Editing IPCC:AR6/SROCC/Chapter-6 (section)

==== 6.4.2.1 Impacts on Marine Organisms and Ecosystems ====

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Temperature plays an essential role in the biology and ecology of marine organisms (e.g., Pörtner, 2002; Pörtner and Knust, 2007 <sup>[[#fn:r396|396]]</sup> ; Poloczanska et al., 2013 <sup>[[#fn:r397|397]]</sup> ; Hoegh-Guldberg et al., 2014 <sup>[[#fn:r398|398]]</sup> ), and therefore extreme high ocean temperature can have large impacts on marine ecosystems. Recent studies show that MHWs have strongly impacted marine organisms and ecosystem services in all ocean basins (Smale et al., 2019 <sup>[[#fn:r399|399]]</sup> ) over the last two decades. Impacts include coral bleaching and mortality (Hughes et al., 2017b <sup>[[#fn:r400|400]]</sup> ; Hughes et al., 2018a <sup>[[#fn:r401|401]]</sup> ; Hughes et al., 2018b <sup>[[#fn:r402|402]]</sup> ), loss of seagrass and kelp forests (Smale et al., 2019 <sup>[[#fn:r403|403]]</sup> ), shifts in species range (Smale and Wernberg, 2013 <sup>[[#fn:r404|404]]</sup> ), and local (Wernberg et al., 2013 <sup>[[#fn:r405|405]]</sup> ; Wernberg et al., 2016 <sup>[[#fn:r406|406]]</sup> ) and potentially global extinctions of coral species (Brainard et al., 2011 <sup>[[#fn:r407|407]]</sup> ).

A growing number of studies have reported that MHWs negatively affect corals and coral reefs through bleaching, disease, and mortality (see Chapter 5 for an extensive discussion on coral reefs and coral bleaching). The recent (2014–2017) high ocean temperatures in the tropics and subtropics triggered a pan-tropical episode of unprecedented mass bleaching of corals (100s of km 2 ), the third global-scale event after 1997–1998 and 2010 (Heron et al., 2016 <sup>[[#fn:r408|408]]</sup> ; Eakin et al., 2017 <sup>[[#fn:r409|409]]</sup> ; Hughes et al., 2017b <sup>[[#fn:r410|410]]</sup> ; Eakin et al., 2018 <sup>[[#fn:r411|411]]</sup> ; Hughes et al., 2018a <sup>[[#fn:r412|412]]</sup> ). The heat stress during this event was sufficient to cause bleaching at 75% of global reefs (Hughes et al., 2018a; Figure 6.3b) and mortality at 30% (Eakin et al., 2017 <sup>[[#fn:r414|414]]</sup> ), much more than any previously documented global bleaching event. In some locations, many reefs bleached extensively for the first time on record, and over half of the reefs bleached multiple times during the three year event. However, there were distinct geographical variations in bleaching, mainly determined by the spatial pattern and magnitude of the MHW (Figure 6.3b). For example, bleaching was extensive and severe in the northern regions of the Great Barrier Reef, with 93% of the northern Australian Great Barrier Reef coral suffering bleaching in 2016, but impacts were moderate at the southern coral reefs of the Great Barrier Reef (Brainard et al., 2018 <sup>[[#fn:r415|415]]</sup> ; Stuart-Smith et al., 2018 <sup>[[#fn:r416|416]]</sup> ).

