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

==== 3.4.2.8 Shelf Seas ====

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Shelf seas overlie the continental margin, often with maximum depths of &lt;200 m, and represent 7% of the global ocean by area ( [[#Simpson--2012|Simpson and Sharples, 2012]] ). These ecosystems are found offshore of every continent, generate 10–30% ( [[#Mackenzie--2000|Mackenzie et al., 2000]] ; [[#Andersson--2004|Andersson and Mackenzie, 2004]] ) of global marine net primary production and play a key role in global biogeochemical cycling, including the export of land-borne carbon and nutrients ( [[#Johnson--1999|Johnson et al., 1999]] ; [[#Nishioka--2011|Nishioka et al., 2011]] ; [[#Li--2019|Li et al., 2019]] ) to the deep ocean and recycling of fixed nitrogen back to the atmosphere via denitrification ( [[#Devol--2015|Devol, 2015]] ). The shelf seas are home to several of the world’s major industrial capture fisheries, such as those of the mid-Atlantic Bight, Scotian Shelf, Eastern Bering Sea Shelf and North Brazil Shelf ( [[#Barange--2018|Barange et al., 2018]] ), and support other marine industries, including aquaculture, extractive industries (oil, gas and mining), shipping and renewable energy installations.

'''Table 3.11 |''' Summary of past IPCC assessments of shelf seas

{| class="wikitable"
|-
! Observations

! Projections

|-
| ''AR5 ( [[#Hoegh-Guldberg--2014|Hoegh-Guldberg et al., 2014]] )''

|
|-
| ‘Primary productivity, biomass yields and fish capture rates have undergone large changes within the ECS [East China Sea] over the past decades ( ''limited evidence, medium agreement, low confidence'' ).’

‘Changing sea temperatures have influenced the abundance of phytoplankton, benthic biomass, cephalopod fisheries and the size of demersal trawl catches in the northern SCS [South China Sea] observed over the period 1976–2004 ( ''limited evidence, medium agreement'' ).’

‘Concurrent with the retreat of the ‘cold pool’ [...] on the northern Bering Sea shelf, [...] bottom trawl surveys of fish and invertebrates show a significant community-wide northward distributional shift and a colonisation of the former cold pool areas by sub-Arctic fauna ( ''high confidence'' ).’

‘Observed changes in the phenology of plankton groups in the North Sea over the past 50 years are driven by climate forcing, in particular regional warming ( ''high confidence'' ).’

| ‘Global warming will result in more frequent extreme events and greater associated risks to ocean ecosystems ( ''high confidence'' ). In some cases, [...] projected increases will eliminate ecosystems, and increase the risks and vulnerabilities to coastal livelihoods [and the vulnerabilities for food security including that of Southeast Asia] ( ''medium to high confidence'' ). Reducing stressors not related to climate change represents an opportunity to strengthen the ecological resilience within these regions, which may help them [biota] survive some projected changes in ocean temperature and chemistry.’

Changes in eutrophication and hypoxia are ''likely'' to influence shelf seas, but there is ''low confidence'' in the understanding of the magnitude of potential changes and impacts on ecosystem functioning, fisheries and other industries.

|-
|
|-
| ''SROCC ( [[#Bindoff--2019a|Bindoff et al., 2019a]] )''

|
|-
| ‘Species composition of fisheries catches since the 1970s in many shelf seas ecosystems of the world is increasingly dominated by warm-water species ( ''medium confidence'' ).’

‘Estuaries, shelf seas and a wide range of other intertidal and shallow-water habitats play an important role in the global carbon cycle through their primary production by rooted plants, seaweeds (macroalgae) and phytoplankton, and also by processing riverine organic carbon. However, the natural carbon dynamics of these systems have been greatly changed by human activities ( ''high confidence'' ).’

| ‘Direct anthropogenic impacts include coastal land-use change; indirect effects include increased nutrient delivery and other changes in river catchments, and marine resource exploitation in shelf seas. There is ''high confidence'' that these human-driven changes will continue, reflecting coastal settlement trends and global population growth.’

