Why Lakes Turn Green: How Phosphorus and Nutrient Pollution Work

Phosphorus is the nutrient that controls algae

In most freshwater lakes, phosphorus is the limiting nutrient — the resource in shortest supply relative to what algae need to grow. Algae need light, carbon, nitrogen, and phosphorus; in a typical lake they run out of phosphorus first. That is why phosphorus is the lever: add a little, and algae that were held in check suddenly have everything they need to multiply. A lake that gets clearer or greener year to year is usually responding to how much phosphorus is reaching it, not to any single dramatic event.

Where the phosphorus comes from

The nutrients that green a lake arrive from the land around it. The largest external sources are:

• Fertilizer and manure from farm fields and lawns, washed off by rain and irrigation.
• Failing or undersized septic systems near the shoreline, which leak nutrient-rich wastewater into groundwater and the lake.
• Urban stormwater that carries lawn fertilizer, pet waste, and street grime straight into storm drains and downstream waters.
• Eroded soil, because phosphorus binds tightly to soil particles — so anything that increases erosion (bare fields, cleared shoreline, construction) moves phosphorus with it.

The U.S. EPA's nutrient pollution program documents these pathways in detail and treats nitrogen and phosphorus loading as one of the most widespread water-quality problems in the country.

Eutrophication, step by step

Eutrophication is the process of a lake becoming over-enriched with nutrients, and it follows a predictable chain. Excess phosphorus enters the lake. Algae and cyanobacteria bloom, tinting the water green and cutting clarity. The bloom eventually dies and sinks. Bacteria decomposing that dead algae consume oxygen, and deep water can go hypoxic — low or empty of oxygen. Low oxygen stresses or kills fish and, critically, changes the chemistry at the lake bottom in a way that releases still more phosphorus. What began as a nutrient input becomes a self-reinforcing cycle.

Internal versus external loading

Managers split phosphorus sources into two kinds. External loading is everything washing in from the watershed — the fertilizer, septic, stormwater, and eroded soil above. Internal loading is phosphorus already in the lake's bottom sediment being released back into the water, which happens when deep water loses oxygen during a summer bloom. Internal loading is why some lakes stay green for years even after a community reduces the runoff coming in: the lake is, in effect, fertilizing itself from its own sediment until that stored phosphorus works its way out.

How green shows up in the grade

Because phosphorus drives algae, two of the three measurements behind a LakeQuality grade are direct readouts of this process: total phosphorus (the fuel) and chlorophyll-a (a proxy for how much algae the fuel produced). A lake loaded with phosphorus and thick with chlorophyll grades poorly; a clear, nutrient-poor lake grades well. Our grading methodology explains exactly how those thresholds map to letter grades. The lakes below carry the highest measured chlorophyll-a in our dataset — the clearest picture of nutrient stress in action.

Nutrients and impaired waters

When nutrient pollution pushes a lake past legal water-quality standards, it can land on a state's Clean Water Act 303(d) list of impaired waters (tracked nationally in EPA's ATTAINS system), often with nutrients or excess algal growth named as the cause. You can see which lakes carry that designation on our impairment pages — for example, impaired lakes in Minnesota. To understand the full spectrum from clear to green, our companion explainer on trophic state shows how nutrient enrichment moves a lake through recognized productivity categories.