Apart from strong impacts on corals, recent MHWs have demonstrated their potential impacts on other marine ecosystems and ecosystems services (Ummenhofer and Meehl, 2017 <sup>[[#fn:r417|417]]</sup> ; Smale et al., 2019 <sup>[[#fn:r418|418]]</sup> ). Two of the best studied MHWs with extensive ecological implications are the Western Australia 2011 MHW and the Northeast Pacific 2013–2015 MHW. The Western Australia 2011 MHW resulted in a regime shift of the temperate reef ecosystem (Wernberg et al., 2013 <sup>[[#fn:r419|419]]</sup> ; Wernberg et al., 2016 <sup>[[#fn:r420|420]]</sup> ). The abundance of the dominant habitat-forming seaweeds ''Scytohalia dorycara'' and ''Ecklonia radiata'' became significantly reduced and ''Ecklonia'' kelp forest was replaced by small turf-forming algae with wide ranging impacts on associated sessile invertebrates and demersal fish. The sea grass ''Amphibolis antarctica'' in Shark Bay underwent defoliation after the MHW (Fraser et al., 2014 <sup>[[#fn:r421|421]]</sup> ), and together with the loss of other sea grass species, these lead to releases of 2–9 Tg CO 2 to the atmosphere during the subsequent three years after the MHW (Arias-Ortiz et al., 2018 <sup>[[#fn:r422|422]]</sup> ). In addition, coral bleaching and adverse impacts on invertebrate fisheries were documented (Depczynski et al., 2013 <sup>[[#fn:r423|423]]</sup> ; Caputi et al., 2016 <sup>[[#fn:r424|424]]</sup> ). The Northeast Pacific 2013–2015 MHW also caused extensive alterations to open ocean and coastal ecosystems (Cavole et al., 2016 <sup>[[#fn:r425|425]]</sup> ). Impacts included increased mortality events of sea birds (Jones et al., 2018 <sup>[[#fn:r426|426]]</sup> ), salmon and marine mammals (Cavole et al., 2016 <sup>[[#fn:r427|427]]</sup> ), very low ocean primary productivity (Whitney, 2015 <sup>[[#fn:r428|428]]</sup> ; Jacox et al., 2016 <sup>[[#fn:r429|429]]</sup> ), an increase in warm water copepod species (Di Lorenzo and Mantua, 2016) and novel species compositions (Peterson et al., 2017 <sup>[[#fn:r430|430]]</sup> ). In addition, a coast wide bloom of the toxigenic diatom ''Pseudo-nitzschia'' resulted in the largest ever recorded outbreak of domoic acid along the North American west coast (McCabe et al., 2016 <sup>[[#fn:r431|431]]</sup> ). Domoic acid was detected in many marine mammals, such as whales, dolphins, porpoises, seals and sea lions. The elevated toxins in commercially harvested fish and invertebrates resulted in prolonged and geographically extensive closure of razor clam and crab fisheries.

Other MHWs also demonstrated the vulnerability of marine organisms and ecosystems to extremely high ocean temperatures. The Northwest Atlantic 2012 MHW strongly impacted coastal ecosystems by causing a northward movement of warm water species and local migrations of some species (e.g., lobsters) earlier in the season (Mills et al., 2013 <sup>[[#fn:r432|432]]</sup> ; Pershing et al., 2015) <sup>[[#fn:r433|433]]</sup> . The Mediterranean Sea 2003 MHW lead to mass mortalities of macro-invertebrate species (Garrabou et al., 2009 <sup>[[#fn:r434|434]]</sup> ) and the Tasman Sea 2015–2016 MHW had impacts on sessile, sedentary and cultured species in the shallow, near-shore environment including outbreaks of disease in commercially viable species (Oliver et al., 2017 <sup>[[#fn:r435|435]]</sup> ). ''Vibrio'' outbreaks were also observed in the Baltic Sea in response to elevated SSTs (Baker-Austin et al., 2013 <sup>[[#fn:r436|436]]</sup> ). The Alaskan Sea 2016 MHW favoured some phytoplankton species, leading to harmful algal blooms, shellfish poisoning events and mortality events in seabirds (Walsh et al., 2018 <sup>[[#fn:r437|437]]</sup> ; see chapter 3 for more details). Also, lower than average size of multiple groundfish species were observed including Pollock, Pacific cod, and Chinook salmon (Zador and Siddon, 2016 <sup>[[#fn:r438|438]]</sup> ). The Yellow Sea/East China Sea 2016 MHW killed a large number of different marine organisms in coastal and bay areas around South Korea (Kim and Han, 2017 <sup>[[#fn:r439|439]]</sup> ) and the Southwest Atlantic 2017 MHW lead to toxic algal blooms (Manta et al., 2018 <sup>[[#fn:r440|440]]</sup> ). The Coastal Peruvian 2017 MHW affected anchovies, which showed decreased fat content and early spawning as a reproductive strategy (IMPARPE, 2017), a behaviour usually seen during warm El Niño conditions (Ñiquen and Bouchon, 2004 <sup>[[#fn:r442|442]]</sup> ).

Based on the examples described above we conclude with ''very high confidence'' that a range of organisms and ecosystems have been impacted by MHWs across all ocean basins over the last two decades. Given that MHWs will ''very likely'' increase in intensity and frequency with further climate warming, we conclude with ''high confidence'' that this will push some marine organisms, fisheries and ecosystem beyond the limits of their resilience. These impacts will occur on top of those expected from a progressive shift in global mean ocean temperatures.

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