The state Department of Health said the cause of a mysterious brownish-green plume off Kauai appeared to be an algae bloom.

The water discoloration prompted a brown water advisory for coastal waters between Moloaa and Anahola bays.

The Health Department’s Clean Water Branch said cyanobacteria, which is known to cause algae blooms in tropical marine waters, may be the culprit.

As a precaution, the public is advised to stay out of affected waters to avoid the risk of swimmer’s itch.