Scientists zap dangerous caulerpa seaweed with ultraviolet light

It's used to make drinking water safe, zap bugs, and reveal blood spatters at crime scenes. Could ultraviolet light also help to save New Zealand's kaimoana and "boatie" way of life?

There are promising signs for UV to play a role in containing the pest seaweed caulerpa that is spreading rapidly, choking the life out of seafloor habitats around the northeast coast of the upper North Island.

Diving down seven to eight metres at Rakino Island in the Hauraki Gulf, scientists from Waipapa Taumata Rau / University of Auckland have successfully trialled an experimental unit that blasts the seaweed with intense ultraviolet light.

Most of the caulerpa in five small test areas was rapidly destroyed, with follow-up treatments targeting regrowth from fragments of plant surviving beneath the sediment.

"Invasive exotic caulerpa is one of the biggest threats to our marine life, smothering everything in its path, so it's exciting to have another potential tool to destroy it," says Liz Brooks, Team Manager Pathways and Marine at Auckland Council, which invested around $26,500 in the experiment.

In the right conditions, a piece of caulerpa spreads quickly, creating a thick mat and suffocating all forms of life.

"The field tests were in line with our lab results, so the next step would be testing at a much larger scale to confirm it is good for controlling this difficult seaweed," says Professor Andrew Jeffs, of the University's Institute of Marine Science.

Since arriving in the upper North Island in 2021 caulerpa has rapidly spread to cover more than 1,500 hectares of seabed.

The pest seaweed has spread from Aotea / Great Barrier Island to places including Ahuahu / Great Mercury Island, the Mokohinau Islands, Kawau Island, Waiheke Island, Te Rāwhiti Inlet in Northland, Fantail Bay in the Coromandel Peninsula, and not far from the Leigh Marine Reserve.

This year, Biosecurity NZ concluded it would not be feasible to eradicate caulerpa, advising long-term management through containment, suppression and local elimination.

At the test site, a single dose of ultraviolet light was found to kill over 90 per cent of Caulerpa parvifolia, the type of caulerpa identified in the lab as seeming most vulnerable to the treatment. The other caulerpa species causing problems, Caulerpa brachypus, is also vulnerable but wasn't included in the experiment.

Ultraviolet light has some potential advantages over the limited methods currently available for controlling Caulerpa, such as suction dredging, including minimal disturbance of the seafloor and avoiding fragmentation of the seaweed, which can lead to the spread of the pest.

The experimental treatment unit was imported from the United States, where high power ultraviolet light has been used successfully for the treatment of lakeweed. Three parallel ultraviolet lamps in a plastic frame are powered from the surface via cables.

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