Editing IPCC:AR6/SROCC/Chapter-3 (section)

==== 3.2.3.2 3.2.3.2 Southern Ocean ====

<div id="section-3-2-3-2-southern-ocean-block-1"></div>

Marine ecosystem dynamics in the Antarctic region are dominated by the ACC and its frontal systems (Cross-Chapter Box 7 in Chapter 3), subpolar gyres, polar seasonality, the annual advance and retreat of sea ice (Section 3.2.1.1) and the supply of limiting micronutrients for productivity (most commonly iron) (Section 5.2.2.5). Antarctic krill ( ''Euphausia superba'' ) play a central role in Southern Ocean foodwebs as grazers and as prey items for fish, squid, marine mammals and seabirds (Schmidt and Atkinson, 2016 <sup>[[#fn:r683|683]]</sup> ; Trathan and Hill, 2016 <sup>[[#fn:r684|684]]</sup> ) (SM3.2.6). This is due in part to the high abundance and circumpolar distribution of Antarctic krill, although the abundance and importance of this species varies between different regions of the Southern Ocean (Larsen et al., 2014 <sup>[[#fn:r685|685]]</sup> ; Siegel, 2016 <sup>[[#fn:r686|686]]</sup> ; McCormack et al., 2017 <sup>[[#fn:r687|687]]</sup> ). Recent work has characterised the nature of habitat change for Southern Ocean biota at regional and circumpolar scales (Constable et al., 2014 <sup>[[#fn:r688|688]]</sup> ; Gutt et al., 2015 <sup>[[#fn:r689|689]]</sup> ; Constable et al., 2016 <sup>[[#fn:r690|690]]</sup> ; Hunt et al., 2016 <sup>[[#fn:r691|691]]</sup> ; Gutt et al., 2018 <sup>[[#fn:r692|692]]</sup> ), and the direct responses of biota to these changes (Constable et al., 2014 <sup>[[#fn:r693|693]]</sup> ) (summarised in Figure 3.6). These findings indicate that overlapping changes in key ocean and sea ice habitat characteristics (temperature, sea ice cover, iceberg scour, mixed layer depth, aragonite undersaturation; Sections 3.2.1, 3.2.2) will be important in determining future states of Southern Ocean ecosystems (Constable et al., 2014 <sup>[[#fn:r694|694]]</sup> ; Gutt et al., 2015 <sup>[[#fn:r695|695]]</sup> ) ( ''medium confidence'' ). However, there is a need to better characterise the nature and importance of indirect responses to physical change using models and observations. Important advances have also been made since AR5 in (i) identifying key variables to detect and attribute change in Southern Ocean ecosystems, as part of long-term circumpolar modelling designs (Constable et al., 2016 <sup>[[#fn:r696|696]]</sup> ), and (ii) refining methods for using sea ice projections from global climate models in ecological studies and ecosystem models for the Southern Ocean (Cavanagh et al., 2017 <sup>[[#fn:r697|697]]</sup> ).

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<span id="plankton-and-pelagic-primary-production"></span>
===== 3.2.3.2.1 Plankton and pelagic primary production =====

Changes in column-integrated phytoplankton biomass for the Southern Ocean are coupled with changes in the spatial extent of ice-free waters, suggesting little overall change in biomass per area at the circumpolar scale (Behrenfeld et al., 2016 <sup>[[#fn:r698|698]]</sup> ). Arrigo et al. (2008) <sup>[[#fn:r699|699]]</sup> also report no overall trend in remotely-sensed column-integrated primary production south of 50 ° S from 1998 to 2006. At a regional scale, local-scale forcings (e.g., retreating glaciers, topographically steered circulation and sea ice duration) and associated changes in stratification are key determinants of phytoplankton bloom dynamics at coastal stations on the West Antarctic Peninsula (Venables et al., 2013 <sup>[[#fn:r700|700]]</sup> ; Schofield et al., 2017 <sup>[[#fn:r701|701]]</sup> ; Kim et al., 2018 <sup>[[#fn:r702|702]]</sup> ; Schofield et al., 2018 <sup>[[#fn:r703|703]]</sup> ) ( ''medium confidence'' ). For example, a shallowing trend in mixed layer depth in the southern part of the Peninsula (as opposed to no trend in the north) associated with changes in sea ice duration over a 24-year period (from 1993 to 2017) has been linked to enhanced phytoplankton productivity (Schofield et al., 2018 <sup>[[#fn:r704|704]]</sup> ). The phenology of Southern Ocean phytoplankton blooms in this region may also be shifting to earlier in the growth season (Arrigo et al., 2017a <sup>[[#fn:r705|705]]</sup> ). However, the effect of climate change on Southern Ocean pelagic primary production is difficult to determine given that the length of time series data is insufficient (less than 30 years) to enable the climate change signature to be detected and attributed; and that, even when records are of sufficient length, data trends are often reported as being driven by climate change when they are due to a combination of climate change and variability.

