The Rhode Island Department of Environmental Management (DEM) has observed and received reports of rust-colored patches of water (known as “rust tide”) in Narragansett Bay, Mt. Hope Bay and coastal salt ponds. Rust tide, not to be confused with Red Tide, poses no public health risk and will not lead to shellfish closures. It is expected to decline as water temperatures fall to 60°F or lower.

Recent water sampling reveals the rust tide is being caused by a large bloom of naturally-occurring phytoplankton called Cochlodinium polykrikoides. This organism contains red photosynthetic pigments that cause the rust color visible on the water surface. 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.

Accumulations or blooms of Cochlodinium appear to be a regional phenomenon this summer. Rust tide has been observed in Peconic Bay in New York off the eastern end of Long Island, in Buzzards Bay, and off Martha’s Vineyard in Massachusetts. DEM will continue to monitor the bloom over the next several weeks.

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