Calcifying invertebrates succeed in a naturally CO<sub>2</sub>-rich coastal habitat but are threatened by high levels of future acidification

Abstract

Abstract. CO 2 emissions are leading to an acidification of the oceans. Predicting marine community vulnerability towards acidification is difficult, as adaptation processes cannot be accounted for in most experimental studies. Naturally CO 2 enriched sites thus can serve as valuable proxies for future changes in community structure. Here we describe a natural analogue site in the Western Baltic Sea. Seawater pCO 2 in Kiel Fjord is elevated for large parts of the year due to upwelling of CO 2 rich waters. Peak pCO 2 values of >230 Pa (>2300 µatm) and pH NBS values of <7.5 are encountered during summer and autumn, average pCO 2 values are ∼70 Pa (∼700 µatm). In contrast to previously described naturally CO 2 enriched sites that have suggested a progressive displacement of calcifying auto-and heterotrophic species, the macrobenthic community in Kiel Fjord is dominated by calcifying invertebrates. We show that blue mussels from Kiel Fjord can maintain control rates of somatic and shell growth at a pCO 2 of 142 Pa (1400 µatm, pH NBS = 7.7). Juvenile mussel recruitment peaks during the summer months, when high water pCO 2 values of ∼100 Pa (∼1000 µatm) prevail. Our findings indicate that calcifying keystone species may be able to cope with surface ocean pH NBS values projected for the end of this century when food supply is sufficient. However, owing to non-linear synergistic effects of future acidification and upwelling of corrosive water, peak seawater pCO 2 in Kiel Fjord and many other productive estuarine habitats could increase to values >400 Pa (>4000 µatm). These changes will most likely affect calcification and recruitment, and increase external shell dissolution.

How will Ocean Acidification Affect Baltic Sea Ecosystems? An Assessment of Plausible Impacts on Key Functional Groups

“…Similar small negative effects on larval shell growth have also been reported in populations of M. edulis from the North Sea (Gazeau et al 2010;Bechmann et al 2011), and in related Mytilus species around the world (Kurihara et al 2009;Gaylord et al 2011;Sunday et al 2011). Early reports of the effects of ocean acidification on shell growth in adult M. edulis showed negative impacts (Gazeau et al 2007), a result that contrasts with those of Melzner and co-workers in the Kiel fjord (Thomsen et al 2010;Melzner et al 2011), although the latter might be expected to be a result of local adaptation to seasonally low pH-especially at such extreme levels (Melzner et al 2009b;Thomsen et al 2010).…”

“…Most of these come from the Kiel fjord, where summer upwelling can drive the pH down to 7.5 (Thomsen et al 2010), well below the levels typically predicted for the open oceans by end of the century (B1000 latm CO 2 ; Cao et al 2007;IPCC 2007). Here, the mussel Mytilus edulis, a dominant macrobenthic species in the Baltic Sea, was found to maintain shell and somatic growth rates at ca.…”

“…M. edulis was also observed to recruit actively at 1000 latm CO 2 (&pH 7.75; Thomsen et al 2010). Thomsen et al (2010) suggest that this unusual ability to maintain physiological function despite substantial levels of acidification was due in part to an abundance of food, which provided the energy needed to offset the physiological costs of maintenance and growth at such low ambient pH (Melzner et al 2011). More sensitive early life-history stages of M. edulis from the Skagerrak have been shown to respond differently to ocean acidification: fertilization success increased at reduced pH (induced by high pCO 2 ) whereas subsequent larval shell growth was negatively affected, albeit only slightly (Renborg and Havenhand, unpublished results).…”

“…1400 latm CO 2 (&pH 7.6). M. edulis was also observed to recruit actively at 1000 latm CO 2 (&pH 7.75; Thomsen et al 2010). Thomsen et al (2010) suggest that this unusual ability to maintain physiological function despite substantial levels of acidification was due in part to an abundance of food, which provided the energy needed to offset the physiological costs of maintenance and growth at such low ambient pH (Melzner et al 2011).…”

“…Data from coastal regions in the southern Baltic Sea reflect this: pH of the Kiel fjord varies by ca. 0.7 pH units seasonally (Thomsen et al 2010), and diurnal fluctuations of ±0.15 pH units are common in shallow bays of the Skagerrak (personal observation). Similar observations have been made in a variety of open ocean and coastal locations (Wootton et al 2008;Hofmann et al 2011).…”