Recent studies on the ecological effects of acidification in coastal waters near the Antarctic continent indicate a detrimental effect of acidification on primary production and changes to the structure and function of microbial communities (Hancock et al., 2017 <sup>[[#fn:r706|706]]</sup> ; Deppeler et al., 2018 <sup>[[#fn:r707|707]]</sup> ; Westwood et al., 2018 <sup>[[#fn:r708|708]]</sup> ) ( ''medium confidence'' ). Trimborn et al. (2017) report that Southern Ocean diatoms are more sensitive to ocean acidification and changes in irradiance than the prymnesiophyte ''Phaeocystis antarctica'' , which may have implications for biogeochemical cycling because diatoms and prymnesiophytes are generally considered key drivers of these cycles. Both laboratory manipulations and ''in situ'' experiments indicate that sea ice algae are tolerant to acidification (McMinn, 2017 <sup>[[#fn:r709|709]]</sup> ) ( ''medium confidence'' ). Model projections of trends in primary production in the Southern Ocean due to climate change from Leung et al. (2015) <sup>[[#fn:r710|710]]</sup> are summarised in Table 3.2.

<span id="section-2"></span>
<!-- START TABLE -->
'''Table 3.2:''' Model projections of trends due to climate change driven alteration of phytoplankton properties under RCP8.5 from 2006 to 2100 across three zones of the Southern Ocean, from Leung et al. (2015) <sup>[[#fn:r711|711]]</sup> . There is ''low confidence '' in predicted zonal changes in phytoplankton biomass due to  ''low confidence '' regarding future changes in iron supply in the Southern Ocean (Hutchins and Boyd, 2016 <sup>[[#fn:r712|712]]</sup> ). Acidification was not reported as an important driver in this modelling experiment.

<!-- TABLE -->
{| class="wikitable"
|-
| Zonal Band
| Predicted change in phytoplankton biomass
| Drivers
| Mechanisms
|-
| 40 ° S–50 ° S
| [[File:702f6140bb33c093f79f5159ebebed41 arrowup.png]]
| Higher mean underwater irradiance
More iron supply

| Shallowing of the summertime mixed layer depth
Change in iron supply mechanism

|-
| 50 ° S–65 ° S
| [[File:afb97dd099f055cf73abda49d9421cff arrowdown.png]]
| Lower mean underwater irradiance
| Deeper summertime mixed layer depth
Decreased summertime incident radiation (increased cloud fraction)

|-
| South of 65 ° S
| [[File:702f6140bb33c093f79f5159ebebed41 arrowup.png]]
| More iron supply
Higher mean underwater irradiance

