It looked like something straight out of Dr. Seuss's world, but this is no imaginary scene.
In 2012, a form of algae spread across Australia's eastern coast, turning the water an alarming shade of red and forcing at least ten beaches, including Sydney's popular Bondi Beach, to close.
Known as an algae bloom or "red tide," the event occurs when unicellular organisms—in this case dinoflagellates from the genus Noctiluca—find optimal conditions (including sunlight and nutrients) and reproduce quickly.
"It's sort of like the rapid growth of bacteria," said Stanford University marine biologist William F. Gilly.
Large red tides can be harmful to fish, said Lauren Freeman, a Ph.D. candidate at Scripps Institution of Oceanography in San Diego, California. "As the algae die and sink in the shallow coastal water, they decompose and oxygen is taken from the water column. This can lead to temporary low oxygen zones," she added. These zones can kill marine animals if the oxygen depletion is severe enough.
So can red tides hurt humans? It depends on the species that's blooming, said Gilly.
Blooms of certain dinoflagellates are associated with saxitoxin, a shellfish toxin that can cause paralytic shellfish poisoning in people. "If it's in a harbor or contained bay, it's probably a serious matter," he said.
But as for the notion that this is some kind of harbinger of doom, don't worry. Said Gilly: "I've seen patches like this in the Gulf of California and Monterey Bay, so such events are not rare. They certainly don't merit 'end of days' status."