What causes algae blooms in ponds?

Short Answer:
Algae blooms are caused by excess nutrients (especially phosphorus), sunlight, and warm water.

Detailed Answer:
Algae growth is driven by a process called eutrophication, where nutrients fuel rapid plant growth.

Primary causes:
High phosphorus (typically > 0.03 mg/L or 30 ppb)
Nitrogen inputs (fertilizer, runoff)
Warm temperatures
Sunlight penetration

Common nutrient sources:
Lawn fertilizer runoff
Fish waste and overfeeding
Decaying leaves and organic matter
Septic or agricultural runoff

Key takeaway:
If phosphorus is not controlled, algae will continue to return—even after treatment.