Temperature

| Melting of sea ice
Warming ocean

|}
<!-- END TABLE -->

Previously reported declines in Antarctic krill abundance in the South Atlantic Sector (Atkinson et al., 2004 <sup>[[#fn:r725|725]]</sup> ) cited in WGII AR5 (Larsen et al., 2014 <sup>[[#fn:r726|726]]</sup> ) may not represent a long-term, climate driven, regional-scale decline (Fielding et al., 2014 <sup>[[#fn:r727|727]]</sup> ; Kinzey et al., 2015 <sup>[[#fn:r728|728]]</sup> ; Steinberg et al., 2015 <sup>[[#fn:r729|729]]</sup> ; Cox et al., 2018 <sup>[[#fn:r730|730]]</sup> ) ( ''medium confidence'' ) but could reflect a sudden, discontinuous change following an episodic period of anomalous peak abundance for this species (Loeb and Santora, 2015 <sup>[[#fn:r731|731]]</sup> ) ( ''low confidence'' ). Recent analyses have not detected trends in long-term krill abundance in the South Atlantic Sector in acoustic surveys (Fielding et al., 2014 <sup>[[#fn:r732|732]]</sup> ; Kinzey et al., 2015 <sup>[[#fn:r733|733]]</sup> ), net-based surveys (Steinberg et al., 2015 <sup>[[#fn:r734|734]]</sup> ) or reanalysis of historical data (Cox et al., 2018 <sup>[[#fn:r735|735]]</sup> ). Nevertheless, the spatial distribution and size composition of Antarctic krill may already have changed in association with change in the sea ice environment (Atkinson et al., 2019 <sup>[[#fn:r736|736]]</sup> ) ( ''medium confidence'' ) and may result in different regional trends in numerical krill abundance (Cox et al., 2018 <sup>[[#fn:r737|737]]</sup> ; Atkinson et al., 2019 <sup>[[#fn:r738|738]]</sup> ) ( ''medium confidence'' ).

The distribution of Antarctic krill is expected to change under future climate change because of changes in the location of the optimum conditions for growth and recruitment (Melbourne-Thomas et al., 2016 <sup>[[#fn:r739|739]]</sup> ; Piñones and Fedorov, 2016 <sup>[[#fn:r740|740]]</sup> ; Meyer et al., 2017 <sup>[[#fn:r741|741]]</sup> ; Murphy et al., 2017 <sup>[[#fn:r742|742]]</sup> ; Klein et al., 2018 <sup>[[#fn:r743|743]]</sup> ). The optimum conditions for krill are predicted to move southwards, with the decreases most apparent in the areas with the most rapid warming (Hill et al., 2013 <sup>[[#fn:r744|744]]</sup> ; Piñones and Fedorov, 2016 <sup>[[#fn:r745|745]]</sup> ) (Section 3.2.1.2.1) ( ''medium confidence'' ). The greatest projected reductions in krill due to the effects of warming and ocean acidification are predicted for the southwest Atlantic/Weddell Sea region (Kawaguchi et al., 2013 <sup>[[#fn:r746|746]]</sup> ; Piñones and Fedorov, 2016 <sup>[[#fn:r747|747]]</sup> ) ( ''low confidence'' ), which is the area of highest current krill concentrations, contains important foraging grounds for krill predators, and is also the main area of operation of the krill fishery. Modelled effects of warming on krill growth in the Scotia Sea and northern Antarctic Peninsula (AP) region resulted in reductions in total krill biomass under both RCP2.6 and RCP8.5 (Klein et al., 2018 <sup>[[#fn:r748|748]]</sup> ). Projections from a food web model for the West Antarctic Peninsula under simple scenarios for change in open water and sea ice-associated primary production from 2010 to 2050 (6, 15, and 41% increases in phytoplankton production with equivalent percentage decreases in ice algal production) indicate a decline in krill biomass with contemporaneous increases in the biomass of gelatinous salps (Suprenand and Ainsworth, 2017 <sup>[[#fn:r749|749]]</sup> ).

Current understanding of climate change effects on Southern Ocean zooplankton is largely based on observations and predictions from the South Atlantic and the West Antarctic Peninsula. Comparison of the mesozooplankton community in the southwestern Atlantic Sector between 1926–1938 and 1996–2013 showed no evidence of change despite surface ocean warming (Tarling et al., 2018 <sup>[[#fn:r713|713]]</sup> ). These results suggest that predictions of distributional shifts based on temperature niches may not reflect the actual levels of thermal resilience of key taxa. Sub-decadal cycles of macrozooplankton community composition adjacent to the West Antarctic Peninsula are strongly linked to climate indices, with evidence of increasing abundance for some species over the period from 1993 to 2013 (Steinberg et al., 2015 <sup>[[#fn:r714|714]]</sup> ). Pteropods are vulnerable to the effects of acidification, and new evidence indicates that eggs released at high CO 2 concentrations lack resilience to ocean acidification in the Scotia Sea region (Manno et al., 2016 <sup>[[#fn:r715|715]]</sup> ) ( ''medium confidence'' ).

