Establishing a baseline of the annual variability of pteropod shell thickness in the Cariaco Basin

Pteropods are considered to be the 'canary in the coal mine' for ocean acidification due to their thin aragonitic shells and high abundance in polar oceans. Culture and incubation studies of calcareous plankton find that calcification rates decrease as seawater carbonate saturation decreases. This decreased calcification be may be manifested in the formation of smaller or thinner shells.

Despite the importance of pteropods in ocean acidification research, there are limited data on the natural variability of shell thickness and other shell properties. This makes it difficult to differentiate between environmentally-caused change and natural variability in pteropod shell properties. Here we address this issue by studying the shells of the thecosome pteropod Heliconoides inflatus from a sediment trap in the Cariaco Basin, deployed as part of the CARIACO timeseries. We use a combination of light microscopy and micro-CT scanning to study how the size and thickness of 45 H. inflatus shells change at eight time-points through a year.

We find that the thickness of the pteropod shells is broadly bi-model, with thicker shells found during times of upwelling and thinner shells found during times with no upwelling. The seawater in the Cariaco Basin was supersaturated with respect to aragonite throughout our study and variation in aragonite saturation does not appear to affect shell size or thickness. Instead, pteropod shell thickness correlates with water temperature and nutrient concentrations with the thinnest shells found when water is warm and nutrient poor. This implies these factors are significant in controlling shell development and should be evaluated in parallel with saturation state in future studies on the impact of the changing ocean on calcareous plankton.

The results from this study provide a baseline of annual variability of H. inflatus shell size and thickness in the Cariaco Basin. In a broader context, these results can help inform studies in different ocean basins, and focused on other species, to determine the key factors that impact pteropod shell properties globally.