Clear Lake and Apsey Lake

Transcription

1 2013 Report of the Milfoil Solution Post Program Survey in Clear Lake and Apsey Lake Prepared for: The Town of Espanola, ON Prepared by: EnviroScience, Inc., 5070 Stow Road, Stow, Ohio (800) October 30, 2013

2 1.0 Introduction The exotic invasive macrophyte, Eurasian watermilfoil (Myriohpyllum spicatum, herein after referred to as milfoil) has become one of the most troublesome aquatic plants in North America. The ability to displace native species creating dense monotypic beds can limit recreational use, reduce biodiversity and cause detrimental changes to water quality. Although milfoil has been present in Ontario since the 1960s, this plant remains a nuisance throughout many waterways in which it has established itself and continues to infest new inland lakes. Within recent years, Clear Lake and Apsey Lakes in Espanola, Ontario have become infested with this invasive plant. To manage portions of this milfoil infestation, EnviroScience implemented the Milfoil Solution process using the milfoil weevil (Euhrychiopsis lecontei). The milfoil weevil is a native insect to North America that began to feed on this invasive plant when it was introduced. The milfoil weevil spends its entire life cycle on the plant through the growing season impacting milfoil in multiple ways. The most significant impacts are caused during their larval lifestage as they feed on the meristem, or growing tip of the plant, and burrow through the stem. This causes nutrient flow within the plant to be disrupted. Additionally, the stem loses buoyancy and collapses in the water column creating a cascading effect which pulls neighboring plants lower into the water column. Stocking programs are typically approached over a 3-5 year program to cause significant declines in nuisance populations of milfoil throughout a lake. The Milfoil Solution program was implemented in Clear Lake from with 35,000 weevils stocked over three years and in Apsey Lake during the summer of 2012 with 35,000 weevils. In August, 2013 we performed lake-wide milfoil surveys at Clear Lake and Apsey Lake to determine the current extent of milfoil infestation. Additionally, we performed follow-up surveys at all stocking and monitoring sites from Clear Lake and Apsey Lake to evaluate the weevil population.

3 The table below outlines the Milfoil Solution program for Clear Lake and Apsey Lake, including site establishment and number of weevils stocked: Lake Year Stocking Dates Sites Established Number of weevils Clear 2008 June 26, 2008 S1, S2 12,000 Clear 2009 July 16, 2009 S1, S2 12,000 Clear 2010 July 8, 2010 S1, S2 16,000 Apsey 2012 July 29, 2012 S1 - S3, M1, M2 35, Survey Methods Stocking and Monitoring Site Survey Surveying the stocking and monitoring sites provide us with the opportunity to compare and monitor changes in the aquatic plant community and the weevil populations between sites and seasons. These surveys are integral in monitoring changes that occur in both the augmented weevil population and the health of the milfoil over the course of the program in order to make informed management decisions. Qualitative observations in these surveys include the overall density and health of milfoil, identification of native plant species present, and the presence of weevils and weevil-induced damage. Quantitative measurements include milfoil density and weevil population density. Milfoil density is determined by randomly collecting stems throughout the milfoil bed using a quadrat. This sample is then converted to the number of stems per square meter (stems/m 2 ). Weevil population density (number of weevils per stem) is determined through lab analysis of 30 stems sampled from three transect lines at each site. Lake-Wide Milfoil Presence Survey In addition to returning to stocking sites at Clear and Apsey Lake, we performed lake-wide milfoil presence surveys to determine the current extent of the milfoil infestation at these two

4 lakes. Following methods used in the 2012 milfoil presence survey at Clear Lake, a team of biologists performed visual searches for milfoil by boat. Once located, the density of milfoil was estimated relative to the entire infestation and recorded as sparse, moderate, or dense. Sparse stands were noted where milfoil composed less than 50% of the plant community. Moderate stands consisted of milfoil that was the dominant plant present (roughly 50-75% of the plant community). Dense stands included populations where milfoil appeared to consist of greater than 75% of the plant community. Once identified, the perimeter of the milfoil beds were traced using a GPS unit and used to produce a map outlining the areas of concern throughout the lake. 3.0 Results 3.1 Clear Lake Stocking Site Survey Clear Lake is a 74 ha lake that was stocked with 35,000 milfoil weevils over a three year period from to decrease the amount of nuisance milfoil. Over the three year program, two stocking sites were established and milfoil density decreased to be considered at a nonnuisance level by the end of In 2012 a lake wide milfoil survey was performed and showed that milfoil was still not considered a nuisance throughout the lake. Although milfoil was not troublesome in 2012, continued monitoring was recommended by ES biologist in order to implement the Milfoil Solution program before nuisance populations are observed. On August 25, 2013 ES biologists visited Clear Lake to perform the lake-wide milfoil presence/absence survey and stocking site surveys. The following are the results of the survey: S1 At the time of the survey, milfoil was observed to compose 70% of the plant community within S1. Milfoil density consisted of stems/m 2 with 10% of the plants at the surface of the water. Milfoil was reddish in colour with roughly 40% of the plants showing larval damage from the milfoil weevil. Weevils were observed at S1 in the adult and pupal lifestages with a weevil density of 0.30 weevils/stem. Additionally, five native species were also observed at S1 including: large leaf pondweed (Potamogeton amplifolius), northern waterweed (Elodea nuttalli), Robin s pondweed (P. robinsii), small-leaf pondweed (P. pusillus) and tape grass (Vallisneria americana).