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<span id="benthic-communities-1"></span>
===== 3.2.3.2.2 Benthic communities =====

Carbon uptake and storage by Antarctic benthic communities is predicted to increase with sea ice losses, because across-shelf growth gains from longer algal blooms outweigh ice scour mortality in the shallows (Barnes, 2017 <sup>[[#fn:r716|716]]</sup> ). Bentho-pelagic coupling and vertical energy flux will also influence Southern Ocean ecosystem responses to climate change (Jansen et al., 2017 <sup>[[#fn:r717|717]]</sup> ). Benthic communities in shallow water habitats mostly consist of dark-adapted invertebrates and rely on sea ice to create low-light marine environments. Increases in the amount of light reaching the shallow seabed under climate change may result in ecological regime shifts, in which invertebrate-dominated communities are replaced by macroalgal beds (Clark et al., 2015 <sup>[[#fn:r718|718]]</sup> ; Clark et al., 2017 <sup>[[#fn:r719|719]]</sup> ) ( ''low confidence'' ) (Table 6.1). Griffiths et al. (2017a) <sup>[[#fn:r720|720]]</sup> modelled distribution changes for 963 benthic invertebrate species in the Southern Ocean under RCP8.5 for 2099. Their results suggest that 79% of Antarctica’s endemic species will face a reduction in suitable temperature habitat (an average 12% reduction) over the current century. Predicted reductions in the number of species are most pronounced for the West Antarctic Peninsula and the Scotia Sea region (Griffiths et al., 2017a <sup>[[#fn:r720|720]]</sup> ).

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<span id="fish-1"></span>
===== 3.2.3.2.3 Fish =====

Many Antarctic fish have a narrow thermal tolerance as a result of physiological adaptations to cold water (Pörtner et al., 2014 <sup>[[#fn:r721|721]]</sup> ; Mintenbeck, 2017 <sup>[[#fn:r722|722]]</sup> ), which makes them vulnerable to the effects of increasing temperatures (Mueller et al., 2012 <sup>[[#fn:r723|723]]</sup> ; Beers and Jayasundara, 2015 <sup>[[#fn:r724|724]]</sup> ). Increasing water temperatures may displace icefish (family ''Channichthyidae'' ) in marginal habitats (e.g., shallow regions around subantarctic islands) as they lack haemoglobin and are unable to adjust blood parameters to an increasing oxygen demand (Mintenbeck et al., 201250‹) ( ''low confidence'' ). Future warming may also reduce the planktonic duration and increase egg and larval mortality for fish species, which is predicted to affect dispersal patterns, with implications for population connectivity and the ability of fish species to adapt to ongoing environmental change (Young et al., 2018 <sup>[[#fn:r751|751]]</sup> ). The Antarctic silverfish ( ''Pleuragramma antarctica'' ) is an important prey species in some regions of the Southern Ocean, and has an ice-dependent life cycle (Mintenbeck et al., 2012 <sup>[[#fn:r752|752]]</sup> ; Vacchi et al., 2012 <sup>[[#fn:r753|753]]</sup> ). Documented declines in the abundance of this species in some parts of the West Antarctic Peninsula may have consequences for associated food webs (Parker et al., 2015 <sup>[[#fn:r754|754]]</sup> ; Mintenbeck and Torres, 2017 <sup>[[#fn:r755|755]]</sup> ) ( ''low confidence'' ).

