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

=== CCP6.2.1 Marine and Coastal Ecosystems ===

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==== CCP6.2.1.1 Warming and sea ice retreat cause shifts in distribution ranges of species ====

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In Arctic seas, warming and other climate impact drivers, primarily sea ice retreat, have led to range contractions of Arctic marine and ice-associated species and poleward expansions of boreal species ( ''very high confidence'' ) (Table CCP6.2) ( [[#Bouchard--2020|Bouchard and Fortier, 2020]] ; [[#Huntington--2020|Huntington et al., 2020]] ; [[#Mueter--2020|Mueter et al., 2020]] ) even though light and energetics at seasonal extremes may limit some range shifts ( ''limited evidence'' ) ( [[#Ljungström--2021|Ljungström et al., 2021]] ). Altered conditions allow more microorganisms to move poleward and provide opportunities for invasive species ( [[#Cavicchioli--2019|Cavicchioli et al., 2019]] ; [[#Nielsen--2020|Nielsen et al., 2020]] ; Mustonen, 2021). Phytoplankton communities harbour increasing numbers of taxa, including harmful species ( [[#Lovejoy--2017|Lovejoy et al., 2017]] ) and the coccolithophore ''Emiliania huxleyi'' , which meanwhile forms regular blooms in the Barents Sea ( [[#Neukermans--2018|Neukermans et al., 2018]] ; [[#Silkin--2020|Silkin et al., 2020]] ). Northward shifts of pelagic, benthic and demersal species and subsequent changes in Arctic community composition have been observed in the Bering, Greenland and Barents Seas ( [[#Grebmeier--2018|Grebmeier et al., 2018]] ; [[#Mueter--2020|Mueter et al., 2020]] ), as have higher numbers of economically important boreal species such as haddock and Pacific and Atlantic cod (CCP6.2.3). Cold-adapted Arctic fish species such as polar cod ( ''Boreogadus saida'' ) are expected to decline further and lose spawning habitats at GWL &gt;1.5°C, mainly due to a lack of phenotypic plasticity, as well as increasing interspecific competition with and predation from invading boreal species ( [[#Dahlke--2018|Dahlke et al., 2018]] ; [[#Marsh--2020|Marsh and Mueter, 2020]] ). Numerous mammals and sea birds respond to changes in the distribution of their preferred habitats and prey by shifting their range, altering the timing or pathways for migration or switching prey ( ''very high confidence'' ) ( [[#Hamilton--2017|Hamilton et al., 2017]] ; [[#Loseto--2018|Loseto et al., 2018]] ; [[#Meredith--2019|Meredith et al., 2019]] ). Ice-breeding seals (e.g., harp seals – ''Pagophilus groenlandicus'' ) often have little scope to shift distribution, leading to increases in strandings and pup mortality in years with little ice cover ( ''medium confidence'' ) (Table CCP6.2) ( [[#Boveng--2020|Boveng et al., 2020]] ). Recent studies confirm that polar bears ( ''Ursus maritimus'' ) are negatively affected by changing ice and snow conditions with decreases in denning, foraging, reproduction, genetic diversity and survival rates ( ''very high confidence'' ) (Table CCP6.2) ( [[#Boonstra--2020|Boonstra et al., 2020]] ; [[#Johnson--2020|Johnson and Derocher, 2020]] ; [[#Maduna--2021|Maduna et al., 2021]] ).

