Coventry Lake Hydrilla Management Program 2019 Treatment Report

Northeast Aquatic Research &

Coventry LakeNortheast Aquatic Hydrilla Management Program

2019 Treatment Report

Prepared for the Connecticut Department of Energy and Environmental Protection & the Town of Coventry

January 2020

Northeast Aquatic Research, LLC ⁘ 74 Higgins Highway, Mansfield, CT 06250 ⁘ 860-456-3179

Introduction Coventry Lake contains the invasive species Hydrilla (Hydrilla verticillata), which was first found in 2015. Efforts to control the species have been undertaken over the past four years. In 2016 and 2017, the individual Hydrilla patches were treated using the herbicide Aquathol-K (endothall). In 2017, a number of isolated hydrilla plants were found and patches smaller than 3’ in diameter were covered with benthic barriers. One large patch was also covered with benthic barriers at this time. Despite herbicide treatments of Aquathol-K (endothall) in 2016 and 2017, the invasive species Hydrilla (Hydrilla verticillata) continued to expand in density and range in Coventry Lake through October 2017 (Map 1). Based on these results, the decision was made to engage in a whole-lake management approach using the systemic herbicide fluridone, a product with multiple cases of success against Hydrilla.

In 2018 and 2019, Hydrilla was treated with the systemic herbicide Sonar One (fluridone) for a 90+ day period. After completion of the 2018 treatment, a scattering of Hydrilla plants were found along the northeastern shoreline, between Knoll Drive Bay and Island Coves (Map 2). These plants were generally in poor condition, showing effects of the herbicide. Due to the slow-acting nature of Fluridone, it is likely that the herbicide continued to affect the plants after the time of the post-treatment survey.

The 2019 treatment was administered by SOLitude Lake Management on July 18th. Two booster treatments were administered on August 28th and September 30th. Northeast Aquatic Research (NEAR) conducted three aquatic plant surveys of Coventry Lake in 2019. The first survey, on June 21st, was a detailed search for the Connecticut state-listed Threatened species Potamogeton vaseyi and Lipocarpha micrantha in locations where these species had been historically documented. The whole-lake pre-herbicide treatment survey was conducted on July 15th and 17th. The post-herbicide treatment survey was conducted on October 1st and 2nd.

Map 1. 2017 Hydrilla locations and named bays.

Northern Patches

Knoll Drive Bay

Island Outlet Coves Cove

Boat Ramp Cove

Map 2. 2018 Hydrilla locations.

2019 Management Approach The 2019 management approach consisted of maintaining a lake-wide concentration of Sonar One (Fluridone), capable of controlling Hydrilla in Coventry Lake for a 90+ day period. The initial herbicide application was performed by SOLitude on July 18th, with a dose of 4 ppb applied to the 178-acre littoral zone, using an airboat equipped with a calibrated electric spreader. The first booster treatment was administered on August 28th, using a lake-wide dose of 2 ppb applied to the littoral zone. The second booster treatment was administered on September 30th, again using a 2 ppb lake wide dose. Map 3 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.

Map 3. Coventry Lake 2019 Treatment Area.

To monitor fluridone concentrations, water samples were collected from the surface and 1-foot off the bottom from six different locations as shown in Map 3. Samples were collected multiple times over the course of the project and results are displayed in Table 1 below.

Table 1. 2019 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' 7/29/2019 2.7 2.4 2.3 2.8 2.5 2.7 3.1 5.3 2.2 3 1.9 2.7 8/14/2019 1.2 1.6 1.9 1.9 <1 2.3 2 1 2.4 2.1 1.7 1 9/13/2019 2 1.9 2.1 1.8 2.2 2.3 1.9 2 1.7 1.5 1.8 1.1 11/5/2019 2.2 1.9 2.5 2.2 1.8 2.5 2.5 2.2 2.3 1.8 2.6 2.1

