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

=== 5.3.1 Estuaries ===

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Estuarine ecosystems are defined by the river-sea interface that provides high habitat heterogeneity and supports high biodiversity across freshwater and subtidal zones (Basset et al., 2013 <sup>[[#fn:r858|858]]</sup> ). AR5 WGII (Wong et al., 2014a <sup>[[#fn:r859|859]]</sup> ) and SR15 (Hoegh-Guldberg et al., 2018 <sup>[[#fn:r860|860]]</sup> ) concluded that estuarine ecosystems have been impacted by SLR and human influences that drive salinisation, resulting in increased flooding, land degradation and erosion of coastal areas around estuaries.

Observations since AR5 provide further evidence that SLR increases seawater intrusions and raises salinity in estuaries. Salinisation of estuaries can be exacerbated by droughts and modifications of drainage area by human activities (Ross et al., 2015 <sup>[[#fn:r861|861]]</sup> ; Cardoso-Mohedano et al., 2018 <sup>[[#fn:r862|862]]</sup> ; Hallett et al., 2018 <sup>[[#fn:r863|863]]</sup> ; Zahid et al., 2018 <sup>[[#fn:r864|864]]</sup> ). The changing salinity gradients in estuaries have been linked to the observed upstream expansion of brackish and marine benthic and pelagic communities, and a reduction in the diversity and richness of freshwater fauna (Robins et al., 2016 <sup>[[#fn:r865|865]]</sup> ; Raimonet and Cloern, 2017 <sup>[[#fn:r866|866]]</sup> ; Hallett et al., 2018 <sup>[[#fn:r867|867]]</sup> ; Addino et al., 2019 <sup>[[#fn:r868|868]]</sup> ) ( ''medium confidence'' ). However, because the distribution of benthic species in estuaries is strongly determined by sediment properties like grain size, the gradient of sediment types in estuaries can be a barrier to upstream shifts of brackish and marine benthic biota, leading to a reduction in species richness in mid- to upper- estuarine areas and altering food webs (Little et al., 2017 <sup>[[#fn:r869|869]]</sup> ; Hudson et al., 2018 <sup>[[#fn:r870|870]]</sup> ; Addino et al., 2019 <sup>[[#fn:r871|871]]</sup> ). Similarly, estuarine wetlands (Section 5.3.2) and tidal flats (Murray et al. 2019) have reduced their extent and productivity in response to increased salinity, inundation and wave exposure, especially in areas with limited capacity for soil accretion or inland migration due to coastal squeezing (Sections 4.3.2.3, 5.3.2) ( ''high confidence'' ). Poleward migration of tropical and sub-tropical biota between estuaries has been observed in response to warming (Hallett et al., 2018 <sup>[[#fn:r872|872]]</sup> ) ( ''medium confidence'' ), in agreement with the global trend of biogeographic shifts of marine organisms (Sections 5.2.3.1.1; 5.3.2 − 5.3.6).

Intensive human activities around estuaries and river deltas worldwide has substantially increased nutrient and organic matter inputs into such systems since the 1970s (Maavara et al., 2017 <sup>[[#fn:r873|873]]</sup> ). Increased organic matter accumulation has been shown to interact with warming, resulting in intensification of bacterial degradation and eutrophication (Maavara et al., 2017 <sup>[[#fn:r874|874]]</sup> ; Chen et al., 2018 <sup>[[#fn:r875|875]]</sup> ; Fennel and Testa, 2019 <sup>[[#fn:r876|876]]</sup> ), contributing to an increase in the frequency and extent of hypoxic zones (Breitberg et al., 2015 <sup>[[#fn:r877|877]]</sup> ; Gobler and Baumann, 2016 <sup>[[#fn:r878|878]]</sup> ). The interaction between warming, increased nutrient loading, and hypoxia has shown to be related to the increased occurrences of HABs (Anderson et al., 2015 <sup>[[#fn:r879|879]]</sup> ; Paerl et al., 2018 <sup>[[#fn:r880|880]]</sup> ) (Box 5.4) ( ''high confidence'' ), pathogenic bacteria such as ''Vibrio'' species (Baker-Austin et al., 2017 <sup>[[#fn:r881|881]]</sup> ; Kopprio et al., 2017 <sup>[[#fn:r882|882]]</sup> ) (Section 5.4.2) ( ''low confidence'' ), and mortalities of invertebrates and fish communities (Jeppesen et al., 2018 <sup>[[#fn:r883|883]]</sup> ; Warwick et al., 2018 <sup>[[#fn:r884|884]]</sup> ) ( ''medium confidence'' ).

