Fungus causes Delmarva rivers to turn yellow

The Broadkill River erupted with a yellow-orange coating earlier this week, worrying residents and sending scientists on a mission to identify the source.

At first glance, Ed Whereat, program coordinator at the University of Delaware's Citizens Monitoring Program, thought it might have been an algae bloom, based on photos taken May 22 when the river turned gold. 

By the next day, the river was nearly clear, with small remnants of tiny yellow flecks congregating along the water's edge.

Over on the Nanticoke River in Seaford, the same thing happened. Scientists from the Department of Natural Resources and Environmental Control, who could not be reached for comment, and Judith Stribling, a Salisbury University biology professor, eventually discovered the yellow film was not algae, but a common, nontoxic fungus.

That's great news for Milton residents. Some worried the film was caused by industrial waste or a malfunction at the municipal sewer plant, which discharges directly into the river.

“It was suspicious because it was near the treatment plant,” said Miltonian Roy Fleming.

Stribling said the fungus is hyphomycetes, commonly found on decaying leaves or organic matter that falls in the water.

Stribling said hyphomycetes are harmless. The fungi plays an important role in breaking down leaves decaying in or near waterways.

“Leaves can't really be consumed by many organisms. They have to be broken down,” she said. “This is not an uncommon critter.”

The fungi - seen on the Broadkill and Nanticoke rivers as well as Broad Creek, Choptank River and Marshyhope Creek - was the result of a perfect storm, Stribling said.

“My guess is – and this is speculation – was when we had all that rain, high flows generated turbulence and mixing of organic matter, trapped these things and lit up the river,” she said. “They were pretty much gone by the next day. Once that condition relaxed, they sank back to the bottom.”

Stribling, who normally focuses on nutrient management and wetland biogeochemistry, said a friend who is a botanist helped identify the organisms. But it made for a fun day in the lab, she said.

“It was really fascinating,” she said.