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

==== 5.2.4.2 Open Ocean Seafloor - Abyssal Plains (3000-6000 m) ====

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Abyssal communities (3000–6000 m) cover over 50% of the ocean’s surface and are considered to be extremely food limited (Gage and Tyler, 1992; Smith et al., 2018 <sup>[[#fn:r684|684]]</sup> ). There is a strong positive relationship between surface primary production, export flux, and organic matter supply to the abyssal seafloor (Smith et al., 2008 <sup>[[#fn:r685|685]]</sup> ), with pulses of surface production reflected as carbon input on the deep seafloor in days to months (Thomsen et al., 2017 <sup>[[#fn:r686|686]]</sup> ). Both vertical and horizontal transport contribute organic matter to the sea floor (Frischknecht et al., 2018 <sup>[[#fn:r687|687]]</sup> ). Food supply to the seafloor regulates faunal biomass, explaining the strong positive relationships documented between surface production and seafloor faunal biomass in the Pacific Ocean (Smith et al., 2013 <sup>[[#fn:r688|688]]</sup> ), Gulf of Mexico (Wei et al., 2011 <sup>[[#fn:r689|689]]</sup> ) and north Atlantic Ocean (Hartman et al., 2015 <sup>[[#fn:r690|690]]</sup> ). Extended time series and broad spatial coverage reveal strong positive relationship between annual POC flux and abyssal sediment community oxygen consumption (Rowe et al., 2008 <sup>[[#fn:r691|691]]</sup> ; Smith et al., 2016a <sup>[[#fn:r692|692]]</sup> ). Observed reduction in in POC flux at the abyssal seafloor enhances the relative importance of the microbial loop and reduces the importance of benthic invertebrates in carbon transfer (Dunlop et al., 2016 <sup>[[#fn:r693|693]]</sup> ) (single study, ''limited evidence'' ). However, changes in the overlying mesopelagic and bathypelagic communities (see Section 5.2.3.2) will also affect food flux to the deep seafloor, as nekton and zooplankton transfer energy to depth through diel (daily day-night) vertical migrations, ontogenetic (life staged-based) migrations and falls of dead carcasses (Gage, 2003 <sup>[[#fn:r694|694]]</sup> ). Therefore, climate change impacts on organic carbon export from the epipelagic (Section 5.2.3.1) and deeper pelagic systems (Section 5.2.3.2) can affect the energy available to support the abyssal seafloor ecosystems ( ''medium confidence'' ). However, because observations on historical changes in POC flux in abyssal seafloor ecosystems are limited to a few locations, long-term records show high variability, and mechanistic understanding of factors affecting the biological carbon pump is incomplete, there is ''limited evidence'' that the abyssal seafloor ecosystem has already been affected by changes in POC flux as a result of climate change. The metabolic rate of deep seafloor ectotherms, and consequently their demand for food, increases with temperature. Thus, observed warming in deep sea ecosystems (Hoegh-Guldberg et al., 2014 <sup>[[#fn:r695|695]]</sup> ) (Section 5.2.2.2.1) is expected to increase the sensitivity of deep seafloor biota to decrease in food supplies associated with a change in POC flux ( ''high confidence'' ). However, there is ''limited evidence'' of observed changes in abyssal biota. Small deep sea biota demonstrate increased efficiency (effective use of food energy for growth and metabolism with minimal loss) at low food inputs (due to small size and dominance by prokaryotic taxa) (Gambi et al., 2017 <sup>[[#fn:r696|696]]</sup> ). Adaptation to low food availability in abyssal ecosystems may confer higher capacity to adjust to reduced food availability than for shallow biota ( ''limited evidence).'' Overall, the risk of impacts of climate change on abyssal ecosystems through reduction in food supplies from declining POC flux in the present day is low with ''low confidence'' .

