Editing IPCC:AR6/SROCC/Chapter-5 (section)

=== 5.3.5 Rocky Shores ===

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Rocky shore ecosystems span the intertidal and shallow subtidal zones of the world’s temperate coasts and are typically dominated by calcareous mussels or seaweeds (macroalgae). Other organisms that inhabit rocky shores are coralline algae (i.e., maerl beds), polychaetes, molluscs, bryozoans and sponges. Intertidal habitats are characterised by strong environmental gradients, and are exposed to marine and atmospheric climate regimes (Hawkins et al., 2016 <sup>[[#fn:r1106|1106]]</sup> ). IPCC AR5 (Wong et al., 2014a <sup>[[#fn:r1107|1107]]</sup> ) concluded that rocky shores are among the better-understood coastal ecosystems in terms of potential impacts of climate variability and change. The high sensitivity of sessile organisms (e.g., barnacles, mussels) to extreme temperature events (e.g., mass mortality and drastic biodiversity loss of mussels beds), and to acidification (widely observed in manipulative experiments) gives ''high confidence'' that rocky shore species are at high risk of changes in distribution and abundance from these two drivers. SR15 (Hoegh-Guldberg et al., 2018 <sup>[[#fn:r1108|1108]]</sup> ) concluded that rocky coasts are already experiencing large-scale changes, and critical thresholds are expected to be reached at warming of 1.5°C and above ( ''high confidence'' ).

More observational and empirical evidence since AR5 and SR15 confirms that climate change poses high risk to rocky shore ecosystems’ biodiversity, structure and functioning through warming, acidification, SLR and extreme events (Agostini et al., 2018 <sup>[[#fn:r1109|1109]]</sup> ; Duarte and Krause-Jensen, 2018 <sup>[[#fn:r1110|1110]]</sup> ; Ullah et al., 2018 <sup>[[#fn:r1111|1111]]</sup> ; Milazzo et al., 2019 <sup>[[#fn:r1112|1112]]</sup> ). Immobile intertidal organisms are especially vulnerable to warming, due to the potential for extreme heat exposure during low tide emersion and prolonged desiccation events (Hawkins et al., 2016 <sup>[[#fn:r1113|1113]]</sup> ; Zamir et al., 2018 <sup>[[#fn:r1114|1114]]</sup> ) ( ''high confidence'' ). This effect is expected to lower the upper vertical limit of intertidal communities (Hawkins et al., 2016 <sup>[[#fn:r1115|1115]]</sup> ), reducing their suitable habitat (Harley, 2011 <sup>[[#fn:r1116|1116]]</sup> ), and accompanied by temperature-induced increases in predation by consumers (Sanford, 1999 <sup>[[#fn:r1117|1117]]</sup> ). While previous studies have documented a poleward shift in species distributions of rocky intertidal and reef algae (Duarte et al., 2013 <sup>[[#fn:r1118|1118]]</sup> ; Nicastro et al., 2013 <sup>[[#fn:r1119|1119]]</sup> ) and faunal species (Barry et al., 1995 <sup>[[#fn:r1120|1120]]</sup> ; Mieszkowska et al., 2006 <sup>[[#fn:r1121|1121]]</sup> ; Lima et al., 2007 <sup>[[#fn:r1122|1122]]</sup> ), local extinctions at the equatorial or warm edge of species ranges are increasingly being attributed to climate change (Yeruham et al., 2015 <sup>[[#fn:r1123|1123]]</sup> ; Sorte et al., 2017 <sup>[[#fn:r1124|1124]]</sup> ) ( ''high confidence'' ). Extreme heat waves are expected to cause mortality among rocky shore species (Gazeau et al., 2014 <sup>[[#fn:r1125|1125]]</sup> ; Jurgens et al., 2015 <sup>[[#fn:r1126|1126]]</sup> ) and subsequent declines or losses in important species can have cascading effects on the whole intertidal community and the services it provides (Gatti et al., 2017 <sup>[[#fn:r1127|1127]]</sup> ; Sorte et al., 2017 <sup>[[#fn:r1128|1128]]</sup> ; Sunday et al., 2017 <sup>[[#fn:r1129|1129]]</sup> ). Coralline fauna adapted to narrow environmental conditions seem especially vulnerable to heat waves, with observed mass mortalities in the Adriatic Sea in response to extreme summer temperatures (Kružić et al., 2016 <sup>[[#fn:r1130|1130]]</sup> ). The loss of thermal refugia associated with continued warming could exacerbate the impacts of heat stress on rocky intertidal communities (Lima et al., 2016 <sup>[[#fn:r1131|1131]]</sup> ). Nevertheless, experimental data indicate that some coralline algae that are well adapted to highly variable transitional environments can tolerate the warming projected for 2100 under RCP8.5; for these species, ocean acidification will constitute the main hazard (Nannini et al., 2015 <sup>[[#fn:r1132|1132]]</sup> ).

