Scum on our beaches
The brown scum seen on occasion on the beaches around Dunedin is a naturally occurring phenomenon common to most beaches around New Zealand at some time or another.
It may look unsightly, and may even smell bad, but it is not raw sewage. The scum first appears as a yellow, or brownish foam in the surf. This foam is stranded on the beach in more condensed lines or clumps as the tide recedes. When examined microscopically the scum contains unicellular algae such as diatoms and dinoflagellates, along with tiny particles of larger decaying algae (seaweed). Also in the mix are tiny crustaceans and a variety of body parts of other small planktonic animals, all bound together in a mucus film.
In addition to these organic components tiny particles of sand and finer sediment that have been caught up in the foam are also present. Chemical analysis of similar scums from beaches around the world reveals the presence of polysaccharides (the main component of the mucus) and proteins, both of which are released as algae die and break down.
The foam usually appears during times of heavy surf (see pictures below) and may be more common in the summer when algal growth is at its peak. The runoff of nutrients, such as fertiliser, from land may exacerbate the problem by encouraging algae to flourish in near-shore waters. When algae are present in high enough densities the water may become discoloured and manifest themselves as a "bloom". The algae eventually die off as conditions become less favourable and their breakdown products are whipped into foam by the action of the surf. The foam, in turn, traps other debris and forms the familiar brown scum that gets left on the beach.
These scums are not to be confused with toxic the algal blooms that may occur in warmer waters further to the north.
When does the scum appear?
The foam usually appears when there is heavy surf and is more common in summer when algal growth is at its peak. The runoff of nutrients, such as fertiliser, from land may worsen the problem by encouraging algae to flourish in near-shore waters.
When enough algae are present, they may discolour the water and manifest themselves as a "bloom". The algae die off as conditions become less favourable and their breakdown products are whipped into foam by the action of the surf. The foam, in turn, traps other debris and forms the familiar brown scum that gets left on the beach.
Scum Test
The DCC has had samples of the scum tested for faecal contamination in various occasions in the past, usually at times when concerns have been voiced by members of the public. These tests usually show very low concentrations of bacteria in the scum.
On 21st April 2009, an extremely concentrated sample of scum was collected and tested. Bacterial levels in the sample reached 360 MPN/100ml (Most probable number/100ml).
The water the scum was floating on, however, returned a count of just 10 MPN/100ml. If the water had been contaminated by sewage, one would have expected an MPN in the order of hundreds or thousands. The higher number for the scum will be attributable to the fact the foam traps small particles, including bacteria, concentrating them into a relatively small volume floating on the waters surface.
A comparable scum of sewage would have an MPN in the tens of thousands.
Last reviewed: 21 May 2009 4:51pm
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