Rust Tide in Narragansett, Mt. Hope Bays not Harmful, DEM Says

RHODE ISLAND—State officials said Tuesday that rust-colored patches of water observed in Narragansett and Mt. Hope Bays and coastal salt ponds do not pose a risk to public health and is not to be confused with red tide, a highly toxic type of algae bloom.

Instead, the patches are known as a rust tide, the state Department of Environmental Management said in a news release. The rust tide is caused by a large bloom of phytoplankton called Cochlodinium polykrikoides, an organism with red photosynthetic pigments that cause the rust color.

"Rust tide is not harmful to humans; however, Cochlodinium can damage delicate finfish and shellfish exposed to concentrated populations. Rust tide is not associated with the red tide that caused shellfish closures in other New England states," the DEM said.

The rust tide should clear up once water temperatures fall below 60 degrees.

Rust tides "appear to be a regional phenomenon this summer," the DEM said and has been observed in Peconic Bay in New York on the eastern end of Long Island and in Buzzards Bay and off Martha's Vineyard in Massachusetts.

The DEM will monitor the bloom over the next several weeks.

Photo: Rust tide caused by Cochlodinium in Flanders Bay, New York, in 2013 (courtesy of Auxiliary Coast Guard)