The chemical compound that can cause algal blooms and cultural eutrophication in streams is phosphorus.
Phosphorus is a key nutrient that promotes the excessive growth of algae and aquatic plants in water bodies when present in high concentrations. The main sources of excess phosphorus include:

1. Agricultural runoff containing phosphorus-rich fertilizers
2. Sewage and wastewater effluents containing phosphates from detergents and industrial processes
3. Soil erosion carrying phosphorus from land into water bodies

The oversupply of phosphorus leads to a process called cultural eutrophication, where water becomes overly enriched with nutrients. This triggers rapid growth and blooms of algae and aquatic vegetation.
Algal blooms deplete dissolved oxygen, reduce water clarity, can produce toxins harmful to aquatic life and humans, and disrupt ecosystems. Controlling phosphorus inputs from sources like farms, sewage, and erosion is critical for preventing cultural eutrophication and nuisance algal blooms in streams, lakes and other waters.

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