Pteropods as early‐warning indicators of ocean acidification

Aragonite undersaturation ( 1) events are projected to rapidly increase in frequency and duration in the Antarctic Weddell Sea by 2050. Thecosome pteropods (pelagic snails) are bioindicators of ocean acidification (OA) because their aragonite shell dissolves easily at low saturation states. Here, we describe the shell dissolution state of the pteropod Limacina helicina antarctica in relation to the water column in the southern Weddell Sea during austral summer 2018 as benchmark for future monitoring of ongoing OA. depth profiles at the sampling sites were consistently close to or in the range of threshold levels ( 1.1–1.3) for pteropod shell dissolution. Pteropods contributed up to 69% of total mesozooplankton biomass, and their distribution correlated positively with and chlorophyll a concentration. When analyzed with scanning electron microscopy, 78% of the investigated shells exhibited dissolution, and 50–69% showed the more severe Type II dissolution exceeding current projections of pteropod shell dissolution for the Southern Ocean. But importantly, in our study, only two specimens had the most severe Type III dissolution. Dissolution often co‐occurred with and occurred in scratch marks of unclear origin supporting notions that an intact periostracum protects the shell from dissolution. Where dissolution occurred in the absence of scratches or absence of evidence of periostracum breaches, microscale/nanoscale breaches may have been an important pathway for dissolution commencement supporting recent findings of a reduction of the organic shell content caused by low /low pH. The dissolution benchmark we provide here allows future application of pteropods as early‐warning indicators of presumably progressing OA in the Weddell Sea.