The Toxic Stew of Human Waste and Warming Water on Cape Cod

Ashley K. Fisher made her way to the edge of the boat, slipped into a pair of thick black waders, and plunged into the river to search for carcasses.

She quickly discovered them: the decomposed shells of ribbed mussels, enshrouded in a gray-black sludge that emitted a foul odor akin to rotting garbage and had the consistency of mayonnaise.

The sediment at the bottom of the Mashpee River thickens each year, increasingly smothering the river’s flora and fauna. It rose up to Ms. Fisher’s waist, making it challenging for her to free herself and return to the boat.

“I wasn’t expecting to sink so deeply,” Ms. Fisher, Mashpee’s director of natural resources, said with a laugh.

She recalled an incident where her team had to rescue a resident ensnared in the muck by securing him to a motorboat and revving the engine to pull him out.

This sludge results from harmful algae blooms, which are progressively overtaking more of Cape Cod’s rivers and bays each summer.