Red algal bloom at Waipū Cove reappears, origin baffles locals

Dave Johnston and son-in-law Raynard Van Zyl with dog Luma who avoids the water when the algal bloom is present. Photo / Michael Cunningham

The reappearance of thick crimson algal bloom carpeting Waipū Cove seems to have become the norm with locals but its origins still leaves many baffled.

What's more, Northland Regional Council (NRC) experts are not entirely sure themselves.

The east coast beach, south of Whangārei, is again a mass of the red seaweed, believed to be called Spyridia filamentosa, after increasingly being plagued by its presence over the past decade. It is particularly prominent after storms. As the tide recedes, it becomes trapped, stagnates, rots then smells in the sun.

In March, the algal bloom became trapped in the nearby estuary, depleting oxygen levels, which led to hundreds of thousands of marine life deaths, threatening a wildlife refuge within. The ensuing putrid stench, described as rotten eggs and dead bodies, drove nearby residents from their homes.

Eels that have beached themselves to escape their water. Photo / David Lourie

However, this week's deposit is not thought to have reached the estuary, which had subsequently flushed out and replenished itself, but the layer currently blanketing Waipū Cove is almost half a metre deep in places.

Waipū Cove resident of six years Raynard Van Zyl was walking his dog Luma with visiting father-in-law Dave Johnson, this week, and said Luma instinctively avoided the ocean when the algal bloom is present.

"Usually I can't keep him out of the water but he won't go near it."

Van Zyl said he often fished off the beach or nearby rocks but he had driven to Ruakākā to fish for most of the summer.

"Fishing off the beach is a no-go because every time you reel in, you get a whole lot of red seaweed."

He had recently witnessed a surfer brave the ocean only to emerge with the red seaweed throughout his long blond hair and covering his wetsuit.

The thick crimson algal bloom carpeting Waipū Cove this week. Photo / Michael Cunningham

"It's amazing how quickly it appears – I'll be down here and it's just a nice white sandy beach and when I come back, it's like this within 12 hours."

Van Zyl was unsure of its cause and disagreed with a broadly held view that it arrived in ballast water discharged from international ships.

NRC coastal and water quality field operations manager Ricky Eyre and coastal resource scientist Richie Griffiths confirmed this was not the case. They said numerous samples had been sent to Niwa for identification and only native species had been identified.

A 2018 Niwa report identified Spyridia filamentosa as native to northern New Zealand but widely distributed in warm to tropical waters worldwide. The New Zealand specimens that have been studied are most closely related to strains also known from Hawaii, Western Australia and Japan.

"Spyridia filamentosa has been washed ashore in very large quantities at sites in Northland, the Hauraki Gulf, Manukau Harbour and Coromandel. In some cases, these accumulations break up quickly, but in other situations the accumulations persist, and are resuspended by tides and then deposited again in following tidal cycles," the report stated.

Said Eyre: "Though we have previously engaged Niwa to investigate the causes of the nuisance algae events at Waipū, we are still not fully clear on the full range and/or combination of conditions to cause the algae to get washed up in nuisance. It's likely a combination of tide and winds."

A NIWA report concluded natural processes such as local currents, tides, and weather and storm activities likely result in the algae being washed ashore. Photo / Michael Cunningham

New Zealand northern subtropical and Tasman Sea waters typically have a classical cycle of spring and autumn algae blooms. However, NRC records indicate the algae is present in Bream Bay throughout spring, summer and autumn.

"We are not surprised there is still algae in the water given the recent warm wet weather we have experienced."

Eyre said that when NRC receive complaints, in most cases they send an officer to investigate.

"This may include making water quality measurements and collecting algae samples for analysis by Niwa."

A fish that died gasping for breath. Photo / David Lourie

In March, the Waipū estuary lagoon was described as a "dead zone". What remained alive was said to be beaching themselves, gasping for breath in an attempt to escape the black oxygen-depleted waters of their environment from the trapped and rotting algal bloom. The smell was created by a gas called hydrogen sulphide generated by the break-down of organic matter.

A wildlife refuge situated within the affected area, an integral feeding zone for the protected bird species, was under threat from the lack of food source.

None of samples sent to Niwa were found to be toxic or harmful and Eyre said the NRC were unaware of any algae entering the estuary this time.

Decaying algal bloom in the southern lagoon of the Waipū river mouth Wildlife Refuge killed off hundreds and thousands of small fish and other marine organisms in March.

"As far as we know there is still some algae remaining in the estuary but it is currently not causing a nuisance.

"We made regular visits to the estuary after the March event and dissolved oxygen levels returned to normal relatively quickly. The black colouration of the water and the unpleasant odour also dissipated relatively quickly. Staff observed the rapid return of bait fish to the affected area as soon as oxygen levels returned to normal levels."

Waipū local David Lourie took this photo at Waipū Cove yesterday morning.