Editing IPCC:AR6/WGII/Cross-Chapter-Paper-6 (section)

==== CCP6.2.1.3 Impacts of ocean acidification vary spatially and among biotas ====

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In Arctic seas, areas with acidification levels corrosive to organisms forming CaCO 3 shells or skeletons expanded between the 1990s and 2010 ( ''high confidence'' ), with instances of extreme aragonite under-saturation ( [[#Ding--2017|Ding et al., 2017]] ; [[#Zhang--2020|Zhang et al., 2020]] ). Key species of diatom and picoeukaryote phytoplankton species yet appear relatively resilient to decreasing pH levels over a range of temperature and light conditions ( ''medium confidence'' ) (Table CCP6.2) ( [[#Thoisen--2015|Thoisen et al., 2015]] ; [[#Wolf--2018|Wolf et al., 2018]] ; [[#White--2020|White et al., 2020]] ). In contrast, there is evidence for species- and stage-specific sensitivities of zooplankton, pteropods and fishes ( ''high confidence'' ) (Table CCP6.2) ( [[#Bailey--2016|Bailey et al., 2016]] ; [[#Dahlke--2018|Dahlke et al., 2018]] ; [[#Thor--2018|Thor et al., 2018]] ). Warming, rising river-sediment discharge and coastal erosion in Arctic shelf regions are expected to increase the input of labile, often permafrost-derived organic carbon, the remineralisation of which further increases acidification rates ( ''medium confidence'' ) ( [[#Semiletov--2016|Semiletov et al., 2016]] ; [[#AMAP--2018b|AMAP, 2018b]] ; [[#Bröder--2018|Bröder et al., 2018]] ). Interactions with other physical changes, such as warming or freshening, are expected to aggravate the impacts of ocean acidification (Chapter 3) ( [[#Falkenberg--2018|Falkenberg et al., 2018]] ).

In the Southern Ocean, calcifying organisms are also most vulnerable to ocean acidification ( ''high confidence'' ) (Table CCP6.2), as evidenced by rates of calcification declining by 3.9% between 1998 and 2014 ( [[#Freeman--2015|Freeman and Lovenduski, 2015]] ). Calcifying species with low-magnesium calcite or mechanisms to protect their skeletons are less vulnerable to the corrosive effects of acidification than those using aragonite or high-magnesium calcite ( ''high confidence'' ) ( [[#Figuerola--2021|Figuerola et al., 2021]] ). In diatom-dominated communities, silicification diminishes with reduced pH levels, albeit with rates differing among taxa ( ''low confidence'' ) ( [[#Petrou--2019|Petrou et al., 2019]] ). Species-specific responses exist regarding growth and primary production, which are further strongly modulated by iron and light availability ( ''high confidence'' ) ( [[#Hoppe--2013|Hoppe et al., 2013]] ; [[#Trimborn--2013|Trimborn et al., 2013]] ; [[#Hoppe--2015|Hoppe et al., 2015]] ; [[#Henley--2020|Henley et al., 2020]] ; [[#Seifert--2020|Seifert et al., 2020]] ). A meta-analysis yielded different CO 2 thresholds for Antarctic organismal groups; for example, negative impacts emerged at &gt;1000 μ atm CO 2 in phytoplankton and at &gt;1500 μ atm CO 2 in invertebrates, whereas bacterial abundance was positively affected by ocean acidification ( [[#Hancock--2020|Hancock et al., 2020]] ). Species sensitivity can also differ strongly between life-cycle stages (Chapter 3.3.2). For instance, eggs and embryos of Antarctic krill are negatively impacted at &gt;1250 μ atm CO 2 whereas adults can thrive even at 1000–2000 µatm CO 2 over one year ( [[#Kawaguchi--2013|Kawaguchi et al., 2013]] ; [[#Ericson--2018|Ericson et al., 2018]] ).

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