In the Southern Ocean, southward range shifts are expected to result from increased warming coupled with the narrow thermal tolerance of cold-adapted Antarctic species ( [[#Convey--2019|Convey and Peck, 2019]] ; [[#Morley--2019|Morley et al., 2019]] ; [[#Gutt--2021|Gutt et al., 2021]] ). Such shifts have so far only been detected for Antarctic krill ( ''Euphausia superba'' ), with a poleward contraction of the highest densities of krill in the Atlantic sector ( ''medium confidence'' ) (Table CCP6.2); ( [[#Atkinson--2019|Atkinson et al., 2019]] ). Ocean warming is expected to put pressure on Antarctic phytoplankton ( [[#Pinkerton--2021|Pinkerton et al., 2021]] ) and fish species unable to move further south in shelf areas, including waters off sub-Antarctic islands ( ''low confidence'' ) (Table CCP6.2) ( [[#Caccavo--2021|Caccavo et al., 2021]] ). Off the Antarctic Peninsula and sub-Antarctic islands, invasive benthic invertebrates and macroalgae have already been detected ( ''medium confidence'' ) ( [[#Fraser--2018|Fraser et al., 2018]] ; [[#Avila--2020|Avila et al., 2020]] ; [[#Brasier--2021|Brasier et al., 2021]] ), and projected changes will further favour the spread of invasive species ( [[#Fraser--2020|Fraser et al., 2020]] ; [[#Macaya--2020|Macaya et al., 2020]] ). On a local to regional scale, the benthic recolonisation of the newly exposed seabed after the disintegration of ice shelves shows typical succession patterns, with mass occurrences of few pioneer species followed by gradual shifts to a more diverse typical shelf community, driven by increasing pelagic primary production upon ice-shelf collapse and strengthening of the pelagic–benthic coupling ( ''high confidence'' ) ( [[#Brasier--2021|Brasier et al., 2021]] ; [[#Gutt--2021|Gutt et al., 2021]] ). Range changes of Antarctic birds and marine mammals have been observed, which vary among sub-regions and are mostly attributable to changes in sea ice extent and food availability ( ''high confidence'' ) (Table CCP6.2) ( [[#Gutt--2018|Gutt et al., 2018]] ; [[#Convey--2019|Convey and Peck, 2019]] ; [[#Bestley--2020|Bestley et al., 2020]] ). With projected sea ice retreat and associated change in prey distribution ( [[#Henley--2020|Henley et al., 2020]] ), foraging areas of sub-Antarctic sea birds and marine mammals will shift southwards, leading to elevated pressure on populations due to higher foraging costs during the breeding season ( ''medium confidence'' ) ( [[#Ropert-Coudert--2018|Ropert-Coudert et al., 2018]] ; [[#Bestley--2020|Bestley et al., 2020]] ; [[#Hindell--2020|Hindell et al., 2020]] ; [[#Hückstädt--2020|Hückstädt et al., 2020]] ; [[#Wege--2021|Wege et al., 2021]] ). These changes are particularly impacting emperor penguins ( ''Aptenodytes forsteri'' ) (Table CCP6.2), with the projected population declining close to extinction by 2100 under Business-As-Usual climate scenarios ( ''medium confidence'' ) ( [[#Jenouvrier--2020|Jenouvrier et al., 2020]] ; [[#Trathan--2020|Trathan et al., 2020]] ; [[#Jenouvrier--2021|Jenouvrier et al., 2021]] ), whereas population decline is halted by 2060 under the 1.5°C climate scenario ( ''low confidence'' ) ( [[#Jenouvrier--2020|Jenouvrier et al., 2020]] ).

'''Table CCP6.2 |''' Summary of observed impacts (and projected risks of climate change for polar marine, terrestrial and freshwater ecosystems identified in [[IPCC:Wg2:Chapter:Chapter-3#3.2.3|Section 3.2.3]] and Box 3.4 in [[IPCC:Wg2:Chapter:Chapter-3|Chapter 3]] of the IPCC SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ).

{| class="wikitable"
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! '''Affected system'''

! '''Hazard'''

'''*Cascading effect'''

! '''Observed impacts, future risks and natural adaptations identified in SROCC (confidence level)'''

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| colspan="3"| ''Arctic marine ecosystems''

|-
| Primary producers (PP-1)

| Sea ice loss

\* Freshening

\* Stratification

| Impact: timing (earlier and later blooms), distribution and magnitude (&gt;30% increase in annual net primary production since 1998) ( ''high confidence'' )

|-
|
| Acidification

| Adaptation: phytoplankton may compensate for decrease in pH

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| Zooplankton

| \* PP-1

| Impact: changing production and community composition ( ''medium confidence'' )

|-
| Benthos

| \* PP-1

| Impact: changing production and biodiversity ( ''medium confidence'' )

|-
|
| Acidification

| Risk: effects on zooplankton and pteropods depends on climate scenario and species’ sensitivity/adaptive capacity

|-
| Fish

| Warming

\* Prey changes

| Impact: northward expanding ranges of sub-Arctic/boreal species (e.g., Atlantic cod) in Bering Sea (Detection— ''high confidence'' , Attribution— ''medium confidence'' ) negatively affecting Arctic polar cod ( ''medium confidence'' )

|-
|
| \* Prey declines

| Risk: decreasing production of walleye pollock, Pacific cod and arrowtooth flounder, due to declines in large copepods ( ''medium confidence'' )

|-
| Birds and marine mammals

| Sea ice loss

| Impact: phenological, behavioural, physiological and distributional changes; endemic marine mammals have little scope to move northwards in response to warming ( ''high confidence'' )

|-
| Polar bears

| Sea ice timing, distribution, thickness

| Impact: phenological shifts, and changes in distribution, denning, foraging behaviour and survival rates ( ''hi'' ''gh co'' ''nfidence'' )

|-
| colspan="3"| ''Antarctic marine ecosystems''

|-
| Primary productivity

| Sea ice loss

\* Freshening

\* Stratification

| Impact: little overall change in biomass at circumpolar scale from 1998 to 2006, but sub-regional differences ( ''medium confidence'' ); changes difficult to detect and attribute to climate change

