Algae blooms are not triggered by a single factor, but rather, variation in some combination of the following conditions:

  • Nutrients: Nutrients, like nitrogen and phosphorus, fuel algae growth; however, excess nutrients from human activities can trigger or exacerbate algae blooms.
  • Temperature: Generally speaking, warmer water leads to increased algae growth. The temperature of the water may even favor the growth of certain types of algae over others.
  • Streamflow: Algae prefer stable conditions. Low water levels create slower, more uniform conditions that encourage algae growth.
  • Consecutive years of low spring runoff: High or prolonged spring runoff can scour algae basal cells off of rocks and set the growing season back. Multiple years of low runoff can produce early algae blooms.
  • Light: Like land plants, algae need sunlight to grow. Sunny reaches tend to have more algae growing in them than shaded reaches.
  • Turbidity (water clarity): Particles suspended in streams affect how light travels through the water. More sunlight penetrates clear water contributing to algae blooms.
  • Size of rocks on the stream bottom: Long stringy algae (usually Cladophora) prefer stable cobble stream bottoms because they don’t shift during low flow and are typical of riffle areas that have preferable water velocities.

For more information, visit the Center for Earth and Environmental Science at Indiana University website.