Sea ice

Sea ice is a globally unique ecosystem that hosts a diversity of endemic flora and fauna, and whose presence in fjords provides an array of services to society (Eamer et al., Reference Eamer, Donaldson, Gaston, Kosobokova, Lárusson, Melnikov, Reist, Richardson, Staples and von Quillfeldt2013). Indeed, the presence of sea ice, or lack thereof, forms the basis through which many of the drivers in this review interact with one another.

The primary conditions for the formation of sea ice are air temperature and salinity (Pavlov et al., Reference Pavlov, Tverberg, Ivanov, Nilsen, Falk-Petersen and Granskog2013), but other complex factors also play an important role. Wind stress can fragment forming sea ice and prevent water stratification, freshwater inputs allow freezing at less negative temperatures, and snow cover can insulate against colder air temperatures which prevents further growth (Merkouriadi et al., Reference Merkouriadi, Cheng, Graham, Rösel and Granskog2017). The amount of sea ice formation and its location in late winter and spring determines the bottom temperature over the shelf when melted water is mixed with bottom water by storms (Hunt et al., Reference Hunt, Coyle, Eisner, Farley, Heintz, Mueter, Napp, Overland, Ressler, Salo and Stabeno2011), which has implications for benthic life (see section “Biomass”).

Large pulses of warm and salty Atlantic water (AW) have been increasing in the fjords along the North/West Svalbard Archipelago over the last three decades (Skogseth et al., Reference Skogseth, Olivier, Nilsen, Falck, Fraser, Tverberg, Ledang, Vader, Jonassen, Søreide, Cottier, Berge, Ivanov and Falk-Petersen2020). The combination of AW with increased air temperatures (e.g., winter trend of +3°C dec⁻¹; Maturilli et al., Reference Maturilli, Hanssen-Bauer, Neuber, Rex, Edvardsen, Hop and Wiencke2019) have severely restricted sea ice formation (Kongsfjorden: Cottier et al., Reference Cottier, Nilsen, Inall, Gerland, Tverberg and Svendsen2007; Tverberg et al., Reference Tverberg, Skogseth, Cottier, Sundfjord, Walczowski, Inall, Falck, Pavlova, Nilsen, Hop and Wiencke2019; Isfjorden: Muckenhuber et al., Reference Muckenhuber, Nilsen, Korosov and Sandven2016; Skogseth et al., Reference Skogseth, Olivier, Nilsen, Falck, Fraser, Tverberg, Ledang, Vader, Jonassen, Søreide, Cottier, Berge, Ivanov and Falk-Petersen2020; Gronfjorden: Zhuravskiy et al., Reference Zhuravskiy, Ivanov and Pavlov2012). Pronounced warming in the temperature of AW inflow (see section “Seawater temperature”) itself has been recorded during the summer from 1912 to 2019 (Bloshkina et al., Reference Bloshkina, Pavlov and Filchuk2021).

Unlike North/West Svalbard, most of Greenland is not exposed to rapidly warming ocean currents. The tidewater glaciers (see section “Glacier mass balance”) of Nuup Kangerlua (W Greenland) introduce large amounts of icebergs to the fjord, creating a dense ice melange stretching over several kilometres and freezing together in winter (Mortensen et al., Reference Mortensen, Rysgaard, Bendtsen, Lennert, Kanzow, Lund and Meire2020). As of this writing, there was a scarcity of in situ time series measuring sea ice cover for West Greenland, but satellite measurements (NSIDC, Reference Fetterer, Savoie, Helfrich and Clemente-Colón2022) from 2006 to 2020 show trends of increasing cover within fjords and embayments (Schlegel and Gattuso, Reference Schlegel and Gattusoin review). The ice-free season in Young Sound (E Greenland) has been increasing, primarily driven by later formation of sea ice in autumn, accompanied by increased interannual variability since 2000 (Middelbo et al., Reference Middelbo, Møller, Arendt, Thyrring and Sejr2019). On the southern border of the Barents Sea, Porsangerfjorden (N Norway) does not freeze over in the winter, with only the very inner reaches of the fjord occasionally covered by seasonal sea ice (Petrich et al., Reference Petrich, O’Sadnick and Dale2017).

