Increasing atmospheric CO2 vs Ocean acidification

Human activities are responsible for a 36% increase in atmospheric CO2 since the beginning of the industrial era (1800). This increase is due to the CO2 emissions from a variety of sources like the combustion of fossil fuels (coal, oil, and natural gas), or from industrial processes such as the production of iron, steel, and cement. Mass deforestation also contributes by reducing the amount of CO2 that is captured.

1 1The atmosphere’s level of CO2, measured as the partial pressure (pCO2), rose from 280 ppm (part per million) before the start of the industrial era to 385 ppm in 2008. Atmospheric CO2 will continue to increase in coming decades, as predicted under all the Intergovernmental Panel on Climate Change (IPCC) fossil-fuel emission scenarios, with levels that may well reach 700 ppm or more by 2100.

Only 45% of the CO2 emitted by human activities during the industrial era has remained in the atmosphere. Approximately 30%, or 118 (± 19) billion tons (Gt) of C were absorbed by the oceans between 1800 and 1994. Currently, the oceans take up 2.2 ± 0.5 Gt C atmospheric annually (IPCC, 2007)

Ocean acidification is the term used to describe the decrease in4 1 seawater pH due to ocean’s absorption of anthropogenic carbon dioxide (CO2) from the atmosphere. The average surface-ocean pH, which is currently hovering around 8.1, has already fallen by 0.1 unit since the beginning of the industrial era, and it is likely to decline by another 0.2 to 0.4 unit by the end of this century (Source: Caldeira and Wickett, 2003). By limiting the accumulation of CO2 in the atmosphere, and therefore climate change, the ocean CO2 uptake has a beneficial environmental effect.

However, this CO2 dissolves in the surface water and reacts with the water molecules (H2O), forming carbonic acid (H2CO3). Most of this acid dissociates into hydrogen ions (H+) ions and bicarbonate ions (HCO3-). The increase in the concentration of H+ ions reduces pH (pH = -log10[H+]) as well as the carbonate ion concentration (CO32-), which join with H+ ions to form HCO3- via the reaction CO2 + H2O + CO32- → 2HCO3-.

Effect of ocean acidification on marine organisms

Ocean acidification affects marine organisms through changes in pH as well as through changes in other carbonate system variables.

5 1On one hand, the decreased availability of CO32- as induced by ocean acidification, has an impact of many species that make use of these CO32- ions to build calcareous shells and skeletons. The phenomenon of calcification occurs in a large number of marine species, such as algae, corals, mollusks, foraminifera, echinoderms, crustaceans, and bryozoans.

On the other hand, increasing seawater CO2 concentrations could affect carbon fixation by5 2 photosynthesis. Both benthic and pelagic photosynthetic organisms have major biogeochemical and ecological roles. They provide more than 99% of the organic material used in marine food webs. In the terrestrial environment, increasing atmospheric CO2 generally has a beneficial effect on plant photosynthesis, which is often limited by the atmospheric CO2 concentration. In the ocean, increasing CO2 also appears to be beneficial to the growth of certain species of marine seagrasses and seaweeds.

The need for in situ long-term experiments

Most of the experiments to date have been obtained through short- to medium- timescale laboratory studies. Progress in our understanding of possible impacts of ocean acidification on marine life is partly limited by the scarcity of information on responses at the community and ecosystem level. 6 1To close this gap two approaches are particularly promising: (1) community-level studies in natural high-CO2 environments and (2) CO2 perturbation experiments at the community and ecosystem level.

The best know example for a natural high CO2 environment is a CO2 venting site in the Gulf of Naples, where a community shift was observed along a pCO2 gradient, with calcifying organisms successively disappearing from the community approaching the CO2 venting site (Hall-Spencer et al. 2008)
Natural high CO2 environments capture the full scope of ecosystem interactions over long time scales, thereby providing crucial information on the effects on ocean acidification on trophic and competitive interactions and the potential for adaptation. However, the high spatial and temporal variability in pCO2 and pH makes it difficult to determine a reliable dose-response relationship, complicating the use of this information in projecting future high CO2 scenarios. The interpretation is also complicated by the uncontrolled advection and recruitment from unperturbed adjacent areas. Last but not least, the number of these sites is obviously limited and does not allow assessing the effects of ocean acidification on a large variety of communities and ecosystems.

Recently, systems have been developed at the Monterey Bay Aquarium Research Institute (MBARI) in order to study the effects of ocean acidification on benthic communities by injecting CO2-saturated water. While the original system was designed for a deployment in the deep-sea, worldwide projects, similar to eFOCE, are presently adapting the system study shallow water areas.foce3 cc 600