A decade of science has revealed how climate change is slowly shifting the chemistry of New Zealand's oceans, threatening the multitude of life found in our waters.
But, in a sweeping new stocktake, Kiwi scientists say there's still much uncertainty about how our marine species will fare in a warmer world.
Measured by a reduction in sea water pH, ocean acidification is mostly driven by oceans absorbing and storing the increasing load of carbon dioxide that humans are pumping into the atmosphere.
Between 1909 and 2009, New Zealand's sea-surface temperatures had warmed by a statistically significant 0.71C, while pH levels of subantarctic waters had dropped by 0.0015 units per year since 1998.
Globally, the oceans' average pH is currently 8.1, which is 0.1 lower than it was 250 years ago.
While that might not sound much, a decrease of one pH unit represented a 10-fold increase in the acidity.
The decline in pH was projected to continue in line with the increase in atmospheric CO2, leading to the most rapid decrease in ocean pH in the past 50 million years.
The effect is associated with decreases in nutrients such as nitrate and phosphate in the surface ocean, where most marine organisms live.
Even small shifts had big consequences: mussels and paua might struggle to build their carbonate shells, while some fish species could experience changes in behaviour, physiology and even habitat distribution.
Niwa scientists estimate that perhaps 25 per cent or less of the existing cold water coral locations around New Zealand will be able to sustain their growth by 2100 due to ocean acidification.
Early research on juvenile paua had shown the species grew more slowly under acidic conditions and their shells showed clear signs of being dissolved, with similar effects observed in flat oysters.
Another study suggested suitable habitat regions would shrink for many coral species in our part of the planet, although the Chatham Rise would likely remain a suitable coral habitat in a high-carbon world.
'It's got more and more complicated'
Ten years after Kiwi scientists first started exploring ocean acidification, the same experts have spelt out a pressing need for new research.
"It's important that after a decade of research, we identify where the research is going and pinpoint the knowledge gaps," explained Professor Cliff Law, a Niwa marine biogeochemist and lead author of the review, published in the New Zealand Journal of Marine and Freshwater Research.
"Ten years ago we were doing basic experiments, now we're looking at everything together - how changing temperatures, pH levels, nutrient run-off and turbidity for example, are affecting our coastal waters.
"It's got more and more complicated as it's gone on but what we know is that New Zealand waters are already exposed to ocean acidification and will be subject to further pH stress in the future."
Niwa scientists have been aided by the Munida transect time series, a 20-year record of pH measurements taken along a 65km line in the open ocean off Otago.
This, the only time series of its kind in the Southern Hemisphere, had shown the water acidifying at the same rate as CO2 levels have risen in the atmosphere.
While the paper showed species were meeting ocean acidification with a variety of responses, there had been only limited research into the resilience of marine organisms.
Current research included a large four-year, Niwa-led collaboration monitoring spots like the Firth of Thames, Karitane and Nelson bays, with experiments focused on species such as green-lipped mussels, paua and snapper.
"We want to understand whether different life stages of these key species are affected by lower pH and how other factors in the environment might influence this impact," Law said.
"Coastal waters are the most variable in their natural pH levels; they are where we get the most benefits in terms of food, recreation and other amenities, yet also where we affect the ocean most."
There was a need to better understand whether our coastal areas would grow more resilient or vulnerable, and whether measures like selective breeding of shellfish might help.
"We are looking for tools and solutions as well as conducting research to determine if there is something we can do at the local level," Law said.
"The outcome will be better models, allowing more accurate predictions of the impacts of acidification in coastal waters, as well as management options for stakeholders."
Can some of our species stand change?
Meanwhile, a Victoria University marine botanist is investigating why some New Zealand species may be able to cope more easily with ocean acidification.
Because of their highly soluble calcium carbonate skeletons, reef-building algae are widely considered to be among the species most at risk.
But Dr Christopher Cornwall challenged this idea, suggesting certain species of calcifying algae might pack the physiological machinery needed to tolerate change.
He aimed to find out whether the resilience seen in some populations of local coralline algae was due to them having evolved in more variable pH environments.
Coralline algae are ecologically important calcifying algae that create and bind together rocky reefs and act as nurseries for species important to fisheries in New Zealand and worldwide.
Our underwater kelp forests are a common habitat for coralline algae, which are exposed to large daily shifts in pH as a result of fluctuating CO2 concentrations in the surrounding seawater.
This fluctuation was created by the kelp taking up CO2 during daytime photosynthesis and releasing it at night during respiration.
The variability in sea water pH in these forests could be extreme, with pH dropping at night to levels often lower than those estimated to occur by the end of this century.
In his five-year study, supported with an $800,000 Rutherford Discovery Fellowship, Cornwall will draw on cutting-edge geochemical techniques and other measures to find the factors at play.
Cornwall also wanted to reveal whether any tolerance is maintained after successive generations in constant pH conditions.
The findings would boost our understanding of how climate change might affect rocky reefs and whether kelp forest habitats could protect resident organisms, helping us plan for shallow reef systems in years to come.