Glacier mass balance

Glaciers are mountainous bodies of land-borne ice that have formed and persisted over millennia. Glaciers have such a dominating downstream effect on fjords that the ecosystems therein are generally defined by whether there is a glacier present and, if there is, whether it is land-terminating or marine-terminating (Lydersen et al., Reference Lydersen, Assmy, Falk-Petersen, Kohler, Kovacs, Reigstad, Steen, Strøm, Sundfjord, Varpe, Walczowski, Weslawski and Zajaczkowski2014).

Most of the large reservoirs of glacial ice in the Arctic, including the Greenland ice sheet (GrIS), are losing mass by surface melt, basal ice melt, and solid ice discharge at marine-terminating glacier fronts (Kochtitzky and Copland, Reference Kochtitzky and Copland2022). The rate of this loss is projected to double by 2100 (Geyman et al., Reference Geyman, van Pelt, Maloof, Aas and Kohler2022). While the GrIS gained mass between 1972 and 1980 (+47 ± 21 Gt yr⁻¹), since 1980 the GrIS has lost mass at an accelerating rate until a peak of 286 ± 20 Gt yr⁻¹ between 2010 and 2018 (Mouginot et al., Reference Mouginot, Rignot, Bjørk, van den Broeke, Millan, Morlighem, Noël, Scheuchl and Wood2019). This process of ice loss (in both solid and liquid form) has also been well documented for fjord glaciers on Svalbard, such as those in Kongsfjorden (Schuler et al., Reference Schuler, Kohler, Elagina, Hagen, Hodson, Jania, Kääb, Luks, Małecki, Moholdt, Pohjola, Sobota and Van Pelt2020).

Terrestrial runoff

The Arctic Ocean holds ca. 1% of the world’s seawater, but receives 11% of global freshwater runoff (Shiklomanov, Reference Shiklomanov1997). Meltwater from land-terminating glaciers enters the fjord at the surface, resulting in strong stratification that drives estuarine circulation. This also increases turbidity (Konik et al., Reference Konik, Darecki, Pavlov, Sagan and Kowalczuk2021), which may have consequences for benthic life (see section “Biomass”). At marine-terminating glaciers, freshwater input comes mostly from below as subglacial discharge, often several hundred metres below the sea surface (Hopwood et al., Reference Hopwood, Carroll, Dunse, Hodson, Holding, Iriarte, Ribeiro, Achterberg, Cantoni, Carlson, Chierici, Clarke, Cozzi, Fransson, Juul-Pedersen, Winding and Meire2020). Due to its low density, the subglacial meltwater can drive upwelling, thereby resupplying nutrient-rich but potentially warmer deep water to shallower depths (Meire et al., Reference Meire, Mortensen, Rysgaard, Bendtsen, Boone, Meire and Meysman2016; Hopwood et al., Reference Hopwood, Carroll, Dunse, Hodson, Holding, Iriarte, Ribeiro, Achterberg, Cantoni, Carlson, Chierici, Clarke, Cozzi, Fransson, Juul-Pedersen, Winding and Meire2020) and stimulating primary production (see section “Primary production”; Hopwood et al., Reference Hopwood, Carroll, Dunse, Hodson, Holding, Iriarte, Ribeiro, Achterberg, Cantoni, Carlson, Chierici, Clarke, Cozzi, Fransson, Juul-Pedersen, Winding and Meire2020). Icebergs, which originate from the calving of marine-terminating glaciers, can add freshwater at the surface, increasing stratification. As the cryosphere warms, glaciers do not melt at a linearly increasing rate, rather the melt rate eventually slows as they lose mass (Huss and Hock, Reference Huss and Hock2018). On Svalbard, glacial meltwater is already decreasing due to mass loss below a critical tipping point (Nowak et al., Reference Nowak, Hodgkins, Nikulina, Osuch, Wawrzyniak, Kavan, Łepkowska, Majerska, Romashova, Vasilevich, Sobota and Rachlewicz2021).

