These diatoms are small marine plants that have a silica casing (and are therefore heavier than water). The diatoms mainly live in the sand but use the foam in the surf zone as a means to stay afloat and spread. As the foam washes ashore, the diatoms slide off and/or the bubbles of foam burst leaving a characteristic ring pattern in the sand. Large accumulations of diatoms are naturally occurring and the diatom themselves are a major food source for prawns, fish and filter-feeding molluscs (such as pipis).
The EPA (see link below) suggest these "Blooms" of surf diatoms may be related to weather conditions, particularly onshore winds. Similar blooms on the pacific coast of USA are linked to upwelling in cold water currents (the suggestion is that the change of temperature precipates iron from the sea water and feeds the bloom).
So a nice side observation project for us is to record when the blooms are visible and how extensive they are, also the wind conditions and temperature (hot, warm, cool, cold, very cold is probably sufficient for now, but there is a link to the Pound Creek weather station over on the right-hand column of this blog). The most important observation is to distinguish them from other sludge/pollutants such as oil spills, untreated sewerage or other effluents.
- They have a distinct golden coffee colour
- They do not smell (of oil fumes or in fact any distinct smell)
- No iridescent (rainbow-coloured) slick on the water surface
- They are not sticky and if you collect them just on the stained surf foam it feels gritty
- They do not form blobs or lumps in the sand
Much of this information comes from the EPA's Surf Diatom Page