Aphanizomenon: The Bloom You Don’t Want to See

Ecology and Habitat: Where and Why Aphanizomenon Blooms Occur

Aphanizomenon is a genus of filamentous cyanobacteria—often referred to as “blue-green algae,” though it’s technically bacteria, not algae. Found in freshwater bodies across the globe, it tends to dominate in nutrient-rich (eutrophic) lakes and ponds, especially during the warmer months (Wacklin et al., 2009).

What gives Aphanizomenon its competitive edge? For one, it can fix atmospheric nitrogen. That means even when other types of phytoplankton struggle due to low nitrogen levels, Aphanizomenon can thrive by pulling nitrogen gas directly from the atmosphere and converting it into a usable form (Paerl & Otten, 2013). Add high phosphorus levels—often the result of runoff from agriculture or lawn fertilizers—and you’ve got a recipe for a bloom.

It prefers:

• Warm temperatures (20–30°C / 68–86°F) 

• High phosphorus concentrations 

• Calm, stable water columns (little mixing) 

• Long daylight hours 

• Low nitrogen-to-phosphorus (N:P) ratios 

These conditions often occur in midsummer, making July through September prime bloom season in many temperate lakes (Dokulil & Teubner, 2000).

Aphanizomenon and Toxins: A Dangerous Brew

Fatal Encounters: When Blooms Turn Deadly

Unfortunately, blooms of Aphanizomenon have been linked to numerous animal deaths and even some human health concerns. Dogs are often the first victims—because they drink from, swim in, or lick scum off their fur after exposure to contaminated water.

Documented Cases:

• Minnesota, USA (2021): Several dogs died within hours of swimming in a lake with a dense Aphanizomenon bloom. Toxin analysis confirmed anatoxin-a concentrations exceeding 1,000 µg/L—well above the WHO’s guideline of 1 µg/L for drinking water (Minnesota Pollution Control Agency [MPCA], 2021).

• Germany (2000s): Waterfowl die-offs linked to saxitoxin-producing Aphanizomenon flos-aquae blooms in shallow lakes (Ballot et al., 2008).

• California, USA (2010): A dog died after ingesting water from a lake where Aphanizomenon was present; anatoxin-a was detected at concentrations >500 µg/L (Backer et al., 2013).

Humans typically don’t ingest bloom water in the same way animals do, but recreational exposure (swimming, boating) can cause rashes, eye irritation, gastrointestinal symptoms, or respiratory distress—especially in sensitive individuals (Stewart et al., 2006).

 Management Strategies: What Can Be Done? 

A Final Word

Aphanizomenon may be ancient—cyanobacteria have been around for billions of years—but its role in modern waterbodies is a growing concern. As climate change lengthens warm seasons and nutrient pollution continues unchecked, blooms are becoming more frequent, more intense, and more dangerous.

The good news? Awareness is growing, too. With smart monitoring, better land use practices, and targeted lake management, we can reduce the frequency and impact of Aphanizomenon blooms. So the next time you see a greenish tinge on your favorite lake, you’ll know what to watch for—and why it matters.