Impact of aragonite saturation state changes on migratory pteropods

Abstract

Thecosome pteropods play a key role in the food web of various marine ecosystems and they calcify, secreting the unstable CaCO(3) mineral aragonite to form their shell material. Here, we have estimated the effect of ocean acidification on pteropod calcification by exploiting empirical relationships between their gross calcification rates (CaCO(3) precipitation) and aragonite saturation state Ω(a), combined with model projections of future Ω(a). These were corrected for modern model-data bias and taken over the depth range where pteropods are observed to migrate vertically. Results indicate large reductions in gross calcification at temperate and high latitudes. Over much of the Arctic, the pteropod Limacina helicina will become unable to precipitate CaCO(3) by the end of the century under the IPCC SRES A2 scenario. These results emphasize concerns over the future of shelled pteropods, particularly L. helicina in high latitudes. Shell-less L. helicina are not known to have ever existed nor would we expect them to survive. Declines of pteropod populations could drive dramatic ecological changes in the various pelagic ecosystems in which they play a critical role.

Figure 1.

Location of the nine study sites.

Figure 2.

Aragonite saturation state (Ω_a) simulated by the model (red circles with solid line) and calculated from the field data (blue circles with solid line) for (a) study site 1 (Arctic Ocean; 83.5° N, 98.5° W), (b) study site 2 (Svalbard; 79° N, 11° E), (c) study site 3 (Southern Ocean; 62° S, 60° E) and (d) study site 4 (Sargasso Sea; 33.5° N, 69.5° W). Points on the colour curves indicate sample depths (blue) and model levels (red). A figure, similar to the one presented here, is available in the electronic supplementary material for the other study sites.

Figure 3.

Mean annual aragonite saturation state (Ω_a) projected by the IPSL model (corrected for the modern model-data bias) during 1860–2100, and G_gross during 1860–2100 relative to the preindustrial rate at the four study sites. Lines are given for projections at 10 m (red), the mean over the 200 m vertical migration range (green) and at 200 m (blue). Dashed lines correspond to the limit between supersaturation and undersaturation with respect to aragonite. Site locations are given in the caption to figure 3. A figure, similar to the one presented here, is available in the electronic supplementary material for the other study sites. (a) Ω_a; (b) % of calcification.