FORT PIERCE, Fla. — Edith Widder presented a handful of greenish muck that had been pulled from the shallows of the Indian River Lagoon and cupped it in her palm.

“See that?” she asked. “That’s a lot of decayed organic matter. It’s just a great holding area for pollutants.”

Collecting mud is a new calling for Dr. Widder, a marine biologist who is known around the world for her work in much larger bodies of water.