Dave Allen

Acid seas

The chemistry of seawater is changing, becoming more acidic, and this transformation is most profound along our coastlines. In this delicate borderland between land and sea, some places are experiencing a surge in acidity, peaking at levels that the open ocean isn’t predicted to reach until the end of this century. What does this mean for marine life?

Written by       Photographed by Dave Allen



Whenever a strong wind blew in from the sea, Lucy Tukua’s grandfather would get a trailer and head to Kaiaua to fill it up with horse mussels that had washed up on the beach. The ocean always provided enough food for him to distribute to whānau all the way up to Auckland.

“Today, we don’t have those same experiences with our children,” says Tukua.

We’re preparing to launch a boat from Kawakawa Bay, where the firth opens into the Hauraki Gulf, Tikapa Moana. It’s a week after the Tasman Tempest dumped a month’s worth of rain on the top of the North Island in a day. The water is calm, but even more turbid than usual. The small jetty is deserted, as is the boat club a hundred metres down the coastal road. Low-lying cloud flattens the view across to Pakihi Island and Waiheke in the distant haze.

Our small aluminium boat, the Inanga, takes course towards the inner Firth of Thames, past two headlands and along its western shore to a group of mussel-farm buoys.

From each buoy hang dropper ropes laden with growing Greenshell mussels. One rope also carries a steel frame festooned with scientific instruments, submerged six metres deep to monitor changes in the water’s temperature, salinity and acidity. The latter will rise dramatically over autumn and winter.

Lucy Tukua seems completely at home at sea, stroking the water’s surface and taking in the breeze. She shares stories of her iwi Ngāti Paoa’s war canoe and its voyages across the gulf, and speaks of the firth itself as a pātaka kai, or food basket.

“To think that my grandson will potentially never taste a horse mussel is pretty sad,” she says.

Tukua has come to see for herself what causes such marked changes in acidity, and what this means for the firth and the kai moana it produces. On behalf of her iwi, Ngāti Paoa, she’s an advocate for the environment—in 2016, she helped to develop a marine spatial plan for the Hauraki Gulf—and she’s acutely aware of what may be lost in her lifetime.

“I learned about ocean acidification, and straight away I felt sad for the fact that outside the front of my marae, at the bottom end of Tikapa Moana, this issue is most prominent,” she says. “As a seafaring people we want to undertake our own research projects … and one way of doing that is by getting our butts wet and protecting what’s there.”

The most dramatic changes in carbon chemistry and acidity levels take place in coastal seas. Ngāti Paoa is part of a nationwide project to monitor the changes along New Zealand’s coastlines: CARIM, or Coastal Acidification: Rate, Impacts and Management. It’s investigating how changes in acidity and carbonate chemistry affect ecosystems, and it’s particularly focused on three iconic species: paua, Greenshell mussels and snapper.

Like a thermometer for a patient, a CTD profiler is a cage of instruments that measures conductivity, temperature and depth to determine contributing factors of ocean acidification, and collect samples for laboratory comparison.

CARIM began about six years ago following a visit from a group of American oyster farmers. Mass die-offs of oyster larvae in hatcheries along the coast of the Pacific Northwest had stumped aquaculture farmers and scientists. Tests for various pathogens came back negative. Finally, the cause was identified: ocean acidification.

“Their coast is affected by upwelling waters that are very low in oxygen and high in carbon dioxide, so the pH is very low,” says Cliff Law, a marine scientist at the National Institute of Water and Atmospheric Research (NIWA) and the programme leader for CARIM. “Combined with the input of anthropogenic carbon dioxide from the atmosphere, their coastal waters hit a threshold where the oyster larvae could no longer grow.”

Land, air and water meet at the coast, and here, the ocean absorbs much of what runs off the land or has been released into the air. The ocean soaks up more than a third of the carbon dioxide produced by human activities, and as the proportion of carbon dioxide in the atmosphere increases, the water becomes more acidic. Along many stretches of New Zealand’s coastline, the ocean swallows nutrients from fertiliser that washes off the land, and this boosts the growth of algae, which in turn adds more carbon to the water. Even heavy rainfall contributes, because fresher water is more acidic.

