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

==== 3.2.3.3 Changes in Nutrient Availability ====

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The availability of nutrients in the surface ocean often limits primary productivity, with implications for marine food webs and the biological carbon pump. Nitrogen availability tends to limit phytoplankton productivity throughout most of the low-latitude ocean, whereas dissolved iron availability limits productivity in high-nutrient, low-chlorophyll regions, such as in the main upwelling region of the Southern Ocean and the Eastern Equatorial Pacific ( ''high confidence'' ) ( [[#Moore--2013|Moore et al., 2013]] ; [[#IPCC--2019b|IPCC, 2019b]] ). Phosphorus, silicon, other micronutrients such as zinc, and vitamins can also co-limit marine phytoplankton productivity in some ocean regions ( [[#Moore--2013|Moore et al., 2013]] ). Whereas some studies have shown coupling between climate variability and nutrient trends in specific regions, such as in the North Atlantic ( [[#Hátún--2016|Hátún et al., 2016]] ), North Pacific ( [[#Di%20Lorenzo--2009|Di Lorenzo et al., 2009]] ; [[#Yasunaka--2014|Yasunaka et al., 2014]] ) and tropical ( [[#Stramma--2021|Stramma and Schmidtko, 2021]] ) Oceans, very few studies have been able to detect long-term changes in ocean nutrient concentrations (but see [[#Yasunaka--2016|Yasunaka et al., 2016]] ).

Future changes in nutrient concentrations have been estimated using ESMs, with future increases in stratification generally leading to decreased nutrient levels in surface waters ( [[#IPCC--2019b|IPCC, 2019b]] ). CMIP6 models project a decline in the nitrate concentration of the upper 100 m in 2080–2099 relative to 1995–2014 of −0.46 ± 0.45 ( ''very likely range'' ), −0.60 ± 0.58, −0.80 ± 0.77 and −1.00 ± 0.78 mmol m –3 under SSP1-2.6, SSP2-4.5 and SSP5-8.5, respectively (Figure 3.5; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). These declines in nitrate concentration are greater than simulated by the CMIP5 models in their RCP analogues, a ''likely'' consequence of enhanced surface warming and stratification in CMIP6 models (Figure 3.5; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). It is concluded that the surface ocean will encounter reduced nitrate concentrations in the 21st century ( ''medium confidence'' ).

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