8.1. SR 06-17-2019 - Laserfiche WebLink

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the predicted more intense rainfalls when planning sedimentation removal projects, because such rainfall patterns will increase TSS, and therefore sediment accumulation, in Lake Orono. Invasive Plants Management Lake Orono has three invasive plant species: the submersed plant curly -leaf pondweed (CLP) and two emergent plants, purple loosestrife and yellow iris. CLP has been present in the lake since 2003 and currently covers approximately 33% of the lake. The LID proposes to reduce abundance of CLP with herbicide applications and possibly mechanical removal. The two emergent aquatic invasive species, purple loosestrife and yellow iris, can be managed by manual removal and replacement with native wetland plantings and shoreline vegetation restoration. Herbicide treatment of CLP started in 2017 and was repeated in 2018. In 2017, 4.27 acres were treated, and in 2018, 22 acres were treated. CLP was found at a far lower frequency in 2018 than in 2017 (35% of the littoral zone vs. 71% of the littoral zone). With so few data points, no conclusions can be drawn. Pesticide treatments also may not be as effective if flow rates are high and the pesticides move downstream. Therefore, timing and concentration exposure time is essential when considering a pesticide treatment. Full lake point intercept surveys of aquatic vegetation in Lake Orono have been conducted in May 2012, by MN DNR staff, and in 2017 and 2018 (early June to evaluate CLP, and July to evaluate the native plant community) by a private consultant. The 2012 DNR survey was conducted to learn the extent of CLP, while the two later surveys were conducted to assess the native aquatic plant community in addition to CLP. The 2012 DNR survey found CLP at 14% of sampling points, and the 2017 and 2018 surveys found higher occurrences of CLP. The July surveys found aquatic plant growth at 46% and 72% of points sampled, with 18 to 22 native species. The surveys found plants to a maximum depth of 8 feet in 2017 and 5.5 feet in 2018, with the vast majority between 3 and 5 feet. The dominant native plant species are coontail (Ceratophyllum demersum), 'common waterweed' (Elodea canadensis) and duckweeds. The DNR notes that the dredging and partial lake drawdown, which are expected to be implemented as soon as 2019-2020, may have a profound effect on the distribution and abundance of CLP. Additional CLP plants and turions would be removed during sediment dredging. The DNR notes that the heaviest CLP growth occurs in Upper Orono, where most sediment deposition occurs and which will be most heavily dredged. If done properly, the dredging could help to alleviate the CLP abundance. The proposed lake drawdown could also help to reduce CLP. A winter drawdown would expose sediments to freezing and kill many of the exposed plants and turions, likely reducing the "seed bank" for CLP. Areas that do not maintain a hard freeze may be more suitable for pesticide treatments. It is important to note that, while a winter drawdown would freeze out CLP, it can also have an undesirable effect on other plants as well as hibernating animals. Drawdowns should be timed to avoid or minimize these effects, especially any possibility of taking endangered or threatened species, or impacting Species of Special Concern. The DNR suggests that the LID re-evaluate the lake's vegetation community after any drawdown and dredging, then base its vegetation management activities on the new community structure. The drawdown and dredging may significantly reduce the need for herbicide application. In addition, CLP has unique life history patterns (emergence and growth early in the spring, followed by midsummer senescence) that must be taken into account when treatments and monitoring surveys are conducted. DNR Advisory Report on Lake Orono Lake Improvement District Page 5