In shallow, warm, coastal waters, these factors combine to raise acidification to levels that the open ocean isn’t predicted to reach until the end of the century. In these same waters live many of the fish and shellfish we like to eat and export.

But marine creatures in coastal environments already manage natural fluctuations in acidity across days and seasons. Seaweeds fix carbon dioxide during the day and release it at night, while algae bloom in the spring and die in autumn. Because of this, coastal organisms may have evolutionary reservoirs for adapting to increased acidification.

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The longest-running record of ocean acidification in the Southern Hemisphere is called the Munida time series, after the boat that first sailed out of Dunedin Harbour to follow a 65-mile transect of ocean. That voyage has been made every two months for almost two decades.

The keeper of the data is NIWA marine chemist Kim Currie, who was embarking on her doctoral project when the series began. Its goal: to discover whether the Southern Ocean is a sink or a source of carbon dioxide.

“We knew then that the oceans play an important part in taking up carbon dioxide from the atmosphere,” she says. “But it wasn’t until about 10 years ago that ocean acidification emerged as one of the consequences of this process.”

Since then, Currie’s research has taken her on a scientific and personal journey. The better she understood the complexities of climate change, the more she looked for ways of eliminating her own contribution. With her partner, Bill, she built an off-grid, low-carbon home in Sawyers Bay near Port Chalmers, made of heat-retaining slabs of concrete and powered by a single-bladed wind turbine Bill designed and commercialised.

To get a better idea of how ocean acidification plays out along different parts of the coastline, Currie set up a nationwide network of 14 coastal monitoring stations. The New Zealand Ocean Acidification Observing Network tracks pH as well as the amount of carbonate in the water.

The network includes long-standing data collection sites, such as those maintained by regional councils, but Currie has also found willing participants among remote fishermen, who regularly post water samples from the Chatham Islands or the West Coast to her lab at the University of Otago.

Of the 14 sites, three are under scrutiny by CARIM: Tasman Bay, with its expanding aquaculture industry, Karitane, just north of Dunedin, which has a taiapure in place to protect pāua stocks and the Firth of Thames, a marine habitat which has been changed dramatically by human activities.

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A century ago, dense mussel beds covered 500 square kilometres of the firth’s muddy seabed. In the 1920s, a dredge fishery began rolling up these biological carpets. The annual mussel take peaked at 2800 tonnes in the late 1950s, and then it crashed. The dredges returned 180 tonnes in 1965, and nothing in 1969.

Mussel farms took over, but the natural beds have shown no sign of recovery, even decades later. Before the fishery began, mussels were the firth’s cleaning crew, with each one filtering the equivalent of three bathtubs of water a day.

It’s not the only problem for the firth, which lies like an appendix to the south of the Hauraki Gulf. Enclosed on three sides, it is a repository for everything that flows off the land surrounding it. Sediment runs into the sea, smothering any regenerating mussel reefs. Heavy metals such as copper, lead and zinc frequently exceed health guidelines in the south-eastern corner of the firth. Dairy farms, which have some of the highest stocking rates in the country, suffuse the coastal waters with nutrients.

On the coast of Karitane, Otago, Kim Currie (right) and Judith Murdoch analyse levels of nutrients, chlorophyll and the acidity of seawater on board NIWA’s Beryl Brewin. The same process is also conducted in the Firth of Thames and Tasman Bay.

Acidification arrives on top of all these other stresses, says Emily Frost, a doctoral student in marine science at the University of Auckland. She leads the fieldwork for CARIM in the firth, retrieving data from the array of instruments suspended from the mussel buoys.

Her passion for science is clear from the moment I first meet her. A tattoo on her wrist depicts the chemical structures of two of the four main building blocks of DNA.

“I’m a Winston triplet,” she tells me later, referring to the British IVF expert who helped her parents to have a family. By the time she and her sisters were eight, the Frosts left the metropolitan buzz of London for the serenity of Lake Tekapo, where her Dad took on the role of resident superintendent at the Mount John Observatory. Living in New Zealand reinforced her love of the oceans.