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| Microbes

| Acidification

| Impact: detrimental effect on primary production and changes to the structure and function of microbial communities ( ''medium confidence'' )

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| Antarctic krill

| Warming

| Impact: declines in abundance in the South Atlantic sector ( ''medium confidence'' ); may not represent a long-term, climate-driven trend but a decline following a period of anomalous peak abundance ( ''low confidence'' )

|-
|
| Risk: southward range shift due to changes in the location of the optimum conditions for growth and recruitment, with decreases most apparent in the areas with the most rapid warming, such as the southwest Atlantic/Weddell Sea region ( ''medium confidence'' )

|-
| Zooplankton

| Acidification

| Risk: vulnerability of pteropods through effects on eggs ( ''medium confidence'' )

|-
| Benthos

| Sea ice loss

| Risk: increase of biomass on the Antarctic continental shelf as productivity from longer phytoplankton blooms outweighs ice-scour mortality ( ''low confidence'' )

|-
|
| Sea ice loss

| Risk: shallow-water communities may become dominated by macroalgae due to increases in the amount of light (possible loss of endemic species by 12% due to warming temperatures) ( ''low confidence'' )

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| Fish

| Warming

| Risk: icefish may be displaced from shallow regions around sub-Antarctic islands ( ''low confidence'' )

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| Birds and marine mammals

| Sea ice cover

| Impact: predictability of foraging grounds and sea ice cover associated with climate are main drivers of population changes: increases for gentoo penguins (decreases for Adélie, chinstrap, king and Emperor penguins) ( ''high confidence'' )

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| colspan="3"| ''Arctic terrestrial and freshwater ecosystems''

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| Vegetation

| Warming

| Impact: greening ( ''high confidence'' )

|-
|
| Risk: decrease in tundra areal extent &gt;50% by 2050; wood shrubs expected to increase ( ''medium confidence'' )

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| Vertebrates

| Warming

| Impact: expanding range into Arctic

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| Freshwater primary productivity

| \* Increased runoff

\* Increased permafrost thaw

| Impact: increased productivity in rivers, lakes and coastal areas

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|
| Risk: expected to mobilise stores of pollutants

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| Pathogens

| Warming

| Impact: expanding range into Arctic

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|
| Risk: mobilisation may increase in high latitudes, including anthrax from frozen carcasses possibly released from permafrost

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| Fish

| \* Freshwater winter habitat

\* Increased discharge

| Risk: disruption of the life history of Arctic freshwater fish

|-
|
| \* Warming freshwater

| Risk: may make some surface waters inhospitably warm for cold water fish species

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| Biodiversity

| Warming

| Impact: sub-Arctic biodiversity expanding into Arctic

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| Reindeer/caribou

| Climate factors

| Impact: reindeer/caribou declined overall without adaptation ( ''high confidence'' ), with climate affecting many aspects of their life history ( ''medium confidence'' )

|-
|
| Risk: domesticated reindeer/caribou can be affected by fire, which reduces pasture, as well as by increased ice-on-snow, which can cause starvation

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| colspan="3"| ''Antarctic terrestrial and freshwater ecosystems''

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| Terrestrial biota

| \* Increased coastal ice melt

| Impact: increasing coastal ice-free areas available for colonisation ( ''high confidence'' )

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| Alien species

| Warming

| Risk: barriers to alien species reduce, affecting terrestrial biodiversity ( ''medium confidence'' )

|}

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==== 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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==== 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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==== CCP6.2.1.4 Climate change alters food web dynamics ====

