Brown sludge that appeared off Oahu’s South Shore this week is a natural algae bloom, the state Department of Health says.

The substance, sometimes called "sea sawdust," is not considered harmful, but could be an irritant like pollen, said Michael Tsuji, supervisor of the Enforcement Section of the department’s Clean Water Branch.

"Yesterday it looked like sawdust floating in the waters off of Point Panic and Waikiki," Tsuji said.

The scum appears to be Trichodesmium, an important nitrogen-fixing bacterium, and was seen about a month ago at Kailua Beach. It occurs in periods of low winds and warm temperatures.

A sample was taken Wednesday, and the bacteria level was found to be low, confirming it was not sewage, Tsuji said. Examination under a microscope showed it to be algae, he said.

When large, the brown algae blooms have been mistaken for sandbars by ships’ crews.

The blooms are a natural part of marine ecology and not harmful to the ecosystem, but it is not fully understood why they occur, health officials said.

"Our recommendation is whenever the water looks discolored or unusual, it’s safer not to go into the water, particularly because different people react differently," said Janice Okubo, spokeswoman for the Health Department.