Fluctuations in estuarine salinity, turbidity and nutrient gradients are influenced by changes in precipitation and wind-stress caused by large-scale climatic variations such as the ENSO, the NAO and the South Atlantic Meridional Overturning Circulation (SAMOC) which have shown persistent anomalies associated with climate change since the 1970s (Wang and Cai, 2013 <sup>[[#fn:r885|885]]</sup> ; Delworth and Zeng, 2016 <sup>[[#fn:r886|886]]</sup> ; García-Moreiras et al., 2018 <sup>[[#fn:r887|887]]</sup> ). Similarly, storm surges and heat waves have increased nutrients and sediment loads in estuaries (Tweedley et al., 2016 <sup>[[#fn:r888|888]]</sup> ; Arias-Ortiz et al., 2018 <sup>[[#fn:r889|889]]</sup> ; Chen et al., 2018 <sup>[[#fn:r890|890]]</sup> ). Sustained long-term observations (15 − 40 years) provide evidence that large-scale climatic variations and extreme events affect plankton phenology and composition in estuaries worldwide with regional differences in the characteristics of the responses (Thompson et al., 2015 <sup>[[#fn:r891|891]]</sup> ; Abreu et al., 2017 <sup>[[#fn:r892|892]]</sup> ; Marques et al., 2017 <sup>[[#fn:r893|893]]</sup> ; Arias-Ortiz et al., 2018 <sup>[[#fn:r894|894]]</sup> ; López-Abbate et al., 2019 <sup>[[#fn:r895|895]]</sup> ) ( ''high confidence'' ). Although these changes in ecosystem components may be attributed to climate variability (Box 5.1), they demonstrate the sensitivity of estuarine ecosystems to climate change. Also, these large-scale climate events are ''likely'' to be intensified in the 21st century (Stocker, 2014 <sup>[[#fn:r896|896]]</sup> ) (Section 6.5.1).

Salinisation in estuaries is projected to continue in response to SLR, warming and droughts under global warming greater than 1.5°C ( ''high confidence'' ), and will pose further risks to ecosystems biodiversity and functioning (Zhou et al., 2017 <sup>[[#fn:r897|897]]</sup> ; Hallett et al., 2018 <sup>[[#fn:r898|898]]</sup> ; Zahid et al., 2018 <sup>[[#fn:r899|899]]</sup> ; Elliott et al., 2019 <sup>[[#fn:r900|900]]</sup> ) (Section 4.3.3.4, Cross-Chapter Box 7) ( ''medium confidence'' ). Estuarine wetlands are resilient to modest rates of SLR due to their sediment relocation capacity, but such adaptation is not expected to keep pace with projected rates of SLR under the RCP8.5 climate scenario (Section 5.3.2) ( ''high confidence'' ). Moreover, human activities that inhibit sediment movement and deposition in coastal deltas increase the likelihood of their shrinking as a result of SLR (Brown et al., 2018b <sup>[[#fn:r901|901]]</sup> ; Schuerch et al., 2018 <sup>[[#fn:r902|902]]</sup> ) ( ''medium'' ''confidence'' ).

