Shell dissolution observed in Limacina helicina antarctica from the Ross Sea, Antarctica: paired shell characteristics and in situ seawater chemistry

Abstract. The euthecosome (shelled) Antarctic pteropod, Limacina helicina antarctica, is a dominant member of the Southern Ocean macrozooplankton community, and due to its aragonitic shell, is thought to be particularly vulnerable to ocean acidification and under-saturation conditions that are predicted in the future. Notably, pteropods in surface waters and near the continental shelf in the Ross Sea are highly vulnerable as these regions are predicted to be seasonally under-saturated within 2–3 decades. Carbonate chemistry data are rare for this region and here we present the results of a 6-week field study and report patterns of dissolution of juvenile pteropods along with carbonate chemistry of seawater at the time of collection. Conducted in McMurdo Sound in the south Ross Sea in the Pacific sector of the Southern Ocean, L. h. antarctica was successfully collected in plankton tows through the fast sea ice at a single station at 50 m. During the 6-week field study, ocean pH was relatively stable, ranging from 7.988 in October to 8.029 by early December. Calculated saturation states for aragonite (Ω_arag) over the 6-week study period ranged from 1.16 to 1.24. Pteropods collected at each sampling time point were prepared for SEM and analysis revealed that roughly 63 % of the shells displayed some degree of shell irregularities suggesting that active dissolution of the aragonitic shell was ongoing under in situ conditions. These results add to the accumulating evidence that shelled pteropods of the Southern Ocean are experiencing aragonite under-saturation events in the present-day that lead to a majority of individuals displaying shell dissolution. Predicted changes to the carbonate system in the Southern Ocean from ocean acidification will likely expand the intensity and duration of these under-saturation events, increasing the need to better understand how pteropods will fare in response to ocean acidification.