Myctophids and toothfish are important fish groups from both a food web (myctophids) and fishery (toothfish) perspective. Species distribution models for ''Electrona antarctica'' , a dominant myctophid species in the Southern Ocean, project habitat loss for this species under RCP4.5 (6.2 ± 6.0% loss) and RCP8.5 (13.1 ± 10.2% loss) by 2090, associated with increased sea surface temperature (Freer et al., 2018 <sup>[[#fn:r756|756]]</sup> ) ''.'' There have been no observed effects of climate change on the two species of toothfish that are found in the Southern Ocean: Patagonian and Antarctic toothfish ( ''Dissostichus eleginoides'' and ''D. mawsoni'' ), but recruitment is inversely correlated with sea surface temperature for Patagonian toothfish at South Georgia (Belchier and Collins, 2008 <sup>[[#fn:r757|757]]</sup> ). Given differences in temperature tolerances for Patagonian toothfish (with a wide temperature tolerance) and Antarctic toothfish (limited by a low tolerance for water temperatures above 2°C), the latter may be faced with reduced habitat and potential competition with southward-moving Patagonian toothfish under climate change (Mintenbeck, 2017 <sup>[[#fn:r758|758]]</sup> ) ( ''very low confidence'' ).

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<span id="seabirds-and-marine-mammals-1"></span>
===== 3.2.3.2.4 Seabirds and marine mammals =====

Since AR5, there has been an increasing body of evidence of climate-induced changes in populations of some Antarctic higher predators such as seabirds and marine mammals. These changes vary between different regions of the Southern Ocean and reflect differences in key drivers (Bost et al., 2009 <sup>[[#fn:r759|759]]</sup> ; Gutt et al., 2015 <sup>[[#fn:r760|760]]</sup> ; Constable et al., 2016 <sup>[[#fn:r761|761]]</sup> ; Hunt et al., 2016 <sup>[[#fn:r762|762]]</sup> ; Gutt et al., 2018 <sup>[[#fn:r763|763]]</sup> ), particularly sea ice extent and food availability ( ''high confidence)'' across regions (Sections 3.2.1.1.1, 5.2.3.1, 5.2.3.2, 5.2.4). The predictability of foraging grounds and ice cover are associated with variations in climate (Dugger et al., 2014 <sup>[[#fn:r764|764]]</sup> ; Youngflesh et al., 2017 <sup>[[#fn:r765|765]]</sup> ; Abrahms et al., 2018 <sup>[[#fn:r766|766]]</sup> ) (Section 3.2.1.1) and are the main drivers of observed population changes of Southern Ocean higher predators ( ''high confidence'' ) (Descamps et al., 2015 <sup>[[#fn:r767|767]]</sup> ; Jenouvrier et al., 2015 <sup>[[#fn:r768|768]]</sup> ; Sydeman et al., 2015 <sup>[[#fn:r769|769]]</sup> ; Abadi et al., 2017 <sup>[[#fn:r770|770]]</sup> ; Bjorndal et al., 2017 <sup>[[#fn:r771|771]]</sup> ; Fluhr et al., 2017 <sup>[[#fn:r772|772]]</sup> ; Hinke et al., 2017a <sup>[[#fn:r773|773]]</sup> ; Hinke et al., 2017b <sup>[[#fn:r774|774]]</sup> ; Pardo et al., 2017 <sup>[[#fn:r775|775]]</sup> ). The suitability of breeding habitats and the location of environmental features that facilitate the aggregation of prey are also influenced by climate change, and in turn influence the distribution in space and time of marine mammals and birds (Bost et al., 2015 <sup>[[#fn:r776|776]]</sup> ; Kavanaugh et al., 2015 <sup>[[#fn:r777|777]]</sup> ; Hindell et al., 2016 <sup>[[#fn:r778|778]]</sup> ; Santora et al., 2017 <sup>[[#fn:r779|779]]</sup> ) ( ''medium confidence'' ). Finally, biological parameters (reproductive success, mortality, fecundity and body condition), life history traits, morphological, physiological and behavioural characteristics of top predators in the Southern Ocean, as well as their patterns of activity (migration, distribution, foraging and reproduction) are also changing as a result of climate change (Braithwaite et al., 2015a <sup>[[#fn:r780|780]]</sup> ; Whitehead et al., 2015 <sup>[[#fn:r781|781]]</sup> ; Seyboth et al., 2016 <sup>[[#fn:r782|782]]</sup> ; Hinke et al., 2017b <sup>[[#fn:r783|783]]</sup> ) ( ''high confidence'' ).