5 S2 Milfoil at S2 was dense and composed 90% of the plant community with roughly 1/3rd of the plants topped out or matted at the surface. Milfoil density at the time of the survey consisted of stems/m 2. Weevils were also observed at the site in the adult and pupal lifestages with 10% of the plants showing signs of weevil damage. Weevil density at the time of the survey consisted of 0.37 weevils/stem. Additionally, three native species were also observed at S1 including: Canada waterweed (Elodea canadensis), Robin s pondweed and tape grass. In comparison to previous seasons, milfoil density is still lower than what was observed in August of 2008 when the milfoil solution program was implemented, however densities in 2013 were higher than what were observed in 2010 (See Table 2). 3.2 Apsey Lake Stocking Site Surveys Apsey Lake is a 285 ha lake that was originally stocked in 2012 using the Milfoil Solution program. In total, 35,000 weevils were stocked at three sites (10,000 at S1 and S3; 15,000 at S2) and two monitoring sites were established. The following outlines the status of the stocking sites during the August 29 th visit by ES biologists: S1 Milfoil at S1 was dense with 20% of the plants flowering at the surface. Milfoil density at the site was stems/m 2 making up roughly 90% of the plant community. Weevils were not observed at the site or in collected samples to determine weevil density. Four native plant species were present at the site including: Canada waterweed, flat-stemmed pondweed (P. zosteriformis), large-leaf pondweed and Robin s pondweed. S2 - Milfoil made up 99% of the plant community at S2 with a density of stems/m 2. The overall area covered by milfoil at S2 appeared to have expanded in comparison to Weevils were not observed at the site or in collected in samples to determine weevil density. Large-leaf pondweed was the only native plant observed at the site.

6 S3 Milfoil at S3 was dense and consisted of stems/m 2 and composed 95% of the plant community. At the time of the survey, 50% of the milfoil bed was topped out at the surface with 20% of the plants flowering. Weevils were not observed at the site or in collected in samples to determine weevil density. Large-leaf pondweed, Richardson s pondweed (P. richardsonii), watershield (Brasenia schreberi) and white water lily (Nymphea odorata) were observed at the site. M1 M1 was set up as a monitoring site to compare to S1-S3. Weevils were not stocked at this site. The milfoil M1 was healthy, dense and composed 95% of the plant community. At the time of the survey, 80% of the plants were topped out at the surface with 75% flowering. Weevil density at this site consisted of 0.03 weevils/stem. Large leaf pondweed, Robin s pondweed and tape grass were observed at this site. M2 Similar to M1, M2 was set up as a monitoring site to compare to S1-S3. Milfoil at the time of the survey was dense and composed 98% of the plant community. Twenty five percent of the milfoil was flowering and milfoil density consisted of stems/m 2. Weevils were not observed in the site however roughly 5% of the plants showed weevil damage. Weevil density at the time of the initial survey was 0.03 weevils/stem. No other plant species were present during the survey. In comparison to the 2012 stocking season, milfoil density is much lower than what was observed at all sites in the 2012 follow-up surveys (S1: stems/m 2 ; S2: stems/m 2 ; S3: stems/m 2 ; M1: stems/m 2 ; M2: stems/m 2 ). Additionally, weevil density at the stocking sites has decreased at all stocking and monitoring sites and were only detected in low densities at the M1. Weevil densities in 2013 are similar to what was observed in July 2012 prior to stocking.

7 4.0 Lake-Wide Milfoil Presence Results 4.1 Clear Lake Milfoil beds were identified throughout the extent of the lake with most occurring within 30m of the shoreline. Similar to the 2012 survey, milfoil was below the surface of the water except for several sites. Although most of the milfoil throughout the lake was below the surface, dense sites often had milfoil reaching the surface of the water and flowering. In comparison to the 2012 survey, the number of locations within the lake with moderate to dense milfoil presence has increased. Figure 1 shows the extent of the milfoil beds throughout Clear Lake. 4.2 Apsey Lake The 2013 presence/absence survey in Apsey Lake provides the first lake-wide snapshot of milfoil presence throughout the lake. Milfoil appears to be a major issue throughout several areas of Apsey Lake, most notably in the northwest corner and throughout the areas surrounding the three stocking sites. Areas with dense milfoil populations were often observed to be topped out at the surface of the water and flowering. The map from this survey provides a baseline information regarding problematic areas of the lake which can be used to inform management decisions. Figure 2 shows the extent of the milfoil bed throughout Apsey Lake. 5.0 Discussion and Recommendations Overall, the results of the 2013 surveys show mixed results for Clear Lake and Apsey Lake. However, these results provide great insight into the success of the programs to date and future recommendations. The ongoing success of both lakes are discussed below: 5.1 Clear Lake Discussion and Recommendations Since the initial implementation S2 of the Milfoil Solution process in 2008 we have seen an overall increase in weevil density and decrease in milfoil density throughout Clear Lake. Initial milfoil densities at Clear Lake were well above 300 stems/m 2. Throughout the duration of the stocking program we saw a major decline in milfoil density and by late summer 2012 further S2

