Unsightly but harmless algae at Caroline Bay

Algae at Caroline Bay, Timaru

Image of diatom (Attheya armatus) causing foam and brown staining on Caroline Bay

A harmless plant plankton species with a brown scummy appearance has been turning up at Caroline Bay in Timaru recently.

The naturally occurring algae, which some people mistake for contamination, is a surf diatom and creates brownish foam on the waves and on the beach, particularly after swell events and as the tide recedes. 

 Surface water team leader Shirley Hayward said members of the public often had questions about the brown substance, which appears in Caroline Bay a few times each year.

 “Although it’s harmless, the diatom bloom can look unsightly, and we do often get queries about the state of the beach. 

 “The primary causes of surf diatom blooms are wave action pounding on the beach releasing nutrients that promote bloom development.  Wave action also traps air and creates foam, leaving bubbles and stains on the beach.” 

Find out more 

For more information on surf diatoms, read NIWA's article The forces behind beach foam.

Or, for the latest water quality information at Caroline Bay, visit Land, Air, Water Aotearoa.