Pteropods make thinner shells along a natural ocean acidification gradient

Rising atmospheric carbon dioxide concentrations lead to ocean acidification (OA), which affect marine calcifying organisms. Shelled pteropods, a group of calcifying zooplankton, are widely regarded as bio-indicators of ocean acidification. Their delicate aragonitic shell can dissolve in waters with low aragonite saturation conditions, but pteropods can also counter shell damage through repair mechanisms. Most studies are based on short-term acidification experiments, whereas little is known about net effects of OA on pteropod shells in natural ecosystems. Here, we show with high-resolution Micro-CT technology that shell thickness of the pteropod Limacina helicina declines by 38% with lowering pH along an ocean acidification gradient generated by upwelling of deep CO₂-rich waters in the northern California Current Ecosystem (CCE). Shell thickness was not related to body size, and dissolution did not impact the thickness of the shells. Based on these results, we infer that the decrease in shell thickness at lower pH is not the result of enhanced dissolution, but rather a decline in calcification. Making thinner shells may be an adaptive strategy to cope with OA, though it is unclear how much longer pteropods in the CCE will be able to sustain this. Reduced calcification of pteropods is likely to have major implications for the global CaCO₃ export from oceanic surface waters to the deep sea.