Abstract
Ocean acidification is expected to impact the high latitude oceans first, as CO2 dissolves more easily in colder waters. At the current rate of anthropogenic CO2 emissions, the sub-Antarctic Zone will start to experience undersaturated conditions with respect to aragonite within the next few decades, which will affect marine calcifying organisms. Shelled pteropods, a group of calcifying zooplankton, are considered to be especially sensitive to changes in carbonate chemistry because of their thin aragonite shells. Limacina retroversa is the most abundant pteropod in sub-Antarctic waters, and plays an important role in the carbonate pump. However, not much is known about its response to ocean acidification. In this study, we investigated differences in calcification between L. retroversa individuals exposed to ocean carbonate chemistry conditions of the past (pH 8.19; mid-1880s), present (pH 8.06), and near-future (pH 7.93; predicted for 2050) in the sub-Antarctic. After 3 days of exposure, calcification responses were quantified by calcein staining, shell weighing, and Micro-CT scanning. In pteropods exposed to past conditions, calcification occurred over the entire shell and the leading edge of the last whorl, whilst individuals incubated under present and near-future conditions mostly invested in extending their shells, rather than calcifying over their entire shell. Moreover, individuals exposed to past conditions formed larger shell volumes compared to present and future conditions, suggesting that calcification is already decreased in today’s sub-Antarctic waters. Shells of individuals incubated under near-future conditions did not increase in shell weight during the incubation, and had a lower density compared to past and present conditions, suggesting that calcification will be further compromised in the future. This demonstrates the high sensitivity of L. retroversa to relatively small and short-term changes in carbonate chemistry. A reduction in calcification of L. retroversa in the rapidly acidifying waters of the sub-Antarctic will have a major impact on aragonite-CaCO3 export from oceanic surface waters to the deep sea.
Highlights
Since the start of the industrial revolution, the oceans have absorbed approximately 30% of anthropogenic CO2 emissions (Le Quéré et al, 2015; Gruber et al, 2019), which has caused a lowering in surface-ocean pH of ∼0.1 units from ∼8.2 to ∼8.1, a process referred to as “ocean acidification” (Caldeira and Wickett, 2003; Feely et al, 2004)
We found pronounced differences in calcification of L. retroversa during short-term exposure to ocean carbonate chemistry conditions representative of the past, present, and near-future
Our results show that active calcification shifted from whole shell calcification in many pteropods exposed to past conditions toward a predominance of apertural calcification in pteropods exposed to present and near-future conditions
Summary
Since the start of the industrial revolution, the oceans have absorbed approximately 30% of anthropogenic CO2 emissions (Le Quéré et al, 2015; Gruber et al, 2019), which has caused a lowering in surface-ocean pH of ∼0.1 units from ∼8.2 to ∼8.1, a process referred to as “ocean acidification” (Caldeira and Wickett, 2003; Feely et al, 2004). The decline of [CO32−] results in lower saturation states of the carbonate minerals calcite and aragonite (Gruber et al, 2019). This will have adverse consequences for a broad variety of marine organisms, those that precipitate calcium carbonate (CaCO3) shells or skeletons, such as coccolithophores, foraminifers, and molluscs (Riebesell et al, 2000; Orr et al, 2005; Hoegh-Guldberg et al, 2007; Moy et al, 2009; Gazeau et al, 2013; Kroeker et al, 2013; Waldbusser et al, 2015). The rise in atmospheric CO2, and, in turn, decreasing sea surface carbonate concentration are expected to have a profound impact on the calcification efficiency of marine calcifiers, and subsequently affect the strength of the carbonate pump
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