Pteropods make thinner shells in the upwelling region of the California Current Ecosystem

Lisette Mekkes, Willem Renema, Nina Bednaršek, Simone R. Alin, Richard A. Feely, Jef Huisman, Peter Roessingh & Katja T. C. A. Peijnenburg - Naturalis Biodiversity Center, Leiden, The Netherlands; Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, The Netherlands; Southern California Coastal Water Research Project, Costa Mesa, CA, USA; National Institute of Biology, Ljubljana, 1000, Slovenia; Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, WA, USA

Shelled pteropods are widely regarded as bioindicators for ocean acidification, because their fragile aragonite shells are susceptible to increasing ocean acidity. While short-term incubations have demonstrated that pteropod calcification is negatively impacted by ocean acidification, we know little about net calcification in response to varying ocean conditions in natural populations. Here, we examine in situ calcification of Limacina helicina pteropods collected from the California Current Ecosystem, a coastal upwelling system with strong spatial gradients in ocean carbonate chemistry, dissolved oxygen and temperature. Depth-averaged pH ranged from 8.03 in warmer offshore waters to 7.77 in cold CO2-rich waters nearshore. Based on high-resolution micro-CT technology, we showed that shell thickness declined by ~ 37% along the upwelling gradient from offshore to nearshore water. Dissolution marks covered only ~ 2% of the shell surface area and were not associated with the observed variation in shell thickness. We thus infer that pteropods make thinner shells where upwelling brings more acidified and colder waters to the surface. Probably the thinner shells do not result from enhanced dissolution, but are due to a decline in calcification. Reduced calcification of pteropods is likely to have major ecological and biogeochemical implications for the cycling of calcium carbonate in the oceans.

How Amira-Avizo Software is used

Reconstructed files for each specimen were used to compute a 3D rendering in AVIZO 9.2 software, a program we used for shell visualization and quantification. First, shell material was segmented from the reconstructed radiographs by using a threshold of 11–13, an arbitrary value embedded in AVIZO to distinguish shell from non-shell material. Then, the embedded thickness-measuring tool ‘Thickness map’ in Avizo was used to calculate shell thickness (µm) along the complete surface in the binary image, defined as the diameter of the largest ball containing the voxels. (…) Diameter and height (µm) of the shells were measured using the simple measuring tool in AVIZO.