Pteropods make thinner shells along a natural ocean acidification gradient

Lisette Mekkes1, Willem Renema1, Nina Bednarsek2, Simone R Alin3, Richard A Feely4, Jef Huisman5, Peter Roessingh6 and Katja Peijnenburg1, (1)Naturalis Biodiversity Center, Marine Biodiversity, Leiden, Netherlands, (2)Southern California Coastal Water Research Project, Biogeochemistry, Costa Mesa, United States, (3)NOAA Pacific Marine Environmental Laboratory, Seattle, United States, (4)NOAA Pacific Marine Environmental Laboratory, Seattle, WA, United States, (5)University of Amsterdam, Department of Freshwater and Marine Ecology, Amsterdam, Netherlands, (6)University of Amsterdam, Department of Evolutionary & Population Biology, Amsterdam, Netherlands
Abstract:
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.