8 management was not warranted. However, in the 2013 surveys, we observed that milfoil densities are beginning to increase. Additionally, the number of sites with moderate to dense milfoil populations has increased slightly between the 2012 and 2013 milfoil presence/absence surveys. It is our recommendation that re-stocking occur at the original sites at Clear Lake. Although milfoil density is slowly increasing at these sites, building upon the current weevil populations will provide the opportunity to manage nuisance populations of milfoil before greater management is needed. Depending on the aggressiveness of the stocking program, new sites with dense milfoil populations could also be established (such as the dense milfoil population located along the southern shoreline). 5.2 Apsey Lake Discussion and Recommendations Milfoil density across all sites was observed to decline in comparison to the summer of 2012 in Apsey Lake. However, decreases in weevil density were also observed in This may be due in part by the expansive size of the milfoil beds at the stocking sites in Apsey Lake. Milfoil weevils returning from overwintering sites would be able to spread out within the milfoil beds and become spatially isolated from each other for reproductive purposes. As observed by ES biologists, Milfoil Solution programs are most effective when stocking occurs over several seasons. This allows weevil populations to build and deal with year to year fluctuations. Although milfoil density was lower at the stocking sites in the 2013 season, it is our recommendation to continue stocking throughout Apsey Lake in the 2014 season. This can be done by re-establishing previous weevil stocking sites and focusing on areas such as the northwest section of the lake to reduce the amount of nuisance milfoil.

9 5.3 General Recommendations In addition to continuation of the Milfoil Solution program, there are many practices that can contribute to the success of the stocking program and decrease the potential of increasing milfoil density such as: Limiting the amount of boat traffic in stocked sites and dense milfoil beds. Milfoil weevils typically reside in the upper 60cm of the plant, if heavy boat traffic occurs throughout the patch these plants can be damaged and impact the weevils ability to flourish. Driving through dense patches should also be limited since fragments of the plant can be dispersed throughout the lake capable of starting a new population. Providing a natural shoreline can increase weevil habitat for overwintering. In the fall months, weevils move to shore to overwinter in loose soils and leaf litter, leaving a 1-3m buffer adjacent to shore where grasses can grow and leaves can collect provides suitable habitat for weevils through these winter months. Milfoil stems that are floating in the water or washed up on shore can be collected and removed. If left alone, these fragments can move freely through the waterbody and can root to start a new plant. Once dried or composted, milfoil is often used as mulch in gardens and flower beds. As part of a lake-wide strategy to manage Eurasian watermilfoil infesting Clear Lake and Apsey Lake, it is key to recognize that suppressing this invasive species will be a gradual process that will require continued support from the Township of Espanola. Thank you for choosing our natural program to manage Eurasian watermilfoil safely and sustainably. Please contact EnviroScience/Milfoil Solution LLC. at (800) , or at kborrowman@enviroscienceinc.com with questions regarding this report. EnviroScience, Inc. Lake Management Division Milfoil Solution

10 Site # S1 Table 1. Average Weevil Population Density (weevils/stem) in Clear Lake Parameter measured 6/27/08 8/21/08 7/16/09 8/26/09 7/8/10 9/2/10 8/28/13 Total weevils Total stems Avg. weevils/stem S2 Total weevils Total stems Avg. weevils/stem * * Due to heavy weevil damage, the stems from Transect 3 were too destroyed to accurately assess. Table 2. Average Density data of Eurasian Watermilfoil Collected During the and 2013 Initial and Follow-up Surveys of Clear Lake Site # 6/27/08 8/21/08 7/16/09 8/26/09 7/8/10 9/2/10 8/28/13 S S

11 Table 3. Average Weevil Population Density (weevils/stem) in Apsey Lake Site Parameter measured 7/29/2012 9/5/2012 8/29/2013 S1 Total weevils Total stems Avg. weevils/stem S2 S3 M1 M2 Total weevils Total stems Avg. weevils/stem Total weevils Total stems Avg. weevils/stem Total weevils Total stems Avg. weevils/stem Total weevils Total stems Avg. weevils/stem

12 Table 2. Average Density of EWM (stems/m 2 ) in Apsey Lake Site 7/29/2012 9/5/2012 8/29/2013 S S S M M

13 S2 S1 Figure 1. Clear Lake Espanola, Ontario Sparse milfoil Moderate milfoil Dense milfoil S1-S2 Weevil stocking sites 150 m

14 M2 S3 S1 M1 S2 Figure 2. Apsey Lake Espanola, Ontario Sparse milfoil Moderate milfoil Dense milfoil S1-S3 Weevil stocking sites M1-M2 Monitoring Sites 300 m