Coventry Lake Hydrilla Management Program 2020 Treatment Report

Home , Ceratophyllum demersum, Elodea nuttallii, Hydrilla, Potamogeton

Northeast Aquatic Research & Northeast Aquatic

Coventry Lake Hydrilla Management Program

2020 Treatment Report

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

December 2020

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

Introduction Over the past six years, Northeast Aquatic Research (NEAR) and SOLitude Lake Management (SOLitude) have been involved in the management of the invasive species Hydrilla verticillata (Hydrilla) in Coventry Lake. Following discovery of Hydrilla near the public boat ramp in 2015, the individual patches of the invasive species were treated with the herbicide Aquathol-K (endothall) in 2016 & 2017. In 2017, several small patches of Hydrilla were found along the lake’s shoreline. Hydrilla patches smaller than 3’ in diameter, along with one large patch, were covered with benthic barriers. After two consecutive years of contact herbicide treatments using Aquathol-K, Hydrilla was continuing to spread through the lake, and individual beds were becoming larger and denser (Map 1 & 2). As a result, the program shifted to a whole-lake management approach using the systemic herbicide fluridone, a product with multiple cases of success against Hydrilla.

In 2018, 2019, and 2020, Hydrilla was treated with the systemic herbicide Sonar One (fluridone) for a 90+ day period each season.

The 2020 fluridone treatment was administered by SOLitude Lake Management on July 21st. Booster treatments were administered on August 25th and September 28th. Northeast Aquatic Research (NEAR) conducted three aquatic plant surveys of Coventry Lake in 2020. On June 15th, NEAR conducted a thorough search for the Connecticut state-listed Threatened species Potamogeton vaseyi (Vasey’s Pondweed) and Lipocarpha micrantha (Dwarf Bulrush) in locations where these species had been historically documented. The lake-wide pre-herbicide treatment survey was conducted on July 6th and 7th. The post-herbicide treatment survey was conducted on October 6th and 7th.

Map 1. 2017 Hydrilla locations and named bays.

Northern Patches

Knoll Drive Bay

Island Coves Outlet Cove

Boat Ramp Cove

Map 2. 2018 Hydrilla locations.

2020 Management Approach The move was made to a whole lake treatment approach in 2018 to provide more complete, systemic control of Hydrilla throughout the lake and well as eliminate the need to base treatment areas on survey data. Surveying Hydrilla in Coventry Lake is challenging given the size of the waterbody and the abundance of the look-alike plant Elodea sp. (waterweed). Therefore, as seen in 2015-2017, it was difficult to locate and target all instances of Hydrilla growth with a spot-treatment approach. Like previous fluridone treatments in prior years at Coventry Lake, the 2020 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 21st, with a lake-wide dose of 4ppb applied to the 178-acre littoral zone, using an airboat equipped with a calibrated electric spreader. The first booster treatment was administered on August 25th, using a lake-wide dose of 2 ppb applied to the littoral zone. The second booster treatment was administered on September 28th, 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 2020 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. 2020 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' 8/5/2020 2.5 1.7 1.9 2.4 1.3 1.2 2.2 1.9 2.1 1.2 1.3 1.1 8/25/2020 2.5 2.4 2.7 3.8 3.2 2.2 3.4 4.2 2.5 2.8 3 2.3 9/22/2020 2.4 2.3 2.1 2.7 2.6 2.8 2.7 6.7 2.3 2.5 2.3 3.4 10/22/2020 2.7 2.5 2.5 2 3.6 2 2.5 2.1 2.3 1.8 2 1.8

Fluridone testing results showed an expected range of concentrations at the various stations between less than 1.1 ppb and 6.7 ppb. The average of all this year’s FasTEST data was 2.47 ppb and slightly higher than last year’s average of 2.15 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. 2020 Aquatic Plant Survey Results Northeast Aquatic Research made four visits to Coventry Lake in 2020 – on June 15th for an NDDB survey and removal of shallow-water benthic barriers, on July 6th and 7th for the pre-treatment full-lake aquatic plant survey, on July 14th to inspect the Hydrilla beds, and on October 6th and 7th for the post-treatment full-lake aquatic plant survey.

Hydrilla Results

One Hydrilla bed was found during the 2020 pre-treatment survey, in a location where hydrilla had been found previously in 2017 and 2018 (Map 4, Map 5). This bed was located near the inlet stream in ~31” deep water. The bed was located alongside a dock and was ~40 feet long by 10 feet wide. The density of plants within the bed was moderate, with some stragglers outside the main bed. The plants were generally ~8-10” tall. On June 15th, NEAR removed the shallow-water benthic barriers. Dr. Kortmann of Ecosystem Consulting Services and his team removed the deep-water barriers. These barriers were installed in 2017 to limit the growth of Hydrilla and to stop the spread of the species via plant fragments. The barriers were removed prior to the herbicide treatment in 2020 so that the herbicide could come in contact with any plants that remained in the areas where the barriers had been placed. The Hydrilla was inspected again on July 14th to verify the size of the bed found during the pre-treatment survey. During the July 14th visit, the water was clearer, allowing for easier viewing of the Hydrilla.

