Ocean acidification

What is ocean acidification?

Ocean acidification is the term given to the reduction in pH of seawater caused by the absorbtion of carbon dioxide (CO2) from the atmosphere.

The acidity of a liquid is described by the term pH which ranges in value from one to 14; where one is highly acid and 14 highly alkaline. Pure water has a pH of seven – neutral. Seawater pH varies naturally around the world and over annual cycles but is usually in the region of pH 7.9 to 8.4; that is, slightly alkaline. This range is created through a balance between biological activity and seawater chemistry. As far as we can tell seawater pH has been in this range for at least the past 20 million years.

When CO2 dissolves in seawater it reacts with water to produce a weak acid (carbonic acid). This lowers the pH. Thus ocean acidification is the decrease in the pH of seawater – hence the definition above. But, confusingly, because seawater is naturally a weak alkali (as noted above), strictly speaking the process should be called a reduction in alkalinity because the ocean pH is highly unlikely to become less than seven, that is an acid.

Whilst accurate, this is rather pedantic and it is simpler to use “ocean acidification” because this relates to the cause. Whilst the chemistry involved (and the basic biological effects) of ocean acidification have been known for decades, attention to the processes – both in professional circles and the public – has only recently come to the fore. See question three below.

Why does this matter to me?

It matters because only small changes in the pH of seawater can lead to very significant changes to the ecology and functioning of the seas all around the world.

How much of it is due to us and how much of it is nature?

The basic process of the absorption of CO2 by seawater is entirely natural and is a fundamental property of the interaction of the world’s atmosphere and oceans. The process provides a balance in the earth system.

We know the amount of CO2 in the atmosphere has varied markedly over time. Ice-core records show that CO2 varied between about 180 ppm during ice ages and 280-300 ppm during the warm interglacials. The records show remarkably similar atmospheric cycles over the past 800,000 years. Because the amount of CO2 dissolved in seawater is dependent upon the amount of CO2 in the atmosphere (more accurately the difference between the atmosphere and the sea) it follows that there would have been variations in seawater pH with lower pHs during the warm interglacials.

However the concern is that the levels of CO2 in the atmosphere now (more than 380 ppm) far exceed anything we have seen for the past nearly 1 million years because of human use of fossil fuels and land-use change (primarily deforestation). The ocean pH is estimated already to be 0.1 unit lower because of human activity and this can only decrease further as the levels of CO2 continue to increase in the atmosphere. Predictions suggest that ocean pH could be lower at the end of this century than at any time in the past 20 million years, at least, if we continue with the IPCC business as usual scenario.

Are oxygen levels affected by this?

Not directly except to a very small theoretical degree. However, oxygen levels in the ocean are part of a delicate balance involving complex biogeochemical processes, thus large but as yet unknown, shifts in these cycles caused by lowered pH could theoretically cause shifts in oxygen levels. However, this is not something that causes much concern at the moment.
There are concerns about large scale reductions in oxygen in certain parts of the world ocean but these are the result of over-enrichment of the ocean due to land derived (fertilizers and sewage) nutrients. This is a different story.

Are there any benefits to ocean acidification?

It is hard to see any. As in all environments some organisms will thrive at the expense of others when conditions change. Almost certainly there will be some organisms that will do well in seawater with a lower pH. One group that probably will do well are the sea-grasses. These appear to do well in naturally CO2 enriched waters caused by submarine vents, examples of which have been studied off the coast of Italy by researchers at the University of Plymouth (Jason M. Hall-Spencer et al., 2008. Nature 454, doi: 10.1038/nature 07051). It is possible that some phytoplankton might benefit but this is speculation and might arise simply by the removal of competition rather than direct benefits.

Overall, one would have to say that ocean acidification, in the sense of its description here, is a bad thing!

How is sea life affected?

The principal affect is that lowering pH results is a reduction of the availability of carbonate minerals (for example aragonite) that many marine animals need to create their shells. The amount of carbonate available is determined by the saturation values of the minerals. At high pHs the chemical equilibrium in seawater allows for high saturation values. At lower pHs (i.e. due to acidification) the chemical equilibrium shifts to lower the saturation. This equilibrium can shift such that seawater can become undersaturated. Animals with carbonate skeletons (for example corals) and shells (for example snails), find it more difficult to make them the lower the saturation, and in theory the skeletons and shells dissolve in seawater undersaturated in carbonate minerals. Clearly this is fundamental to the survival of many groups of marine organisms.