Trends of populations of Antarctic penguins affected by climate change include both increases for gentoo penguins, ( ''Pygoscelis papua'' ) (Lynch et al., 2013 <sup>[[#fn:r784|784]]</sup> ; Dunn et al., 2016 <sup>[[#fn:r785|785]]</sup> ; Hinke et al., 2017a <sup>[[#fn:r786|786]]</sup> ), and decreases for Adélie ( ''P. adeliae),'' chinstrap ( ''P. antarctica),'' king ( ''Aptenodytes patagonicus'' ) and Emperor ( ''A. forsteri'' ) penguins (Trivelpiece et al., 2011 <sup>[[#fn:r787|787]]</sup> ; LaRue et al., 2013 <sup>[[#fn:r788|788]]</sup> ; Jenouvrier et al., 2014 <sup>[[#fn:r789|789]]</sup> ; Bost et al., 2015 <sup>[[#fn:r790|790]]</sup> ; Southwell et al., 2015 <sup>[[#fn:r791|791]]</sup> ; Younger et al., 2015 <sup>[[#fn:r792|792]]</sup> ; Cimino et al., 2016 <sup>[[#fn:r793|793]]</sup> ) ( ''high confidence'' ). Yet population shifts in Adélie penguins (Youngflesh et al., 2017 <sup>[[#fn:r794|794]]</sup> ) may have resulted from strong interannual environmental variability in good and bad years for prey and breeding habitat rather than climate change ( ''low confidence'' ). New evidence suggests that present Emperor penguin population estimates should be evaluated with caution based on the existence of breeding colonies yet to be discovered/confirmed (Ancel et al., 2017 <sup>[[#fn:r795|795]]</sup> ) as well as studies that draw conclusions based on trend estimates from single colonies (Kooyman and Ponganis, 2017 <sup>[[#fn:r796|796]]</sup> ).

Evidence for climate change impacts on Antarctic flying birds indicates that contraction of sea ice (seasonally and in specific regions), increases in sea surface temperatures, extreme events (snowstorms) and wind regime shifts can reduce breeding success and population growth rates in some species: southern fulmars ( ''Fulmarus glacialoides'' ), Antarctic petrels ( ''Thalassoica antarctica'' ) and black-browed albatrosses ( ''Thalassarche melanophris'' ) (Descamps et al., 2015 <sup>[[#fn:r797|797]]</sup> ; Jenouvrier et al., 2015 <sup>[[#fn:r798|798]]</sup> ; Pardo et al., 2017 <sup>[[#fn:r799|799]]</sup> ) ( ''low confidence)'' . Poleward population shifts with increased intensity and frequency of westerly winds affect functional traits, demographic rates, foraging range, rates of travel and flight speeds of flying birds (Weimerskirch et al., 2012 <sup>[[#fn:r800|800]]</sup> ; Jenouvrier et al., 2018 <sup>[[#fn:r801|801]]</sup> ) but also increase overlap with fisheries activities thus increasing the risk of bycatch and the need for mitigation measures (Krüger et al., 2018 <sup>[[#fn:r802|802]]</sup> ) ( ''medium confidence)'' .