|}

Similar to other coastal ecosystems, evidence since SROCC (Table 3.11) suggests that shelf-sea ecosystems and the fisheries and aquaculture they support are sensitive to the interactive effects of climate-induced drivers, as well as non-climate drivers, including nutrient pollution, sedimentation, fishing pressure and resource extraction (Table 3.12; Figure 3.12). Changes in freshwater, nutrient and sediment inputs from rivers due to both climate-induced and non-climate drivers can influence productivity and nutrient limitation, ecosystem structure, carbon export and species diversity and abundance ( [[#Balch--2012|Balch et al., 2012]] ; [[#Picado--2014|Picado et al., 2014]] ), and can result in reduced water clarity and light penetration ( [[#Dupont--2013|Dupont and Aksnes, 2013]] ; [[#McGovern--2019|McGovern et al., 2019]] ). Seasonal bottom-water hypoxia occurs in some shelf seas (e.g., northern Gulf of Mexico, Bohai Sea, East China Sea) due to riverine inputs of freshwater and nutrients, promoting stratification, enhanced primary production and organic carbon export to bottom waters ( ''high confidence'' ) ( [[#Zhao--2017|Zhao et al., 2017]] ; [[#Wei--2019|Wei et al., 2019]] ; [[#Del%20Giudice--2020|Del Giudice et al., 2020]] ; [[#Große--2020|Große et al., 2020]] ; [[#Jarvis--2020|Jarvis et al., 2020]] ; [[#Rabalais--2020|Rabalais and Baustian, 2020]] ; [[#Song--2020a|Song et al., 2020a]] ; [[#Xiong--2020|Xiong et al., 2020]] ; [[#Zhang--2020a|Zhang et al., 2020a]] ).

'''Table 3.12 |''' Synthesis of interactive effects and their influence on shelf-sea ecosystems and the fisheries and aquaculture they support

{| class="wikitable"
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! Factor

! Example of effect

! Example references

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| Temperature

| Altered habitats for species, change in plankton, fish and macrofauna community structure, influence on species growth, thermal stress, altered diversity, altered productivity and altered phenology

| Liang et al. (2018); [[#Maharaj--2018|Maharaj et al. (2018)]] ; [[#Ma--2019|Ma et al. (2019)]] ; [[#Meyer--2019|Meyer and Kröncke (2019)]] ; [[#Yan--2019|Yan et al. (2019)]] ; [[#Bargahi--2020|Bargahi et al. (2020)]] ; [[#Bedford--2020|Bedford et al. (2020)]] ; [[#Denechaud--2020|Denechaud et al. (2020)]] ; [[#Friedland--2020b|Friedland et al. (2020b)]] ; [[#Mérillet--2020|Mérillet et al. (2020)]] ; [[#Nohe--2020|Nohe et al. (2020)]]

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| pH

| Acidification with hypoxia

| [[#Zhang--2019|Zhang and Wang (2019)]]

|-
| Salinity

| Change in species distribution due to altered salinity front distribution

| [[#Liu--2020c|Liu et al. (2020c)]]

|-
| Oxygen concentration

| Deoxygenation

| [[#Wei--2019|Wei et al. (2019)]] ; Del Giudice et al. (2020)

|-
| River discharge

| Change in plankton community structure

| [[#Shi--2020|Shi et al. (2020)]]

|-
| Nutrient pollution

| Enhanced primary production, change in plankton community structure

| Kong et al. (2019); [[#Nohe--2020|Nohe et al. (2020)]]

|-
| Sedimentation

| Modified ocean chemistry

| [[#Hallett--2018|Hallett et al. (2018)]]

|-
| Fishing pressure

| Increased vulnerability leading to changes in community structure

| [[#Maharaj--2018|Maharaj et al. (2018)]] ; [[#Wang--2019c|Wang et al. (2019c)]] ; [[#Hernvann--2020|Hernvann and Gascuel (2020)]]

|-
| Resource extraction

| Contamination, change in benthic community structure

| [[#Hall--2002|Hall (2002)]]

