Dissolution dominating calcification process in polar pteropods close to the point of aragonite undersaturation

Sea snails that build aragonite shells are also known as pteropods. They are a prolific upper-ocean zooplankton, especially abundant in high-latitudinal environments in the Arctic and the Southern Ocean but also found in highly productive upwelling regimes such as the California Current System. They represent a food source for higher trophic levels, including varieties of fish, birds, and whales, and they play a key role in energy transfer and carbon fluxes in these regions by exerting a high grazing pressure with large feeding webs, faeces, and pseudofaeces sinking rapidly and transferring carbon to the ocean interior.

Ocean acidification lowers aragonite saturation levels by decreasing the concentration of carbonate ions in the water column, which results from buffering increased uptake of atmospheric CO2. Consequently, several incubation studies with pteropods have shown rates of calcification decrease and shell dissolution increase in these organisms. After discovering severe dissolution on pteropod shells as a consequence of ocean acidification along the US West Coast and the Southern Ocean, scientists then considered the questions: (1) Can these shelled species offset severe shell dissolution by more intense calcification? and (2) What impacts would it have on them?

We measured gross dissolution in the pteropod Limacina helicina antarctica in the Scotia Sea (Southern Ocean) by incubating living pteropod specimens in high CO2 conditions over a range of plausible future aragonite saturation states (Ω) for 14 days. Specimens started dissolving almost immediately upon exposure to undersaturated conditions, even at transitional levels of Ω ≈ 1.0, with much higher dissolution rates as Ω decreased to 0.8, where we recorded the loss of 1.4% of shell mass per day. Further analyses showed that dissolution became the dominating process around the transitional level of 1.0, net shell growth ceased at Ω = 1.03, and gross dissolution increasingly dominated net change in shell mass as saturation levels decreased below 1.0.

Even under optimistic emission scenarios, in some of the upwelling regimes and polar oceanic regions, calcium carbonate dissolution will be 30–50% higher by 2050 than in pre-industrial times. In such conditions, our results indicate that pteropods would not be capable of calcifying enough to offset dissolution and to grow in shell mass. The consequence is a severe loss of suitable habitat for aragonite calcifiers, as well as depletion of the rate of carbon and carbonate flux to the deep ocean. As confidence intervals of future projections increasingly narrow, the argument is progressing beyond whether suitable habitat will be lost to when it will be lost, and to what extent. The application of results obtained in this study enable regions of imminent habitat loss to be identified and monitored, and consequences to be estimated.

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