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Published: April 1, 2009
First published in IGBP's Global Change Newsletter Issue 73, April 2009

Natural CO2 vents reveal ecological tipping points due to ocean acidification

Features |
The Ocean in a High-CO2 World: Science highlights from the symposium
Jason M. Hall-Spencer
Marine Institute
University of Plymouth
Plymouth PL4 8AA, UK
jason.hall-spencer@plymouth.ac.uk
Investigation into the long-term biological effects of permanent exposure to high CO2 concentrations in a natural ecosystem has taken research into ocean acidification an important step forwards.  Effects were studied on rocky and sedimentary marine communities around underwater volcanic vents that release millions of litres of CO2 per day.  The vents lacked the poisonous sulphur compounds that characterise many vents. The high CO2 levels had major impacts on marine life including 30% reductions in species diversity at average pH 7.8, compared with normal seawater (pH 8.1).

This work provides the first confirmation of modelling and short-term laboratory experiments which predict severe reductions in the ability of marine organisms to build shells or skeletons from calcium carbonate due to the dramatic effects of CO2 on seawater chemistry. Seagrasses thrived at increased CO2 (Figure) levels but major groups such as corals, sea urchins and calcified algae were removed from the ecosystem and replaced by invasive species of algae. Such studies will help us to predict the future effects of ocean acidification and demonstrate, for the first time, what happens to marine ecosystems when key groups of species are killed due to rising CO2 levels.

Venting of CO2 at a Mediterranean site provides the opportunity to observe changes in ecosystems along gradients of decreasing pH close to the vents. Sea grasses and brown algae grow well at the vents but groups such as sea urchins, coralline algae and stony corals are killed by the acidified water.
References
Hall-Spencer, J.M., Rodolfo-Metalpa, R., Martin, S., Ransome, E., Fine, M., Turner, S.M., Rowley, S.J., Tedesco, D., M.-C. Buia (2008). Volcanic carbon dioxide vents reveal ecosystem effects of ocean acidification. Nature, 454: 96-99.

Martin, S., Rodolfo-Metalpa, R., Ransome, E., Rowley, S., Buia, M.-C., Gattuso, J.-P. & J.M. Hall-Spencer (2008). Effects of naturally acidified seawater on seagrass calcareous epibionts. Biology Letters, 4: 689-692.

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