Out on the boat, as we haul the steel frame on board, the extent of the excess nutrients that have flooded the firth becomes obvious. The gear is completely encrusted by a thick coat of tube-worm casings. “Biofouling is a huge problem,” says Frost. “We have to retrieve the instruments more often than in other places, because they get so overgrown that it could affect the data collection.”

She scrapes off tube worms and little crabs, reciting a list of organisms she’s previously found growing on the science kit: barnacles, algae, bryozoa, small crustaceans.

Frost’s CARIM team has been working for a year, and the results confirm a trend seen in a 20-year record of ocean-monitoring data from the firth, collected by John Zeldis at NIWA. “What we have seen is an enrichment of the firth,” he says. “By that I mean an increase in stocks of nutrients, primarily nitrate and phosphorous, occurring through the 2000s.”

Bill Dickson hauls in a ‘rosette’ of instruments that are part of the Munida time series—a transect of measurements from coastal, subtropical and subantarctic currents within 65 kilometres of Otago Harbour. It has been sampled every two months since 1998, making it the southern hemisphere’s longest running record of pH measurements.

Agriculture is the source of 70 per cent of the added nitrogen, which drives blooms of phytoplankton, feeding the mussels in aquaculture but also adding carbon to the system.

“The firth is well known as an area that grows mussels very well. But the other consequence of this productivity is that in spring, you’ll see the growth of phytoplankton. When it dies back in autumn and sinks in the water column, it decomposes, basically composting.”

These seasonal changes are behind the wild oscillations in pH, which can mean several weeks of high acidity levels in winter.

Frost has seen the consequences of the firth’s acidification in her laboratory experiments with kina larvae. They need carbonate to construct their shells, and so they struggle to grow in more acidic waters. Now, she’s figuring out if pāua and Greenshell mussels are as sensitive as kina, or more robust.

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If you follow the coastal road after State Highway 6 takes an inland turn towards Hira, just north of Nelson, a series of ponds announces Cawthron Institute’s aquaculture park. A shallow bund separates the ponds from the sea, and a pair of pied stilts nest on the narrow strip of land.

The ponds concentrate phytoplankton, which is fed to Greenshell mussels in experimental tanks nearby. I’ve arrived on the one quiet day separating two large experiments. Yesterday, Norman Ragg’s team spawned several lines of mussels that had been kept at different acidity levels. Tomorrow, exactly 42 hours after spawning, the team will begin to analyse whether the environment of the parent mussel has affected its offspring.

Greenshell mussels are endemic to New Zealand, and grown in aquaculture for export and the domestic market. Ragg says at first mussels were selectively bred for faster growth, better meat yield and stronger shells. After that, a suite of other characteristics became important—flavour, appearance, and the ability to remain productive in the face of different stresses.

Ocean acidity is one stress.

The seawater flowing through the industry’s mussel-breeding tanks is filtered to return its acidity to pre-industrial levels, because this makes mussel larvae perform better.

A few months ago, Ragg’s team completed a trial in which they randomly crossed mussels from several families and then tested the offspring under extreme pH conditions, to see if any turned out to be more robust, and if those that did had a genetic advantage.

“We made 96 new families,” says Ragg. “The results show a huge range of variation between resilient families and susceptible families—and a lot of that is heritable. So it looks like they do have the potential to adapt and evolve. The parents of these new families are now undergoing more research to look at the specific mechanisms behind that.”

Inside the laboratory, the adult mussels spawned yesterday have been kept in one of three acidity conditions for two months: normal seawater, or a future scenario where the atmosphere has 850 parts per million of carbon dioxide, and a third scenario representing the “aggressive acidification” that would happen at 1100 parts per million of carbon dioxide. The larvae were now developing in replicate tanks under the same conditions as their parent generation, ready to be analysed at different time intervals.

“We really want to know how long-term exposure affects the adults’ reproduction and if they give any resilience to the offspring.”


