ORIGINAL PAPER Tolerance of juvenile barnacles (Amphibalanus improvisus) to warming and elevated pCO 2 Christian Pansch • Ali Nasrolahi • Yasmin Shirin Appelhans • Martin Wahl Received: 29 December 2011 / Accepted: 4 September 2012 / Published online: 23 September 2012 Ó Springer-Verlag 2012 Abstract We investigated the impacts of warming and elevated pCO 2 on newly settled Amphibalanus improvisus from Kiel Fjord, an estuarine ecosystem characterized by significant natural pCO 2 variability. In two experiments, juvenile barnacles were maintained at two temperature and three pCO 2 levels (20/24 °C, 700–2,140 latm) for 8 weeks in a batch culture and at four pCO 2 levels (20 °C, 620–2,870 latm) for 12 weeks in a water flow-through sys- tem. Warming as well as elevated pCO 2 hardly affected growth or the condition index of barnacles, although some factor combinations led to temporal significances in enhanced or reduced growth with an increase in pCO 2 . While warming increased the shell strength of A. improvisus individuals, elevated pCO 2 had only weak effects. We demonstrate a strong tolerance of juvenile A. improvisus to mean acidifi- cation levels of about 1,000 latm pCO 2 as is already naturally experienced by the investigated barnacle population. Introduction Anthropogenically, increased atmospheric CO 2 causes a reduction in seawater pH, commonly referred to as ocean acidification (‘‘OA’’; Caldeira and Wickett 2003; IPCC 2007; Gattuso and Hansson 2011). This process can have substantial impacts on single marine species (summarized in, e.g., Fabry et al. 2008; Kurihara 2008; Doney et al. 2009; Dupont et al. 2010; Kroeker et al. 2010; Gattuso and Hansson 2011) as well as on entire ecosystems (Riebesell 2008; Hale et al. 2011). Coastal habitats have been shown to differ substantially from the open ocean conditions. Not only absolute mean values and the amplitude of annual or daily fluctuations (e.g., Blackford and Gilbert 2007; Shim et al. 2007; Feely et al. 2008; Salisbury et al. 2008; Wootton et al. 2008; Miller et al. 2009; Feely et al. 2010) but even future pre- dictions differ considerably from open ocean norms (e.g., Wootton et al. 2008; Melzner et al. 2012). The Kiel Fjord (western Baltic Sea) is characterized by strong fluctuations in water pCO 2 and pH. Heterotrophic degradation of organic material in deeper waters is directly linked to oxygen consumption and CO 2 production (Rabalais et al. 2002; Diaz and Rosenberg 2008), and upwelling events will expose organisms in shallow marine habitats to these corrosive waters. Annual mean pCO 2 values of about 700 latm can be measured within Kiel Fjord already today (Jo ¨rn Thomsen personal communication), with occasional pCO 2 peaks of up to *2,300 latm (Thomsen et al. 2010). Investigations into the vicinity of volcanic CO 2 vents in the Mediterranean show a close correlation of the degree of CO 2 -mediated acidification and the occurrence of calcify- ing organisms (Hall-Spencer et al. 2008; see also Fabricius et al. 2011). In such small and gradually acidified areas, adaptation to increased CO 2 may be hindered by gene flow from adjacent non-impacted areas. Adaptation, however, seems possible when entire regions such as the Baltic Sea with restricted genetic exchange with the oceans feature temporarily acidified conditions. Adaptation over many generations might thus have led to populations with a reduced sensitivity to future OA. In nature, ocean acidification is not an isolated stre- ssor but will always act together with other predicted Communicated by S. Dupont. C. Pansch (&)  A. Nasrolahi  Y. S. Appelhans  M. Wahl Department of Marine Ecology, GEOMAR—Helmholtz Centre for Ocean Research Kiel, Du ¨sternbrooker Weg 20, 24105 Kiel, Germany e-mail: cpansch@geomar.de 123 Mar Biol (2013) 160:2023–2035 DOI 10.1007/s00227-012-2069-4