Christmas Lake Milfoil Weevil Stocking Program: Aquatic Plant Evaluation for 2012

Transcription

1 Eurasian Watermilfoil in an Area Stocked with Weevils in Christmas Lake, 2012 Christmas Lake Milfoil Weevil Stocking Program: Aquatic Plant Evaluation for 2012 Survey Dates: Pre-Weevil Survey: May 23, 2012 Weevil Stocking: May 23, May 30, June 4, 2012 Post Weevil Survey: August 9, 2012 Prepared for: Minnehaha Creek Watershed District Prepared by: Steve McComas Blue Water Science December 19, 2012

2 Christmas Lake Milfoil Weevil Stocking Program: Aquatic Plant Evaluation for 2012 Steve McComas, Blue Water Science, St. Paul, Minnesota Abstract: Christmas Lake is a 267 acre lake in Hennepin County, Minnesota. Starting on May 23, 2012 and over a 12-day span 13,000 milfoil weevil eggs and larvae were stocked into an area of about on acre in size dominated with heavy growth of Eurasian watermilfoil (EWM) located on the northwest side of Christmas Lake. Aquatic plant evaluations were conducted prior to milfoil weevil introduction in May and then in August in the stocked area and in two reference areas. In May, EWM was abundant in all three study areas. In August EWM was still relatively abundant, but EWM biomass and stem densities were lower in the stocked weevil area compared to the two reference areas. A September fish survey recorded low to moderate bluegill abundance based on a trapnet survey. Bluegill densities were 17.4 fish/net which are at or below a threshold density where bluegills have been found to adversely impact milfoil weevil numbers. Bluegills likely have some predation impact on the weevil density in Christmas Lake, but it appears weevil stocking has likely decreased EWM biomass and stem densities to a degree. This was the first year of a weevil stocking program and although there were modest positive results, it would be of interest to determine if stocking in a second year would build on gains made from year 1. Its is common for biomanipulations to take several years to produce an effective impact. EWM in the stocked weevil area on August 8, 2012.

3 Introduction Christmas Lake is a 267 acre lake located in Hennepin County, Minnesota. Eurasian watermilfoil (EWM) was first documented in Christmas Lake in Currently, EWM is widely distributed around Christmas Lake. It is found as scattered stems within native plant communities as well as the dominant plant in some plant beds. EWM is distributed over about 60 acres in Christmas Lake and exhibits heavy growth that varies between 10 to 20 acres in a given year depending on growing conditions. Figure 1. Eurasian watermilfoil distribution and density for plant surveys conducted in 2007 (left) and 2011 (right). Green circles = light growth, yellow circles = moderate growth, and red circles = heavy growth. Points were spaced 60 meters apart. Each point represents about 0.9 acres (source: Blue Water Science) Objectives The objectives of this project were to determine if stocking native milfoil weevils could reduce Eurasian watermilfoil to light to moderate growth and if native plant diversity would increase. Eurasian watermilfoil along with native plants were assessed in the area that was stocked with weevils and in two reference areas. Christmas Lake Weevil Stocking Program 1

