Recent research reflects concepts in understanding lake phosphorus and changes in their ‘state’.
Vitense, K., M.A. Hanson, B.R. Herwig, K.D. Zimmer, and J. Fieberg. 2019.
Predicting total phosphorus levels as indicators for shallow lake management. Ecological Indicators 96:278-287. This recent study suggests that phosphorus levels can determine the potential success of shallow lake management such as drawdowns. Little Rock Lake total phosphorus (TP) levels were once as high as 600 parts per billion (ppb) in 2006. Thanks to aggressive efforts by both Morrison and Benton county Soil, Water Conservation Districts, working with cooperative agricultural producers, that level is now down to less than 130 ppb in 2017. The study used classifications of TP of less than 50 ppb and greater than 50 ppb. Some lakes higher than 350 ppb were found to have a permanent turbid state, while lakes with less than 50 ppb were in a permanently clear state. Little Rock Lake sits at the lower end of the spectrum of what is known as a “bi-stable state”. As suggested in the study ” These estimates provide valuable information to managers because a lake’s TP level in relation to the tipping points has implications for identifying effective management actions. Clear lakes with TP levels consistently below the lower tipping point are highly resilient, and management efforts for these lakes should typically focus on watershed and shoreline protection. Highly dynamic lakes with TP levels frequently in the bistable region are those for which active in-lake management, such as water level drawdowns or fish removal, is most sensible.”
Other drawdowns happening around Minnesota
Another common question was asked during the process of planning: Has a drawdown like this been done before anywhere? Quite a few have been done around Minnesota, however, each system and situation is unique. In the early 2000s, the Mississippi River near the Iowa state line had a few pools that were drawn down to improve waterfowl and fish habitat. Lake Orono (near Elk River) had a winter drawdown done to help an excavation project and minimize curly-leaf pondweed. Lakes Freeborn, Fulda, and Long in southern Minnesota have had drawdowns for various purposes with impressive results. Major water clarity improvements were observed, which increases invaluable emergent plant growth as well. Lakes Ocheda and High Island, also in southern Minnesota, are in the process of planning drawdowns in the near future. Interestingly, many well-known lakes north-central Minnesota, are currently experiencing a drought cycle. This lower water level has allowed plants like wild rice, bulrush, and cattail to expand in the shallow flats. This validates the presumption that a drawdown is intended to simulate a drought cycle.
For more information and to stay up to date on the DNR efforts of the drawdown go to:
https://www.dnr.state.mn.us/areas/fisheries/littlefalls/drawdown.html