As bright green plumes of toxic algae spread over Lake Erie in the summer of 2014, suffocating one of the largest lakes on earth, reporter Dan Egan was there. He had arrived in Toledo, Ohio, to investigate what had sickened the water — and how treatment plants might not be able to purify it.

Indeed, that’s exactly what happened. The day after he returned home to Wisconsin, Toledo warned people to stop drinking, boiling or bathing in tap water. Ohio’s governor declared a state of emergency. And Egan soon published an expansive report in the Milwaukee Journal Sentinel about how we got to a place where people living by such an abundant source of life-giving freshwater could not drink it or even touch it.

As the Journal Sentinel’s Great Lakes reporter for nearly 20 years, where he was twice a finalist for the Pulitzer Prize, and now writing magazine stories, Egan has long explored the tension between people and place. From invasive species to the multibillion-dollar recreational fishing industry to Chicago’s fraught relationship with Lake Michigan, he serves as a watchdog for the massive inland seas. The narrative power of his first book, “The Death and Life of the Great Lakes,” helped it reach a wide audience. A New York Times bestseller, it won both the Los Angeles Times Book Prize and the J. Anthony Lukas Award.

Egan’s new book, “The Devil’s Element: Phosphorus and a World Out of Balance,” tells the urgent story of the 13th element to be discovered. (It’s the 15th element on the periodic table.) The unchecked flow of phosphorus into our waterways — often from farm runoff — contributes to “dead zones” and toxic algae blooms. At the same time, as an essential ingredient in fertilizer, phosphorus turns vast swaths of land green, nourishing crops and animals. It makes life possible for billions of people.

Phosphorus, he writes, isn’t only essential to us; it is us. It’s found in our bones, teeth, even our DNA. In the naturally replenishing cycle, animals eat phosphorus-rich plants and then return the element to the soil when they defecate or die and decay. The soil then grows the next generation of plant life. Thanks to the remnants of long-dead organisms, phosphorus is also found in rare caches of sedimentary rocks on ancient seabeds. But in the 19th century, humans figured out how to break the cycle — systematically taking rocks, guano and even bones from one place to fertilize the soil of another place. Today, the world’s food supply depends on diminishing phosphorus reserves in places like Bone Valley, Florida, and the Western Sahara. At the same time, excess phosphorus from both plant and animal farms spills into our water and spoils it.

Egan, now the Brico journalist in residence at the Center for Water Policy in the University of Wisconsin-Milwaukee’s School of Freshwater Sciences, spoke with ProPublica about phosphorus, algae and the perils and possibilities of book-length journalism. This interview was edited for length and clarity.

You’ve spent nearly 30 years covering environmental stories, first in Idaho and Utah and then at the Milwaukee Journal Sentinel. What are the earliest stories you remember writing about toxic algae blooms?

I come at this without a background in science or environmental studies. But being out in Idaho, I was thrown to the wolves, literally, because wolf reintroduction was a huge issue. I also covered salmon recovery and grizzly bear recovery. That was a crash course in environmental journalism.

But I don’t remember writing about algae until 2014. I was in Toledo the week before they lost their water, doing a story on what would happen if Toledo lost their water.

What did you come across in your reporting that surprised you?

When I was writing about the algae blooms in Lake Erie, I was mostly reading about the algae blooms. I was just introduced to phosphorus along the way. I didn’t put much of it in my first book. But the idea that we need rocks to sustain modern agriculture — somebody was saying, “Yeah, it comes out of Florida, it comes up on trains to the various fertilizer factories.” “Rocks? Any rocks?” “No, special rocks.”

And then, the whole stuff about grinding bones and spreading them on crops. I wasn’t bored writing this book, I will tell you that.