4 Methods Three study areas were delineated in the summer of 2012 based on site inspections by staff from EnviroScience. A map showing the three study areas is shown in Figure 2. An area of about 1 acre was stocked with 13,000 weevils over three dates starting on May 23, 2012 then on May 30 and finishing on June 4. Weevils were propagated and stocked by staff of EnviroScience. An adjacent area of about 0.9 acres where no weevils were stocked was monitored as well. The stocked area and adjacent reference area are shown in Figure 3. The third area was a reference area and was located about 600 meters south of the weevil stocking area with the center of the 1 acre area located at an UTM coordinate of North and is referred to as a remote reference area (Ref 300)(Figure 2). Figure 2. Three monitoring locations for the weevil survey are shown with black circles. The colored circles represent EWM growth in Green circles = light growth, yellow circles = moderate growth, and red circles = heavy growth. The grid lines are 100 meters apart. Figure 3. Close-up of the weevil stocking area (1.0 acre) and an adjacent unstocked area of about 0.9 acres (Ref 1). The grid lines are 50 meters apart. Aquatic Plant Assessment by Blue Water Science: At each of the three areas, three samples at three depths (7 feet, 9 feet, and 11 feet) were collected by scuba diving for a total of nine quadrat samples per area. The quadrat size was 0.1 m 2. Aquatic plant biomass and stem densities were collected on May 23, 2012 prior to the weevil introduction. Weevils were introduced on three dates starting on May 23, 2012 to the weevil stocking area. Aquatic plant samples were collected using quadrat sampling by scuba diving at all three areas on August 9, 2012 representing post weevil impacts. For the two surveys, within each area, transects parallel to shore at 7, 9, and 11 feet were conducted with the quadrat randomly placed within an Eurasian watermilfoil bed. All plants within the quadrat were cut at the sediment/water interface and collected. At these depths, the EWM beds were nearly continuous with few gaps in the distribution of EWM. For all three depths in all three areas, because quadrat samples were randomly placed within milfoil areas, there was 100% occurrence of milfoil in all the study areas. A total of 27 quadrat samples were collected in each of the two surveys. Christmas Lake Weevil Stocking Program 2

5 Aquatic Plant Assessment by EnviroScience: EWM stem density was determined by randomly tossing a 0.09m 2 quadrat ahead of the investigator and counting all EWM stems within the quadrat. Six quadrat samples per area were collected and averaged. The range of depths for each area was 7 to 14 feet for the stocked weevil area, 8 to 12 feet for the adjacent reference area, and 4 to 11 feet for the remote reference area. Milfoil Weevil Assessment by EnviroScience: Milfoil weevils were propagated by EnviroScience and a total of 13,000 eggs, larvae, and adults were stocked into the stocked weevil area on three dates (May 23, May 30, and June 4). A pre-stocking weevil survey and a follow-up weevil survey were conducted. At each area, plants for weevil presence were collected by swimming perpendicular to shore on three transects. The tops of two randomly selected plants were removed at five evenly spaced intervals for a total of ten plants per transect and 30 plants per area. Plants were examined using a microscope for the presence of weevils and number of meristems. Fish Trapnet Survey by Blue Water Science: Six standard trapnets were set over two nights to survey fish in Christmas Lake. The trapnet was a MnDNR-style with a 4 x 6 feet square frame with two funnel mouth openings and 50-feet lead. Net mesh size was 3/8 inch (bar length). Six standard trap nets were set on Wednesday afternoon, September 26, Six nets were fished for the following 2 days (September 27 and 28). Trapnet locations are shown in Figure 4. Figure 4. Map of trapnet sets are shown with blue squares. Net 3 was moved on Day 2. The numbers along the shoreline are aquatic plant sampling sites from the 2011 point intercept survey. The colored circles indicate Eurasian watermilfoil density, where green circles = light growth, yellow circles = moderate growth, and red circles = heavy growth. Christmas Lake Weevil Stocking Program 3

6 Results and Discussion Eurasian Watermilfoil Conditions in the Three Study Areas: Examples of Eurasian watermilfoil (EWM) in the weevil stocking area and in the remote reference area are shown in Figure 5. In the weevil stocking area, Eurasian watermilfoil (EWM) biomass (g-dry wt/m 2 ) decreased 22% from May to August (Figure 6). In the adjacent reference area, EWM biomass increased 38% and in the remote reference area, EWM biomass increased 123% (Figure 6). The EWM biomass in the stocked weevil area in August was the lowest biomass of the three areas. EWM stem densities (stems/m 2 ) were also monitored. In the weevil stocking area, EWM stem density decreased 22% from May to August (Figure 7). In the adjacent reference area, EWM stem density decreased 9%. However, in the remote reference area EWM stem density increased 48% from May to August (Figure 7). The EWM stem densities in the stocked weevil area in August were the lowest of the three areas, but at 212 stems/m 2 this is still a relatively high density. The individual weight of an EWM stem was calculated and results are shown in Figure 8. The average weight of an EWM stem did not increase from May to August but stem weights did increase in the two reference areas. Figure 5. [top] EWM in the weevil stocking area on August 9, [bottom] EWM in the remote reference area (Ref 300) on August 9, Christmas Lake Weevil Stocking Program 4

