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Size matters for ocean acidification effects

14 May 2012, by Tom Marshall

The microscopic organisms on which most marine life depends could be more vulnerable to ocean acidification than scientists realised, according to a new study.

Emiliana huxleyi

Coloured scanning electron micrograph (SEM) of Emiliania huxleyi.

Previous experiments may have given an unduly optimistic view of the phenomenon's impact on plankton. It turns out that the methods used may have biased their results.

'Plankton often grow in clumps or aggregates,' says Professor Kevin Flynn of Swansea University, lead author of the study. 'But the way they are handled tends to break these clumps up. When scientists starts working on a plankton sample in the lab, the first thing many of them do is give it a good shake.'

How changes in water chemistry affect microbes depends greatly on the size of the aggregate they're in, so studying plankton whose groupings have been disrupted may not give an accurate picture of the conditions they will face in the wild.

To correct this potential distortion, UK and Australian researchers used computer simulations to examine the chemical environment immediately around the bodies of plankton in varying conditions. They found that under projected levels of atmospheric CO2, many kinds of plankton will face bigger changes in their chemical environment than previously realised, and more variation in conditions over the day. These changes will be beyond anything in recent history.

Ocean acidification (OA) is happening as the carbon dioxide we release into the atmosphere dissolves in seawater to form a weak acid. Scientists predict that by the end of the century, seawater's average pH level will drop by 0.3 units - the lower the value, the greater the concentration of hydrogen ions in the water, and hence the more acidic the conditions.

At present the ocean's average pH is on the alkaline side of the neutral value of 7, and OA isn't expected to take it past this threshold, so it's a question of getting closer to acidity rather than actually becoming acidic.

A 0.3-unit drop in pH doesn't sound like a big change. But it equates to doubling the concentration of hydrogen ions, and will be the biggest shift in ocean conditions in 55 million years. Many organisms may struggle to deal with it, particularly those that grow external skeletons out of calcium carbonate.

Organisms from crustaceans and corals to microscopic algae could be at risk; this study focused on photosynthetic plankton. These are among the most important living things on Earth; they form the base of the ocean food pyramid, turning sunlight into nourishment for grazing animals and ultimately predators.

They also play a major part in the global carbon cycle, and hence in regulating the climate. Some of the carbon they absorb through photosynthesis is taken out of the cycle for long periods and stored in sediments on the seabed.

Flynn explains that understanding the effects of OA on marine life is much more complex than it might seem. Even apart from the effects of their tendency to clump together, the metabolic processes of very small organisms can cause major changes in the conditions close to them. This means these organisms don't simply face the conditions found in seawater at a large scale; they create their own environment.

'Different types of plankton will experience different conditions depending on their sizes and metabolic rates,' Flynn says. Living things that are photosynthesising absorb carbon dioxide from the water around them, raising its pH; conversely those that are respiring absorb oxygen and emit CO2, lowering pH.

The results suggest that ocean acidification could change the mix of plankton living in the ocean, with species that can't deal with changed conditions losing out to those that can. Calcifying plankton are likely to feel the greatest effects, because the chemical processes involved in forming their chalky shells themselves contribute to lowering the pH of the water around them.

But Flynn says OA's impact will be complex. While calcifying plankton are alive, it's conceivable they won't be too badly affected; a more important consideration for the wider environment may be what happens after they die. If their skeletons dissolve instead of sinking to the seabed and being locked up in sediments, there are important implications for the carbon cycle.

The researchers now plan to turn their attention to other marine animals that may be at risk, including the early stages of fish and shellfish - these are microscopically tiny, forming part of the plankton, so are subject to similar small-scale environmental effects. This work on commercial fisheries is part of the UK Ocean Acidification Programme, supported by the Natural Environment Research Council (NERC), the Department for Environment, Food and Rural Affairs (Defra) and the Department of Energy and Climate Change (DECC).

NERC provided the primary funding for the study, which appears in Nature Climate Change. Its authors included scientists from Plymouth Marine Laboratory, the Marine Biological Association and the universities of Swansea and Dundee in the UK, and from the University of Technology Sydney and Monash University, Victoria in Australia.

Changes in pH at the exterior surface of plankton with ocean acidification, Kevin J. Flynn, Jerry C. Blackford, Mark E. Baird, John A. Raven, Darren R. Clark, John Beardall,Colin Brownlee, Heiner Fabian & Glen L. Wheeler. Nature Climate Change (2012) doi:10.1038/nclimate1489

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