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

==== 3.4.2.2 Rocky Shores ====

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Rocky shore ecosystems refer to a range of temperate intertidal and shallow coastal ecosystems that are dominated by different foundational organisms, including mussels, oysters, fleshy macroalgae, hard and soft corals, coralline algae, bryozoans and sponges, which create habitat for species-rich assemblages of invertebrates, fish, marine mammals and other organisms. Rocky shores provide services including wave attenuation, habitat provision and food resources, and these support commercial, recreational and Indigenous fisheries and shellfish aquaculture.

'''Table 3.4 |''' Summary of previous IPCC assessments of rocky shores

{| class="wikitable"
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! Observations

! Projections

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

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| ‘Rocky shores are among the better-understood coastal ecosystems in terms of potential impacts of climate variability and change. The most prominent effects are range shifts of species in response to ocean warming ( ''high confidence'' ) and changes in species distribution and abundance ( ''high confidence'' ) mostly in relation to ocean warming and acidification.’

‘The dramatic decline of biodiversity in mussel beds of the Californian coast has been attributed to large-scale processes associated with climate-related drivers [...] ( ''high confidence'' ).’

| ‘The abundance and distribution of rocky shore species will continue to change in a warming world ( ''high confidence'' ). For example, the long-term consequences of ocean warming on mussel beds of the northeast Pacific are both positive (increased growth) and negative (increased susceptibility to stress and of exposure to predation) ( ''medium confidence'' ).’

‘Observations performed near natural CO 2 vents in the Mediterranean Sea show that diversity, biomass and trophic complexity of rocky shore communities will decrease at future pH levels ( ''high confidence'' ).’

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| ''SR15 ( [[#Hoegh-Guldberg--2018a|Hoegh-Guldberg et al., 2018a]] )''

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| ‘Changes in ocean circulation can have profound impacts on [temperate] marine ecosystems by connecting regions and facilitating the entry and establishment of species in areas where they were unknown before (‘tropicalization’ ...) as well as the arrival of novel disease agents ( ''medium agreement, limited evidence'' ).’

| ‘In the transition to 1.5°C, changes to water temperatures are expected to drive some species (e.g., plankton, fish) to relocate to higher latitudes and cause novel ecosystems to assemble ( ''high confidence'' ). Other ecosystems (e.g., kelp forests, coral reefs) are relatively less able to move, however, and are projected to experience high rates of mortality and loss ( ''very high confidence'' ).’

‘In the case of ‘less mobile’ ecosystems (e.g., coral reefs, kelp forests, intertidal communities), shifts in biogeographic ranges may be limited, with mass mortalities and disease outbreaks increasing in frequency as the exposure to extreme temperatures increases’ ( ''high agreement, robust evidence'' ).

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

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| Intertidal rocky shores ecosystems are highly sensitive to ocean warming, acidification and extreme heat exposure during low tide emersion ( ''high confidence'' ).

‘Sessile calcified organisms (e.g., barnacles and mussels) in intertidal rocky shores are highly sensitive to extreme temperature events and acidification ( ''high confidence'' ), a reduction in their biodiversity and abundance have been observed in naturally acidified rocky reef ecosystems ( ''medium confidence'' ).’

| ‘Intertidal rocky shores are also expected to be at very high risk (transition above 3°C) under the RCP8.5 scenario ( ''medium confidence'' ). These ecosystems have low to moderate adaptive capacity, as they are highly sensitive to ocean temperatures and acidification.’

‘Benthic species will continue to relocate in the intertidal zones and experience mass mortality events due to warming ( ''high confidence'' ). Interactive effects between acidification and warming will exacerbate the negative impacts on rocky shore communities, causing a shift towards a less diverse ecosystem in terms of species richness and complexity, increasingly dominated by macroalgae ( ''high confidence'' ).’

|}

Observations since AR5 and SROCC (Table 3.4) find increased impacts of ocean warming on rocky shores. This includes extirpation of species at the warm edge of their ranges ( [[#Yeruham--2015|Yeruham et al., 2015]] ; [[#Martínez--2018|Martínez et al., 2018]] ), extension of poleward range boundaries ( [[#Sanford--2019|Sanford et al., 2019]] ), mortality from climate extremes ( [[#Seuront--2019|Seuront et al., 2019]] ), reduction in survival at shallower depths ( [[#Sorte--2019|Sorte et al., 2019]] ; [[#Wallingford--2019|Wallingford and Sorte, 2019]] ) and reorganisation of communities ( [[#Wilson--2019|Wilson et al., 2019]] ; [[#Mulders--2020|Mulders and Wernberg, 2020]] ; [[#Albano--2021|Albano et al., 2021]] ). Data collected after MHWs find ecological phase shifts ( ''moderate evidence, high agreement'' ) (e.g., California; [[#Rogers-Bennett--2019|Rogers-Bennett and Catton, 2019]] ; [[#McPherson--2021|McPherson et al., 2021]] ) and homogenisation of communities ( ''limited evidence'' ) (e.g., Alaska; [[#Weitzman--2021|Weitzman et al., 2021]] ). For example, the collapse of sea star populations in the Northeast Pacific due to a MHW-related disease outbreak ( [[#Hewson--2014|Hewson et al., 2014]] ; [[#Menge--2016|Menge et al., 2016]] ; [[#Miner--2018|Miner et al., 2018]] ; [[#Schiebelhut--2018|Schiebelhut et al., 2018]] ), including 80–100% loss of the common predatory sunflower star, ''Pycnopodia helianthoides'' ( ''very high confidence'' ) ( [[#Harvell--2019|Harvell et al., 2019]] ), triggered shifts from kelp- to urchin-dominated ecosystems ( [[#Schultz--2016|Schultz et al., 2016]] ; [[#Gravem--2017|Gravem and Morgan, 2017]] ; [[#McPherson--2021|McPherson et al., 2021]] ).