Is some sea life more affected than others?

Yes, as noted above it is organisms that have body structures made totally, or in part, of carbonate based minerals. These are principally animals and include: corals, molluscs and echinoderms. However, there are some important microscopic plant species, for example coccolithophores that have calcium carbonate structures.

However, the exact interaction between reduced seawater pH and the impact on marine organisms is only now being determined experimentally. The basic mechanisms are reasonably well known but some experiments are showing conflicting results. For example, theoretically coccolithophores should experience increasing difficulty making their external carbonate plates as seawater pH decreases – and indeed this has been shown (for example Ulf Riebesell et al. 2000. Nature 407, 364-367. doi: 10.1038/35030078). On the other hand some experiments have been conducted that found conflicting results (M. Debora Iglesias-Rodriguez et al. 2008. Science 320, 336-340. doi: 10.1126/science.1154122). There is still a lot to learn.

What are the impacts on fishing communities?

This is a complex question to answer. We are only now beginning to understand the impact of acidification on individual marine organisms so it is impossible to predict how the effects might be translated ‘upwards’ into communities and ecosystems and thus fishing communities. At an individual level we know all sorts of organisms are vulnerable to changes in seawater pH. Some of these are themselves part of a fishery – for example some shellfish – or support fishery indirectly. See below for further information.

Are some regions of the world more affected than others?

Yes. The calcium carbonate chemistry of seawater is such that colder and deeper waters have naturally lower levels of carbonate saturation than warmer and shallow waters. So with reference to Q6 above cold, deep waters are already more challenging environments for organisms with carbonate skeletons and will be impacted first as a result of ocean acidification. Polar waters are already close to saturation equilibrium for carbonate and are predicted to become undersaturated sometime this century. This might be the demise of many planktonic organisms – for example pteropods (a planktonic swimming snail with a calcium carbonate shell). We can only speculate what this might mean for polar marine ecosystems because we do not know the full role of these and similar organisms in the ecosystem.

In contrast, the shallow and warmer waters of tropical coastal waters exhibit high levels of carbonate saturation. Corals have evolved in this high carbonate environment but are very vulnerable to reductions that will result from acidification. Predictions of future pH in seawater show that these tropical regions will suffer decreases in carbonate saturation that will adversely affect coral growth. This, combined with increases in seawater temperature because of warming (corals live close to their thermal maximum), are a likely significant threat to corals worldwide.

How does this impact on humans?

Ocean acidification has a potentially profound indirect impact on humans. The affect will be manifest through the negative impact on marine ecosystems and the subsequent reduction in the value of the services provided to humans by the marine environment.
Some impacts are obvious and impact humans directly – for example fisheries. Humans worldwide depend very heavily on protein from the sea – some of these fisheries will be affected badly – most notably shell fisheries. However, all fisheries depend fundamentally on healthy ecosystems and every fishery is potentially at risk due to potentially fundamental changes to the marine ecosystem.

However, there are also less obvious indirect consequences for humans. For example, coral reefs are highly effective physical barriers that provide vital protection to low-lying lands and islands throughout the tropics. The loss of these reefs would result in the degradation of key sea defences that are depended upon by hundreds of thousands of people worldwide. Coral reefs are also important nursery grounds for countless species of organisms that are used directly for food or support fisheries; for example, as nursery grounds for adult fish that are found offshore.
Also, we are only now beginning to try to quantify the benefits that the marine environment provides for humans. Economists call this the ‘goods and services’ provided by the environment. Some of these are relatively obvious and direct for example, fisheries and coastal protection – although still difficult to quantify in monetary terms. However, others are far more difficult to assess; for example, the value of an ecosystem that is capable of rendering harmless some pollutant or other.

Given that ocean acidification could potentially change entire marine ecosystems we are only at the beginning of estimating its potential consequences.

Is anything being done to fix this problem?

A casual web search shows that ocean acidification is something that is only now being addressed. For example, Google™ searches reveal nearly 100 times more hits for the search term global warming than ocean acidification. However, increasingly ocean acidification is being linked into the wider debate about climate change and global warming. Clearly ocean acidification is neither of these things but is a result of a high CO2 world.

Nevertheless, the ocean acidification problem is now attached to the ‘apron strings’ of the climate change/global warming lobby and pragmatically I am happy with this because it means the subject gets an airing. However, I would prefer to use the terms global environment change – because this encompasses the range of drivers (land use changes) – or consequences of a high CO2 world; however, the latter is only partially right because this ignores the significant human induced changes in other greenhouse gases.

