Assessing the Impact of Ocean Acidification
Authors:   , Koester Bryce , Sessa Jocelyn 
Published: November 2022

Since the onset of the Industrial Revolution, the world's oceans have absorbed approximately one third of all anthropogenic CO₂ emissions and are experiencing acidification as a result. Pteropods are a marine group of snails that are vulnerable to acidification due to their thin shells composed of aragonite, which is 50% more soluble than calcite. Due to their vulnerability and ubiquity throughout the world's oceans, pteropods are considered bioindicators of ocean acidification; their responses include decreased size, reduced shell thickness, and increased shell dissolution. Shell dissolution has been measured using a variety of metrics involving light microscopy, scanning electron microscopy (SEM), and computed tomography (CT). Assessing which method(s) effectively capture acidification's impact on pteropod shells is still an active area of research. While CT and SEM metrics offer high resolution imaging, these analyses are cost- and time-intensive relative to light microscopy analyses and may be inaccessible for ocean monitoring projects and research. This research compares light microscopy, CT, and SEM shell dissolution metrics across three pteropod species: Limacina helicina, Limacina retroversa, and Heliconoides inflatus. Sourced from multiple localities, these taxa lived in tropical to subpolar environments and were exposed to varying aragonite saturations states due to stark oceanographic differences in these environments. Specimens were evaluated using light microscopy for the Limacina Dissolution Index (LDX), using SEM for average and maximum dissolution type, and using CT for shell thickness. Spearman correlation tests were run among the dissolution metrics within each species dataset and significance was assessed both before and after Bonferroni correction. Before Bonferroni correction, LDX and SEM average dissolution type were highly correlated for both the Limacina retroversa (rho = 0.81, p > 0.001) and Heliconoides inflatus (rho = 0.79, p > 0.001) datasets, and remained significant after Bonferroni correction. For Limacina retroversa, LDX was also significantly correlated to SEM maximum dissolution type (rho = 0.77, p > 0.001). The CT metrics for shell thickness were not significantly correlated to any other dissolution metrics for any species. However, severely dissolved (type 3) areas apparent in SEM were also visually discernible in CT thickness heatmaps. Although the genera Heliconoides and Limacina have different shell microstructures, the relationship between LDX and SEM average dissolution type did not vary by species. Additionally, the Heliconoides inflatus specimens were sourced from both the aragonite-undersaturated California Current and the aragonite-oversaturated Cariaco Basin; however, the differing localities and their respective oceanographic conditions did not have a significant influence on the relationship between LDX and SEM average dissolution. Overall, these findings show that the cheaper and faster LDX method, which needs only a light microscope, is a promising method for detecting dissolution resulting from ocean acidification across multiple species and oceanographic conditions