The globally integrated export flux of carbon is projected to decrease in the open ocean in the 21st century under RCP2.6 (by 1.6–4.9%) and RCP8.5 (by 8.9–15.8%) relative to 2000 ( ''medium confidence'' ) (Section 5.2.2.6). This change in export flux of carbon is projected to yield declines in POC flux at the abyssal seafloor (representing food supply to benthos) of up to –27% in the Atlantic and up to –31 to –40% in the Pacific and Indian Oceans, with some increases in polar regions (Sweetman et al. 2017 <sup>[[#fn:r697|697]]</sup> ). In some models, additional dissolution of calcium carbonate due to ocean acidification further lowers POC flux, causing the projected export production declines to be up to 38% at the northeast Atlantic seafloor (Jones et al., 2014 <sup>[[#fn:r699|699]]</sup> ). Lower POC fluxes to the abyss reduce food supply and have been projected to cause a size-shift towards smaller organisms (Jones et al., 2014), resulting in rising respiration rates, lower biomass production efficiency, and lesser energy transfer to higher trophic levels (Brown et al., 2004 <sup>[[#fn:r700|700]]</sup> ) ( ''medium confidence'' ). Changes are projected to be largest for macrofauna and lesser and similar for megafauna and meiofauna (Jones et al., 2014) ( ''limited evidence'' , ''low confidence'' ). Projections using outputs from seven CMIP5 models suggest that 97.8 ± 0.6% (95% CI) of the abyssal seafloor area will experience a biomass decline by 2091–2100 relative to 2006–2015 under RCP8.5. The projected decreases in overall POC flux to the abyssal seafloor are projected to cause a 5.2–17.6% reduction in seafloor biomass in 2090–2100, relative to 2006–2015 under RCP8.5 (Jones et al., 2014 <sup>[[#fn:r701|701]]</sup> ). The projected impacts on abyssal seafloor biomass are significantly larger under RCP8.5 than RCP4.5 (Jones et al., 2014 <sup>[[#fn:r703|703]]</sup> ). However, existing estimates are based on total POC flux changes and do not account for changes in the type or quality of the sinking material, to which macrofaunal and meiofaunal invertebrates are highly sensitive (Smith et al., 2008 <sup>[[#fn:r704|704]]</sup> ; Smith et al., 2009 <sup>[[#fn:r705|705]]</sup> ; Tittensor et al., 2011 <sup>[[#fn:r706|706]]</sup> ). The projections also do not account for direct faunal responses to changes in temperature, oxygen or the carbonate system, all of which will influence benthic responses to changing food availability (AR5 Chapter 30.5.7), reducing to ''medium confidence'' the risk assessment that is based on these projections (Figure 5.16).

Regionally, while reductions in POC flux are projected at low and mid latitudes in the Pacific, Indian and Atlantic Oceans, increases are projected at high latitudes associated in part with reduction in sea ice cover (Yool et al., 2013 <sup>[[#fn:r707|707]]</sup> ; Rogers, 2015 <sup>[[#fn:r708|708]]</sup> ; Sweetman et al., 2017 <sup>[[#fn:r709|709]]</sup> ; Yool et al. 2017 <sup>[[#fn:r710|710]]</sup> ; FAO 2019 <sup>[[#fn:r711|711]]</sup> ) (see Chapter 3) ( ''medium confidence'' ). Notably, Arctic and Southern Ocean POC fluxes at the abyssal seafloor are projected to increase by up to 38% and 21%, respectively by 2100 under RCP8.5 (Sweetman et al., 2017 <sup>[[#fn:r712|712]]</sup> ). While an increase in food supply may yield higher benthic biomass at high latitudes, warmer temperatures and reduced pH projected for the polar regions (Chapter 3) would elevate faunal metabolic demands, likely diminishing the benefit of elevated food supply to an unknown extent (Sweetman et al., 2017 <sup>[[#fn:r713|713]]</sup> ). Overall, given the limited food availability for fauna in the abyssal plains and the projected warming (Section 5.2.2.2.2) that increases the demand for food to support the elevated metabolic rates, the projected decrease in influx of organic matter and seafloor biomass will result in high risks of impacts to abyssal ecosystems by the end of the 21st century under RCP8.5 ( ''medium confidence'' ) (Figure 5.16). The risk of impacts is projected to be substantially lower under RCP4.5 or RCP2.6 ( ''high confidence'' ). The impacts on abyssal seafloor ecosystems affect functions that are important to support ecosystem services (see Section 5.4.1). For example, smaller-sized organisms exhibit reduced bioturbation intensity and depth of mixing causing reduced carbon sequestration (Smith et al., 2008 <sup>[[#fn:r714|714]]</sup> ) (Figure 5.15).

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'''Figure 5.15'''

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'''Figure 5.15 | A conceptual diagram illustrating how climate drivers are projected to modify deep sea ecosystems as discussed in Section 5.2.4.'''
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[[File:31366bf5606fb7d2d2844e775476da50 IPCC-SROCC-CH_5_15-3000x2465.jpg]]

Figure 5.15 | A conceptual diagram illustrating how climate drivers are projected to modify deep sea ecosystems as discussed in Section 5.2.4.

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