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Climate change has transformed Arctic marine ecosystems from sea ice-associated to open-water production regimes, with profound impacts on trophic energy transfer efficiencies and pathways ( ''high confidence'' ) ( [[#Behrenfeld--2017|Behrenfeld et al., 2017]] ; [[#Meredith--2019|Meredith et al., 2019]] ; [[#Huntington--2020|Huntington et al., 2020]] ) as well as benthic–pelagic coupling ( ''medium confidence'' ) ( [[#Birchenough--2015|Birchenough et al., 2015]] ; [[#Degen--2016|Degen et al., 2016]] ; [[#Solan--2020|Solan et al., 2020]] ). Shifts in bloom phenology favour small phytoplankton and smaller zooplankton over large lipid-rich macro-zooplankton, leading to longer, less efficient food chains ( ''medium confidence'' ) ( [[#Aarflot--2018|Aarflot et al., 2018]] ; [[#Feng--2018|Feng et al., 2018]] ; [[#Kimmel--2018|Kimmel et al., 2018]] ; [[#Weydmann--2018|Weydmann et al., 2018]] ; [[#Møller--2020|Møller and Nielsen, 2020]] ). In the Beaufort Sea and Svalbard waters, earlier spring phytoplankton blooms have resulted in a mismatch in dynamics between microalgae and herbivorous copepods ( [[#Renaud--2018|Renaud et al., 2018]] ; [[#Dezutter--2019|Dezutter et al., 2019]] ). In the Bering Sea, zooplankton declines following the particularly pronounced sea ice retreats in 2017 and 2018 were associated with reduced forage fish production ( [[#Duffy-Anderson--2019|Duffy-Anderson et al., 2019]] ) as well as multi-trophic mortality of ctenophore, fish, bird and mammal species, coupled with severe emaciation, reproductive failure, disease and high mortality rates of sea bird predators ( [[IPCC:Wg2:Chapter:Chapter-14#14.4|Section 14.4.4.2]] ) ( [[#Jones--2019|Jones et al., 2019]] ; [[#Maekakuchi--2020|Maekakuchi et al., 2020]] ; [[#Piatt--2020|Piatt et al., 2020]] ; [[#Romano--2020|Romano et al., 2020]] ). Species range shifts have restructured higher trophic levels in Arctic food webs ( ''high confidence'' ) (Table CCP6.2; CCP6.2.3.3 Chapter 3) ( [[#Huntington--2020|Huntington et al., 2020]] ). In the northern Barents Sea, increased predation mortality for key species and incursions of boreal fish have induced entire ecosystem reorganisation ( [[#Degen--2016|Degen et al., 2016]] ; [[#Pecuchet--2020a|Pecuchet et al., 2020a]] ; [[#Pecuchet--2020b|Pecuchet et al., 2020b]] ). Regional taxonomic and functional diversity increased with immigration of boreal species, although the ongoing decline in Arctic species suggests high species turnover (Table CCP6.2) ( [[#Frainer--2017|Frainer et al., 2017]] ). Recent marine heatwaves induced rapid and profound food web changes unprecedented over the last four decades ( [[#Siddon--2020|Siddon et al., 2020]] ).

Climate impacts on Arctic marine food webs will be profound and intensify with GWL ( ''high confidence'' ), regardless of mitigation scenarios due to multi-decadal lags in sea ice extent and atmospheric carbon (WGI) ( [[#Jones--2020|Jones et al., 2020]] ). However, the exact nature of these impacts remains unclear due to attenuating and amplifying dynamics of both top-down and bottom-up processes in polar food webs and the management of fisheries ( ''high confidence'' ) (Chapter 3) ( [[#Cavicchioli--2019|Cavicchioli et al., 2019]] ; [[#Meredith--2019|Meredith et al., 2019]] ). Projected sea ice loss is associated with a &gt;50% decline in the density of large zooplankton species by 2100 (relative to early 21st century levels) in the southern Bering Sea and a net increase in large zooplankton in the Northern Bering Sea in scenarios without carbon mitigation (Representative Concentration Pathway (RCP) 8.5), whereas these declines are roughly half the magnitude under moderate mitigation scenarios (RCP4.5) ( [[#Hermann--2019|Hermann et al., 2019]] ; [[#Kearney--2020|Kearney et al., 2020]] ). Warming is expected to reduce the quantity and quality of lipid-rich copepod prey ( ''high confidence'' ) ( [[#Aarflot--2018|Aarflot et al., 2018]] ; [[#Kimmel--2018|Kimmel et al., 2018]] ; [[#Bouchard--2020|Bouchard and Fortier, 2020]] ; [[#Møller--2020|Møller and Nielsen, 2020]] ; [[#Mueter--2020|Mueter et al., 2020]] ), leading to declines in survival and growth of multiple upper-trophic level fish species; these impacts are amplified over time under low mitigation scenarios (RCP8.5) ( ''high confidence'' ) (CCP6.2.1.1) ( [[#Dahlke--2018|Dahlke et al., 2018]] ; [[#Holsman--2020|Holsman et al., 2020]] ; [[#Mueter--2020|Mueter et al., 2020]] ; [[#Oke--2020|Oke et al., 2020]] ; [[#Reum--2020|Reum et al., 2020]] ; [[#Thorson--2020|Thorson et al., 2020]] ; [[#Whitehouse--2021|Whitehouse et al., 2021]] ). Marine mammals and sea birds will continue to attenuate climate change impacts by shifting their diets and behaviour ( ''medium confidence'' ) (Table CCP6.2) ( [[#Hamilton--2017|Hamilton et al., 2017]] ; [[#Lowther--2017|Lowther et al., 2017]] ; [[#Lydersen--2017|Lydersen et al., 2017]] ; [[#Vihtakari--2018|Vihtakari et al., 2018]] ; [[#Boveng--2020|Boveng et al., 2020]] ). However, sea birds generally have low temperature-mediated plasticity of reproductive timing, making them vulnerable to mismatches with their prey and limiting long-term adaptation ( ''medium confidence'' ) ( [[#Keogan--2018|Keogan et al., 2018]] ; [[#Kharouba--2020|Kharouba and Wolkovich, 2020]] ; [[#Piatt--2020|Piatt et al., 2020]] ; [[#Samplonius--2021|Samplonius et al., 2021]] ).

