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’s reporting takes him not only to the Great Lakes, which hold about a fifth of all the freshwater on the face of the planet, but also to Germany, where an alchemist first isolated the combustible element and where traces of phosphorus cast down by World War II firebombers still wash ashore — with alarming results. We follow him to the saltwater beaches of the Mississippi Gulf Coast, once thought safe from the telltale shock of green, and to Ontario’s Experimental Lakes Area, where scientists discovered the connection between phosphorus and algae, much to the chagrin of detergent makers of the era. Along the way, Egan explores the Clean Water Act’s “yawning exemption” for agriculture and how some scientists fear we’ll hit “peak phosphorus” in a few decades.

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.

Can you share more about how phosphorus is uniquely lethal and life-giving?

What really caught my eye was how phosphorus doesn’t exist on its own in the environment. It’s always bound with oxygen atoms to make phosphates, which are stable, or noncombustible. But when they first isolated pure phosphorus in the late 1600s, it was magical stuff. It got above 80 Fahrenheit, and it just burst into flames and will not stop. Nothing will stop it. I guess you saw this in the book — a guy that’s burned goes into the water, and then he comes out of the water, and it flares up again.

And then you see that it was used as a weapon. But it’s also this essential fertilizer. Of the three big [elements in] fertilizers — nitrogen, potassium and phosphorus — phosphorus is the limiting [or least available] element.

There’s this paradox of how we’re just squandering these relatively scarce deposits and at the same time we’re overdosing our waters to the point where sometimes you can’t drink them, you can’t swim in them, it kills dogs, it threatens people.

How does the nuance here compare to our relationship with other materials that have proved vexing, such as lead, or PFAS, or even the vinyl chloride recently unleashed on East Palestine, Ohio?

With any toxin or element that we exploit and pollute the environment, there was a reason we did it. But phosphorus is so essential and also just so potentially harmful. Managing this stuff was hard enough when we had a billion people, but now we’re zooming toward 9 billion.

We need to change the way we’re using this, or there’s two consequences, and they’re not exclusive. We’re going to poison the crap out of our waters, or we’re going to run out of easily accessible deposits and have food shortages.

That’s the story. There’s a lot unfolding fast here. And I think it’s only going to accelerate.

What is slowing people down in restoring what you call “the virtuous cycle” of phosphorus?

It’s probably the agriculture lobby. They know there’s a problem, but it’s not being adequately addressed or we wouldn’t have these chronic blooms in every state in the union.

As far as slowing down people, I don’t know. It’s just not something people talk about. People would ask, “Are you writing another book?” I’d say, “Yep.” “What’s it about?” “Phosphorus.” And they’d look at me like I just told them I was diagnosed with something really bad.

And these are your book fans, asking what you’re writing next!

That alone is daunting. On the other hand, when you start telling people about how we mined the battlefield of Waterloo for all the [human] bones to grind them up to throw them on crops to grow turnips in England? That gets people’s attention.

There’s so much that goes into modern food production that we’re just divorced from. There’s been books written about this, and very good books, but I don’t think anybody has written a book for popular consumption that connects the dots between the food on a table and the poisoned waters. And also the lengths we’ve gone to find this precious substance that nobody thinks about.

Your book discusses a number of 20th century wins, such as the revival of Lake Erie after it was virtually declared dead and the pushback against the detergent industry’s overuse of phosphorus. Do you see a blueprint here for how to tackle problems with phosphorus today?

It’s useful to look at when we first tangled with phosphorus as a pollutant in the 1960s and ’70s and solved the problem, largely by banning phosphates in detergents. But it’s not entirely applicable. Today it’s a much bigger problem. It’s more diffuse. When we could plug a pipe or cap a smokestack to stop the pollutants, that’s easy. But now that it’s spread on the landscape, we’ve got these legacy phosphorus deposits. They’re going to be leaching into the water for decades. Even if we clamp down on CAFOs [concentrated animal feeding operations] tomorrow, there’s so much inertia in the system. It’s like climate: Things are going to get worse before they get better.

But it’s also important that we do look back and see that we have been successful. And we also have an obligation to just try. We have a chance to make things better for future generations. We should take advantage of it.

Before we leave off, is there any part of the book that you’d like to underline? Water or fertilizer, mining or politics, what would you like to make sure gets through to the public?

It’s a deep question and requires something of a deeper answer. But I think it’s the circle of life. It’s not just “The Lion King.” It’s real. And the thing that stitches it together in this case is phosphorus. We’ve got to learn that you don’t use it and chuck it. You use it again and use it again and use it again and use it again, if we’re going to stay fed and have waters that are safe enough to fish in and swim in and drink from and have your pets play in. This book is about the circle of life, manifested in phosphorus.