New MPCA report: Cleaner tributaries, less phosphorus in discharges will help reduce algae blooms in Lake of the Woods

Dan Olson, 218-846-8108

Famous for its fishing and other recreation, Lake of the Woods at the northern tip of Minnesota should experience reduced algae blooms, as long as phosphorus levels in tributaries that feed the lake continue at current levels or decrease, according to a recent study by the Minnesota Pollution Control Agency (MPCA) and its partners.

Located in Lake of the Woods and Roseau counties and straddling the U.S.-Canada border (with portions of the lake in the provinces of Manitoba and Ontario), the Minnesota portion of Lake of the Woods covers 320,000 acres.

The MPCA report, known as a total maximum daily load (TMDL) study, determines the reductions needed from pollutant sources to the lake to enable the lake to meet water quality standards. Phosphorus needs to decrease by 17.3 percent in Lake of the Woods to meet water quality standards and decrease the frequency of algal blooms that affect recreation. This reduction is based on studies by the MPCA, University of Minnesota, University of Wisconsin-Stout, Science Museum of Minnesota’s St. Croix Research Station, and United States Geological Survey. (Read more about one of these cooperative efforts that is helping explain an algae mystery in the lake.)

Two primary sources of phosphorus to the lake are:

  • Tributary rivers that contribute 39 percent of the lake’s phosphorus level
  • Deposits collected on the lake bottom over time that contribute 35 percent of the lake’s phosphorus level and come from the tributaries and other sources, such as industries, cities, and runoff.

A recent report by the MPCA shows that the Rainy River, the largest river flowing into Lake of the Woods, has had significant phosphorus reductions from municipal and industrial wastewater sources, due to widespread efforts by partners. Thanks to these long-term efforts, the Rainy River now meets water quality standards for nutrients. However, that legacy of phosphorus pollution is still affecting Lake of the Woods. Phosphorus collected over time on the lake bottom can become re-suspended in lake water due to fish activity or wave motion (known as internal loading).

A 2015 study estimates that phosphorus from internal loading in the lake is decreasing by 1 percent per year; cleaner water entering the lake is helping to flush out the stored phosphorus. As a result, the MPCA’s new report is not calling for additional reductions from most sources.

The 2015 study also identified excess sediment and phosphorus coming from the Little Fork River Watershed. A study is planned to identify the source of the sediment (i.e., stream banks, in-stream channel stream beds, floodplains, or uplands).

The TMDL report is part of the MPCA’s approach to gauging the health of Minnesota’s 80 major watersheds, each of which will have an approved comprehensive watershed management plan by 2025. After intensive water monitoring, the agency and partners evaluate water quality and biological conditions in lakes and streams. The MPCA places waters that fail to meet standards on the Impaired Waters List, and develops information and strategies to restore impaired waters and protect healthy ones.

The draft Lake of the Woods Nutrient TMDL report is available on the MPCA's website. Submit comments to or request information from Cary Hernandez (218-846-8124, 800-657-3864), MPCA, 714 Lake Ave. Ste. 220, Detroit Lakes MN, 56501 by 4:30 p.m. CST on Wednesday, March 24, 2021.

Two virtual public information meetings will be held via Webex on Thursday, March 4 from 1 – 3 p.m. and 6 – 8 p.m.

Visit the Lake of the Woods TMDL web page for more information and options for joining a meeting.