The Pacific oyster, Crassostrea gigas, shows negative correlation to naturally elevated carbon dioxide levels: Implications for near-term ocean acidification effects

We report results from an oyster hatchery on the Oregon coast, where intake waters experienced variable carbonate chemistry (aragonite saturation state , 0.8 to . 3.2; pH , 7.6 to . 8.2) in the early summer of 2009. Both larval production and midstage growth (, 120 to , 150 mm) of the oyster Crassostrea gigas were significantly negatively correlated with the aragonite saturation state of waters in which larval oysters were spawned and reared for the first 48 h of life. The effects of the initial spawning conditions did not have a significant effect on early-stage growth (growth from D-hinge stage to , 120 mm), suggesting a delayed effect of water chemistry on larval development. Rising atmospheric carbon dioxide (CO2) driven by anthropogenic emissions has resulted in the addition of over 140 Pg-C (1 Pg 5 1015 g) to the ocean (Sabine et al. 2011). The thermodynamics of the reactions between carbon dioxide and water require this addition to cause a decline of ocean pH and carbonate ion concentrations ([CO3 ]). For the observed change between current-day and preindustrial atmospheric CO2, the surface oceans have lost approximately 16% of their [CO3 ] and decreased in pH by 0.1 unit, although colder surface waters are likely to have experienced a greater effect (Feely et al. 2009). Projections for the open ocean suggest that wide areas, particularly at high latitudes, could reach low enough [CO3 ] levels that dissolution of biogenic carbonate minerals is thermodynamically favored by the end of the century (Feely et al. 2009; Steinacher et al. 2009), with implications for commercially significant higher trophic levels (Aydin et al. 2005). There is considerable spatial and temporal variability in ocean carbonate chemistry, and there is evidence that these natural variations affect marine biota, with ecological assemblages next to cold-seep high-CO2 sources having been shown to be distinct from those nearby but less affected by the elevated CO2 levels (Hall-Spencer et al. 2008). Coastal environments that are subject to upwelling events also experience exposure to elevated CO2 conditions where deep water enriched by additions of respiratory CO2 is brought up from depth to the nearshore surface by physical processes. Feely et al. (2008) showed that upwelling on the Pacific coast of central North America markedly increased corrosiveness for calcium carbonate minerals in surface nearshore waters. A small but significant amount of anthropogenic CO2 present in the upwelled source waters provided enough additional CO2 to cause widespread corrosiveness on the continental shelves (Feely et al. 2008). Because the source water for upwelling on the North American Pacific coast takes on the order of decades to transit from the point of subduction to the upwelling locales (Feely et al. 2008), this anthropogenic CO2 was added to the water under a substantially lowerCO2 atmosphere than today’s, and water already en route to this location is likely carrying an increasing burden of anthropogenic CO2. Understanding the effects of natural variations in CO2 in these waters on the local fauna is critical for anticipating how more persistently corrosive conditions will affect marine ecosystems. The responses of organisms to rising CO2 are potentially numerous and include negative effects on respiration, motility, and fertility (Portner 2008). From a geochemical perspective, however, the easiest process to understand conceptually is that of solid calcium carbonate (CaCO3,s) mineral formation. In nearly all ocean surface waters, formation of CaCO3,s is thermodynamically favored by the abundance of the reactants, dissolved calcium ([Ca2+]), and carbonate ([CO3 ]) ions. While oceanic [Ca 2+] is relatively constant at high levels that are well described by conservative relationships with salinity, ocean [CO 3 ] decreases as atmospheric CO2 rises, lowering the energetic favorability of CaCO3,s formation. This energetic favorability is proportional to the saturation state, V, defined by