Ocean acidification is expected to decrease the net calcification ( ''high confidence'' ) and abundance ( ''medium confidence'' ) of rocky intertidal and reef-associated species (Kroeker et al., 2013 <sup>[[#fn:r1133|1133]]</sup> ), and the dissolution of calcareous species has already been documented in tide-pool communities (Kwiatkowski et al., 2016 <sup>[[#fn:r1134|1134]]</sup> ; Duarte and Krause-Jensen, 2018 <sup>[[#fn:r1135|1135]]</sup> ). Recent experimental and field studies, however, have demonstrated the importance of food resources in mediating the effects of ocean acidification on vulnerable rocky shores species (Ciais et al., 2013 <sup>[[#fn:r1136|1136]]</sup> ; Ramajo et al., 2016 <sup>[[#fn:r1137|1137]]</sup> ), suggesting that species’ vulnerability to ocean acidification may be most pronounced in areas of high heat stress and low food availability ( ''medium confidence'' ) (Kroeker et al., 2017 <sup>[[#fn:r1138|1138]]</sup> ). There is increasing evidence that the interactions between multiple climate drivers will determine species vulnerability and the ecosystem impacts of climate change (Hewitt et al., 2016 <sup>[[#fn:r1139|1139]]</sup> ).

Studies on naturally acidified rocky reef ecosystems suggest ocean acidification will simplify rocky shore ecosystems, due to an overgrowth by macroalgae, a reduction in biodiversity and a reduction in the abundance of calcareous species ( ''medium confidence'' ) (Kroeker et al., 2013 <sup>[[#fn:r1140|1140]]</sup> ; Linares et al., 2015 <sup>[[#fn:r1141|1141]]</sup> ). These shifts in community structure and function have been observed in CO 2 seep communities (Hall-Spencer et al., 2008 <sup>[[#fn:r1142|1142]]</sup> ), already exposed to levels of pCO 2 expected to generally occur by the end of the century (Agostini et al., 2018 <sup>[[#fn:r1143|1143]]</sup> ). Reductions in the abundance of calcareous herbivores that can create space for rarer species by grazing the dominant algae, are expected to contribute to the overgrowth of fleshy macroalgae on rocky shores (Baggini et al., 2015 <sup>[[#fn:r1144|1144]]</sup> ). This shift towards macroalgae is associated with a simplification of the food web at lower trophic levels (Kroeker et al., 2011 <sup>[[#fn:r1145|1145]]</sup> ).

At the local scale, warming and ocean acidification are expected to change energy flows within rocky shores ecosystems ( ''medium confidence'' ). Experiments indicate that both climate drivers may boost primary productivity in some cases (Goldenberg et al., 2017 <sup>[[#fn:r1146|1146]]</sup> ); however, increased metabolic demands and greater consumption by predators under warmer temperature increase the strength of top-down control (predation mortalities of herbivores) and thus counteracts the effects of increased bottom-up productivity (Goldenberg et al., 2017 <sup>[[#fn:r1147|1147]]</sup> ; Kordas et al., 2017 <sup>[[#fn:r1148|1148]]</sup> ). Ocean acidification could also increase species energetic costs and the grazing rate of herbivores, affecting ecosystem responses to increased primary productivity (Ghedini et al., 2015 <sup>[[#fn:r1149|1149]]</sup> ). Although these increasingly complex experiments have highlighted the potential for species interactions to mediate the effects of climate change, our understanding of the effects on intact, functioning ecosystems is limited. Despite predictions for increased production and herbivory with warming and acidification, an experimental study of a more complex food web revealed an overall reduction in the energy flow to higher trophic levels and a shift towards detritus-based food webs (Ullah et al., 2018 <sup>[[#fn:r1150|1150]]</sup> ).

Overall, intertidal rocky shores ecosystems are highly sensitive to ocean warming, acidification and extreme heat exposure during low tide emersion ( ''high confidence'' ). More field and experimental evidence shows that these ecosystems are at a moderate risk at present and this level is expected to rise to very high under the RCP8.5 scenario by the end of the century (see Section 5.3.7). 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'' ).

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