A successful larva is one that reaches a development stage known as D-shell within the first 42 hours of its life. Preliminary results suggest that larvae from mussels raised in normal conditions or at 850 parts per million do well when raised in normal seawater, but the offspring of parents raised at 1100 parts per million are slightly compromised.

But if the larvae are raised in acidified seawater, those with parents that spent two months in higher acidity did better than those from normal conditions.

“We can use that to investigate what resilience is present in the species,” says Ragg. “If we didn’t see much variability, that would be challenging for the species because it would suggest that there isn’t much ability to adapt, but what the preliminary analysis suggests is quite the opposite.”

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It could be a different story for pāua. About 20 years ago, elders at Puketeraki marae in Karitane, the centre of Ngāi Tahu hapu Kāti Huirapa, were concerned about pāua stocks along their stretch of the East Otago coast. They called for a temporary halt to harvests to allow time for stocks to recover. Eventually, in 1999, a taiapure was put in place in recognition of the area’s customary significance. Brendan Flack, who is part of the local community fisheries management group, says the decline of pāua was initially blamed on overfishing—but increasingly people realise that isn’t the full story.

“Pāua are part of a much bigger ecosystem, and it’s not just the overfishing that’s causing the depletion but also things that happen in the ocean and on the land—because they are in this mixing zone on the coast.”

At the top of Puketeraki, we look across to Huriawa Peninsula, where pāua is off limits, and to Taiaroa Head at the tip of the Otago Peninsula. The community has joined forces with oceanography researchers from the University of Otago to study kelp forests along the coast and to carry out pāua and habitat surveys.

“We understand our coast, but we’re still seeing a decline in our pāua,” says Flack. “This community has done as much as a community could do to stem the flow but I think we’re just slowing down the demise.

“We’ve lost a lot of pāua since we started doing those surveys. I don’t think we’re alone, I think it’s happening up and down the coast, but we’re the one place where we actually have a history of these surveys so we can see that decline despite the reduced bag limits.”

NIWA marine ecologist Vonda Cummings has been putting pāua adults and larvae through an acid test, checking shell development and tracking respiration rates to see if they have to work harder to grow, as well as scouring areas of their DNA for any changes.

She says the biggest challenge in the lifecycle of marine molluscs—clams and snails—is metamorphosis, the brief window in which they transform from soft-bodied larvae to shelled juveniles. Oysters, for example, have to form their shell within the first 48 hours of their life.

“The transition is quite complex—and it has to be successful,” says University of Otago marine biologist Miles Lamare, who studies this moment of transformation. “It’s a fast process and it determines where they’ll settle down for the rest of their lives. It’s important that they choose the right place.”

Marine ecologist Vonda Cummings cleans a paua tank at NIWA’s ocean acidification facility at Greta Point, Wellington, where scientists are examining the influence of seawater acidity on adult paua, and the survival and development of their offspring.

Lamare is testing how coastal acidification could affect the settlement process for marine organisms. Acidification delays development, he says, but if larvae stay floating in the water column for longer, they could become less selective. In addition, the changing chemistry of the water could be influencing the ability of the larvae to decipher chemical clues about the best spots to settle down. Will they still recognise their favourite substrate?

“Metamorphosis in marine species is a very quick process, and it has to be. It’s a very dangerous time in their lives; they come out of the water column and they’re living in a completely different environment. Suddenly they are on the seafloor, in a different ecology, feeding on new things, with lots of new predators around. It needs to happen quickly, otherwise their chances of survival are slim.”

Greenshell mussels are intertidal and have a broad geographic range, so they have long been coping with different habitat conditions and are likely to be more flexible. Other species, including pāua, can be extremely selective, and depend on chemical clues to trigger metamorphosis. Preferred habitat for pāua is a turf or knob of coralline algae, marine plants that act as master builders on the seafloor. Coralline algae grow in branched structures, lie down in a dense carpet, or form calcified crusts or free-rolling rocks. Some of them produce chemicals that prompt pāua and other larvae to settle and develop quickly. But coralline algae also rely on carbonate as a building block in order to grow.


