Surf diatoms
From time to time, visitors to surf beaches along the coast of Victoria notice patches of brown discolouration along the shore line. Although oil may occasionally be washed up on our beaches, this discolouration may be due to natural accumulations of large numbers of microscopic plants called diatoms.
The diatoms which usually make up these patches are not toxic, although they may cause some irritation to the human body. It is advisable to avoid swimming in dense patches or at least shower after swimming or surfing.
What do accumulations of surf diatoms look like?
 Surf diatoms viewed from the shore. |
Accumulations of surf diatoms turn the water a golden coffee colour, with froth or foam. Where diatoms cause the discolouration, there will be:
- no smell of oil
- no iridescent (rainbow-coloured) slick on the water surface
- no sticky patches
- no particles or blobs in the sand.
Often the colour will disappear at night, only to reappear in the morning.
Where will these accumulations be?
 Surf diatoms viewed from the air. |
The brown patches will always be in the broad shallow surf zone, where waves are breaking, often adjacent to rip channels. These accumulations only occur in particular locations – usually on long beaches, those greater than 4 kilometres, with fine sand, and often backed by dunes.
Weather conditions may play a role – onshore winds may contribute to the accumulations. They have been recorded from a number of sites in Victoria including Inverloch, Waratah Bay and Discovery Bay. They have also been recorded from many places around the world, including California, New Zealand, Hawaii and South Africa.
What are surf diatoms?
 Surf diatoms viewed under a microscope. |
Diatoms are single-celled plants with brown chloroplasts and glass-like silica walls. They are very abundant in marine and freshwaters around the world. When conditions are suitable in some sandy beach surf zones, large numbers of diatoms can accumulate in dense patches, with as many as 12,000,000cells/mL of seawater. There are only a small number of species which form patches restricted to this environment – these are known as surf diatoms.
The diatom pictured to the left, Anaulus australis, is one of the most common species of surf diatoms on Victorian coasts. These diatoms are found in the sand at night, and migrate into the water column during the day, where they attach to bubbles of surface foam formed by wave action. Because of the physical characteristics of the surf zone, the diatoms remain in this region of the beach. These accumulations are a natural phenomenon, and are not 'blooms' caused by pollution. In fact, diatoms are a major food source for prawns, fish and filter-feeding molluscs.
What should I do if I see brown patches on the beach?
If the brown patches form sticky particles or blobs, smell of oil or form an iridescent slick on the water surface, contact the EPA Pollution Watch Line.
If the water is a golden coffee colour and the foam feels gritty when you rub it between your fingers, it is most likely to be a natural accumulation of surf diatoms.