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

==== 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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