the State of CT boat ramp in 2015, was treated locally with two partial lake treatments with the herbicide Aquathol-K (endothall) in 2016 and 2017. Isolated Hydrilla plants and beds smaller than 3’ in diameter were addressed with benthic barriers in 2017. Our surveys in October 2017 showed varying success from the endothall treatments but more importantly located several new areas with Hydrilla, indicating that the plant was no longer confined to the beds found in 2015 but was spreading throughout the lake. Based on these results, project partners discussed moving to a whole-lake management approach that included using the systemic herbicide Sonar (fluridone). The management plan for controlling Hydrilla in Coventry Lake calls for annual investigations and ongoing adaptive control efforts. 2018 Management Approach The 2018 management approach consisted of maintaining a lake-wide concentration of Sonar (fluridone), capable of controlling Hydrilla in Coventry Lake, for a 90+ day period. To achieve this, a total lake-wide dose of 8 parts per billion (ppb) was applied in pellet form to the littoral area of the lake over three separate applications. Applying the lake-wide dose only to the littoral area would result in an increased exposure to the plants, and use of the pellet formulation (Sonar ONE) would release fluridone over a period of weeks directly at the base of the target plants. Herbicide applications were performed on July 12th, August 8th, and September 17th using an airboat equipped with a calibrated electric spreader. The figure to the left (Figure 1) shows the littoral zone treatment area where the pellets were applied during each application. Prior to each application, signs were posted around the shoreline of the lake warning of the temporary water use restrictions. Swimming was restricted either for the full day or until the afternoon on the days of treatment. Restrictions on certain types of irrigation were imposed for 30-days after each treatment. 1 To monitor fluridone concentrations, water samples were collected from the surface and 1-foot off the bottom from six different locations as shown in Figure 1. Samples were collected multiple times over the course of the project and results are displayed in Table 1 below. Table 1 - Coventry Lake - FasTEST Data Results (ppb) Date Station ("a" is surface sample, "b" is bottom sample) 1a 1b 2a 2b 3a 3b 4a 4b 5a 5b 6a 6b Water Depth 8' 10' 12' 9' 12' 32' 7/16/2018 1.3 <1 1.3 <1 1.5 <1 2 2.4 1.1 1.3 NS <1 7/23/2018 1.1 <1 2.3 1.3 2.2 1.3 1.5 1.4 1.7 1.5 1 <1 8/2/2018 1.6 <1 <1 <1 1.7 1.1 2.1 1.4 2 3.1 1.4 <1 8/22/2018 1.9 1.3 3.6 2.2 2.1 2.2 2 2.1 1.2 3 1.5 <1 10/17/2018 1.8 3 2.7 2.6 2.2 3.2 3.5 2.7 1.9 2.9 2.3 2.5 Fluridone testing results showed an expected range of concentrations at the various stations between less than 1 ppb and 3.6 ppb. Based on prior experience in hundreds of other lakes, this range of concentrations has been effective in controlling Hydrilla. Due to the characteristics of the pellet formulation of herbicide, even higher concentrations are expected to be present at the bottom of the lake and in the sediment pore water, which is in direct contact with the Hydrilla. These results also show that desirable herbicide concentrations were maintained for over 90 days from the first application and were sufficient to continue to provide herbicidal affect after the final sampling. Coventry Lake Aquatic Plant Survey Results Despite herbicide treatments in 2016 and 2017, the invasive species Hydrilla continued to expand in density and range in Coventry Lake through October 2017. Multiple new Hydrilla patches were found in the Knoll Drive Bay and the Outlet Cove in October 2017. These beds were not found during the 2017 August survey and were assumed to have spread and grown from Hydrilla fragments that year. Figure 2 shows all locations where Hydrilla was found during the 2017 season. Many of these 2017 Hydrilla locations were identified by underwater SCUBA surveys in the Boat Ramp and Island Coves. Figure 2 also identifies the major named areas that are referenced in this report. 