Why do harmful algal blooms occur?

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•  Levels of nutrients such as phosphorus and nitrogen increase in water. Sources include: 
  •  Fertilizer (for example, from home lawns and agricultural land) 
  •  Sewage from people and animals 
  •  Run-off from cities and industrial buildings 
•  Water flow is low and moves slowly, such as during a drought. 
• Water conditions like pH or turbidity (how much “stuff” is floating in the water) change. When turbidity is low, light can shine through the water easily. This helps phytoplankton grow.
• There are changes in the local ecology. Ecology is how organisms interact with the environment and with each other.

Source: Centers for Disease Control