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

==== 5.2.2.5 Changing Ocean Nutrients ====

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Changes to ocean nutrient cycling are driven by modifications to ocean mixing and transport (Section 5.2.2.2.2), internal biogeochemical cycling and fluctuations in external supply, particularly from rivers and the atmosphere. This assessment will focus on the main nutrients important for driving microbial growth (Section 5.2.2.6), namely nitrogen, phosphorus and iron.

Diverse studies (including shipboard experiments and use of protein biomarkers) have highlighted nitrogen and phosphorus limitation in the stratified tropical ocean regions accompanied by widespread iron limitation at high latitudes and in upwelling regions that typically have elevated levels of productivity (Figure 5.11) (Moore et al., 2013 <sup>[[#fn:r288|288]]</sup> ; Saito et al., 2014 <sup>[[#fn:r289|289]]</sup> ; Browning et al., 2017 <sup>[[#fn:r290|290]]</sup> ; Tagliabue et al., 2017 <sup>[[#fn:r291|291]]</sup> ). Moreover, more extensive experimental work has demonstrated overlapping nitrogen-iron co-limitation at the boundaries between gyre and upwelling regimes (Browning et al., 2017 <sup>[[#fn:r292|292]]</sup> ). There is ''high confidence'' arising from ''robust evidence'' and ''high agreement'' across different types of studies that the main limiting nutrient is either iron (in most major upwelling regions and the Southern, north Atlantic and sub-Arctic Pacific Oceans) or nitrogen and phosphorus (in the low productivity tropical ocean gyres).

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'''Figure 5.11'''

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'''Figure 5.11 | Map of the dominant limiting resource (Moore et al. 2013), updated to include new experiments from the north Pacific, tropical Atlantic and south east Atlantic (Browning et al. 2017; Shilova et al. 2017). The background is depth integrated primary productivity using the Vertically Generalized Production Model algorithm. Colouring of the circles indicates […]'''
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Figure 5.11 | Map of the dominant limiting resource (Moore et al. 2013), updated to include new experiments from the north Pacific, tropical Atlantic and south east Atlantic (Browning et al. 2017; Shilova et al. 2017). The background is depth integrated primary productivity using the Vertically Generalized Production Model algorithm. Colouring of the circles indicates the primary limiting nutrients inferred from chlorophyll and/or primary productivity increases following artificial amendment of: N (blue), P (black), Fe (red), Co (yellow) and Zn (cyan). Divided circles indicate potentially co-limiting nutrients, for example, a red-blue divided circle indicates Fe-N co-limitation.

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There is ''limited evidence'' on contemporary trends in nutrient levels, either from time series sites or broader meta-analyses. Increasing inputs of anthropogenic nitrogen from the atmosphere are perturbing ocean nutrient levels (Jickells et al., 2017 <sup>[[#fn:r293|293]]</sup> ). In the North Pacific in particular, additional atmospheric nitrogen input has raised the nitrogen to phosphorus ratio between 1988–2011 and induced a progressive shift towards phosphorus limitation in this region (Kim et al., 2011 <sup>[[#fn:r294|294]]</sup> ; Kim et al., 2014 <sup>[[#fn:r295|295]]</sup> ; Ren et al., 2017 <sup>[[#fn:r296|296]]</sup> ). This tendency is supported by modelling experiments that find enhanced atmospheric nitrogen input only has a small influence on productivity due to expanded phosphorus limitation (Yang and Gruber, 2016) and other nitrogen cycle feedbacks (Somes et al., 2016 <sup>[[#fn:r298|298]]</sup> ; Landolfi et al., 2017 <sup>[[#fn:r299|299]]</sup> ).

In general, future increases in stratification (Dave and Lozier, 2013 <sup>[[#fn:r300|300]]</sup> ; Talley et al., 2016 <sup>[[#fn:r301|301]]</sup> ; Kwiatkowski et al., 2017 <sup>[[#fn:r302|302]]</sup> ; and see also Section 5.2.2.2) will trap nutrients in the ocean interior and reduce upper ocean nutrient levels, alongside an additional local impact from changes to atmospheric delivery. However, no CMIP5 models accounted for changes in nutrient delivery from dust and anthropogenic aerosols during their experiments, which could be an important component of regional change (Wang et al., 2015b <sup>[[#fn:r303|303]]</sup> ; Somes et al., 2016 <sup>[[#fn:r304|304]]</sup> ; Yang and Gruber, 2016 <sup>[[#fn:r305|305]]</sup> ). ESMs project a decline in the nitrate content of the upper 100 m of 9–14% or 1.5–6% (across 90% confidence intervals) for the RCP8.5 or RCP2.6 scenario, respectively, by 2081–2100 relative to 2006–2015 (Figure 5.8g). The largest absolute declines in nitrate content is projected in the present day upwelling zones (Figure 5.8h). Projected changes to upper 100 m nitrate concentrations are significantly different to zero for both RCP8.5 and RCP2.6 at the 90% confidence level, but are overall lower for the RCP2.6. Scenario, internal variability and inter-model variability contribute roughly equally to the overall projection uncertainty in 2100 (Figure 5.8i) and there is no clear separation of nitrate trends between RCP8.5 and RCP2.6 outside the model uncertainty (Figure 5.8h).

Iron concentrations are projected to increase in the future from ESM simulations, due to enhanced lateral transport into high-latitude oceans and reduced biological consumption in regions of declining nitrate (Misumi et al., 2013 <sup>[[#fn:r314|314]]</sup> ). Other modelling efforts also suggest greater levels of the more biologically available Fe(II) species in a warmer and more acidic ocean (Tagliabue and Völker, 2011 <sup>[[#fn:r315|315]]</sup> ). These modelling studies tend to indicate greater ocean iron availability in the future overall, but the very limited skill of contemporary global ocean iron models in reproducing observations available from the new basin scale datasets from the international GEOTRACES program and neglect for parallel dust supply changes lower the confidence in the models’ projected changes (Tagliabue et al., 2016 <sup>[[#fn:r316|316]]</sup> ).

Overall, nitrate concentrations in the upper 100 m are ''very likely'' to decline by 9–14% by 2081–2100, relative to 2006–2015 for RCP8.5 or 1.5–6% for RCP2.6, in response to increased stratification, with ''medium confidence'' in these projections due to the ''limited evidence'' of past changes that can be robustly understood and reproduced by models. Surface ocean iron levels is projected to increase in the 21st century with ''low confidence'' due to systemic uncertainties in these models.

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