Editing IPCC:AR6/SROCC/Chapter-2 (section)

==== 2.3.3.2 Freshwater Biota ====

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Biota in mountain freshwater ecosystems is affected by cryospheric change through alterations in both the quantity and timing of runoff from glaciers and snowmelt. Where melt water from glaciers decreases, river flows have become more variable, with water temperature and overall channel stability increasing and habitats becoming less complex (Giersch et al., 2017 <sup>[[#fn:r782|782]]</sup> ; Milner et al., 2017 <sup>[[#fn:r783|783]]</sup> ) ( ''medium evidence, medium agreement'' ).

Analysis of three invertebrate datasets from tropical (Ecuador), temperate (Italian Alps) and sub-Arctic (Iceland) alpine regions indicates that a number of cold-adapted species have decreased in abundance below a threshold of watershed glacier cover varying from 19 – 32%. With complete loss of the glaciers, 11–38% of the regional species will be lost (Jacobsen et al., 2012 <sup>[[#fn:r784|784]]</sup> ; Milner et al., 2017 <sup>[[#fn:r785|785]]</sup> ) ( ''medium confidence'' ). As evidenced in Europe (Pyrenees, Italian Alps) and North America (Rocky Mountains) (Brown et al., 2007 <sup>[[#fn:r786|786]]</sup> ; Giersch et al., 2015 <sup>[[#fn:r787|787]]</sup> ; Giersch et al., 2017 <sup>[[#fn:r788|788]]</sup> ; Lencioni, 2018 <sup>[[#fn:r789|789]]</sup> ) the loss of these invertebrates — many of them endemic — as glacier runoff decreases and transitions to a regime more dominated by snowmelt leading to a reduction in turnover between and within stream reaches (beta diversity) and regional (gamma) diversity ''(very high confidence'' ). Regional genetic diversity within individual riverine invertebrate species in mountain headwater areas has decreased with the loss of environmental heterogeneity (Giersch et al., 2017 <sup>[[#fn:r790|790]]</sup> ), as decreasing glacier runoff reduces the isolation of individuals permitting a greater degree of genetic intermixing (Finn et al., 2013 <sup>[[#fn:r791|791]]</sup> ; Finn et al., 2016 <sup>[[#fn:r792|792]]</sup> ; Jordan et al., 2016 <sup>[[#fn:r793|793]]</sup> ; Hotaling et al., 2018 <sup>[[#fn:r794|794]]</sup> ) ( ''medium evidence, high agreement'' ). However, local (alpha) diversity, dominated by generalist species of invertebrates and algae, has increased (Khamis et al., 2016 <sup>[[#fn:r795|795]]</sup> ; Fell et al., 2017 <sup>[[#fn:r796|796]]</sup> ; Brown et al., 2018 <sup>[[#fn:r797|797]]</sup> ) ( ''very'' ''high confidence'' ) in certain regions as species move upstream, although not in the Andes, where downstream migration has been observed (Jacobsen et al., 2014 <sup>[[#fn:r798|798]]</sup> ; Cauvy-Fraunié et al., 2016 <sup>[[#fn:r799|799]]</sup> ).

Many climate variables influence fisheries, through both direct and indirect pathways. The key variables linked to cryospheric change include: changes in air and water temperature, precipitation, nutrient levels and ice cover (Stenseth et al., 2003 <sup>[[#fn:r800|800]]</sup> ). A shrinking cryosphere has significantly affected cold mountain resident salmonids (e.g., brook trout, ''Salvelinus fontinalis'' ), causing further migration upstream in summer thereby shrinking their range (Hari et al., 2006 <sup>[[#fn:r801|801]]</sup> ; Eby et al., 2014 <sup>[[#fn:r802|802]]</sup> ; Young et al., 2018 <sup>[[#fn:r803|803]]</sup> ). Within the Yanamarey watershed of the Cordillera Blanca in Peru, fish stocks have either declined markedly or have become extinct in many streams, possibly due to seasonal reductions of fish habitat in the upper watershed resulting from glacier recession (Bury et al., 2011 <sup>[[#fn:r804|804]]</sup> ; Vuille et al., 2018 <sup>[[#fn:r805|805]]</sup> ). In contrast, glacier recession in the mountains of coastal Alaska and to a lesser extent the Pacific northwest have created a large number of new stream systems that have been, and could continue to be with further glacier retreat, colonised from the sea by salmon species that contribute to both commercial and sport fisheries (Milner et al., 2017 <sup>[[#fn:r806|806]]</sup> ; Schoen et al., 2017 <sup>[[#fn:r807|807]]</sup> ) ( ''medium confidence'' ). Changes in water temperature will vary seasonally, and a potential decreased frequency of rain-on-snow events in winter compared to rain-on-ground would increase water temperature, benefiting overwintering survival (Leach and Moore, 2014 <sup>[[#fn:r808|808]]</sup> ). Increased water temperature remaining below thermal tolerance limits for fish and occurring earlier in the year can benefit overall fish growth and increase fitness (Comola et al., 2015 <sup>[[#fn:r809|809]]</sup> ) ( ''medium evidence'' , ''medium agreement'' ).

In the future, increased primary production dominated by diatoms and golden algae will occur in streams as glacier runoff decreases, although some cold-tolerant diatom species will be lost, resulting in a decrease in regional diversity (Fell et al., 2017 <sup>[[#fn:r810|810]]</sup> ; Fell et al., 2018 <sup>[[#fn:r811|811]]</sup> ). Reduced glacier runoff is projected to improve water clarity in many mountain lakes, increasing biotic diversity and the abundance of bacterial and algal communities and thus primary production (Peter and Sommaruga, 2016 <sup>[[#fn:r812|812]]</sup> ) ( ''limited evidence'' ). Extinction of range-restricted prey species may increase as more favourable conditions facilitate the upstream movement of large bodied invertebrate predators (Khamis et al., 2015 <sup>[[#fn:r813|813]]</sup> ) ( ''medium confidence'' ). Modelling studies indicate a reduction in the range of native species, notably trout, in mountain streams, (Papadaki et al., 2016 <sup>[[#fn:r814|814]]</sup> ; Vigano et al., 2016 <sup>[[#fn:r815|815]]</sup> ; Young et al., 2018 <sup>[[#fn:r816|816]]</sup> ) ( ''medium evidence, high agreement'' ), which will potentially impact sport fisheries. In northwest North America, where salmon are important in native subsistence as well as commercial and sport fisheries, all species will potentially be affected by reductions in glacial runoff from mountain glaciers over time (Milner et al., 2017 <sup>[[#fn:r817|817]]</sup> ; Schoen et al., 2017 <sup>[[#fn:r818|818]]</sup> ), particularly in larger systems where migratory corridors to spawning grounds are reduced ( ''medium confidence'' ).

In summary, cryospheric change will alter freshwater communities with increases in local biodiversity but range shrinkage and extinctions for some species causes regional biodiversity to decrease ( ''robust evidence'' , ''medium agreement'' , i.e., ''high confidence'' ).

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