City officials reopened Hanauma Bay Nature Preserve on Thursday after testing showed bacteria in the water of the beach park had dropped to safe levels.

The state Department of Health earlier this week found enterococci levels had exceeded advisory levels and the park was closed on Wednesday.

"We appreciate the patience and understanding while this treasured beach location was closed to the public," said Andrew Pereira, spokesman for Honolulu Mayor Kirk Caldwell.

Pereira said by email the health department does not identify the source of the bacteria but that enterococci is naturally occuring.

High levels of enterococci indicate potentially harmful microorganisms like bacteria, viruses, protozoa, or parasites could be present in the water and could make swimmers sick.

The young, the elderly, and those with weakened immune systems are most at risk if they swim in polluted water.

Residents and visitors can sign up for beach closing advisories on the health department's environmental health website.