The post-treatment survey was conducted on October 6th, 11 weeks after the initial fluridone application, and one week after the final booster treatment. During the post-treatment survey, the 40’ x 10’ bed was no longer present, though several very small, scattered clumps of Hydrilla were located in the same area near the inlet stream. Most plants were in less than two feet of water, some in less than one foot. The plants were brittle, but the herbicide did not kill them completely, and they will likely regrow next season. The Hydrilla likely persisted in this location due to the dilution by the stream that enters the lake in that area. In addition to the small, scattered patches near the inlet stream, two pieces of Hydrilla were raked up ~200 feet to the southeast, in water ~1.3 feet deep. These two pieces were white, suggesting they were dying as a result of contact with the herbicide.

Additional Aquatic Plant Species

The June 15th NDDB survey focused on searching for the Connecticut state-listed Threatened species Potamogeton vaseyi and Lipocarpha micrantha in areas of the lake where the two species have historical records (Map 6). The Connecticut state-listed Threatened species Potamogeton vaseyi was found in two locations near the island at the lake’s eastern end (Map 7). The location to the north of the island was a small bed approximately 3 feet in diameter and consisting of ~7-10 plants. Some of the plants had topped out floating leaves. The second location, southeast of the island, consisted of as single plant. The plant was growing in water ~4 feet deep and had reached ~1 foot from the surface. 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 and shoreline scouring by residents. 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. P. vaseyi was documented during the full-lake pre- treatment from several sites, but not during the post-treatment surveys. P. vaseyi typically begins to die off by mid-July and is completely senesced by late summer, so the plant was not present during the post- treatment survey in October. L. micrantha is a flowering sedge that grows in shoreline habitat. This species was last documented in Coventry Lake in 2017. The area of the lake with a historical record of L. micrantha was searched thoroughly by Dr. Knoecklein, but the species was not found. During the June 2020 NDDB survey, the invasive species Trapa natans (Water Chestnut) was found at the northern end of the lake (Map 8). Two plants were found growing in shallow water close to shore. Both plants were very small and had yet to produce nutlets (seeds). Both water chestnut plants were removed as soon as they were identified. The 2020 full-lake pre-treatment aquatic plant survey was conducted on July 6th and 7th. During the survey, waypoints were created every ~150 feet throughout the lake’s littoral zone. Waypoints where Hydrilla had been found in previous years were revisited (Map 9). The 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 species frequency and density results from the 2020, 2019 and 2018 pre-treatment and post- treatment surveys are listed in Table 2, Table 3, and Table 4. ‘Percent Frequency’ documents the overall frequency as a percentage of the total number of survey waypoints. When a species was found at a waypoint, the density of the species within that area (as seen from the surface, on the grappling rake, and/or on the depth sounder) was recorded. The density class is based on the average of all density estimates across all the locations where the species was found. Density class categories are: Very sparse < 10% cover, Sparse 10-19%, Moderate 20-59%, Dense 60-79%, Very Dense 80-100% cover. During the pre-treatment survey, 25 aquatic plant species were found in the lake (Table 2). Of these, 22 species are native to Connecticut. The three invasive species were Hydrilla verticillata, Potamogeton crispus (Curly-Leaf Pondweed), and Glossostigma cleistanthum (Mud-mat). Nitella (Muskgrass) was the only dominant species in the lake, meaning it was present at greater than 20% frequency (Map 10).

The pondweeds Potamogeton amplifolius (Large-leaf Pondweed) and Potamogeton perfoliatus (Clasping- leaf Pondweed) were present at 12% and 15% frequency respectively, and both were moderately dense where the species were present (Map 11, Map 12). The invasive species Potamogeton crispus (Curly-leaf Pondweed) was found in several locations in the lake during the pre-treatment survey (Map 13). The density of most beds was sparse to moderate; few beds were dense. The invasive species Glossostigma cleistanthum is a very small plant that grows in shallow water with a sandy bottom. Though it rarely becomes a nuisance, it is considered invasive to Connecticut. Glossostigma was found in several locations, mainly along the lake’s southern shoreline (Map 14). The State listed Threatened species Potamogeton vaseyi was found at several locations in the lake during the pre-treatment survey (Map 15), suggesting that this species has not been negatively impacted by the fluridone treatments in prior years. During the post-treatment survey, 21 aquatic plant species were found in the lake, of which 19 are native to Connecticut. As in the pre-treatment survey, Hydrilla, P. crispus, and Glossostigma were the three invasive species. Nitella remained a dominant species post-treatment (Map 16). Zosterella dubia (Water Stargrass) was also dominant, having increased notably since the pre-treatment survey (Map 17). P. amplifolius and P. perfoliatus were both slightly more abundant post-treatment (Map 18, Map 19). P. crispus was found in just one location post-treatment and the bed was very small and sparse (Map 20). Glossostigma was present at only three locations during the post-treatment survey (Map 21). The invasive species Myriophyllum spicatum (Eurasian Milfoil), which has been found in the lake in prior years, was not found during either of the surveys in 2020. This is certainly a direct result of the SONAR treatments. The 2018 herbicide treatment noticeably affected the M. spicatum plants, and the species was significantly reduced in 2019. Eurasian milfoil tends to be very susceptible to low concentrations of fluridone.