Multiple lines of evidence find that foundational calcifying organisms such as mussels are at high risk of decline due to both the individual and synergistic effects of warming, acidification and hypoxia ( ''high confidence'' ) ( [[#Sunday--2016|Sunday et al., 2016]] ; [[#Sorte--2017|Sorte et al., 2017]] ; [[#Sorte--2019|Sorte et al., 2019]] ; [[#Newcomb--2020|Newcomb et al., 2020]] ). Warmer temperatures reduce mussel and barnacle recruitment (e.g., northwest Atlantic; [[#Petraitis--2020|Petraitis and Dudgeon, 2020]] ) and the upper vertical limit of mussels (e.g., northeast Pacific, [[#Harley--2011|Harley, 2011]] ; and southwest Pacific, [[#Sorte--2019|Sorte et al., 2019]] ). Experiments show that ocean acidification negatively impacts mussel physiology ( ''very high confidence'' ), with evidence of reduced growth ( [[#Gazeau--2010|Gazeau et al., 2010]] ), attachment ( [[#Newcomb--2020|Newcomb et al., 2020]] ), biomineralisation ( [[#Fitzer--2014|Fitzer et al., 2014]] ) and shell thickness ( [[#Pfister--2016|Pfister et al., 2016]] ; [[#McCoy--2018|McCoy et al., 2018]] ). Net calcification and abundance of mussels and other foundational species, including oysters, are expected to decline due to ocean acidification ( ''very high confidence'' ) ( [[#Kwiatkowski--2016|Kwiatkowski et al., 2016]] ; [[#Sunday--2016|Sunday et al., 2016]] ; [[#McCoy--2018|McCoy et al., 2018]] ; [[#Meng--2018|Meng et al., 2018]] ), causing the reorganisation of communities ( ''high confidence'' ) ( [[#Kroeker--2013b|Kroeker et al., 2013b]] ; [[#Linares--2015|Linares et al., 2015]] ; [[#Brown--2016|Brown et al., 2016]] ; [[#Sunday--2016|Sunday et al., 2016]] ; [[#Agostini--2018|Agostini et al., 2018]] ; [[#Teixidó--2018|Teixidó et al., 2018]] ). Experiments indicate that acidification can interact with warming and hypoxia to increase the detrimental effects on mussels ( [[#Gu--2019|Gu et al., 2019]] ; [[#Newcomb--2020|Newcomb et al., 2020]] ). In regions where food is readily available to mussels, detrimental effects of ocean acidification may be dampened ( [[#Kroeker--2016|Kroeker et al., 2016]] ); however, recent findings are inconclusive ( [[#Brown--2018a|Brown et al., 2018a]] ).

Coralline algae, foundational taxa that create habitat for sea urchins and abalone, form rhodolith beds in temperate to Arctic habitats and bind together substrates, are expected to be highly susceptible to ocean acidification because they precipitate soluble magnesium calcite ( [[#Kuffner--2008|Kuffner et al., 2008]] ; [[#Williams--2021|Williams et al., 2021]] ). Damage from acidification varies among species and regions, and can be due to direct physiological stress ( [[#Marchini--2019|Marchini et al., 2019]] ) or interactions with non-calcifying competitors such as fleshy macroalgae ( [[#Smith--2020|Smith et al., 2020]] ). Experiments indicate that warming reduces calcification by coralline algae ( ''high confidence'' ) ( [[#Cornwall--2019|Cornwall et al., 2019]] ) and exacerbates the effect of acidification ( [[#Kim--2020|Kim et al., 2020]] ; [[#Williams--2021|Williams et al., 2021]] ).

In contrast to warm-water coral reefs, there are no regional or global numerical models of rocky shore ecosystem response to projected climate change and acidification. Experiments suggest that existing genetic variation could be sufficient for some mussels ( [[#Bitter--2019|Bitter et al., 2019]] ) and coralline algae ( [[#Cornwall--2020|Cornwall et al., 2020]] ) to adapt over generations to ocean acidification. Populations exposed to variable environments often have a greater capacity for phenotypic plasticity and resilience to environmental change [e.g., urchins (Gaitan-Espitia et al., 2017b) and coralline algae ( [[#3.3.2|Section 3.3.2]] ; [[#Rivest--2017|Rivest et al., 2017]] ; [[#Cornwall--2018|Cornwall et al., 2018]] )]. Although parental conditioning within and across generations is an acclimatisation mechanism to global change, there is ''limited evidence'' from experimental studies that this is applicable for marine invertebrates on rocky shores ( [[#Byrne--2020|Byrne et al., 2020]] ).

This assessment concludes that MHWs attributable to climate change ( [[#3.2.2.1|Section 3.2.2.1]] ) can cause fatal disease outbreaks or mass mortality among some key foundational species ( ''high confidence'' ) and contribute to ecological phase shifts ( ''medium confidence'' ) ''.'' The upper vertical limits of some species will also be constrained by climate change ( ''high confidence'' ). Experimental evidence since previous assessments further indicates that acidification decreases abundance and richness of calcifying species ( ''high confidence'' ), although there is ''limited evidence'' for acclimation in some species. Synergistic effects of warming and acidification will promote shifts towards macroalgal dominance in some ecosystems ( ''medium confidence'' ) and lead to reorganisation of communities ( ''medium confidence'' ).

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