7 Figure 6. Christmas Lake Eurasian watermilfoil biomass (g-dry wt/m 2 ) for three sites in The weevil site is where weevils were introduced (Weevil), an adjacent reference site where no weevils were introduced (Ref 1), and a remote reference site several hundred meters away from the site of the weevil introduction is labeled Ref-300. At each area on each date, 9 quadrat samples were collected. Figure 7. Eurasian watermilfoil stem densities (stems/m 2 )for three sites in 2012 (n=9 for each area, for each date. Total samples collected = 54). Figure 8. Eurasian watermilfoil grams per stem for three sites in 2012 (based on dividing biomass by stem density for each area). Christmas Lake Weevil Stocking Program 5

8 Eurasian Watermilfoil by Depth: EWM was monitored at three depths in all three areas in early summer (before weevil impacts) and late summer (three months after weevil stocking). In the stocked weevil area, EWM decreased in the 7 and 9 foot depths but increased at the 11 foot depth from May to August (Figure 9). The overall average biomass in the stocked weevil area was less in August (Figure 9). In the adjacent reference area (Ref 1) and the remote reference area (Ref 300), biomass increased at all three depths from May to August (Figure 9). EWM May 2012 EWM August 2012 Figure 9. Eurasian watermilfoil biomass (g dry/w 2 ) for three depths with the black bars representing the average for May (top) and August (bottom) 2012 surveys. Christmas Lake Weevil Stocking Program 6

9 Eurasian Watermilfoil Stem Density by Depth: EWM stem density was variable over the three sample depths but was less in the stocked weevil area in August compared to May at all three depths (Figure 10). EWM stem density was lower at 7 and 9 feet but not at 11 feet in the adjacent reference area. The average stem densities for all three depths were slightly lower in August compared to May. In the remote reference area stem densities were greater at all three depths in August compared to May (Figure 10). EWM Stem Density May 2012 EWM Stem Density August 2012 Figure 10. Eurasian watermilfoil stem densities (stems/m 2 ) for three depths with the black bar representing the average for May (top) and August (August) 2012 surveys. Christmas Lake Weevil Stocking Program 7

10 Random Sampling for EWM in the Aquatic Plant Beds by EnviroScience: EWM was also evaluated by quadrat sampling by staff from EnviroScience encompassing a wider range of water depths compared to the quadrat sampling results shown in Figure 7. In the EnviroScience survey, EWM stem densities were lower (Figure 11) than what was collected in Figure 7, but the same EWM relationships were found. In both Figure 7 and Figure 11, EWM stems were found to be less in the stocked area and adjacent reference area in August compared to May while EWM stem density was greater in the remote reference area in August compared to May. The lower stem densities found in Figure 11 compared to Figure 7 likely reflect the sampling methods. In Figure 7, sampling occurred within EWM beds with the densest growth between 7 to 11 feet, and in Figure 11, sampling occurred over a broader range of depths and EWM was not as dense within the wider range of depths compared to the 7 to 11 foot range that was used in Figure 7. Figure 11. EWM stem densities in May (red bars) and August (green bars) in 2012 (Site 1 = stocked weevil area, Mon A = adjacent reference area, and Mon B = remote reference area)(source: C. Marquette, EnviroScience). Christmas Lake Weevil Stocking Program 8

11 Milfoil Weevil Densities: Milfoil weevil (Figure 12) densities were evaluated by staff from EnviroScience in the three study areas prior to weevil stocking, and in August, about three months after stocking. Prior to stocking, a population of indigenous milfoil weevils was found at Site 1 (stocked area) and Mon B (remote reference area)(figure 13). In August, milfoil weevils were found in all three areas (Figure 13). The highest August weevil density was found in the adjacent reference area (Mon A). It s possible stocked weevils may have migrated from the stocked area to the adjacent reference area. Figure 12. Milfoil weevil (source: Department of Ecology, State of Washington web site ( html ). Figure 13. Weevil densities in May (red bars) and August (green bars) in 2012 (Site 1 = stocked weevil area, Mon A = adjacent reference area, and Mon B = remote reference area)(source: C. Marquette, EnviroScience). Christmas Lake Weevil Stocking Program 9