|}

Key risks to shelf seas include shifts or declines in marine micro- and macro-organism abundance and diversity driven by eutrophication, HABs and extreme events (storms and MHWs), and consequent effects on fisheries, resource extraction, transportation, tourism and marine renewable energy (Figure 3.12). The combined effects of deoxygenation and warming can affect the metabolism, growth, feeding behaviour and mobility of fish species ( [[#3.3.3|Section 3.3.3]] ). The increasing availability of observations mean that ecosystem changes in shelf seas can be increasingly attributed to climate change ( ''high confidence'' ) ( [[#Liang--2018|Liang et al., 2018]] ; [[#Maharaj--2018|Maharaj et al., 2018]] ; [[#Ma--2019|Ma et al., 2019]] ; [[#Meyer--2019|Meyer and Kröncke, 2019]] ; [[#Bargahi--2020|Bargahi et al., 2020]] ; [[#Bedford--2020|Bedford et al., 2020]] ; [[#Friedland--2020b|Friedland et al., 2020b]] ; [[#Mérillet--2020|Mérillet et al., 2020]] ). Eutrophication and seasonal bottom-water hypoxia in some shelf seas have been linked to warming ( ''high confidence'' ) ( [[#Wei--2019|Wei et al., 2019]] ; [[#Del%20Giudice--2020|Del Giudice et al., 2020]] ) and increased riverine nutrient loading ( ''high confidence'' ) ( [[#Wei--2019|Wei et al., 2019]] ; [[#Del%20Giudice--2020|Del Giudice et al., 2020]] ). Since SROCC, some severe HABs have been attributed to extreme events, such as MHWs ( [[IPCC:Wg2:Chapter:Chapter-14#14.4|Section 14.4.2]] ; [[#Roberts--2019|Roberts et al., 2019]] ; [[#Trainer--2019|Trainer et al., 2019]] ); however, a recent worldwide assessment of HABs attributed the increase in observed HABs to intensified monitoring associated with increased aquaculture production ( ''high confidence'' ) ( [[#Hallegraeff--2021|Hallegraeff et al., 2021]] ).

Since SROCC, changes in the community structure and diversity of plankton, macrofauna and infauna have been detected in some shelf seas, although attribution has been regionally specific (e.g ''.'' , bottom-water warming or hypoxia) ( [[#Meyer--2019|Meyer and Kröncke, 2019]] ; [[#Rabalais--2020|Rabalais and Baustian, 2020]] ). Detection of the picoplankton ''Synechococcus'' spp. in the North Sea is potentially linked to a summer decrease in copepod stocks and declining food-web efficiency ( ''low confidence'' ) ( [[#Schmidt--2020|Schmidt et al., 2020]] ). The seasonally distinct phytoplankton assemblages in the North Sea have begun to appear concurrently and homogenise ( [[#Nohe--2020|Nohe et al., 2020]] ). Changes in abundance, species composition and size of zooplankton have been detected in some shelf seas (Yellow Sea, North Sea, Celtic Sea and Tasman Sea), including a decline in stocks of larger copepods, increased abundances of gelatinous and meroplankton, and a shift to smaller species due to warming, increased river discharge, circulation change and/or extreme events ( ''high confidence'' ) ( [[#Wang--2018a|Wang et al., 2018a]] ; [[#Bedford--2020|Bedford et al., 2020]] ; [[#Evans--2020|Evans et al., 2020]] ; [[#Shi--2020|Shi et al., 2020]] ; [[#Edwards--2021|Edwards et al., 2021]] ).

Ocean warming has shifted distributions of fish ( [[#Free--2019|Free et al., 2019]] ; [[#Franco--2020|Franco et al., 2020]] ; [[#Pinsky--2020b|Pinsky et al., 2020b]] ; [[#Fredston--2021|Fredston et al., 2021]] ) and marine mammal species ( [[#Salvadeo--2010|Salvadeo et al., 2010]] ; [[#García-Aguilar--2018|García-Aguilar et al., 2018]] ; [[#Davis--2020|Davis et al., 2020]] ) poleward ( ''high confidence'' ) or deeper ( ''low to medium confidence'' ) ( [[#3.4.3.1|Section 3.4.3.1]] ; [[#Nye--2009|Nye et al., 2009]] ; [[#Pinsky--2013|Pinsky et al., 2013]] ; [[#Pinsky--2020b|Pinsky et al., 2020b]] ). Warming has also tropicalised the pelagic and demersal fish assemblages of mid- and high-latitude shelves ( ''high confidence'' ) ( [[#Montero-Serra--2015|Montero-Serra et al., 2015]] ; [[#Liang--2018|Liang et al., 2018]] ; [[#Maharaj--2018|Maharaj et al., 2018]] ; [[#Ma--2019|Ma et al., 2019]] ; [[#Friedland--2020a|Friedland et al., 2020a]] ; [[#Kakehi--2021|Kakehi et al., 2021]] ; [[#Punzón--2021|Punzón et al., 2021]] ). Fisheries catch composition in many shelf-sea ecosystems has become increasingly dominated by warm-water species since the 1970s ( ''high confidence'' ) ( [[#Cheung--2013|Cheung et al., 2013]] ; [[#Leitão--2018|Leitão et al., 2018]] ; [[#Maharaj--2018|Maharaj et al., 2018]] ; [[#McLean--2019|McLean et al., 2019]] ). Warming has taxonomically diversified fish communities along a latitudinal gradient in the North Sea but has homogenised functional diversity ( [[#McLean--2019|McLean et al., 2019]] ). However, in some regions, changing predator or prey distributions, temperature-dependent hypoxia, population changes, evolutionary adaptation and other biotic or abiotic processes, including species’ exploitation, confound responses to climate-induced drivers, which must therefore be interpreted with caution ( [[#Frank--2018|Frank et al., 2018]] ). For example, although, most species’ range edges are tracking temperature change on the northeast shelf of the USA ( ''medium confidence'' ) ( [[#Fredston-Hermann--2020|Fredston-Hermann et al., 2020]] ; [[#Fredston--2021|Fredston et al., 2021]] ), range edges of others are not.

