When out on the ocean, you may come across a giant red/brown or greenish "slick" and wonder what it is.

Sea sawdust - Bloom of Trichodesmium sp. off Cape Kimberley, near Snapper Island - Photo by Alex Pawlow

What appears to be a giant slick of red/brown or blue/green pollution, is often a bloom of the algae Trichodemium, which is usually accompanied by a strong fishy odour. It is similar to the blue/green algae blooms you see on freshwater rivers and lakes when too much nutrients are present.

Trichodesmium blooms are a naturally occuring phenomenon, and are beneficial due to the algaes ability to fix nitrogen in the ocean, which is needed for other organisms.

The blooms are actually made up of many very small rice grain shaped particles and do in-fact resemble fine sawdust sprinkled on the water (hence the common name). The blooms can get so large that they are visible from space. See image from the Aqua satellite taken in 2011 of the Great Barrier Reef and a probable Trichodesmium bloom.

The slight red colouration comes from pigments within the algae.

As well as fixing nitrogen, the algae also provides a pseudobenthic structure (something for other organisms to live on) for bacteria, protozoa, copepods and diatoms, and thus helps support complex microenviroments.

It is not believed that the blooms cause any harm to fish or other animals living below the water and fishermen don't seem to notice any detrimental affects on catch.