12 Aquatic Plant Conditions in the Three Study Areas: EWM was the dominant plant in all three study areas in the May and August surveys (Tables 1 and 2). In all three areas in May, native plant biomass was less than 10% of the total plant biomass. In the August survey, native plant biomass increased in all three areas compared to May biomass. The native plant biomass percentage increase was greater in the stocked area (25% of the overall plant biomass) compared to the two reference areas primarily because EWM biomass did not increase in the stocked weevil area from May to August (Table 1). In August, aquatic plant diversity was good in all three areas with the number of species ranging from 9-12 species per area. Although EWM was dominant in biomass and stem density in all three areas, several native plant species increased in biomass and stem density from May to August. Water celery was not observed in May, but in August it increased substantially and was the dominant native plant in biomass and stem density in the adjacent reference area (Ref 1) and was the second dominant plant behind coontail in the remote reference area (Ref 300)(Tables 1 and 2). Whitestem pondweed was the dominant native plant by biomass in the stocked weevil area followed by Illinois pondweed and flatstem pondweed (Table 1). Eurasian watermilfoil did not appear to limit the number of native plant species. In the remote reference areas (Ref 300) the number of native species increased from two in May to eleven in August (Tables 1 and 2). Native plant biomass and stem densities also increased from May to August. The percent of the native plant biomass and the stem densities were slightly greater in the stocked area compared to the reference areas. Data from individual quadrats are shown in Appendix A. Figure 14. Eurasian watermilfoil in the weevil stocking area, prior to weevil stocking on May 23, EWM had substantial growth for late spring which was somewhat. Christmas Lake Weevil Stocking Program 10

13 Table 1. Christmas Lake aquatic plant biomass (g-dry wt/m 2 ) for 2012 based on nine quadrat samples per area for each date. Plant Species Weevil Stocking Site Ref 1 Ref 300 May 23 August 9 May 23 August 9 May 23 Buttercup Cabbage Claspingleaf pondweed Coontail Curlyleaf pondweed Eurasian watermilfoil Fern pondweed Flatstem pondweed Floatingleaf pondweed Illinois pondweed Marsh marigold Northern watermilfoil Sago pondweed Stringy pondweed Water celery Water stargrass Whitestem pondweed Number of Species Total Average (g/m 2 ) Native (g/m 2 ) % EWM 93% 75% 96% 91% 98% 89% % Native 7% 25% 4% 9% 2% 11% August 9 Table 2. Christmas Lake aquatic plant stems (stems/m 2 ) for 2012 based on nine quadrat samples per area for each date. Plant Species Weevil Stocking Site Ref 1 Ref 300 May 23 August 9 May 23 August 9 May 23 Buttercup Cabbage Claspingleaf pondweed Coontail Curlyleaf pondweed Eurasian watermilfoil Fern pondweed Flatstem pondweed Floatingleaf pondweed Illinois pondweed Marsh marigold Northern watermilfoil Sago pondweed Stringy pondweed Water celery 0 54* 0 104* 0 118* Water stargrass Whitestem pondweed Number of Species Total Average (stems/m 2 ) Native (stems/m 2 ) % EWM 77% 58% 83% 64% 94% 67% % Native 23% 42% 17% 36% 6% 33% * leaf blades rather than stems were counted. August 9 Christmas Lake Weevil Stocking Program 11

14 Representative Underwater Photographs of Three Study Areas on August 9, 2012: EWM was the dominant plant in all three study areas. Native plants were present with good diversity but did not grow to the same height as milfoil and native plant biomass was less than EWM. Stocked Weevil Area Adjacent Reference Area (Ref 1) Remote Reference Area (Ref 300) Figure 15. Representative underwater aquatic plant conditions in August in the three study areas. Christmas Lake Weevil Stocking Program 12

