Red sea mystery off Hawke's Bay coast

The sea seen during Art Deco weekend. The discoloured water is now being tested as a possible algal bloom off the coast of Napier. Photo / Ian Cooper

The Hawke's Bay Regional Council is taking tests and awaiting results from a likely algal bloom spreading across the sea off the Hawke's Bay coast.

Coastwatchers say the reddish-brown discolouration has been growing over several weeks – visitors to the Bluff Hill lookout reported what may have been its early signs on January 31, when it appeared to be coming from a ship apparently circling at sea off the Port of Napier.

It was clearly visible from the Marine Parade beach frontage during Art Deco weekend.

Regional Council team leader Marine and Coast Anna Madarasz-Smith told Hawke's Bay Today on Monday staff had been collecting samples for the last couple of days.

"They haven't yet given us a definitive answer, but it certainly looks like an algal bloom although the samples we have taken have not shown a huge amount," she said.

If it remained by morning staff would be "heading out again" for more testing on Tuesday.

In May 2018 samples analysed by the Cawthron Institute identified what the council said at the time were "very high" levels of the algal species Alexandrium catenella.

It was described as a "dinoflagellate species" that produced toxins capable of causing paralytic shellfish poisoning when affected shellfish were eaten. Sampling at that time confirmed warnings that had been issued by Ministry for Primary Industries (MPI) against consuming shellfish from the Bay.

Such blooms were often seen after periods of heavy rain, Madarasz-Smith, said at the time, adding: "As the freshwater moves off the coast, it pulls deeper water up which can feed algal blooms."

The Ministry for Primary Industries has no current warnings in place for algal blooms in the area.