Editing IPCC:AR6/SR15/Chapter-3 (section)

==== 3.4.4.11 Ocean foodwebs (pteropods, bivalves, krill and fin fish) ====

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Ocean foodwebs are vast interconnected systems that transfer solar energy and nutrients from phytoplankton to higher trophic levels, including apex predators and commercially important species such as tuna. Here, we consider four representative groups of marine organisms which are important within foodwebs across the ocean, and which illustrate the impacts and ramifications of 1.5°C or higher levels of warming.

The first group of organisms, pteropods, are small pelagic molluscs that suspension feed and produce a calcium carbonate shell. They are highly abundant in temperate and polar waters where they are an important link in the foodweb between phytoplankton and a range of other organisms including fish, whales and birds. The second group, bivalve molluscs (e.g., clams, oysters and mussels), are filter-feeding invertebrates. These invertebrate organisms underpin important fisheries and aquaculture industries, from polar to tropical regions, and are important food sources for a range of organisms including humans. The third group of organisms considered here is a globally significant group of invertebrates known as ''euphausiid crustaceans'' (krill), which are a key food source for many marine organisms and hence a major link between primary producers and higher trophic levels (e.g., fish, mammals and sea birds). Antarctic krill,  ''Euphausia superba'' , are among the most abundant species in terms of mass and are consequently an essential component of polar foodwebs (Atkinson et al., 2009) <sup>[[#fn:r667|667]]</sup> . The last group, fin fishes, is vitally important components of ocean foodwebs, contribute to the income of coastal communities, industries and nations, and are important to the foodsecurity and livelihood of hundreds of millions of people globally (FAO, 2016) <sup>[[#fn:r668|668]]</sup> . Further background for this section is provided in Supplementary Material 3.SM.3.2.

There is a moderate risk to ocean foodwebs under present-day conditions ( ''medium to high confidence'' ) (Figure 3.18). Changing water chemistry and temperature are already affecting the ability of pteropods to produce their shells, swim and survive (Bednaršek et al., 2016) <sup>[[#fn:r669|669]]</sup> . Shell dissolution, for example, has increased by 19–26% in both nearshore and offshore populations since the pre-industrial period (Feely et al., 2016) <sup>[[#fn:r670|670]]</sup> . There is considerable concern as to whether these organisms are declining further, especially given the central importance in ocean foodwebs (David et al., 2017) <sup>[[#fn:r671|671]]</sup> . Reviewing the literature reveals that pteropods are projected to face high risks of impact at average global temperatures 1.5°C above pre-industrial levels and increasing risks of impacts at 2°C ( ''medium confidence'' ).

As GMST increases by 1.5°C and more, the risk of impacts from ocean warming and acidification are expected to be moderate to high, except in the case of bivalves (mid-latitudes) where the risks of impacts are projected to be high to very high (Figure 3.18). Ocean warming and acidification are already affecting the life history stages of bivalve molluscs (e.g., Asplund et al., 2014; Mackenzie et al., 2014; Waldbusser et al., 2014; Zittier et al., 2015; Shi et al., 2016; Velez et al., 2016; Q. Wang et al., 2016; Castillo et al., 2017; Lemasson et al., 2017; Ong et al., 2017; X. Zhao et al., 2017) <sup>[[#fn:r672|672]]</sup> . Impacts on adult bivalves include decreased growth, increased respiration and reduced calcification, whereas larval stages tend to show greater developmental abnormalities and increased mortality after exposure to these conditions ( ''medium to high confidence'' ) (Q. Wang et al., 2016; Lemasson et al., 2017; Ong et al., 2017; X. Zhao et al., 2017) <sup>[[#fn:r673|673]]</sup> . Risks are expected to accumulate at higher temperatures for bivalve molluscs, with very high risks expected at 1.8°C of warming or more. This general pattern applies to low-latitude fin fish, which are expected to experience moderate to high risks of impact at 1.3°C of global warming ( ''medium confidence'' ), and very high risks at 1.8°C at low latitudes ( ''medium confidence'' ) (Figure 3.18).

Large-scale changes to foodweb structure are occurring in all oceans. For example, record levels of sea ice loss in the Antarctic (Notz and Stroeve, 2016; Turner et al., 2017b) <sup>[[#fn:r674|674]]</sup> translate into a loss of habitat and hence reduced abundance of krill (Piñones and Fedorov, 2016) <sup>[[#fn:r675|675]]</sup> , with negative ramifications for the seabirds and whales which feed on krill (Croxall, 1992; Trathan and Hill, 2016) <sup>[[#fn:r676|676]]</sup> ( ''low-medium confidence'' ). Other influences, such as high rates of ocean acidification coupled with shoaling of the aragonite saturation horizon, are ''likely'' to also play key roles (Kawaguchi et al., 2013; Piñones and Fedorov, 2016) <sup>[[#fn:r677|677]]</sup> . As with many risks associated with impacts at the ecosystem scale, most adaptation options focus on the management of stresses unrelated to climate change but resulting from human activities, such as pollution and habitat destruction. Reducing these stresses will be important in efforts to maintain important foodweb components. Fisheries management at local to regional scales will be important in reducing stress on foodweb organisms, such as those discussed here, and in helping communities and industries adapt to changing foodweb structures and resources (see further discussion of fisheries ''per se'' below; Section 3.4.6.3). One strategy is to maintain larger population levels of fished species in order to provide more resilient stocks in the face of challenges that are increasingly driven by climate change (Green et al., 2014; Bell and Taylor, 2015) <sup>[[#fn:r678|678]]</sup> .

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