A rare red algae bloom off the coast of eastern Australia forced the closure of popular beaches near Sydney on Tuesday.

The thick bloom was caused by noctiluca scintillans, a dinoflagellate marine algae that is also called Sea Sparkle, because it exhibits bioluminescence.
Although the algae is naturally occurring, blooms such as these can be a hazard to swimmers. High levels of ammonia contained in the algae can cause rashes and eye irritation. Those risks caused beaches such as Bondi and Clovelly to close for the day.
A lifeguard at Bondi beach told the Australian Associated Press that the algae had a fishy smell.
Surfers first discovered the red tide at Bondi beach early Tuesday, according to News.com.au. The bloom broke up by evening, and the Bondi reopened. Clovelly, located further south, remained closed. Signs warning swimmers have been installed at affected beaches.