Since the 1990s, Lake Erie has been periodically swamped with algal blooms that turn its waters a bright green color. The plague reached a crescendo in 2011, when pernicious algae covered a record-breaking 2,000 square miles and threatened the drinking water of millions of people. Experts say similar blooms will become more frequent in the near future.
When an excess of nutrients such as nitrogen and phosphorus accumulate in a body of water, algae can grow out of control. There, algal blooms create “dead zones” by greedily consuming the environment’s oxygen and blocking sunlight from reaching underwater plants. Some blooms, called harmful algal blooms because of the danger they pose to humans and other animals if consumed, are produced by a type of algae called cyanobacteria that churn out harmful toxins.
Lake Erie’s algal blooms flare up when rainfall washes the nutrients left behind by human activity — mainly in the form of agricultural fertilizers and untreated sewage — into connecting streams. Freshwater ecosystems around the world share a similar risk, especially as climate change makes intense storms more common during spring and summer.
