What Causes Algae Blooms in Lakes?
Algae blooms are caused by excess phosphorus and nitrogen in the water, warm temperatures, calm winds, and sunny days. When all four conditions line up — usually in mid-to-late summer — algae populations can double in a day, turning lake water green or scummy. The single biggest driver is phosphorus from agricultural runoff, lawn fertilizer, and failing septic systems.
The four drivers
Algae blooms are not random. They appear when four conditions stack up in the same week:
- Phosphorus — the rate-limiting nutrient. Lakes above 30 µg/L are at elevated risk; above 60 µg/L is high risk.
- Nitrogen — secondary fuel, especially for cyanobacteria (blue-green algae).
- Warm water — most blooms peak when surface water exceeds 75°F.
- Calm wind — still water lets cyanobacteria float to the surface and concentrate.
Where the phosphorus comes from
In the Upper Midwest, the dominant phosphorus sources are: (1) agricultural runoff from corn, soy, and dairy operations, (2) lawn fertilizer in shoreline residential developments, (3) failing or aging septic systems near the lakeshore, and (4) internal loading — phosphorus that historical pollution has bound to lake-bottom sediment, which releases under low-oxygen conditions.
Internal loading is why some lakes stay impaired for decades after the original pollution stops. The phosphorus is already in the lake — it just keeps cycling.
Green algae vs. blue-green algae (cyanobacteria)
Not all blooms are dangerous. Green algae (true algae) blooms make water look murky or stringy but rarely produce toxins. Blue-green algae are not algae at all — they are cyanobacteria, photosynthetic bacteria that can produce microcystins, anatoxins, and other toxins capable of causing skin rashes, GI illness, and (in rare cases) liver damage. There is no way to tell by looking. Treat any visible bloom as potentially toxic until tested.
How to predict bloom risk from lake data
Three numbers on any LakeQuality lake page predict bloom risk:
- Chlorophyll-a directly measures algae. Below 5 µg/L is minimal; above 20 µg/L is bloom-prone.
- Phosphorus predicts future blooms — high phosphorus means the fuel is loaded even if today is clear.
- Trophic State Index (TSI) combines clarity, phosphorus, and chlorophyll-a. TSI above 60 (eutrophic or hypereutrophic) flags chronic bloom potential.