More than a dozen water samples collected around the Great South Bay late last week contained the algae that produces Mahogany tide, but the bloom that first showed up in the middle of May appears to be declining.

Foamy residue churned up by winds and boats over Memorial Day weekend was likely due to the algae, known as prorocentrum minimum, dying off, said Chris Gobler, a professor at Stony Brook University’s School of Marine and Atmospheric Sciences who analyzed the samples.

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“The Mahogany tide was in all the samples, but to varying densities and it appears to be waning,” Gobler said. “The foam is an interesting story that’s sort of a consequence of Mahogany tide retreating.”

Mahogany tide has been found in New York waters since the 1980s and can turn water a rusty-mustard color. It is fed by warm temperatures and nutrients, often following heavy rains that can push fertilizers and other runoff into the water. It can lead to low oxygen levels and was partly to blame for a die off of Atlantic Menhaden last year in the Peconic River.

On Long Island, the algae has mostly been found in rivers and tributaries, not the Great South Bay. Gobler said the concentrations of prorocentrum minimum taken Friday were highest in the northern part of Great South Bay. The algae had previously been confirmed this year in the Peconic River, Bellport Bay and Georgica Pond.

Suffolk County Department of Health Services employees plan to sample the western part of the bay Thursday and the eastern section on Friday as part of a routine sampling program, spokeswoman Gioia Knutson said in an email. Gobler’s team will also do some testing this week.