In addition to the melting of glaciers, river runoff is a major input of freshwater into Arctic fjords. River runoff is similar to land-terminating glacier melt in that it decreases the penetration of light, surface heating, stratification, oxygen content, nutrient input, and finally primary production (Wassmann et al., Reference Wassmann, Svendsen, Keck and Reigstad1996; Aksnes et al., Reference Aksnes, Dupont, Staby, Fiksen, Kaartvedt and Aure2009). However, it differs in that the content of terrigenous material in Arctic rivers is highly variable and depends on the catchment type (Slagstad et al., Reference Slagstad, Wassmann and Ellingsen2015; Frigstad et al., Reference Frigstad, Kaste, Deininger, Kvalsund, Christensen, Bellerby, Sørensen, Norli and King2020). Glaciers and ice sheets can dominate catchments in Greenland, Canada, Alaska, and archipelagoes such as Svalbard and Franz Joseph Land, but tundra dominates on the Eurasian and American continents, where catchments extend beyond the Arctic region. The organic carbon content in the large Eurasian rivers can be 10-fold higher than in glacial meltwater, partly reflecting thawing permafrost (Wild et al., Reference Wild, Andersson, Bröder, Vonk, Hugelius, McClelland, Song, Raymond and Gustafsson2019). The nitrogen input (see section “Nutrients”) from land (rivers and eroding coasts combined) is also substantial and has been estimated to sustain a third of the net primary production (see section “Primary production”) of the Arctic Ocean (Terhaar et al., Reference Terhaar, Lauerwald, Regnier, Gruber and Bopp2021).

Summary

The cryosphere, a defining characteristic of the Arctic (Pavlova et al., Reference Pavlova, Gerland, Hop, Hop and Wiencke2019), is vanishing at an alarming rate (Meredith et al., Reference Meredith, Sommerkorn, Cassotta, Derksen, Ekaykin, Hollowed, Kofinas, Mackintosh, Melbourne-Thomas, Muelbert, Ottersen, Pritchard, Schuur, Pörtner, Roberts, Masson-Delmotte and Zhai2019), driven primarily by warming air and seawater temperatures (see section “Seawater temperature”; Isaksen et al., Reference Isaksen, Nordli, Ivanov, Køltzow, Aaboe, Gjelten, Mezghani, Eastwood, Førland, Benestad, Hanssen-Bauer, Brækkan, Sviashchennikov, Demin, Revina and Karandasheva2022). There is also a robust linear relationship between the increase in atmospheric CO₂ and the decrease in sea ice extent (Stroeve and Notz, Reference Stroeve and Notz2018). Many West Svalbard fjords are already experiencing increasingly longer sea ice-free periods (Dahlke et al., Reference Dahlke, Hughes, Wagner, Gerland, Wawrzyniak, Ivanov and Maturilli2020), and given the current emissions trajectory most Arctic fjords will very likely follow this trend in the near future (Meredith et al., Reference Meredith, Sommerkorn, Cassotta, Derksen, Ekaykin, Hollowed, Kofinas, Mackintosh, Melbourne-Thomas, Muelbert, Ottersen, Pritchard, Schuur, Pörtner, Roberts, Masson-Delmotte and Zhai2019). Sea ice volume over the entire Arctic has already diminished by 75% (Overland et al., Reference Overland, Dunlea, Box, Corell, Forsius, Kattsov, Olsen, Pawlak, Reiersen and Wang2019). Within the Svalbard fjords, sea ice has reduced by 50% on average from the periods 1973–2000 to 2005–2019, with a further reduction down to ca. 90% in the next 10 to 20 years (Urbański and Litwicka, Reference Urbański and Litwicka2022).

