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

==== CCP6.2.1.2 Ocean warming and sea ice changes affect marine primary productivity ====

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In the central Arctic Ocean, primary productivity remains low ( ''medium confidence'' ), mostly due to persisting nutrient and light limitations ( [[#Randelhoff--2016|Randelhoff and Guthrie, 2016]] ; [[#Ardyna--2020|Ardyna and Arrigo, 2020]] ). In inflowing (Barents and Chukchi Sea) and interior shelf regions (Laptev, Kara, and Siberian Sea), changes in sea ice extent, thickness and seasonal timing have altered light and mixing regimes, causing increasing overall productivity in open-water and under-ice habitats, and in leads ( ''high confidence'' ) (Table CCP6.2) ( [[#Ardyna--2020|Ardyna and Arrigo, 2020]] ; [[#Lannuzel--2020|Lannuzel et al., 2020]] ). Productivity changes are associated with the earlier-onset phytoplankton spring blooms and the increasing occurrence of autumn blooms, particularly at lower latitudes of the Arctic ( ''high confidence'' ) (Table CCP6.2) ( [[#Tedesco--2019|Tedesco et al., 2019]] ; [[#Ardyna--2020|Ardyna et al., 2020]] ). Ice algal communities are expected to change in productivity and species composition in response to the transition from a predominantly multi-year to a seasonal sea ice pack ( ''high confidence'' ) ( [[#Meredith--2019|Meredith et al., 2019]] ; [[#Tedesco--2019|Tedesco et al., 2019]] ; [[#Lannuzel--2020|Lannuzel et al., 2020]] ). Thinner sea ice increases the likelihood of surface flooding, resulting in the occurrence of snow-infiltration algal communities, which have been described in the Atlantic sector of the Arctic Ocean ( [[#Fernández-Méndez--2018|Fernández-Méndez et al., 2018]] ) and observed by Indigenous Peoples off northern Greenland (Box CCP6.2). The observed transition from marine-terminating to land-terminating glaciers has a negative impact on coastal ecosystems in Greenland ( ''medium confidence'' ) ( [[#Meire--2017|Meire et al., 2017]] ; [[#Hopwood--2018|Hopwood et al., 2018]] ) and Svalbard ( [[#Halbach--2019|Halbach et al., 2019]] ), as land-terminating glacial meltwater input increases stratification, which hinders vertical mixing and lowers local productivity, whereas marine-terminating glaciers can trigger upwelling, which supplies nutrients and enables higher productivity in the summer ( [[#Hopwood--2020|Hopwood et al., 2020]] ). Macroalgae and seagrass are generally expanding in the Arctic ( ''medium confidence'' ), though there are negative trends in some regions, partly due to increased runoff and turbidity from melting glaciers ( [[#Hopwood--2020|Hopwood et al., 2020]] ; [[#Krause-Jensen--2020|Krause-Jensen et al., 2020]] ). In the future Arctic Ocean, higher light availability in response to further sea ice decline and reduced deep mixing is projected to generally increase primary productivity ( ''medium confidence'' ), leading to an increase in phytoplankton biomass from 2000 to 2100 by ~20% for SSP1-2.6 and ~30–40% for SSP5-8.5 (Chapter 3) ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). However, productivity may increase less than predicted and eventually even decrease once nutrient limitation outweighs the benefits of higher light availability ( ''low confidence'' ) ( [[#Randelhoff--2020|Randelhoff et al., 2020]] ; [[#Seifert--2020|Seifert et al., 2020]] ).

Despite large-scale environmental changes in the Southern Ocean, such as the deepening of the summer mixed layer ( ''medium confidence'' ) ( [[#Panassa--2018|Panassa et al., 2018]] ; [[#Sallée--2021|Sallée et al., 2021]] ), and the expected impacts via altered nutrient entrainment, light availability and grazer encounter rates (Chapter 3) ( [[#Behrenfeld--2014|Behrenfeld and Boss, 2014]] ; [[#Llort--2019|Llort et al., 2019]] ), assessments indicated no consistent changes in primary production at the circumpolar scale, as sectors and regions show different trends ( ''medium confidence'' ). Although a global assessment found no overall changes in circumpolar primary production from 1998 to 2015 (Table CCP6.2) ( [[#Gregg--2019|Gregg and Rousseaux, 2019]] ), another study showed an overall increase in phytoplankton biomass in the mixed layer over the period 1997–2019 ( [[#Pinkerton--2021|Pinkerton et al., 2021]] ). Primary productivity has increased in the Pacific sector and decreased in the Atlantic sector and the Ross Sea ( ''low confidence'' ) ( [[#Kahru--2017|Kahru et al., 2017]] ; [[#Henley--2020|Henley et al., 2020]] ; [[#Pinkerton--2021|Pinkerton et al., 2021]] ). Higher productivity has also been observed in regions where rapid environmental changes occurred, such as in the vicinity of retreating IS and declining sea ice cover off the Antarctic Peninsula ( ''medium confidence'' ) ( [[#Henley--2020|Henley et al., 2020]] ; [[#Rogers--2020|Rogers et al., 2020]] ), although diversity of phytoplankton may decrease with warming temperatures and less sea ice ( ''limited evidence'' ) ( [[#Lin--2021|Lin et al., 2021]] ). In the future Southern Ocean, stronger upwelling due to strengthened westerly winds is projected to increase primary productivity at the circumpolar scale in the Antarctic Zone and to the north of the sub-Antarctic Front, but not in the sub-Antarctic Zone ( ''low to medium confidence'' ) (Chapter 3) ( [[#Henley--2020|Henley et al., 2020]] ; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ; [[#Pinkerton--2021|Pinkerton et al., 2021]] ). The largest changes are projected to occur after 2100 at 2–6°C warming of the surface ocean ( [[#Moore--2018|Moore et al., 2018]] ). Such an increase in Southern Ocean productivity will lead to a decline in global ocean productivity ( ''medium confidence'' ), due to nutrient trapping ( [[#Moore--2018|Moore et al., 2018]] ) and altered ocean carbon uptake through ecosystem feedbacks ( [[#Hauck--2018|Hauck et al., 2018]] ).

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