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

===== 3.4.3.2.3 Freshwater =====

Climate-driven changes in seasonal ice and permafrost conditions influence water quality ( ''high confidence'' ). Shortened duration of freshwater ice cover (more light absorption, increased nutrient input) is expected to result in higher primary productivity (Hodgson and Smol, 2008 <sup>[[#fn:r1713|1713]]</sup> ; Vincent et al., 2011 <sup>[[#fn:r1714|1714]]</sup> ; Griffiths et al., 2017b <sup>[[#fn:r1715|1715]]</sup> ) and may also encourage greater methane emissions from Arctic lakes (Greene et al., 2014 <sup>[[#fn:r1716|1716]]</sup> ; Tan and Zhuang, 2015 <sup>[[#fn:r1717|1717]]</sup> ). Thaw slumps, active layer detachments and peat plateau collapse affect surface water connectivity (Connon et al., 2014 <sup>[[#fn:r1718|1718]]</sup> ) and enhance sediment, particulate and solute fluxes in river and stream networks (Kokelj et al., 2013 <sup>[[#fn:r1719|1719]]</sup> ). The transfer of enhanced nutrients from land to water (driven by active layer thickening and thermokarst processes; Abbott et al., 2015 <sup>[[#fn:r1720|1720]]</sup> ; Vonk et al., 2015 <sup>[[#fn:r1721|1721]]</sup> ) has been linked to heightened autotrophic productivity in freshwater ecosystems (Wrona et al., 2016 <sup>[[#fn:r1722|1722]]</sup> ). Still, there is ''low confidence'' in the influence of permafrost changes on dissolved organic carbon, because of competing mechanisms that influence carbon export. Permafrost thaw could contribute to the mobilisation of previously frozen organic carbon (Abbott et al., 2014 <sup>[[#fn:r1723|1723]]</sup> ; Wickland et al., 2018 <sup>[[#fn:r1724|1724]]</sup> ; Walvoord et al., 2019 <sup>[[#fn:r1725|1725]]</sup> ) thereby enhancing both particulate and dissolved organic carbon export to aquatic systems. Increased delivery of this dissolved carbon from enhanced river discharge to the Arctic Ocean (Section 3.4.3.1.2) can exacerbate regionally extreme aragonite undersaturation of shelf waters (Semiletov et al., 2016 <sup>[[#fn:r1726|1726]]</sup> ) driven by ocean uptake of anthropogenic CO 2 (Section 3.2.1.2.4). Conversely, reduced dissolved organic carbon export could accompany permafrost thaw as (1) water infiltrates deeper and has longer residence times for decomposition (Striegl et al., 2005 <sup>[[#fn:r1727|1727]]</sup> ) and (2) the proportion of groundwater (typically lower in dissolved organic carbon and higher in DIC than runoff) to total streamflow increases (Walvoord and Striegl, 2007 <sup>[[#fn:r1728|1728]]</sup> ). Increased thermokarst also has the potential to impact freshwater cycling of inorganic carbon (Zolkos et al., 2018 <sup>[[#fn:r1729|1729]]</sup> ).

Enhanced subsurface water fluxes resulting from permafrost degradation has consequences for inorganic natural and anthropogenic constituents. Emerging evidence suggests large natural stores of mercury (Schuster et al., 2018 <sup>[[#fn:r1730|1730]]</sup> ; St Pierre et al., 2018 <sup>[[#fn:r1731|1731]]</sup> ) and other trace elements in permafrost (Colombo et al., 2018 <sup>[[#fn:r1732|1732]]</sup> ) may be released upon thaw, thereby having effects (largely unknown at this point) on aquatic ecosystems. In parallel, increased development activity in the Arctic is ''likely'' to lead to enhanced local sources of anthropogenic chemicals of emerging Arctic concern, including siloxanes, parabens, flame retardants, and per- and polyfluoroalkyl substances (AMAP, 2017c <sup>[[#fn:r1733|1733]]</sup> ). For legacy pollutants, there is ''high confidence'' that black carbon and persistent organic pollutants (e.g., hexachlorocyclohexanes, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls) can be transferred downstream and affect water quality (Hodson, 2014 <sup>[[#fn:r1734|1734]]</sup> ). Lakes can become sinks of these contaminants, while floodplains can be contaminated (Sharma et al., 2015).

