Nutrients – such as nitrogen – are essential to life, but an overabundance can mean trouble for waterways. Take Chesapeake Bay and the Gulf of Mexico, which are infamous for “dead zones” where closely packed bodies of fish float to the surface or wash ashore by the thousands. These dead zones are caused by nutrient pollution, which makes algae grow too fast. The resulting algal blooms ultimately kill fish and other aquatic creatures by using up the oxygen they breathe.

Nitrogen is high in the San Francisco Bay as well, but so far the Bay has escaped the catastrophic effects of nutrient pollution. That may be about to change, however. 

Besides causing dead zones, some algae produce toxins that kill fish and other aquatic life directly. These harmful algal blooms can also cause health problems in people – including rashes, respiratory issues and liver illness – who swim in or accidentally swallow the water. 

Why has the Bay been immune to nutrient pollution? “There’s inherent protection in the system,” Senn said, explaining that “one of the things keeping a lid on algae is sediment suspended in the water.”

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