Abstract
Abstract. Ocean acidification is a result of the uptake of anthropogenic CO2 from the atmosphere into the ocean and has been identified as a major environmental and economic threat. The release of several thousands of petagrams of carbon over a few hundred years will have an overwhelming effect on surface ocean carbon reservoirs. The recorded and anticipated changes in seawater carbonate chemistry will presumably affect global oceanic carbonate production. Coccolithophores as the primary calcifying phytoplankton group, and especially Emiliania huxleyi as the most abundant species have shown a reduction of calcification at increased CO2 concentrations for the majority of strains tested in culture experiments. A reduction of calcification is associated with a decrease in coccolith weight. However, the effect in monoclonal cultures is relatively small compared to the strong variability displayed in natural E. huxleyi communities, as these are a mix of genetically and sometimes morphologically distinct types. Average coccolith weight is likely influenced by the variability in seawater carbonate chemistry in different parts of the world's oceans and on glacial/interglacial time scales due to both physiological effects and morphotype selectivity. An effect of the ongoing ocean acidification on E. huxleyi calcification has so far not been documented in situ. Here, we analyze E. huxleyi coccolith weight from the NW Mediterranean Sea in a 12-year sediment trap series, and surface sediment and sediment core samples using an automated recognition and analyzing software. Our findings clearly show (1) a continuous decrease in the average coccolith weight of E. huxleyi from 1993 to 2005, reaching levels below pre-industrial (Holocene) and industrial (20th century) values recorded in the sedimentary record and (2) seasonal variability in coccolith weight that is linked to the coccolithophore productivity. The observed long-term decrease in coccolith weight is most likely a result of the changes in the surface ocean carbonate system. Our results provide the first indications of an in situ impact of ocean acidification on coccolithophore weight in a natural E. huxleyi population, even in the highly alkaline Mediterranean Sea.
Highlights
A key question in global climate change research is how the uptake of anthropogenic CO2 from the atmosphere into the ocean will affect the ocean ecosystem in the near future (Bindoff et al, 2007; Kleypas et al, 2006; Kroeker et al, 2013)
Our study demonstrates that long-term and seasonal variability in the average weight of E. huxleyi coccoliths in the Gulf of Lions has different causes
In the long-term the seawater carbonate system displays the strongest change of all environmental parameters
Summary
A key question in global climate change research is how the uptake of anthropogenic CO2 from the atmosphere into the ocean will affect the ocean ecosystem in the near future (Bindoff et al, 2007; Kleypas et al, 2006; Kroeker et al, 2013). Calcifying organisms are most likely to be negatively affected by the process commonly called ocean acidification (OA), and a series of studies have demonstrated the sensitivity of calcification. S. Meier et al.: Emiliania huxleyi coccolith thinning in the Mediterranean Sea in response to OA for a variety of different organism groups (Kleypas, 1999; Lischka et al, 2011; Moy et al, 2009; Riebesell et al, 2000; van de Waal, 2013). Various culturing studies have demonstrated that the most abundant coccolithophore species Emiliania huxleyi reacts to increased CO2 with a decrease in calcification, but the response is highly variable between strains (De Bodt et al, 2010; Langer et al, 2009; Hoppe et al, 2011). The genetic diversity among E. huxleyi (Read et al, 2013) even contains a haplotype that can heavily calcify under elevated CO2 and is environmentally restricted to high latitudes and upwelling regions (Beaufort et al, 2011; Smith et al, 2012), but this has not been reported in the Mediterranean far (Beaufort et al, 2011)
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