There is ''high confidence'' that habitat loss or change due to climate change impact Arctic fishes. Thinning ice on lakes and streams changes the overwintering habitat for aquatic fauna by impacting winter water volumes and dissolved oxygen levels (Leppi et al., 2016 <sup>[[#fn:r1735|1735]]</sup> ). Surface water loss, reduced surface water connectivity among aquatic habitats, and changes to the timing and magnitude of seasonal flows (Section 3.4.1.2) result in a direct loss of spawning, feeding, or rearing habitats (Poesch et al., 2016 <sup>[[#fn:r1736|1736]]</sup> ). Changes to permafrost landscapes have reduced freshwater habitat available for fish and other aquatic biota, including aquatic invertebrates upon which the fish depend for food (Chin et al., 2016 <sup>[[#fn:r1737|1737]]</sup> ). Gullying deepens channels (Rowland et al., 2011 <sup>[[#fn:r1738|1738]]</sup> ; Liljedahl et al., 2016 <sup>[[#fn:r1739|1739]]</sup> ) that otherwise may connect lake habitats occupied by fishes. This can lead to the loss of surface water connectivity, limit fish access to key habitats, and lower fish diversity (Haynes et al., 2014 <sup>[[#fn:r1740|1740]]</sup> ; Laske et al., 2016 <sup>[[#fn:r1741|1741]]</sup> ). Small connecting stream channels, which are vulnerable to drying, provide necessary migratory pathways for fishes, allowing them to access spawning and summer rearing grounds (Heim et al., 2016 <sup>[[#fn:r1742|1742]]</sup> ; McFarland et al., 2017 <sup>[[#fn:r1743|1743]]</sup> ).

Changes to the timing, duration and magnitude of high surface flow events in early and late summer threaten Arctic fish dispersal and migration activities (Heim et al., 2016 <sup>[[#fn:r1744|1744]]</sup> ) ( ''high confidence'' ). Timing of important life history events such as spawning can become mismatched with changing stream flows (Lique et al., 2016 <sup>[[#fn:r1745|1745]]</sup> ). There is regional evidence that migration timing has shifted earlier and winter egg incubation temperature has increased for pink salmon ( ''Oncorhynchus gorbuscha'' ), directly related to warming (Taylor, 2007 <sup>[[#fn:r1746|1746]]</sup> ). While long-term, pan-Arctic data on run timing of fishes are limited, phenological shifts could create mismatches with food availability or habitat suitability in both marine and freshwater environments for anadromous species, and in freshwater environments for freshwater resident species. Changes to the Arctic growing season (Xu et al., 2013a <sup>[[#fn:r1747|1747]]</sup> ) increase the risk of drying of surface water habitats and pose a potential mismatch in seasonal availability of food in rearing habitats.

Freshwater systems across the Arctic are relatively shallow, and thus are expected to warm ( ''high confidence'' ). This may make some surface waters inhospitably warm for cold water species such as Arctic Grayling ( ''Thymallus arcticus'' ) and whitefishes ( ''Coregonus spp'' .), or may increase the risk of ''Saprolegnia'' ''fungus'' that appears to have recently spread rapidly, infecting whitefishes at much higher rates in Arctic Alaska than noted in the past (Sformo et al., 2017 <sup>[[#fn:r1748|1748]]</sup> ). High infection rates may be driven by stress or nutrient enrichment from thawing permafrost, which increases pathogen virulence with fish (Wedekind et al., 2010 <sup>[[#fn:r1749|1749]]</sup> ). Warmer water and longer growing seasons will also affect food abundance because invertebrate life histories and production are temperature and degree-day dependent (Régnière et al., 2012 <sup>[[#fn:r1750|1750]]</sup> ). Increased nutrient export from permafrost loss (Frey et al., 2007 <sup>[[#fn:r1751|1751]]</sup> ), facilitated by warmer temperatures, will ''likely'' increase food resources for consumers, but the impact on lower trophic levels within food webs is not clearly understood.

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