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The food web in the ocean begins with phytoplankton, a complex assemblage of single-celled plants that produce 50 per cent of the oxygen we breathe. It photosynthesises, meaning that extra carbon dioxide boosts its growth—but some phytoplankton species build shells, and some of those need carbonate.

CARIM programme leader Cliff Law wants to know what will happen to phytoplankton if the ocean continues to acidify, and to answer this question he has set up a mesocosm outside NIWA’s wet lab in Wellington: nine giant buckets full of seawater.

Some buckets represent the future, while others mimic the present. Each experimental run takes 20 days, during which time the team collects more than a hundred different measurements each day—from the chemical balance of the seawater to the biological composition of the plankton within it.

“It’s like looking after a baby,” he says.As for the effects of increased acidification—pH at the level expected at the end of the century—the news is fairly good, he says.

“We’re not seeing any major changes in terms of the biomass of the phytoplankton and bacteria, or the amount of particulate carbon,” he says. “What we do see are changes in the composition of the phytoplankton community itself, changes in groups and species. In a nutrient-depleted experiment, we saw differences in the groups of dinoflagellates in low pH. In the second experiment, where nutrients were available throughout, coccolithophores—phytoplankton cells that surround themselves with microscopic plates—disappeared in the low-pH treatment. This is what you’d expect. They have carbonate shells, and like many other organisms they find it more difficult to maintain the structure in low-pH conditions.”

But will these changes in species composition alter the nutritional value of phytoplankton for those further up the chain—including the larval stages of commercial fish species?

In a bid to model the acid seas of the future, NIWA have constructed nine time machines at the Greta Point facility. Each 4000-litre seawater tank—or ‘mesocosm’—simulates projected levels of acidity so that scientists can accurately monitor the effect on water chemistry and plankton communities.

Adult fish can regulate acidity in their blood and tissues, while embryos and larvae also seem relatively resilient. Some studies showed minor or no impacts in many species, while others identified some negative effects on growth and metabolism, albeit at very high carbon dioxide concentrations.

Fish were thought to tolerate changes in seawater acidity—until about a decade ago, when scientists noticed some odd behaviours in reef fish. They became risk takers, swimming straight towards danger, then became lost. Acidification was affecting their brains—or more specifically, a chemical messenger in the brain called GABA, which serves to dampen excitable nerve cells. The GABA disruption muddled the fish’s sense of smell, hearing and vision, reducing survival rates.

But are reef fish the only ones affected? NIWA ecologist Darren Parsons wonders if broadcast-spawning pelagic fish might be similarly incapacitated. In 2017, he began studying yellowtail kingfish, and this summer, his focus will turn to snapper.

Like pāua, snapper are most vulnerable during their transition from floating larvae to semi-transparent juvenile fish, as they look for nursery grounds near the seabed.

“They go to the seabed in inshore shallow areas, looking for seagrass, horse mussels or sponges in sheltered estuaries,” he says. “A lot of their habitats are biogenic in structure. This stage is a bottleneck, and trying to understand all of these effects is potentially more important than any direct effects of ocean acidification.”

Cliff Law agrees. “The real benefit of this project is that a lot of the work that’s been done previously has been done in the laboratory and only on single species, whereas CARIM is looking at ecosystem interactions. We’re focusing on coastal regions because they are the waters that we use a lot and they are under additional acidification stress. We want to understand the factors that drive this because we can manage some of that. If nutrients are an important factor for changing pH in coastal waters, then it’s possible to control that through land-management practices and to mitigate some of the effects of coastal acidification.”

For Mary Sewell, whose research at the University of Auckland has highlighted the effects of climate change on the marine environment, the tension between land and ocean use is reaching a crunch point. “The government wants to push intensification of farming but at the same time it wants to grow aquaculture into a billion-dollar industry. These things are incompatible with each other because, for example in the Firth of Thames, we cannot further intensify agriculture without having an impact on the acidity of the waters offshore. The land and the ocean are connected.”

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