CARE OF THE WATER
DNR Does Pondweed
Turion Survey of Roberds Lake
On October 7th, 2015, the DNR did a survey of pondweed seeds (turions) present in Roberds Lake. The survey is done by bringing up lake bottom sediment by use of a claw type device. The sentiment is washed with lake water to remove soil and rocks. The remaining materials containing the turions are then bagged for further analysis at a later date. This process was repeated 36 times in the areas where pondweed grows. This is done around the entire Lake using GPS locations. The GPS locations are the same as used in prior years. This enables the DNR to compare results for the same locations over a period of years. The results will be included in an updated report that now exists on this website. The update will be completed late this fall or winter. When updated, we will post the report
The survey was done by Allison Gamble of the DNR with assistance from Larry Churchill and Ted Tuma. The picture is one of many pondweed plants seen during the survey. As you can see, the plant is starting to grow now. They will continue to grow throughout the winter and then die off in late June and early July. At the center of the plant in the picture is what is remaining of the turion which is a similar to a small pine cone. The white stems at the bottom of the plant are the roots and the green is the plant's stem and leaves that eventually would reach the Lake's surface.
The surveys help us track and monitor changes over time. In addition, we use the data to help determine the best Lake areas to treat in the future. Thanks to the DNR for completing this survey.
Inspection by the Invasive Species Program
Division of Ecological and Water Resources
Minnesota Department of Natural Resources
Lake: Roberds DOW Number: 66001800 Date of inspection: Sept 20, 2013
County: Rice Observer[s]: Allison Gamble, Nik Myrha-Edwards, Ted Tuma (Roberds Lake)
Time - On Water: 11:20 Off water: 12:30
Type of inspection: Curlyleaf pondweed (Potamogeton crispus) turion sampling
Roberds Lake is located in Rice County near Shieldsville, and is part of the Cannon River Watershed. The lake is 632.28 acres with 393 littoral acres, which represents 63% of the lake. The maximum depth is 43 feet, and there is one concrete boat access managed by the DNR.
The most recent point intercept survey of the lake was in June 2010. Curlyleaf pondweed (Potamogeton crispus) was the dominant plant in 2010, at 44% of all sites, while coontail (Ceratophyllum demersum), Elodea (Elodea canadensis), and filamentous algae were also present at 10-20% of the sites. Curlyleaf pondweed has been chemically treated in Roberds Lake since 2011.
Turion Sampling Methods
Curlyleaf pondweed turion sampling was conducted on Roberds Lake in 2007, 2011, and in 2013 (Fig. 1, Table 1).
The 2007 turion survey was conducted by James Johnson
(Freshwater Scientific Services, LLC, see for survey methods).
Fig 1. Photo of the petite ponar device used to collect turion samples from lake sediments.
Table 1. Curlyleaf pondweed turion densities in Roberds Lake, Rice County, MN in 2007, 2011 and 2013. The data from 2007 are from a Freshwater Scientifc Services, LLC report (“Curlyleaf Pondweed Turion Survey Report –Roberds Lake 2007”). SE stands for standard error, a representation of the amount of uncertainty associated with the estimated average number of turion
Turion Sampling Results
The 2007 Freshwater Sciences, LLC turion survey found that the turion bank in Roberds Lake was generally dense in areas less than 10 feet (see Fig. 3 for estimates). At sites that were 10 feet or less, five sites had turion densities of 2000 per m2 or greater, while 11 sites had densities of 500-2000 turions per m2. In 2010, one site had turion densities of 2000 per m2 or greater, and 25 sites had turion densities of 500 per m2 or less (Fig. 4). In 2013, one site had turion densities of 2000 per m2 or greater, and 29 sites had turion densities of 500 per m2 or less (Fig. 4). The general trend over time appears to be lower numbers of turions per area, although most of the differences are not statistically significant.
Fig. 2. Map of the 2011 and 2013 fall turion sampling locations (green circles) in Roberds Lake, Rice County, MN.
Fig. 3. Estimated turion densities in 2007 in Roberds Lake, Rice County. Density estimates are based on maps created by James Johnson, Freshwater Scientific Services, LLC. Turion estimates are on a per m2 basis.
Fig. 4. Curlyleaf pondweed treatment area (light green) and turion densities in Roberds Lake, Rice County, MN in
2011 (left panel) and 2013 (right panel), based on MNDNR sampling. Turion estimates are on a per m2 basis.