2 Northern Patches Knoll Drive Bay Island Coves Outlet Cove Boat Ramp Cove Figure 2: Coventry Lake 2017 Hydrilla Locations & Named Bays The 2018 pre-treatment survey was conducted by Northeast Aquatic Research on July 2nd and 3rd. The survey consisted of waypoints throughout the entire littoral zone of the lake, including waypoints where Hydrilla had been previously found in 2017. This survey utilized a combination of visual assessments, hand-raking in shallow water, grappling rake tosses, and depth-soundings to view plants growing in deep water. No raking was performed in areas where benthic barriers installed had been installed. During this two day 2018 survey, only one location of Hydrilla was confirmed by the boat survey crew. This location is marked in red in Figure 3 below. At this waypoint, Hydrilla was seen growing around the edges of the benthic barrier that had been placed in 2017. Plant growth in this area was very dense, consisting of mostly native plant species and filamentous algae. Because little to no Hydrilla was documented using traditional survey techniques, a limited SCUBA survey was also performed on July 3rd. This underwater investigation checked three locations around the lake: Boat Ramp Cove, Outlet Cove, and just outside the Island Coves. Hydrilla was found growing in all three locations surveyed by SCUBA divers. These locations are marked in yellow in the map below. 3 Figure 3: Coventry Lake 2018 Hydrilla Locations During the July SCUBA survey, Hydrilla plants were found to be 6 to 24 inches tall. Plants in the Boat Ramp Cove that had been treated with endothall in 2017 were sparsely growing throughout roughly a 100-ft expansive area with shoot heights of ≤6 inches tall. There were dense patches of the plant-like macroalgae Nitella growing in areas that had been previously seen as dense Hydrilla in 2017. The Hydrilla growing in the Outlet Cove, which had not been treated in 2017, was much more prolific. At least six small patches, ranging 2-4ft in diameter, were found with plants up to 24 inches tall. There was one large Hydrilla bed found growing at the end of the Coventry Lakeview Restaurant dock. The Hydrilla beds in the Outlet Cove were surrounded by densely growing Eurasian milfoil (Myriophyllum spicatum), Naiad (Najas guadelupensis), and Water-stargrass (Zosterella dubia). All plant survey data was compiled into the tables below, which documents the overall frequency as a percentage of the total survey waypoints (%Frequency). In areas where the plant was found in the lake, density was also recorded. Density class is based on the average percent coverage estimates over all places where the species was found. Density class categories are: Very sparse < 10% cover, Sparse 10- 20%, Moderate >20-50%, Dense >50% cover as seen from the surface, or on the grappling rake. 4 Table 2 – July 2018 Vegetation Data July 2nd and 3rd, 2018 Most Prevalent Species %Frequency Average Density Zosterella dubia 26 Dense Najas guadalupensis 22 Dense Myriophyllum spicatum 21 Dense Potamogeton crispus 16 Moderate Ceratophyllum demersum 15 Dense Non-dominant Species Potamogeton perfoliatus 12 Dense Nitella sp. 10 Moderate Elodea nuttallii 8 Moderate Potamogeton amplifolius 6 Dense Infrequent Species Hydrilla verticillata Dense (where present) Chara sp. Moderate Potamogeton bicupulatus Less than 5% Very sparse Ludwigia sp. frequency Sparse Glossostigma cleistanthum Dense (where present) Vallisneria americana Sparse A lake-wide post-treatment aquatic plant survey was conducted by NEAR personnel on October 10, 2018. This survey utilized the same methods and density criteria. Native and invasive plant species were recorded at 302 GPS waypoints. Frequency and average density results are listed below in Table 3. Table 3 – October 2018 Vegetation Data October 10th, 2018 Most Prevalent Species %Frequency Average Density Zosterella dubia 43 Moderate Ceratophyllum demersum 31 Sparse Non-dominant Species Myriophyllum spicatum 9 Moderate Potamogeton perfoliatus 7 Sparse Potamogeton amplifolius 6 Sparse 5 Infrequent Species Eleocharis acicularis Sparse Elodea nuttallii Very sparse Filamentous algae Moderate Najas guadalupensis Sparse Nitella sp. Moderate Hydrilla verticillata Dense (where present) Less than 5% Potamogeton bicupulatus frequency Very sparse Sagittaria graminea Moderate Callitriche sp. Moderate Chara sp. Dense Polygonum