Published: 13.06.08
Carbon dioxide and ocean acidification

No carbonate for coral

The observed increase in the amount of CO2 in the atmosphere, due mainly to the burning of fossil fuels, has not only a large effect on the climate. The additional CO2 also causes acidification of the oceans. However, the consequences for the ocean ecosystem are still largely unknown. The EU’s “EPOCA” research program, which has just been launched in Nice with ETH Zurich participation, now plans to research the phenomenon and its results comprehensively.

Peter Rüegg
Gephyrocapsa oceanica, or coccolithophores, could become a victim of ocean acidification. (Photo: wikipedia)
Gephyrocapsa oceanica, or coccolithophores, could become a victim of ocean acidification. (Photo: wikipedia) (large view)

The sea has benefited mankind greatly up to now. One third of the emissions of carbon dioxide has disappeared into the infinite depths of the oceans in the past 200 years, thus removing it from the atmosphere. Climate change and warming would probably have occurred more quickly if part of the additional CO2 had not dissolved in the seawater.

The oceans, which still swallow up about one third of the carbon dioxide caused by humans, are now threatened by acidification. The pH of seawater during the last 10,000 years was about 8.1, i.e. in the slightly alkaline region. It has dropped worldwide by about 0.1 pH units since industrialisation and the increasing release of CO2. Models show that the seawater acidity should decrease down to 7.7 by the end of the 21st century. That doesn’t sound very much, but it can become a problem for some sea creatures. A seawater pH as low as this has not occurred in the last 400,000 years, and possibly not even for 20 million years.

Altered water chemistry

More acidic seawater creates a problem for numerous organisms that form lime shells or skeletons. Gian-Kasper Plattner, Research Associate at the Institute of Biogeochemistry and Pollutant Dynamics, says “If insufficient carbonate is present in the water, it causes problems for organisms that construct lime shells.”

Carbonate is formed in an equilibrium reaction with the CO2 present naturally in seawater. This equilibrium is changing due to the rising input of CO2 into the seawater. Carbonate is “consumed” – and is then not available to the corals, sea-snails or zooplankton organisms that need it to build their lime shells or skeletons. This is shown by an experiment carried out by Israeli scientists who exposed corals to artificially acidified water with a pH of 7.3 to 7.6. These levels of acidity could be reached if the level of CO2 in the atmosphere increases five-fold. The lime shells of the corals began to disintegrate after one month in this acidic water, and in time they disappeared completely. However the coral polyps, i.e. the soft parts, survived even without lime.

Researchers like Gian-Kasper Plattner observe with anxiety the ever-increasing expansion of the carbonate sub-saturation zones. As a rule the upper layers of water are supersaturated, which means that lime (calcium carbonate or coccolithophores) is precipitated, whereas the lower layers of water are sub-saturated. This separation layer is determined by, among other things, the degree of acidity.

This separation layer has shifted upwards in the course of the last 200 years. In the North Pacific, for example, by about one hundred metres from a depth of around 500 metres to 400 metres. In some places this effect of the sub-saturation is also already observable near the surface, for example in the regions of upwelling off the coasts of North America. Lime sub-saturation also develops more rapidly in cold seas around the poles than in warm tropical oceans.

Models suggest that the separation layer will shift up to the surface if atmospheric concentrations of CO2 continue to rise. Cold seas are more severely affected and will become sub-saturated more rapidly, by 2100.

Plattner stresses that “The threshold value from sub- to super-saturation must be understood geochemically, not biologically.” The question of whether or not an organism suffers as a result of lime sub-saturation differs not only from one species to another but even within the same species. Plattner says that therefore not every living creature that builds lime shells will be equally affected by the carbonate deficiency.

He says it is possible that the ecological equilibrium in the future will shift from lime-forming organisms to another group of marine organisms. For example away from lime shell builders to diatoms that construct silicate shells.

However, there is still a yawning gap in our knowledge about the ecological effects of ocean acidification. Through the “EPOCA” research project that was launched in Nice recently in the context of the 7th EU Framework Program, 27 research groups from nine countries are now attempting to discover more about the consequences of the input of carbon dioxide into the sea. The researchers want to use “EPOCA” to document the changes in the chemistry of the oceans and the distribution of marine organisms. In the context of the research project, the plan is also to collect data about whether and how individual creatures, biocenoses (communities of living creatures) and ecosystems are susceptible to ocean acidification.

The European researchers hope that “EPOCA” will provide answers to questions such as the way the pH of seawater will change in the future, how organisms respond to the carbonate saturation level and whether marine organisms will change their productivity. Some research groups are also carrying out work at sea, including using mesocosmos experiments through which they want to discover, under real conditions, how organisms react to an elevated concentration of CO2 in the water and in the air.

The ETH Zurich research group including Gian-Kasper Plattner, doctoral student Claudine Hauri and Professor Nicolas Gruber, which is taking part in “EPOCA”, will focus on integrating the “field data” into numerical models and building simulations of the carbon cycle near the coast. Primarily they would like to discover how the chemistry of the seawater around the Canary Islands and the European continental shelf is changing. On the one hand the coastal regions are some of the sea’s most productive zones, but on the other they are the most vulnerable. Plattner says these regions are interesting from the economic point of view, among other things, because the marine zones down to a depth of 200 metres are among Europe’s most important fishing grounds. If the productivity of these oceans changes, that can have hitherto little understood consequences for the fish and ultimately for the fishing industry.

One focus of the research planned at ETH Zurich will be how the buffer capacity of the waters near the coast develops as the concentration of CO2 in the atmosphere increases. The higher the latter rises, the less carbon dioxide the sea can absorb. Plattner considers that “Today this buffer capacity is already reduced in the coastal regions compared to the open sea.”