Acid Ocean

30% of the CO2 produced by human activity is absorbed by the oceans. However, this is not without consequences, one of them being the gradual acidification of seawater. One result of that is likely to be changes in the ability of shellfish and other calcium dependent marine life to accumulate sufficient calcium into their structures to sustain current numbers and diversity. University of Otago and NIWA are two of several organisations that have research projects under way to monitor the fall in seawater pH around NZ and predict its effects on inshore fisheries and marine farming. A lot more needs to be done to determine the extent of the risk and successful responses to it.

Since the industrial revolution, around 30% of the CO2 emissions from human activity have been absorbed by the oceans and as a consequence there has been an estimated 0.1 unit drop in pH, which translates to a 30% increase in hydrogen ions. Scientists estimate another 0.1 unit drop by mid-century and a further 0.1 to 0.3 by the end of the century.

The increased number of hydrogen ions can tie up some of the carbonate dissolved in seawater, making it more difficult for marine organisms that are dependent on carbonate, such as calcifying algae, mussels, oysters, kina and paua. These organisms use the free carbonate to make their shells and skeletons, but the acidic hydrogen ions reduce the amount available. That may mean a slower growth rate and thinner shells. Other effects may be a lower fertility rate for shellfish, and there may be physiological effects on other fish.

Acidification will proceed faster in the colder waters of the polar and sub-polar regions, and so New Zealand is expected to be one of the first countries impacted by the more acidic oceans.

Dr Christina McGraw, a post-doctoral research fellow at the University of Otago, says that the chemistry of ocean acidification is understood fairly well, and she is developing instruments with NIWA to facilitate lab testing of the effects of pH drop and fluctuating pH on calcification.

“One of the limiting factors at the moment is finding good ways to study these organisms in the lab under conditions that accurately reflect what we expect in the future, so that we can get some reasonable predictions of the effects on shellfish and coralline algae,” she says.

“Some interesting research of this type has been done in Sydney where they are looking at the growth of rock oysters under conditions that they expect by the end of the century, and have seen decreases in reproduction and growth rates and increases in the number of abnormalities.”

Dr McGraw has designed automated equipment in which marine organisms can be grown under conditions that can easily be varied to mimic, say, tidal cycles or predetermined variations in water pH.

Of particular interest are the coralline algae that cover around 80% of the shoreline rocks in Otago and Fiordland, and are present all around NZ and in fact throughout the world. They are a keystone species of calcifying algae, which releases chemicals that are “settlement cues” for larvae of paua, oysters and the like. The larvae detect the chemicals, settle on the coralline algae and grow to maturity.

Prof. Catriona Hurd, a biologist at Otago University, says that ocean acidification could amount a double hit for shellfish.

“If there is a reduction in coralline algae shellfish larvae will lose settlement sites, and those that do settle will have a harder time calcifying. So in the medium term it could affect the harvest of mussels, oysters and other edible species,” she says.

“We don’t yet know whether there is any reduction in coralline algae, but predictions for the Southern Ocean indicate it could happen in perhaps 20 years in the colder waters of New Zealand. We are hopeful that the algae and some of the shellfish may be able to adapt in some way, and some of these algae evolved in quite high concentrations of CO2 in seawater so they may have mechanisms by which they can change the calcification rates, but the rate of change today is 100 times what they have seen before.

Prof. Hurd says that some work is being done in New Zealand looking at the larval and adult phases of shellfish to see whether they are susceptible to pH change, and at what happens at fixed or fluctuating lower pHs.

“Over a ten-year period you may see a pattern developing, as they have done with the coral reefs on the Great Barrier Reef. Monitoring the corallines is key because they are very sensitive, like the canary in the mine,” says Prof. Hurd.

“They are made of high-magnesium calcite which is the most soluble form of calcium carbonate and so they are a good indicator of what might well be happening to other species.”

Seaweeds are another factor in the complex equation. Increased availability of CO2 is likely to increase their growth, and their fragments are an important source of food for many marine species including oysters. Whether or not it will impact oyster growth is just one of the many unknowns of ocean acidification.

While the effects of acidification do not appear to be an immediate threat to aquaculture, the industry should start thinking about ways in which it can minimise any harm so they will be better placed to deal with it in the future.

Says Dr McGraw: “We need to get a better idea of how the pH varies in different parts of the coast because the effect isn’t going to be the same all around the country. When choosing a new location for aquaculture it would be good to consider ocean acidification and perhaps choose areas where the impact is likely to be smaller.”

Prof. Hurd agrees: “There may be changes in species, species dominance or other changes that we can’t predict and we don’t know whether there will be change in the overall productivity of the system in terms of farmed shellfish. The message for marine farmers is that change is happening, and we need to work on determining its extent and how to adapt to it.”

Christina notes further: “Land-based farming will very likely be impacted by climate change. According to the Intergovernmental Panel on Climate Change, the east coast of New Zealand will be hit by more frequent and intense droughts. In the South Island, there will be less run off from the Southern Alps during the summer months, which will result in less water for irrigation.”