Water Quality
Phosphorus
A nutrient that fuels algae growth in lakes, measured as total phosphorus in micrograms per liter.
Phosphorus inputs to lakes come from agricultural runoff, lawn fertilizers, failing septic systems, and atmospheric deposition. Once phosphorus is in a lake, it cycles internally for decades — sediment-bound phosphorus releases back into the water column whenever the bottom is disturbed. LakeGrade weights total phosphorus equally with Secchi depth and chlorophyll-a in the overall grade. Cutoffs: below 20 µg/L = A, below 30 µg/L = B, below 60 µg/L = C, below 90 µg/L = D, above 90 µg/L = F.
The per-lake pages on LakeQuality always show the specific Phosphorus value for that lake, so you can move from the general concept to the specific lake without leaving the site.
What It Means for Your Lake
Phosphorus is the primary nutrient limiting algae growth in most freshwater lakes across Minnesota and Wisconsin. Total phosphorus (TP) is measured in micrograms per liter (ug/L) and includes all forms, dissolved reactive phosphorus that algae can immediately use, and particulate phosphorus bound to sediment or organic matter. In the Upper Midwest, natural background phosphorus levels in lakes range from 10 to 20 ug/L. Lakes with TP below 15 ug/L are typically clear with minimal algae and classified as oligotrophic. Lakes exceeding 40 ug/L almost always experience nuisance algae blooms and may develop harmful cyanobacteria during warm months. Phosphorus enters lakes through agricultural runoff carrying fertilizer, failing septic systems near shorelines, stormwater from developed areas, and internal loading from lake-bottom sediments during low-oxygen conditions. Once phosphorus enters a lake, it can cycle between sediments and the water column for decades, making eutrophication difficult to reverse even after external sources are controlled. LakeQuality weights phosphorus at 30% of the overall grade. Lakes below 15 ug/L earn an A, while those exceeding 60 ug/L receive an F. Reducing phosphorus inputs is the single most effective strategy for improving lake water quality.
Frequently Asked Questions
What is phosphorus?
A nutrient that fuels algae growth in lakes, measured as total phosphorus in micrograms per liter.
Why does phosphorus matter for lake health?
Phosphorus is the primary nutrient limiting algae growth in most freshwater lakes across Minnesota and Wisconsin. Total phosphorus (TP) is measured in micrograms per liter (ug/L) and includes all forms, dissolved reactive phosphorus that algae can immediately use, and particulate phosphorus bound to...
Related Terms
Chlorophyll-a
A green pigment found in all photosynthetic organisms, used as a direct measure of algae concentration in lake water.
Algal Bloom
A rapid increase in algae population in a lake, often visible as green discoloration or surface scums, triggered by warm temperatures and excess nutrients.
Eutrophic
A lake classification indicating high nutrient levels, reduced clarity, and frequent algae blooms, a nutrient-rich and highly productive lake.
Agricultural Runoff
Water flowing off cropland, feedlots, and pastures that carries fertilizer, manure, pesticides, and eroded soil into lakes and streams.
Nonpoint Source Pollution
Water pollution from diffuse sources across the landscape, including agricultural fields, lawns, streets, and construction sites, rather than from a single identifiable discharge point.
Source: EPA National Aquatic Resource Surveys, 2026.