Let me tell you a story about nitrogen. A farmer in western Kentucky applies nitrogen-based fertilizer to his crops. It rains. The rain carries run-off from his fields into a little stream nearby. Hungry fungi and bacteria munch on the nitrogen. And they can do a pretty good job, normally, of filtering that nitrogen from the stream. And that’s important because too much nitrogen can wipe out oxygen and create dead zones where nothing can live or grow.

But a farmer further downstream adds some nitrogen-based fertilizer to his farm. And so on, down the stream, until its microorganisms just can’t eat up all that nitrogen. It gets overloaded with nitrogen. But the stream keeps streaming. It feeds a river. The river pours into an estuary. And the estuary empties into the ocean.

And there, near the beach where you might want to snorkel or hunt for shells, you’ll see a red cloud in the water. That’s an algae bloom, often called a red tide. And it’s dangerous for humans and sea life because it sucks the oxygen out of the water. What made it possible? Nitrogen. This algae eats it. And red tides will probably be washing up this spring. (Here’s a picture of a Florida red tide from the National Oceanic and Atmospheric Administration.)

That’s the story researchers from universities and institutes across the country are telling in the most recent issue of the journal Nature. Here’s the lead author, quoted in the study’s press release:

“‘The filtering [of nitrogen] is a serial process and it’s bigger than any one stream,’ said Patrick Mulholland, the study’s lead author and a researcher with the University of Tennessee, Knoxville, and Oak Ridge National Laboratory. ‘What you see in your backyard, though, matters to the health of coastal oceans.’

Nitrogen doesn’t come just from agricultural sources, of course, there are urban ones, too. And it’s just one of many compounds running off of the land throughout our fragile watershed, ultimately—and, inadvertently—into our oceans.