Algal blooms such as these occur when unicellular organisms, in this case dinoflagellates called noctiluca scintillans, reproduce quickly under optimal conditions. These blooms typically occur when currents bring cold, nutrient-rich water to the surface.
Algal blooms can be harmful to fish. When the algae die they sink in the shallow coastal water; when they decompose they take oxygen from the water column, leading to temporary low oxygen zones. Depending on the amount of oxygen depletion, marine animals can die. Some dinoflagellates are associated with saxitoxin, which is a shellfish toxin that has been known to cause paralytic shellfish poisoning in humans. The algal bloom off the coast of eastern Australia had a fishy odour and irritated the skin and eyes of some people, but was otherwise not harmful to humans.
The beaches reopened on November 30 and were declared safe for swimming by Surf Life Saving NSW. The photo shows a girl checking out the red algal bloom in the water at Clovelly Beach in Sydney.
Image: Newspix / Rex Features