A wide range of responses by fish and invertebrate populations to warming have been observed. The majority of responses have been detrimental, with the direction and magnitude of the response depending on ecoregion, taxonomy, life history and exploitation history ( [[#Free--2019|Free et al., 2019]] ; [[#Yati--2020|Yati et al., 2020]] ). For example, fisheries productivity has strongly decreased in the North Sea ( [[#Free--2019|Free et al., 2019]] ), and fisheries yields have also decreased in the Celtic Sea, attributed primarily to warming and secondarily to long-term exploitation ( [[#Hernvann--2020|Hernvann and Gascuel, 2020]] ; [[#Mérillet--2020|Mérillet et al., 2020]] ). Conversely, fish species diversity and overall productivity have increased in the Gulf of Maine, even with warming ( [[#Le%20Bris--2018|Le Bris et al., 2018]] ; [[#Friedland--2020a|Friedland et al., 2020a]] ; [[#Friedland--2020b|Friedland et al., 2020b]] ). Fisheries yields have decreased in the Yellow Sea, East China Sea and South China Sea partially due to overexploitation ( [[#Ma--2019|Ma et al., 2019]] ; [[#Wang--2019c|Wang et al., 2019c]] ), with warming exerting more influence on the yield of cold-water species than on temperate- and warm-water groups ( [[#Ma--2019|Ma et al., 2019]] ). The combined effects of exploitation and multi-decadal climate fluctuations make it difficult to assess global climate-change impacts on fisheries yields (Chapter 5; [[#Ma--2019|Ma et al., 2019]] ; [[#Bentley--2020b|Bentley et al., 2020b]] ; [[#Johnson--2020|Johnson et al., 2020]] ).

Since AR5, increasing spatio-temporal extent of hypoxia has been projected due to enhanced benthic respiration and reduced oxygen solubility from warming ( [[#Del%20Giudice--2020|Del Giudice et al., 2020]] ). Similar to the open ocean, large shifts in the phenology of phytoplankton blooms have been projected for shelf seas throughout subpolar and polar waters ( ''medium confidence'' ) ( [[#Henson--2018a|Henson et al., 2018a]] ; [[#Asch--2019|Asch et al., 2019]] ). Zooplankton, which are important prey for many fish species and sea birds, are expected to decrease in abundance on the northeast shelf of the USA ( [[#Grieve--2017|Grieve et al., 2017]] ); however, responses vary by shelf ecosystem ( [[#Chust--2014b|Chust et al., 2014b]] ). Trends towards tropicalisation will continue in the future ( ''high confidence'' ) ( [[#Cheung--2015|Cheung et al., 2015]] ; [[#Stortini--2015|Stortini et al., 2015]] ; [[#Allyn--2020|Allyn et al., 2020]] ; [[#Maltby--2020|Maltby et al., 2020]] ; [[#Costa--2021|Costa et al., 2021]] ), but uncertainty of future projections of fisheries production increases substantially beyond 2040 ( [[#Maltby--2020|Maltby et al., 2020]] ). Nevertheless, shelf-sea fisheries at lower latitudes are most vulnerable to climate change ( [[#Monnereau--2017|Monnereau et al., 2017]] ). Under future climate change marked by more frequent and intense extreme events and the influences of multiple drivers, more flexible and adaptive management approaches could reduce climate impacts on species while also supporting industry adaptation ( ''high confidence'' ) ( [[#3.6.3.1.2|Section 3.6.3.1.2]] ; [[#Shackell--2014|Shackell et al., 2014]] ; [[#Stortini--2015|Stortini et al., 2015]] ; [[#Hare--2016|Hare et al., 2016]] ; [[#Stortini--2017|Stortini et al., 2017]] ; [[#Greenan--2019|Greenan et al., 2019]] ; [[#Ocaña--2019|Ocaña et al., 2019]] ; [[#Maltby--2020|Maltby et al., 2020]] ).

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