Many factors have contributed to changes in Antarctic food webs, including historical exploitation of fish and marine mammals as well as changes driven by the ozone hole and climate factors ( [[#Meredith--2019|Meredith et al., 2019]] ; [[#Morley--2020|Morley et al., 2020]] ; [[#Grant--2021|Grant et al., 2021]] ). Most documented changes resulting from warming and sea ice losses relate to shifts in ranges and dynamics of species, with most impacts occurring around the Antarctic Peninsula (CCP6.2.1.1; Table CCP6.2).

The projected general rise in primary production in Antarctic seas by 2100 (CCP6.2.1.2) suggests a concomitant increase in the abundance of higher trophic species, but changes in the structure and function of food webs will vary ( [[#McCormack--2021|McCormack et al., 2021]] ; McCormack, accepted) depending on regional differences in changing drivers ( [[#Morley--2020|Morley et al., 2020]] ; [[#Cavanagh--2021|Cavanagh et al., 2021]] ; [[#Grant--2021|Grant et al., 2021]] ). Primary production in open water habitats is expected to be supported by smaller phytoplankton species in the future ( [[#Henley--2020|Henley et al., 2020]] ), which could increase the relative importance of the copepod-mesopelagic fish pathway (McCormack, accepted), because krill prefer larger diatoms as food ( [[#Siegel--2016|Siegel, 2016]] ). The optimum habitat for Antarctic krill is expected to decline with a shortening of suitable season for krill growth and reproduction, particularly in the northern Scotia and Bellingshausen Seas ( ''medium confidence'' ) ( [[#Veytia--2020|Veytia et al., 2020]] ), although changes may be difficult to distinguish from natural variability until later in the century ( [[#Sylvester--2021|Sylvester et al., 2021]] ). More subtle and unpredictable changes may occur in the structure and relative importance of energy pathways in the food webs ( [[#Trebilco--2020|Trebilco et al., 2020]] ). Small mesopelagic fish are increasingly recognised for their importance as mid-trophic level species in the Southern Ocean, particularly in the sub-Antarctic zone ( [[#Caccavo--2021|Caccavo et al., 2021]] ) and Central Indian Sector ( [[#Subramaniam--2020|Subramaniam et al., 2020]] ; [[#McCormack--2021|McCormack et al., 2021]] ). Although salps have long been considered to be competitors of Antarctic krill ( [[#Suprenand--2017|Suprenand and Ainsworth, 2017]] ; [[#Rogers--2020|Rogers et al., 2020]] ), they provide a third energy pathway in pelagic food webs and, given the changing ocean conditions and their preference for smaller phytoplankton, may increase in importance for copepods ( ''low confidence'' ) ( [[#Plum--2020|Plum et al., 2020]] ; [[#Trebilco--2020|Trebilco et al., 2020]] ; [[#McCormack--2021|McCormack et al., 2021]] ; [[#Pauli--2021|Pauli et al., 2021]] ; McCormack, accepted). Declining ice shelves, such as those off the Antarctic Peninsula, will open up new pelagic and benthic habitats (CCP6.2.1.1) with expected increases in productivity of benthic assemblages in the new areas ( [[#Barnes--2017|Barnes, 2017]] ; [[#Morley--2020|Morley et al., 2020]] ; [[#Brasier--2021|Brasier et al., 2021]] ; [[#Gutt--2021|Gutt et al., 2021]] ).

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