Final Lake of the Woods Excess Nutrients Total Maximum Daily Load

Section 303(d) of the Clean Water Act (CWA) requires the completion of a total maximum daily load
(TMDL) study for water bodies found to not meet a water quality standard and listed as impaired. This
Lake of the Woods (LoW) Nutrient TMDL Study addresses the aquatic recreation impairment of
Minnesota’s portion of the LoW caused by excess nutrients. Minnesota’s portion of the LoW does not
meet water quality standards because of excessive total phosphorus (TP) and Chlorophyll-a (Chl-a)
concentrations (related to nuisance algal blooms) and violation of the Secchi disk (transparency)
standard. Phosphorus (P) is the focus of this TMDL study because it drives a wide array of lake biological responses that affect beneficial uses. While the LoW has a long history of nutrient and organic enrichment, the past several decades have been marked by successes in reducing P loading (particularly
dissolved P, which is more readily available for algal growth) to the Rainy River from wastewater
discharges. Ongoing P-reduction efforts are a management priority in both Minnesota and Canada. This
TMDL study quantifies P reductions that are necessary to satisfy applicable lake TP and response
variable standards, and provides measurable benchmarks to gauge future progress in achieving
reductions required to reduce nuisance algal blooms and achieve designated beneficial uses.