Fluridone testing results showed an expected range of concentrations at the various stations between less than 1 ppb and 3.1 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. 2019 Aquatic Plant Survey Results On June 21st, Northeast Aquatic Research conducted an NDDB survey in search of the Connecticut state- listed Threatened species Potamogeton vaseyi and Lipocarpha micrantha, which both have historical records in Coventry Lake. This survey involved extensive searching for the two species in the NDDB areas of concern (Map 4). During the NDDB survey, the Connecticut state-listed Threatened species Potamogeton vaseyi was found in one location to the east of the island (Map 5). The remainder of the designated P. vaseyi area of concern along the southeast shore contains limited habitat for the species, due to development of boat docks, beaches, and rock piles. The area of concern in the northern corner of the lake may still contain favorable habitat for P. vaseyi, although the species has not been found in this area in recent years. Lipocarpha micrantha is a flowering sedge that grows in shoreline habitat. During the NDDB survey, Dr. Knoecklein searched on the shore in the area with a historical record of L. micrantha but did not find the species. The 2019 pre-treatment survey was conducted over two days, on July 15th and 17th. The survey consisted of creating waypoints throughout the entire littoral zone of the lake and revisiting all waypoints where Hydrilla had been found in previous years (Map 6). 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. The post-treatment survey was conducted on October 1st and 2nd, approximately 11 weeks after the initial fluridone application. This survey utilized the same methods as the pre-treatment survey, as well as an underwater inspection of the three pre-treatment hydrilla locations via SCUBA.

The species frequency and density results from the 2019 and 2018 pre- and post-treatment surveys are listed in Table 2 and Table 3. ‘Percent Frequency’ documents the overall frequency as a percentage of the total number of survey waypoints. 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 10-19%, Moderate 20- 49%, Dense 50-79%, Very Dense 80-100% cover as seen from the surface or on the grappling rake. Three hydrilla beds were found during the 2019 pre-treatment survey – all known locations from previous years, and all in one section of the lake, around the large island on the northeastern shoreline (Map 7). The two beds in the island coves contained small hydrilla plants – likely regrowth following the 2018 herbicide treatment. Off the southern shoreline of the island, small Hydrilla plants were likely growing under the benthic barrier, but no Hydrilla plants were found growing out of or around the edges of the barrier. During the post treatment survey, Hydrilla was found only under the benthic barrier off the island. The survey involved extensive searching via SCUBA, but no Hydrilla plants were found in the other two locations where it was found during the pre-treatment survey. In addition to Hydrilla, the invasive species Myriophyllum spicatum (Eurasian milfoil) and Potamogeton crispus (curly-leaf pondweed) were each found at a handful of locations in the lake, but at a much lower frequency than in previous years’ pre-treatment surveys (Map 8, Map 9). The 2018 herbicide treatment noticeably affected Myriophyllum spicatum, which had been spreading in the lake over the last decade. In 2019, Milfoil was found at only a handful of locations pre-treatment (<5% frequency) and remained scarce post-treatment. The invasive species Potamogeton crispus was present at a low frequency and moderate density pre- treatment. P. crispus was not found in the lake in October. Pre-treatment, 16 native species were found in the lake, all at a low frequency. The most abundant native species were Nitella and Zosterella dubia, both present at approximately 20% frequency (Map 10, Map 11). Post-treatment, the number of native species had reduced to 14, with Nitella and Zosterella dubia remaining the most abundant.

Map 4. Areas where protected aquatic plants have historically been recorded by CT DEEP/NDDB.

Map 5. Locations of Potamogeton vaseyi during June 21st NDDB survey.

Map 6. 2019 pre- and post-treatment survey tracks.

