Foam on the Inlet shore

From time to time, foam appears on Pauatahanui Inlet - foam that can look worrying. Is it soap scum? Is it abnormal? Should it be reported?



While it's always possible that some sort of illegal discharge has occurred, this is now very rare, and the foam is a natural phenomenon. You always get foam where the sea waves break on the beach, or break out at sea. Usually it disperses quickly since the cause is the action of wind on water forcing more air into the water. Hence the foam and hence the rapid dispersal as the bubbles burst. However, in certain conditions the foam production can be much higher and the foam much more persistent.






Around the world, reports abound of huge foaming seas caused by very severe storms out at sea plus the presence of a variety of 'surfactants' - substances that reduce surface tension in the same way that soap does and cause bubbles to form. In Pauatahanui Inlet the most common surfactants are soluble humus compounds from natural organic material that is breaking down. This may be from material produced in the Inlet. such as decomposing algae, or brought down from the catchment of the inflowing streams. Hence you often see lines of foam marking the tidal edge where incoming freshwater meets sea water and the salt (etc.) component of sea water reacts with the humic surfactants to produce foam. This is most likely to be seen following relatively heavy rainfall in the catchment after a prolonged dry spell. A period of strong winds can also produce foam since the winds stir up the shallow Inlet and release into the surface water surfactants that are constantly being produced in the mud on the bottom of the Inlet. (Photos courtesy Alana Bowman.)





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Last Updated: 14/12/2015 12:01am