15 Christmas Lake Fish Survey in 2012: A trapnet fish survey, conducted on September 27 and 28, 2012, sampled nine species of fish in low to moderate quantities (Table 3). Bluegill sunfish were the dominant fish with an average catch of 17.4 fish per net. Previous research investigating the interaction of fish predation on milfoil weevils has found excessive numbers of bluegill sunfish prey on milfoil weevils and reduce the weevil population, thus limiting the weevil s ability to control EWM (Ward and Newman 2006). It appears the threshold density is 25 to 30 bluegills per trapnet (Ward and Newman 2006). When the bluegill density is above the range of fish/net it is likely the weevil density will be low (where a low weevil density is described as <0.1 weevils/stem)(ward and Newman 2006). In Christmas Lake for the September trapnet fish survey, the bluegill density averages 17.4 fish/trapnet with a range of trapnet catches from 3 to 49 fish/net. An additional factor is that 73% of the Christmas Lake bluegill catch was 6 inches or greater in length. Larger bluegills are not as likely to feed on weevils compared to smaller bluegills. However, it is probable bluegills have some impact on weevil density in Christmas Lake. Table 3. Christmas Lake trapnet results for the fish survey conducted in September 2012 (YOY = young-of-the-year). Net Black Crappies Adults Bluegill YOY Bowfin (Dogfish) Green Sunfish Largemouth Bass Northern Pike Pumpkinseed Yellow Bullhead Yellow Perch Thursday (9/27) (Remote Ref Area) (Weevil Area) (Adjacent Ref Area) subtotal fish/net Friday (9/28) (Remote Ref Area) 5 (Weevil Area) (Adjacent Ref Area) subtotal fish/net Total Fish (12 nets) Fish/ Trapnet MnDNR Normal Range* NA NA Christmas Lake Weevil Stocking Program 13

16 Summary of Previous Trapnet Fish Surveys: In the previous seven trapnet surveys, the bluegill sunfish catch per net has been greater than what was recorded in 2012 (Table 4). The bluegill decline measured in 2012 may be real or could be a sampling issue. Several other fish species were sampled at lower densities in 2012 as well. When th last late summer fish survey was conducted in 1986, bluegill numbers were relatively low as well. The next fish survey should probably be scheduled in June to determine if bluegill numbers are still low. Table 4. Christmas Lake historical trapnet fish survey data. Results are shown as number of fish per net. All surveys, except for 2012, were conducted by the MnDNR. TRAPNET DATE Local Average 1976 (6.27) 1981 (6.24) 1986 (8.23) 1991 (6.19) 1996 (6.24) 2001 (6.25) 2007 (6.25) 2012 ( ) (BWS) Bass, largemouth Bowfin (dogfish) Bullhead, black NA Bullhead, yellow Carp Crappie, black Northern pike NA Rainbow trout NA 0.3 Sunfish, bluegill Sunfish, green Sunfish, hybrid NA Sunfish, pumpkinseed Yellow perch Number of fish/net Number of fish species Figure 16. A trapnet is a live fish trap. Fish run into the 50-foot lead net and follow it back through a series of hoops with funnel mouths. Fish end up in the back hoop. Christmas Lake Weevil Stocking Program 14

