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

===== 3.2.3.1.1 Plankton and primary production =====

There is evidence that the combination of loss of sea ice, freshening, and regional stratification (Sections 3.2.1.1 and 3.2.1.2) has affected the timing, distribution and production of primary producers (Moore et al., 2018 <sup>[[#fn:r527|527]]</sup> ) ( ''high confidence'' ). Satellite data show that the decline in ice cover has resulted in a &gt;30% increase in annual net primary production (NPP) in ice-free Arctic waters since 1998 (Arrigo and van Dijken, 2011 <sup>[[#fn:r528|528]]</sup> ; Bélanger et al., 2013 <sup>[[#fn:r529|529]]</sup> ; Arrigo and van Dijken, 2015 <sup>[[#fn:r530|530]]</sup> ; Kahru et al., 2016 <sup>[[#fn:r531|531]]</sup> ), a phenomenon corroborated by both ''in situ'' data (Stanley et al., 2015 <sup>[[#fn:r532|532]]</sup> ) and modelling studies (Vancoppenolle et al., 2013 <sup>[[#fn:r533|533]]</sup> ; Jin et al., 2016 <sup>[[#fn:r534|534]]</sup> ). Ice loss has also resulted in earlier phytoplankton blooms (Kahru et al., 2011 <sup>[[#fn:r535|535]]</sup> ) with blooms being dominated by larger-celled phytoplankton (Fujiwara et al., 2016 <sup>[[#fn:r536|536]]</sup> ). The longer open water season in the Arctic has also increased the incidence of autumn blooms, a phenomenon previously rarely observed in Arctic waters (Ardyna et al., 2017 <sup>[[#fn:r537|537]]</sup> ).

Thinner Arctic sea ice cover has led to the appearance of intense phytoplankton blooms that develop beneath first-year sea ice ( ''medium confidence'' ). Blooms of this size (1000s of km 2 ) and intensity (peaks of approximately 30 mg Chla-m –3 ) were previously thought to be restricted to the marginal ice zone and the open ocean where ample light reaches the surface ocean for rapid phytoplankton growth (Arrigo et al., 2012 <sup>[[#fn:r538|538]]</sup> ). Evidence shows that these blooms can thrive beneath sea ice in areas of reduced thickness, increased coverage of melt ponds (Arrigo et al., 2014 <sup>[[#fn:r539|539]]</sup> ; Zhang et al., 2015 <sup>[[#fn:r540|540]]</sup> ; Jin et al., 2016 <sup>[[#fn:r541|541]]</sup> ; Horvat et al., 2017 <sup>[[#fn:r542|542]]</sup> ), first-year ridges at the snow-ice interface (Fernández-Méndez et al., 2018 <sup>[[#fn:r543|543]]</sup> ), and a large number of cracks (high lead fractions) in the ice (Assmy et al., 2017 <sup>[[#fn:r544|544]]</sup> ), although the latter has not changed significantly in the last three decades (Wang et al., 2016a <sup>[[#fn:r545|545]]</sup> ). Local features including snow-free or thin snow, hummocks and ridges commonly found on multi-year ice also provide habitat for ice algae (Lange et al., 2017 <sup>[[#fn:r546|546]]</sup> ).

The reduction in sea ice area and thickness in the Arctic Ocean appears to be indirectly impacting rates of NPP through increased exposure of the surface ocean to atmospheric forcing ( ''medium confidence'' ) and these indirect impacts will possibly increase in the future ( ''low confidence'' ). Greater wind stress has been shown to increase upwelling of nutrients at the shelf break both over ice-free waters (Williams and Carmack, 2015 <sup>[[#fn:r547|547]]</sup> ) and a partial ice cover (Schulze and Pickart, 2012 <sup>[[#fn:r548|548]]</sup> ), leading to more new production (Williams and Carmack, 2015 <sup>[[#fn:r549|549]]</sup> ). At the same time, enhanced vertical stratification (Section 3.2.1.2.2, SM3.2.2) and decreased upwelling of nutrients into surface waters (Capotondi et al., 2012 <sup>[[#fn:r550|550]]</sup> ; Nummelin et al., 2016 <sup>[[#fn:r551|551]]</sup> ) may reduce Arctic NPP in the future, especially in the central basin (Ardyna et al., 2017 <sup>[[#fn:r552|552]]</sup> ). It could also impact phytoplankton community composition and size structure, with small-celled phytoplankton, which require less nutrients, becoming more dominant as nutrient concentrations in surface waters decline (Yun et al., 2015 <sup>[[#fn:r553|553]]</sup> ).

In addition to its impact on phytoplankton bloom dynamics, the decline in the proportion of multi-year sea ice and proliferation of a thinner first year sea ice cover may favour growth of microalgae within the ice due to increased light availability ( ''medium confidence'' ). Recent studies suggest that the contribution of sea ice algae to total Arctic NPP is higher now than values measured previously (Song et al., 2016 <sup>[[#fn:r554|554]]</sup> ), accounting for nearly 10% of total NPP (ice plus water) and as much as 60% in places like the central Arctic (Fernández-Méndez et al., 2015 <sup>[[#fn:r555|555]]</sup> ).

