Foul-smelling green and brown sea sludge sighted off Port Douglas' Four Mile beach and Alexandra Reefs near Mowbray at the weekend were algal blooms, not the Great Barrier Reef's annual coral spawning.

One Port Douglas resident told The Newsport that people could smell the Four Mile bloom from streets away, with others describing the sight on social media as the reef's annual coral spawn.

However, Dr Andrew Baird, a coral expert and Postdoctoral Fellow at James Cook University's School of Marine Biology and Aquaculture, said as "most of the corals on inshore reefs, such as Magnetic Island, spawned last month, it is most likely that people saw Trichodesmium rather than coral spawn, which is usually red."

Trichodesmium, or 'sea sawdust', is a common naturally occuring cyanobacteria. It can occur along Queensland's coastline between August and December, according to the state's Department of Natural Resources and Mines.

The smelly patches of algal bloom are often mistakenly described as oil slicks or coral spawn, and are not caused by pollution or human impact. 

Dr Baird said though the Four Mile photographs do show "a bit of a reddish tinge and [do] not look like Trichodesmium, I would be very surprised if it was coral spawn on the mainland last week."

"The corals on inshore reefs usually spawn in October just after the full moon [and] the corals on offshore reefs will be spawning later this week."

Read related article:

• Coral spawning dates predicted, Oct 23 2013