Map 4. 2020 pre-treatment and post-treatment Hydrilla locations.

Map 5. Zoomed area of Hydrilla growth.

Map 6. Areas where protected aquatic plants have historically been recorded by CT DEEP/NDDB.

Map 7. Locations of Potamogeton vaseyi during the June 15th NDDB survey.

Map 8. Locations of Trapa natans found on June 15th, 2020. Two plants found; both were removed.

Map 9. 2020 pre-treatment and post-treatment survey tracks.

Table 2. 2020 pre- and post-treatment survey vegetation data. % Frequency Average Density Scientific Name Pre Post Pre Post Brasenia schreberi <5 0 Very Sparse NA Ceratophyllum demersum 10 7.7 Sparse Sparse Chara sp 12 <5 Moderate Very Sparse Elatine sp 0 <5 NA Moderate Eleocharis acicularis 6 6.5 Moderate Sparse Elodea canadensis 0 <5 NA Moderate Elodea nuttallii <5 0 Sparse NA Filamentous algae 5 5.8 Moderate Moderate Glossostigma cleistanthum <5 <5 Sparse Moderate Gratiola sp <5 0 Sparse NA Hydrilla verticillata <5 <5 Moderate Sparse Ludwigia sp <5 <5 Very Sparse Moderate Najas guadalupensis <5 <5 Sparse Dense Nitella sp 22 37.1 Moderate Moderate Potamogeton amplifolius 12 13.9 Moderate Moderate Potamogeton bicupulatus <5 <5 Very Sparse Very Sparse Potamogeton crispus 10 <5 Moderate Very Sparse Potamogeton epihydrus 0 <5 NA Moderate Potamogeton perfoliatus 15 18.4 Moderate Moderate Potamogeton pusillus 0 14.5 NA Moderate Potamogeton spirillus <5 0 Very Sparse NA Potamogeton vaseyi <5 0 Moderate NA Sagittaria graminea <5 <5 Moderate Very Sparse Utricularia radiata 0 <5 NA Very Sparse Vallisneria americana <5 <5 Sparse Very Sparse Zosterella dubia 10 28.4 Sparse Moderate

Table 3. 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 4. 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 10. Pre-treatment survey locations and densities of Nitella sp.

Map 11. Pre-treatment survey locations and densities of Potamogeton amplifolius.

Map 12. Pre-treatment survey locations and densities of Potamogeton perfoliatus.

Map 13. Pre-treatment locations and densities of Potamogeton crispus.

Map 14. Pre-treatment locations and densities of Glossostigma sp.

Map 15. Pre-treatment locations and densities of Potamogeton vaseyi.

Map 16. Post-treatment survey locations and densities of Nitella sp.

Map 17. Post-treatment survey locations and densities of Zosterella dubia.

Map 18. Post-treatment survey locations and densities of Potamogeton amplifolius.

Map 19. Post-treatment locations and densities of Potamogeton perfoliatus.

Map 20. Post-treatment locations and densities of Potamogeton crispus.

Map 21. Post-treatment locations and densities of Glossostigma sp.

Recommendations Overall, the 2020 treatment program was successful in managing the growth of Hydrilla in all areas of Coventry Lake, which achieves the seasonal goal of working to deplete the tuber bank, preventing the formation of additional tubers, stopping the spread of Hydrilla into other areas of the lake, and minimizing foliage that can increase the risk of spread to other waterbodies in the region.

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 continuing the whole-lake fluridone treatment program in 2021. The approach should be the same as in 2020 and the initial fluridone application should be conducted in late June/early July or as determined based on plant surveys.

The interim and post treatment surveys indicated affected but persistent growth of Hydrilla in localized, small areas of Island Cove, possibly because of inflow from a nearby stream. While additional measures were taken this year, in the form of hand-application of herbicide pellets in this area, additional adjustments may be considered for 2021. Such adjustments would include possible treatment with contact herbicides and/or benthic barriers.

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, 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 2020 herbicide treatment. The post-treatment survey should be performed in early October.

Following the removal of all benthic barriers prior to the 2020 herbicide treatment, we recommend inspection of the areas where the deep-water benthic barriers were located via SCUBA next season to determine whether any Hydrilla plants remain in these areas following exposure to fluridone. This SCUBA survey would be conducted in conjunction with the pre-treatment in June.

We recommend tuber sampling in 2021 to determine whether a seed bank of Hydrilla tubers exists in Coventry Lake. This would require one day of diving, one day of sampling via boat, and a half day to search for tubers in the collected sediment samples. Sampling would be conducted at the two largest patches that have occurred in the lake – the patch off the island that was covered by a benthic barrier, and the patch off the boat ramp. This tuber sampling may qualify for state grant money.

The CT DEEP and the Town of Coventry should consider implementing a semi-permanent boat ramp monitoring program 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.