Ongoing changes in NPP will impact the biogeochemistry and ecology of large parts of the Arctic Ocean ( ''high confidence'' ). In areas of enhanced nutrient availability and greater NPP, dominance by larger-celled microalgae increases vertical export efficiency from the surface downwards in both ice covered (Boetius et al., 2013 <sup>[[#fn:r556|556]]</sup> ; Lalande et al., 2014 <sup>[[#fn:r557|557]]</sup> ; Mäkelä et al., 2017 <sup>[[#fn:r558|558]]</sup> ) and open ocean (Le Moigne et al., 2015) areas. However, because exported biomass production may be increasing in some areas but declining in others, the net impact may be small (Randelhoff and Guthrie, 2016 <sup>[[#fn:r559|559]]</sup> ) (Sections 3.2.3.1.2, 5.3.6, SM3.2.6). Phytoplankton may have the capacity to compensate for ocean acidification under a range of temperatures and pH values (Hoppe et al., 2018 <sup>[[#fn:r560|560]]</sup> ).

Increased water temperatures (Section 3.2.1) and shifts in the spatial pattern and timing of the ice algal and phytoplankton blooms, have impacted the phenology, magnitude and duration of zooplankton production with associated changes in the zooplankton community composition ( ''medium confidence'' ). Negative effects of reductions in ice algae on zooplankton may be partially offset by predicted increases in water column phytoplankton production in the Bering Sea (Wang et al., 2015 <sup>[[#fn:r561|561]]</sup> ). Changes in sea ice coverage and thickness may alter the phenology, abundance and distribution of zooplankton in the future. Projected changes will initially have the most pronounced impact on sympagic amphipods, but will subsequently affect food web functioning and carbon dynamics of the pelagic system (Kohlbach et al., 2016 <sup>[[#fn:r562|562]]</sup> ).

At the more southern boundaries of the Arctic such as the southeastern Bering Sea, warm conditions have led to reduced production of large copepods and euphausiids ( ''medium confidence'' ) (Sigler et al., 2017 <sup>[[#fn:r563|563]]</sup> ; Kimmel et al., 2018 <sup>[[#fn:r564|564]]</sup> ). On more northern shelves, the increased open water period has led to increases in large copepods over a 60 year period within the Chukchi Sea (Ershova et al., 2015 <sup>[[#fn:r565|565]]</sup> ) and in recent years also the Beaufort Sea (Smoot and Hopcroft, 2017 <sup>[[#fn:r566|566]]</sup> ), while in the Central Basins zooplankton biomass in general has increased (Hunt et al., 2014 <sup>[[#fn:r567|567]]</sup> ; Rutzen and Hopcroft, 2018 <sup>[[#fn:r568|568]]</sup> ) ( ''medium confidence'' ).

There are inconsistent findings concerning the future development of copepods in the Arctic. Coupled biophysical model results suggest that sea ice loss will increase primary production and that will primarily be consumed pelagically by zooplankton grazers such as ''Calanus hyperboreus'' ; increasing their abundances in the central Arctic (Kvile et al., 2018 <sup>[[#fn:r569|569]]</sup> ). Feng et al. (2018) concluded that ''C. glacialis'' should continue to benefit from a warmer Arctic Ocean. On the other hand, in the transition zone between Arctic and Atlantic water masses, ''C. glacialis'' may face increasing competition from the more boreal ''C. finmarchicus'' (Dalpadado et al., 2016 <sup>[[#fn:r571|571]]</sup> ). Renaud et al. (2018) <sup>[[#fn:r572|572]]</sup> found the lipid content of ''Calanus'' spp. was related to size and not species. This suggests that climate driven shifts in dominant ''Calanus'' species may, because of overlap in size spectrum and contrary to earlier assumptions, not negatively impact their consumers in the Barents Sea.

The effects of ocean acidification on Arctic zooplankton and pteropods (small pelagic molluscs) have been examined for only a few species and these studies reveal that the severity of effects is dependent on emission scenarios and the species sensitivity and adaptive capacity. The copepod ''C. glacialis'' exhibits stage-specific sensitivities to ocean acidification with some stages being relatively insensitive to decreases in pH and other stages exhibiting substantial reductions in scope for growth (Bailey et al., 2017 <sup>[[#fn:r573|573]]</sup> ; Thor et al., 2018 <sup>[[#fn:r574|574]]</sup> ). Although there is strong evidence that pteropods are sensitive to the effects of ocean acidification (Manno et al., 2017 <sup>[[#fn:r575|575]]</sup> ) recent studies indicate they may exhibit some ability to adapt (Peck et al., 2016 <sup>[[#fn:r576|576]]</sup> ; Peck et al., 2018 <sup>[[#fn:r577|577]]</sup> ). However, the metabolic costs of adaptation may be constraining, especially during periods of low food availability (Lischka and Riebesell, 2016 <sup>[[#fn:r578|578]]</sup> ).

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