Part 3: Measurements of CO 2 - sensitive processes 213 Measurements of calcification and dissolution of benthic organisms and 13 communities Chris Langdon 1 , Jean-Pierre Gattuso 2,3 and Andreas Andersson 4 1 Rosenstiel School of Marine and Atmospheric Science (RSMAS), University of Miami, USA 2 Laboratoire d’Océanographie, CNRS, France 3 Observatoire Océanologique, Université Pierre et Marie Curie-Paris 6, France 4 Bermuda Institute of Ocean Sciences (BIOS), Bermuda Introduction 13.1 Approximately 25% of the global calcium carbonate production occurs in coastal environments despite accounting for only 7% of the global ocean area (Mackenzie et al. , 2004). Almost 50% of the calcium carbonate that accumulates in global marine sediments accumulates within this region, with about half of this accumulation in regions of coral reefs. On geological time scales this carbonate production influences the ocean chemistry, the carbon cycle and the Earth’s climate (Ridgwell et al. , 2003). On shorter time scales, cause and effect are reversed and changes in the carbon cycle affect the rates of carbonate production and dissolution in the ocean. Laboratory and mesocosm studies have shown that the calcification rate of many calcium carbonate secreting organisms is negatively impacted by an increase in atmospheric CO 2 . Since calcification is a source of CO 2 on short time scales, a reduction in calcification would constitute a negative feedback on the build up of atmospheric CO 2 . The impact on the global carbon budget, however, is negligible given the mismatch in fluxes, i.e. 0.1 Pg C y -1 vs. 9 Pg C y -1 of anthropogenic CO 2 emissions. Far more important will be the negative impact on the organisms and the ecosystems they support. For this reason, interest in the measurement of calcification of shallow-living benthic organisms is becoming a hot research topic and it has been recognised at several national and international meetings that it is important that the way these measurements are made should be standardised so that the results are of high quality and intercomparable. Approaches and methodologies 13.2 The following sections cover the methods most appropriate to the measurement of calcification of benthic organisms and the calcification/dissolution of benthic ecosystems in the context of studying the impact of ocean acidification. See chapter 7 for some general thoughts about laboratory and benthic mesocosm experiments. The first choice an investigator must make is whether the experiments will be performed in a laboratory setting with well-controlled environmental conditions and good possibilities of replication or to attempt to make the measurements in a mesocosm or in the field where conditions can be less well controlled. In the purest case the investigator makes use of natural variability in the carbonate chemistry to perform the experiment and studies the response against a background of natural variability of many uncontrolled parameters such as light, temperature, salinity, nutrients, flow and food availability. Each approach has its merits. The laboratory studies have the virtue of providing a clear test of a cause and effect relationship. However the results may lack real world applicability if they fail to take into account interactions that in the real world act to mitigate or accentuate the response of interest. Field studies have the virtue of including the full range of factor interactions; however, there are many challenges that need to be addressed. First, there is often no baseline against which to measure a climate-related change. Second, the rate of change due to uptake of fossil fuel CO 2, although fast enough to be of concern over the next decades, is slow enough for it to take as long as ten years to observe a statistically significant change in calcification at present-day levels of precision. Third, diurnal and seasonal variability in carbonate parameters are much larger than the interannual and decadal changes. This poses sampling issues in order to prevent aliasing of the long-term signal. Laboratory studies are needed to investigate the saturation state and pH thresholds for growth and reproduction, i.e. the point below which an individual organism or a population of organisms can not sustain itself in the long term Guide to best practices for ocean acidification research and data reporting Edited by U. Riebesell, V. J. Fabry, L. Hansson and J.-P. Gattuso. 2010, Luxembourg: Publications Office of the European Union.