Scientists Solve Mystery Of Disappearing Salt Marshes

Marshes along streams and estuaries protect land from storm surges. But they're disappearing fast and now scientists have discovered a previously unknown marsh killer: nutrients. Nitrogen from fertilizers and sewage makes marshes grow faster, but the roots grow smaller so the soil can't hold the bigger plants. That means soil banks collapse and marshes turn to mud.

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Let's pay a visit now to one of the crucial parts of our country's ecosystem. Along U.S. coastlines, there are salt marshes that serve as nurseries for fish, crabs and other shellfish. They also protect coastal areas against flooding. Scientists warn that some salt marshes are disintegrating, and researchers have a pretty surprising theory about why that is. Here's NPR's Christopher Joyce.

CHRISTOPHER JOYCE, BYLINE: When you think marsh, you think tall grasses along the muddy banks of streams and bays. Marshes nurture wildlife and also purify water. They sponge up stuff like nitrogen from farm fertilizer or sewage. Marshes like nitrogen. It's plant food. Ecologist Linda Deegan wanted to know how much they like it. She started adding nitrogen to three marshes in Massachusetts.

LINDA DEEGAN: After we were on the ground for a couple of years and we looked around and we said, hey, this isn't going the way we thought it was, we predicted the opposite, that the salt marshes would do well.

JOYCE: They didn't. Instead, they started to fall apart. Deegan's an ecologist with the Marine Biological Laboratory in Massachusetts. She ran the experiment for nine years. The grasses grew taller, and then they fell over.

DEEGAN: The tall cord grass that lines the banks of these creeks has sunk a little and slumped.

JOYCE: Deegan discovered that the plants grew bigger above ground, but not below.

DEEGAN: The plants make fewer roots, and the roots are part of what sort of builds the landscape and stabilizes the marsh and keeps it in place.

JOYCE: There are more problems. The microorganisms in the soil loved the nitrogen, so they multiplied and ate up more of the dead plant matter in the soil. That was like removing the soil's internal skeleton. Deegan describes the phenomenon in the journal Nature.

DEEGAN: I call it a low, slow process. It's the kind of thing where you sort of walk by and you go, huh. Did it look like that last year?

JOYCE: Now, this isn't just something for muddy-boots biologists to ponder. It could mean trouble for New England marshes. And marshes in the Gulf Coast are hugely important. When hurricanes like Katrina hit, the marshes slow down storm surge, except most of the marshes there have been cut down to make navigation channels, or been paved over. Louisiana plans to spend billions of dollars to rebuild these protective marshes by diverting water and sediment from the Mississippi River. But the river is chockfull of nitrogen from farms upstream. Could that undercut the new marshland? Geologist Sam Bentley thinks Deegan's discovery is not a deal-breaker.

SAM BENTLEY: It is a very important study, and it documents a phenomenon that we need to pay very close attention to. But not all mud is the same.

JOYCE: Bentley is at Louisiana State University. He points out that the kind of soil or mud in the Massachusetts marshes is mostly made of organic material, rotting vegetation like peat. You might call it delicate. The Mississippi, once described as too thick to navigate and too thin to plow, carries its own kind of sediment.

BENTLEY: You have lost of silk, clay and sand being deposited. You get a new kind of mud.

JOYCE: A mud that will hang together. Bentley acknowledges that some marshes in the delta will suffer from an influx of nitrogen. How much is a matter of dispute. In fact, there's been lots of dispute about how to rebuild the marshes. Bentley says: enough, already.

BENTLEY: The urgency is so great that we need to move ahead with the best knowledge that we have right now.

JOYCE: There's a master plan now on how to start. The state government hopes to finance it with penalties assessed against BP for its mega oil spill in 2010. Christopher Joyce, NPR News.

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