Table 2. 2019 pre- and post-treatment survey vegetation data. % Frequency Average Density Scientific Name Pre Post Pre Post Ceratophyllum demersum 12 10 Moderate Moderate Chara sp. 5 <5 Moderate Moderate Eleocharis robbinsii <5 0 Very Dense NA Eleocharis acicularis <5 <5 Sparse NA Elodea canadensis <5 0 Moderate NA Elodea nuttallii <5 0 Moderate NA Filamentous algae <5 <5 Dense Moderate Fontinalis sp. 0 <5 NA Very Sparse Hydrilla verticillata <5 <5 Moderate Very Sparse Myriophyllum spicatum <5 <5 Moderate Very Sparse Najas guadalupensis 0 <5 NA Sparse Nitella sp. 21 27 Moderate Moderate Potamogeton amplifolius 10 9 Moderate Moderate Potamogeton bicupulatus <5 <5 Moderate Moderate Potamogeton crispus <5 0 Moderate NA Potamogeton epihydrus <5 0 Moderate NA Potamogeton illinoensis <5 <5 Moderate Moderate Potamogeton perfoliatus 14 14 Dense Dense Sagittaria graminea <5 <5 Sparse Sparse Vallisneria americana <5 0 Very Sparse NA Zosterella dubia 19 38 Moderate Moderate

Table 3. 2018 pre- and post-treatment survey vegetation data. % Frequency Average Density Scientific Name Pre Post Pre Post Callitriche sp. 0 <5 NA Moderate Ceratophyllum demersum 15 31 Dense Sparse Chara sp. <5 <5 Moderate Dense Elatine sp. 0 <5 NA Sparse Eleocharis acicularis 0 <5 NA Sparse Elodea nuttallii 8 <5 Moderate Very sparse Filamentous algae 0 <5 NA Moderate Dense Dense Glossostigma cleistanthum <5 <5 (where present) (where present) Dense Dense Hydrilla verticillata <5 <5 (where present) (where present) Ludwigia sp. <5 0 Sparse NA

Myriophyllum spicatum 21 9 Dense Moderate Najas guadalupensis 22 <5 Dense Sparse Nitella sp. 10 <5 Moderate Moderate Polygonum sp. 0 <5 NA Very sparse Potamogeton amplifolius 6 6 Dense Sparse Potamogeton bicupulatus <5 <5 Very sparse Very sparse Potamogeton crispus 16 0 Moderate NA Potamogeton perfoliatus 12 7 Dense Sparse Sagittaria graminea 0 <5 NA Moderate Vallisneria americana <5 0 Sparse NA Zosterella dubia 26 43 Dense Moderate

Map 7. 2019 Hydrilla locations.

Map 8. July 2019 locations and densities of Myriophyllum spicatum.

Map 9. July 2019 locations and densities of Potamogeton crispus.

Map 10. Locations and densities of Nitella.

Map 11. Locations and densities of Zosterella dubia.

Recommendations Given the reproductive characteristics of Hydrilla and the likely persistence of subterranean tubers and turions, continued management is necessary to eradicate this invasive species from Coventry lake. Thus, we recommend a third whole-lake fluridone treatment in 2020. The approach should be the same as in 2019, and the initial fluridone application should be conducted in mid to late June or as determined by the applicator.

Given the lake-wide nature of the current management approach, the removal of benthic barriers that were installed in 2017 should be considered. The only location at which Hydrilla was found during the 2019 post-treatment survey was under the benthic barrier. The benthic barrier suppresses tuber and turion sprouting, which is needed for the fluridone to affect the plants. Without removing the barrier, any reproductive structure in those areas may escape control and be a source of re-infestation in the future. However, removing the barriers is a difficult, messy process that will disturb a great deal of sediment and potentially cause fragmentation of Hydrilla plants. The water will be turbid due to the disturbed sediment, meaning it will be difficult to see and collect plant fragments. The benefits and drawbacks to benthic barrier removal should be seriously considered prior to the 2020 growing season.

A minimum of two full littoral zone surveys should be conducted in 2020 to search for and document all Hydrilla locations, as well as presence and abundance of all other aquatic species in the lake. The first survey should occur in June or July, prior to the herbicide treatment. A separate survey earlier in the season would be most effective for recording the presence of the state-listed protected species Potamogeton vaseyi and Lipocarpha micrantha and to observe whether the vaseyi (and micrantha if found) were affected by the 2019 herbicide treatment. The post-treatment survey should be performed in early October. In addition to the full-lake pre- and post-treatment surveys, more focused surveys using underwater SCUBA should be considered to monitor the herbicide’s effect on 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. Hydrilla fragments have been found at the ramp and fragmentation and spreading of the plant is still a major concern.