Changes in local- and regional-scale oceanographic features (Section 3.2.1.2) together with bathymetry control prey aggregation and distribution, and affect the ecological responses and biological traits of higher predators (particularly marine mammals) in the Southern Ocean (Lyver et al., 2014 <sup>[[#fn:r803|803]]</sup> ; Bost et al., 2015 <sup>[[#fn:r804|804]]</sup> ; Jenouvrier et al., 2015 <sup>[[#fn:r805|805]]</sup> ; Whitehead et al., 2015 <sup>[[#fn:r806|806]]</sup> ; Cimino et al., 2016 <sup>[[#fn:r807|807]]</sup> ; Hinke et al., 2017a <sup>[[#fn:r808|808]]</sup> ; Pardo et al., 2017 <sup>[[#fn:r809|809]]</sup> ) ( ''medium confidence'' ) and ''likely'' explain most of the observed population shifts (Kavanaugh et al., 2015 <sup>[[#fn:r810|810]]</sup> ; Hindell et al., 2016 <sup>[[#fn:r811|811]]</sup> ; Gurarie et al., 2017 <sup>[[#fn:r812|812]]</sup> ; Santora et al., 2017 <sup>[[#fn:r813|813]]</sup> ). Decadal climate cycles affect access to mesopelagic prey by southern elephant seals ( ''Mirounga leonina'' ) in the Indian Sector of the Southern Ocean and breeding females are excluded from highly productive continental shelf waters in years of increased sea ice extent and duration (Hindell et al., 2016 <sup>[[#fn:r814|814]]</sup> ) ( ''medium confidence)'' . To date there is no unified global estimate of the abundance of Antarctic pack ice seal species (Ross seals ( ''Ommatophoca rossi)'' , crabeater seals ( ''Lobodon carcinophaga)'' , leopard seals ( ''Hydrurga leptonyx)'' and Weddell seals ( ''Leptonychotes weddellii)'' ) as a reference point for understanding climate change impacts on these species (Southwell et al., 2012 <sup>[[#fn:r815|815]]</sup> ; Bester et al., 2017 <sup>[[#fn:r816|816]]</sup> ), although some regional population estimates for pack ice seals are available (Gurarie et al., 2017 <sup>[[#fn:r817|817]]</sup> and references therein). Analysis of long-term data suggests a genetic component to adaptation to climate change ( ''low confidence'' ) in Antarctic fur seals ( ''Arctocephalus gazella'' , Forcada and Hoffman (2014)) and pigmy blue whales ( ''Balaenoptera musculus brevicauda'' , Attard et al. (2015)).

Population trends of migratory baleen whales have been associated with krill abundance in the Atlantic and Pacific sectors of the Southern Ocean which is reflected in increased reproductive success, body condition and energy allocation (milk availability and transfer) to calves (Braithwaite et al., 2015a <sup>[[#fn:r818|818]]</sup> ; Braithwaite et al., 2015b <sup>[[#fn:r819|819]]</sup> ; Seyboth et al., 2016 <sup>[[#fn:r820|820]]</sup> ) ( ''high confidence'' ). There have been predictions of negative future impacts of climate change on krill and all whale species, although the magnitude of impacts differs among populations (Tulloch et al., 2019 <sup>[[#fn:r821|821]]</sup> ) as for other higher predators (Section 5. 2.3 ). Pacific blue (Tulloch et al., 2019 <sup>[[#fn:r822|822]]</sup> ) ( ''Balaenoptera musculus'' ), fin ( ''B. physalus'' ) and southern right whales ( ''Eubalaena australis'' ) are the most at risk but humpback whales ( ''Megaptera novaeangliae'' ) are also at risk, as consequence of reduced prey and increasing interspecific competition. Importantly, climate-related risks for whale populations are a product of environmental conditions and connectivity between whale foraging grounds (Southern Ocean) and breeding grounds (lower latitudes) (Section 5.2.3.1 ).

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<span id="pelagic-foodwebs-and-ecosystem-structure"></span>
===== 3.2.3.2.5 Pelagic foodwebs and ecosystem structure =====

This section assesses the impacts of ocean and sea ice changes on pelagic foodwebs and ecosystem structure. The ecological impacts of loss of ice shelves and retreat of coastal glaciers around Antarctica are assessed in Section 3.3.3.4. Recent syntheses of Southern Ocean ecosystem structure and function recognise the importance of at least two dominant energy pathways in pelagic foodwebs—a short trophic pathway transferring primary production to top predators via krill, and at least one other pathway that moves energy from smaller phytoplankton to top predators via copepods and small mesopelagic fishes—and indicate that the relative importance of these pathways will change under climate change (Murphy et al., 2013 <sup>[[#fn:r823|823]]</sup> ; Constable et al., 2016 <sup>[[#fn:r824|824]]</sup> ; Constable et al., 2017 <sup>[[#fn:r825|825]]</sup> ; McCormack et al., 2017 <sup>[[#fn:r826|826]]</sup> ) ( ''medium confidence'' ). Using an ecosystem model, Klein et al. (2018) <sup>[[#fn:r827|827]]</sup> found that the effects of warming on krill growth off the AP and in the Scotia Sea translated to increased risks of declines in krill predator populations, particularly penguins, under both RCP2.6 and RCP8.5. The relative importance of different energy pathways in Southern Ocean foodwebs has important implications for resource management, in particular the management of krill and toothfish fisheries by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) (Constable et al., 2016 <sup>[[#fn:r828|828]]</sup> ; Constable et al., 2017 <sup>[[#fn:r829|829]]</sup> ) (Sections 3.2.4.1.2, 3.5.3.2.2).