17 Conclusions EWM biomass declined in the stocked weevil area, while it increased in the two reference areas in August compared to May. It appears there may have been possible weevil effects in the adjacent reference area. EWM stem densities decreased in the stocked area and in the adjacent reference area but increased in the remote reference area. However, stem densities in the stocked weevil area could still be considered heavy (EWM growth characteristics are shown in Figure 17 on the next page). EWM biomass and stem densities dominated the plant beds in all three study areas in depths of 7 to 11 feet. However native plant biomass and stem densities were the highest in the stocked weevil area compared to the two reference areas. All three areas had good native plant diversity in August. A September fish survey recorded low to moderate bluegill abundance based on a trapnet survey. Bluegill per trapnet densities are at or below a threshold density where bluegills have been found to adversely impact milfoil weevil numbers (Table 5). Bluegills likely have some predation impact on the weevil density in Christmas Lake, but it appears weevil stocking has likely decreased EWM biomass and stem densities to a degree. Table 5. Christmas Lake bluegill abundance, as fish per trapnet, and milfoil weevil density. Net 5 (stocked weevil area) Net 6 (reference area) Net 4 (remote reference area) Net 1 Net 2 Net 3 Bluegills (#/net) (September 2012) 15 (range 10-20), n= 2 30 (range 10-49), n = 2 13 (range 10-15), n = 2 16 (range 15-16), n= 2 27 (range 5-49), n = 2 5 (range 3-7), n = 2 Weevil Density (weevils/stem) Initial May Survey Follow-up August Survey ND ND ND ND ND ND In May, Eurasian watermilfoil was abundant in all three study areas. In August, EWM was still abundant, but biomass and stem densities were less in the stocked weevil area compared to the two reference areas. This was the first year of a weevil stocking program and although there were modest positive results, it would be of interest to determine if stocking in a second year would build on gains made from year 1. Its is common for biomanipulations to take several years to produce an effective impact. Christmas Lake Weevil Stocking Program 15

18 Figure 17. Light, moderate, and heavy categories of Eurasian watermilfoil growth characteristics based on rake density abundance but quantified with quadrat samples collected by scuba diving. Christmas Lake Weevil Stocking Program 16

19 References Sutter, TJ. and Newman, RM Is predation by sunfish (Lepomis spp.) an important source of mortality for the Eurasian watermilfoil biocontrol agent Euhrychiopsis lecontei? J. Freshwat. Ecol. 12(2): Ward, D.M. and Newman, RM Fish predation on Eurasian watermilfoil (Myriophyllum spicatum) herbivores and indirect effects on macrophytes. Com. J. Fish. Aquat. Sci. 63: Christmas Lake Weevil Stocking Program 17

20 APPENDIX A: Aquatic Plant Data from Quadrat Sampling

21 Christmas Lake plant assessment May 23, is in g-dry wt/0.1 m 2 and stem density is in stems/0.1m 2. ADJACENT REFERENCE Depth (ft) Cabbage Claspingleaf Pondweed Coontail Curlyleaf Pondweed Eurasian Watermilfoil Flatstem Pondweed Illinois Pondweed Marsh Marigold Stringy Pondweed Whitestem Pondweed Ref 1 R Ref 1 R Ref 1 R Ref 1 R Ref 1 R Ref 1 R Ref 1 R Ref 1 R Ref 1 R Average REMOTE REFERENCE Ref 300 R Ref 300 R Ref 300 R Ref 300 R Ref 300 R Ref 300 R Ref 300 R Ref 300 R Ref 300 R Average WEEVIL STOCKED AREA Treat 1 T Treat 1 T Treat 1 T Treat 1 T Treat 1 T Treat 1 T Treat 1 T Treat 1 T Treat 1 T Average

22 Christmas Lake plant assessment August 9, is in g-dry wt/0.1 m 2 and stem density is in stems/0.1m 2. Depth (ft) Buttercup Cabbage Claspingleaf Pondweed ADJACENT REFERENCE Coontail Curlyleaf Pondweed Eurasian Watermilfoil Fern Pondweed Flatstem Pondweed Floatingleaf Pondweed Illinois Pondweed Marsh Marigold Northern Watermilfoil Sago Pondweed Stringy Pondweed Water Celery Ref 1 R Ref 1 R Ref 1 R Ref 1 R Ref 1 R Ref 1 R Ref 1 R Ref 1 R Ref 1 R Average Water Stargrass Whitestem Pondweed REMOTE REFERENCE Ref 300 R Ref 300 R Ref 300 R Ref 300 R Ref 300 R Ref 300 R Ref 300 R Ref 300 R Ref 300 R Average WEEVIL STOCKED AREA Treat 1 T Treat 1 T Treat 1 T Treat 1 T Treat 1 T Treat 1 T Treat 1 T Treat 1 T Treat 1 T Average

23 APPENDIX B 2012 Progress Report of the Milfoil Solution Program in Christmas Lake Prepared by EnviroScience Stow, OH