Oxygen-depleted dead zones in coastal areas are already a problem; they are projected to increase under the co-occurrence and intensification of climate threats and eutrophication (Breitburg et al., 2018 <sup>[[#fn:r903|903]]</sup> ; Laurent et al., 2018 <sup>[[#fn:r904|904]]</sup> ) (Section 5.2.2.4). While warming is the primary climate driver of deoxygenation in the open ocean, eutrophication is projected to increase in estuaries due to human activities and intensified precipitation increasing riverine nitrogen loads under both RCP2.6 and RCP8.5 scenarios, both mid-century (2031–2060) and later (2071–2100) (Sinha et al., 2017 <sup>[[#fn:r905|905]]</sup> ). Moreover, enhanced stratification in estuaries in response to warming is also expected to increase the risk of hypoxia through reduced vertical mixing (Du et al., 2018 <sup>[[#fn:r906|906]]</sup> ; Hallett et al., 2018 <sup>[[#fn:r907|907]]</sup> ; Warwick et al., 2018 <sup>[[#fn:r908|908]]</sup> ).The effects of warming will be more pronounced on high-latitude and temperate shallow estuaries with limited exchange with the open ocean (e.g., Río de La Plata Estuary, Baltic Sea and Chesapeake Bay) and seasonality that already leads to dead zone development when summertime temperatures reach critical values (e.g., Black Sea) (Altieri and Gedan, 2015 <sup>[[#fn:r909|909]]</sup> ) ( ''medium confidence'' ). The coastal acidification related to this expansion of hypoxic zones (Zhang and Gao, 2016 <sup>[[#fn:r910|910]]</sup> ; Cai et al., 2017 <sup>[[#fn:r911|911]]</sup> ; Laurent et al., 2017 <sup>[[#fn:r912|912]]</sup> ) imposes risk for sensitive organisms (Beck et al., 2011 <sup>[[#fn:r913|913]]</sup> ; Duarte et al., 2013 <sup>[[#fn:r914|914]]</sup> ; Feely et al., 2016 <sup>[[#fn:r915|915]]</sup> ; Carstensen et al., 2018 <sup>[[#fn:r916|916]]</sup> ).

The interaction of SLR and changes in precipitation will have a more severe impact on shallow estuaries (&lt;10 m) than on deep basin estuaries (&gt;10 m) (Hallett et al., 2018 <sup>[[#fn:r917|917]]</sup> ; Elliott et al., 2019 <sup>[[#fn:r918|918]]</sup> ) ( ''medium confidence'' ). For a projected SLR of 1 m, climate-related risks for shallow estuaries ecosystems are estimated to increase through increased tidal current amplitudes (by 5% on average), energy dissipation, vertical mixing and salinity intrusion (Prandle and Lane, 2015 <sup>[[#fn:r919|919]]</sup> ). Estuaries with high tidal exchanges and associated well-developed sediment areas are more resilient to global climate changes than estuaries with low tidal exchanges and sediment supply, since the latter are more vulnerable to SLR and changes in river flow (Brown et al., 2018b; Warwick et al., 2018 <sup>[[#fn:r920|920]]</sup> ) ( ''medium confidence'' ).

Overall, this assessment concludes that there is evidence of upstream redistribution of marine biotic communities in estuaries driven by increased sea water intrusion ( ''medium'' ''confidence'' ). Such distribution shifts are limited by physical barriers such as the availability of benthic substrates leading to reduction of suitable habitats for estuarine communities ( ''medium confidence'' ). Warming has led to poleward range shifts of biota between estuaries ( ''medium'' ''confidence'' ). Increased nutrient inputs from intensive human development in deltas increases bacterial respiration, which in turn is exacerbated by warming, leading to an expansion of suboxic and anoxic areas ( ''high confidence'' ). These changes reduce the survival of estuarine animals ( ''medium confidence'' ), and increase the occurrence of HABs and pathogenic microbes ( ''medium confidence'' ). Projected warming, SLR and tidal changes in the 21st century will continue to expand salinisation and hypoxia in estuaries ( ''medium confidence'' ). These impacts will be more pronounced under higher emission scenarios, and in temperate and high-latitude estuaries that are eutrophic, shallow and that naturally have low sediment supply.

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