In summary, advances in knowledge regarding the impacts of climate change on Antarctic marine ecosystems since AR5 are consistent with the impacts described in Larsen et al. (2014) <sup>[[#fn:r830|830]]</sup> (also summarised in Figure 3.6). These advances include further descriptions of local-scale, climate-related influences (sea ice and stratification) on primary productivity, particularly in the West Antarctic Peninsula region (Section 3.2.3.2.1) ( ''medium confidence'' ). At the circumpolar scale, primary production is projected to increase in regions south of 65°S over the period from now to 2100 under RCP8.5 (Leung et al., 2015 <sup>[[#fn:r831|831]]</sup> ) ( ''low confidence'' ). However, ocean acidification may have a detrimental effect on coastal phytoplankton communities around the Antarctic continent (Section 3.2.3.2.1) ( ''medium confidence'' ). Increased information is also available regarding climate-driven changes in Antarctic krill populations in the south Atlantic, including the observed southward shift in the spatial distribution of krill in this region (Atkinson et al., 2019 <sup>[[#fn:r832|832]]</sup> ) ( ''medium confidence'' ) but evidence of a long-term trend in overall abundance in the region is equivocal (Section 3.2.3.2.1). Further habitat contraction for Antarctic krill is predicted in the future ( ''medium confidence'' ) (references detailed in Section 3.2.3.2.1). Under high emissions scenarios the majority of Antarctic seafloor species are projected to be negatively impacted by the end of the century (Griffiths et al., 2017a <sup>[[#fn:r833|833]]</sup> ) ( ''low confidence'' ). Observed changes in the geography of ice-associated habitats (sea ice, ice shelves and polynyas) have both positive and negative effects on seabirds and marine mammals, and will interact with ice dependent changes in Antarctic krill populations to compound the impacts on krill dependent predators (Klein et al., 2018 <sup>[[#fn:r834|834]]</sup> ) (Sections 3.2.3.2.1, 3.2.3.2.4) ( ''medium confidence'' ).

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'''Figure 3.6'''

<span id="schematic-summary-of-key-drivers-that-are-causing-or-are-projected-to-cause-direct-effects-on-southern-ocean-marine-ecosystems.-effects-presented-here-are-described-in-the-main-text-sections-3.2.3.2-3.3.3.4-with-associated-confidence-levels-and-citations.-projected-changes-indicated-by-an-asterisk-are-for-high-emissions-scenarios.-the-cross-sectional-view-of-the-southern"></span>
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'''Schematic summary of key drivers that are causing or are projected to cause direct effects on Southern Ocean marine ecosystems. Effects presented here are described in the main text (Sections 3.2.3.2, 3.3.3.4), with associated confidence levels and citations. Projected changes (indicated by an asterisk) are for high emissions scenarios. The cross-sectional view of the Southern […]'''
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[[File:49e88a41905cd8a2599ddce0e9957444 IPCC-SROCC-CH_3_6.jpg]]

Schematic summary of key drivers that are causing or are projected to cause direct effects on Southern Ocean marine ecosystems. Effects presented here are described in the main text (Sections 3.2.3.2, 3.3.3.4), with associated confidence levels and citations. Projected changes (indicated by an asterisk) are for high emissions scenarios. The cross-sectional view of the Southern Ocean ecosystem shows the association of key functional groups (marine mammals, birds, fish, zooplankton, phytoplankton and benthic assemblages) with Southern Ocean habitats. The configuration of the Southern Ocean foodweb is described in SM3.2.6.

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