Coventry Lake –

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 Survey Results ...... 2 Recommendations ...... 7

Introduction Hydrilla (Hydrilla verticillata) in Coventry Lake was treated with the systemic 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 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.

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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 Survey Results Despite herbicide treatments in 2016 and 2017, the 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.

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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.

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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.

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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 crispus 16 Moderate 15 Dense

Non-dominant Species 12 Dense Nitella sp. 10 Moderate nuttallii 8 Moderate 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) 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

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Infrequent Species Eleocharis acicularis Sparse 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 sp. Very sparse Glossostigma cleistanthum Dense (where present) Elatine sp. Sparse

The survey results indicate that native Naiad (Najas guadalupensis) was impacted the most by the herbicide. In the October post-treatment survey, the remaining Naiad plants were decaying and visibly impacted by the SONAR treatment. Invasive Eurasian milfoil also appeared to be affected by the herbicide treatment, which is an added benefit to the lake because milfoil also appears to be spreading in the lake in the last decade. Native species Coontail (Ceratophyllum demersusm) and Water-stargrass (Zosterella dubia) appeared to be more widespread in the lake during the October survey but grew at lesser densities than in the pre-treatment survey. Invasive Curly- pondweed () was not found during the October survey, but this plant is known to be more of an early-season species. Other native pondweeds appeared not affected by the herbicide treatment, and reduction in growth density can also be attributed to natural pondweed succession late in the season.

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September 11th, 2018 Hydrilla tips have some visible discoloration due to SONAR herbicide, appearing purplish at shoot tips. Hydrilla was very easily fragmented at this time.

As shown in the picture above, Hydrilla plants remaining after completion of the herbicide treatments were generally in poor condition, showing advanced signs of chlorosis (whitening), periphytic algae, and brittleness. Following treatment with fluridone, especially during an initial year of treatment, it is often typical to see some biomass persisting into the fall months due to its slow mode of action. While many areas of Hydrilla growth were absent after treatment, most of the persisting plants were located in shallow areas along the northeastern shoreline, between Knoll Drive Bay and Island Coves. It is expected that the remaining concentration of fluridone in the water and sediment pore-water would continue to effect mortality of the remaining Hydrilla growth, however the slower effects in these areas could indicate the need for a slightly higher dose in these areas or be due to the plants being established in these spots longer than was previously expected.

Recommendations Given the reproductive characteristics of Hydrilla and the likely persistence of subterranean tubers and turions, management of this highly invasive plant must be a multiple year effort. We recommend another whole-lake fluridone treatment in 2019. The general approach will be the same as in 2018; however the initial fluridone application should be conducted earlier in the season – mid-to-late June instead of mid-July. Managers should also consider a slightly higher dosage, especially in the areas with

7 large persistent Hydrilla beds. Discussions in 2018 initially considered a total lake-wide dose of 10 ppb rather than 8 ppb, and we strongly recommend a 10 ppb target concentration in 2019. Yet as in 2018, herbicide treatment details will be discussed among responsible parties.

Given the lake-wide nature of the current management approach, the removal of benthic barriers that were installed in 2017 should be considered. The benthic barrier is likely to suppress tuber and turion sprouting, which is needed for the fluridone to affect the plants. Without removing the barrier, any reproductive structures in those areas may escape control and be a source of re-infestation in the future.

A minimum of two full littoral zone surveys (early season--July & late season--October) should be performed to monitor for any new areas of growth. The first survey will include a census of two State of CT protected species ( and Lipocarpha micrantha), neither species were found during 2018 surveys. In addition, more focused surveys, should be considered to monitor herbicide effects to plants over the course of the treatment period

The CT DEEP and the Town of Coventry should maintain a semi-permanent boat ramp monitor at the state boat launch. Less Hydrilla fragments were found at the boat ramp in 2018 than in 2017, but fragmentation and spreading of the plant is still a major concern.

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