But in relation to the question – the answer is that anything that is being done to reduce the human induced increase in CO2 in the atmosphere also addresses the question of acidification. Is this enough? Probably not but this is all we have.

In recent months the term geoengineering has begun to appear in both the scientific and popular media. Geoengineering is the deliberate engineering of the planet to counter changes to the planet caused by humans. The term is usually used in the context of either reducing the concentrations of greenhouse gases (principally CO2) in the atmosphere or moderating the temperature increases resulting from them. [The Royal Society has recently published an excellent report on geoengineering; Geoengineering the Climate. Science, governance and uncertainty. Sept. 2009. Royal Society, London.].

Where ocean acidification is concerned, however, it is only interventions that reduce CO2 concentrations in the atmosphere that have any relevance. It is theoretically possible to manipulate (geoengineer) the carbonate/CO2/pH chemistry of the ocean through the deliberate dumping of millions of tons of calcium carbonate (e.g. limestone) into the ocean. However the practical limitations of this are huge and the CO2 expended in order to carry out the work suggest this idea is probably best considered a fantasy.

How do we know it is happening, what’s the evidence?

In some respects the evidence is quite simple. The chemistry involved is straightforward and uncontroversial. We have good measures of CO2 in the atmosphere over time; less good – but adequate – measures of CO2 in the sea thus we know how much CO2 has been absorbed by the sea and what the pH changes are.

It is true that beyond this the impacts have all been found through studies in laboratory or, essentially scaled-up, laboratory experiments. I do not know of any directly observed environmental impacts that could be ascribed to ocean acidification. But there is circumstantial evidence. We know that planktonic communities shift in composition for example. There have been significant shifts in plankton communities across the world – of this stage we could not rule out possible acidification effects, but we most certainly cannot do anything more than speculate.

There is a slightly stronger argument to suggest that the decline in the health and growth of tropical corals might be related to ocean acidification. But there are so many other possible factors that could have caused these changes (for example temperature, increased silt due to changed land practices, physical damage) that it is impossible to ascribe cause and affect.

Interesting insights might be gained from the past by the examination of the geological record. Sediment cores from various parts of ocean show that about 55 million years ago there was a major event that apparently resulted in a massive injection of CO2 into the earth system. The original source of the CO2 might have been volcanic but the event has been captured in the marine sediment record which shows the very rapid disappearance of carbonate and carbonate shelled animals at that time. Carbonates simply disappear from the sediments. This can only be explained by a major acidification event. In coarse terms this is exactly what we would expect.

So there is, albeit scant, evidence that acidification events have occurred in the past; theory certainly points to the fact we should expect the consequences of acidification; and experiments, large and small, generally support the theory.

This is too big a problem to ignore and I believe the subject should be ‘up there’ as one of the big environmental concerns of our time: The other CO2 problem.

Links

• Coral reefs in danger of being destroyed. Independent 24 February 2010 

• Ocean acidification rates pose disaster for marine life, major study shows. Guardian 10 December 2009

• Ocean acidification teaching resource

Nicholas was interviewed in December 2009.

Biography of Prof Nicholas J P Owens (Nick)

Nick is a marine biologist by training having studied at the Liverpool University Marine Biological Station, Port Erin, Isle of Man for his first degree and PhD studies at the University of Dundee, under the supervision of Prof. W.D.P. Stewart FRS, who later became Government Chief Scientist.

Nick was then employed at the then relatively newly created Institute for Marine Environmental Research (IMER) in Plymouth. IMER was re-named the Plymouth Marine Laboratory (PML) in 1988 where he remained until 1993. In September 1993 Nick took the position of Professor of Marine Sciences at the University of Newcastle and head of Department (1994-1999) until 2000. During this time Nick was a member of many NERC Committees.

In 2000 Nick became Director of PML – then a component of NERC’s Centre for Coastal and Marine Sciences (CCMS). CCMS was very soon disbanded and Nick, with a valiant and dedicated senior team, took PML out of NERC ownership to create an independent organisation within the NERC family. A fully commercial trading subsidiary was also created in order to maximise diversity of funding and provide a vehicle for exploitation and spin-out of research. Nick became Director of BAS in September 2007.

Nick’s research interests as a biological oceanographer are primarily in the field of biogeochemical cycling, especially the nitrogen cycle and the production and consumption of biogases. He has spent over three years of his career at sea on a variety of research ships, large and small, in almost all of the world’s oceans.