Marine-terminating glaciers in the northern hemisphere have been losing mass at such unprecedented rates that 7% of them have transitioned to land-terminating over the last 20 years (Kochtitzky and Copland, Reference Kochtitzky and Copland2022). Such a change in glacier status restructures the entire local ecosystem and its services. Moreover, it is worth noting that rapid glacial melt may also be driving further increases in atmospheric CO₂ (Wadham et al., Reference Wadham, Hawkings, Tarasov, Gregoire, Spencer, Gutjahr, Ridgwell and Kohfeld2019; Christiansen et al., Reference Christiansen, Röckmann, Popa, Sapart and Jørgensen2021).

Precipitation rates in the Arctic have been increasing, and are projected to continue to increase, and by the end of the century (except Greenland) the majority of this precipitation is projected to be rain rather than snow (Bintanja and Andry, Reference Bintanja and Andry2017). Indeed, from 1979 to 2009, the average trend throughout the Arctic for snow days per year has been −2.49 days per decade (Liston and Hiemstra, Reference Liston and Hiemstra2011). This has resulted in increases of river runoff (Mankoff et al., Reference Mankoff, Noël, Fettweis, Ahlstrøm, Colgan, Kondo, Langley, Sugiyama, van As and Fausto2020), associated with a peak date occurring earlier in the calendar year (Holmes et al., Reference Holmes, Shiklomanov, Suslova, Tretiakov, McClelland, Spencer and Tank2018). This increasing discharge intensifies the freshwater cycle and increases the connectivity between land and sea (Hernes et al., Reference Hernes, Tank, Sejr and Glud2021) through the increased delivery of nutrients, organic matter, sediments, and contaminants. This is especially pronounced for Eurasian rivers (Shiklomanov et al., Reference Shiklomanov, Déry, Tretiakov, Yang, Magritsky, Georgiadi, Tang, Yang and Kane2021). Within Arctic fjords specifically, we see that this process is beginning to affect the surface waters in Greenland fjords (Paulsen et al., Reference Paulsen, Nielsen, Müller, Møller, Stedmon, Juul-Pedersen, Markager, Sejr, Delgado Huertas, Larsen and Middelboe2017), and has a larger impact on Svalbard fjords (Wiedmann et al., Reference Wiedmann, Reigstad, Marquardt, Vader and Gabrielsen2016; Santos-Garcia et al., Reference Santos-Garcia, Ganeshram, Tuerena, Debyser, Husum, Assmy and Hop2022) and their adjacent ecosystems (Delpech et al., Reference Delpech, Vonnahme, McGovern, Gradinger, Præbel and Poste2021), with an even greater effect on northern Norwegian fjords (McGovern et al., Reference McGovern, Pavlov, Deininger, Granskog, Leu, Søreide and Poste2020).

Seawater temperature

One of the primary controlling factors of the extent of the Arctic cryosphere is the earth’s temperature (Meredith et al., Reference Meredith, Sommerkorn, Cassotta, Derksen, Ekaykin, Hollowed, Kofinas, Mackintosh, Melbourne-Thomas, Muelbert, Ottersen, Pritchard, Schuur, Pörtner, Roberts, Masson-Delmotte and Zhai2019). This also has a dominating effect on the presence of species thriving in a given location (see section “Species richness”; Willis et al., Reference Willis, Cottier, Kwasniewski, Wold and Falk-Petersen2006; Vihtakari et al., Reference Vihtakari, Welcker, Moe, Chastel, Tartu, Hop, Bech, Descamps and Gabrielsen2018). It has been established that the rate of warming in the air is four times more rapid in the Arctic than elsewhere (Rantanen et al., Reference Rantanen, Karpechko, Lipponen, Nordling, Hyvärinen, Ruosteenoja, Vihma and Laaksonen2022). However, changes in seawater temperature are not always linear, nor are they uniform in scale temporally or spatially. Rather, disturbances may materialise as non-linear phenomena, such as shifts of ocean currents or the ephemeral appearance of extreme ocean temperature events.

AW, which is warmer and more nutrient-rich than Arctic waters, is circulated to Svalbard via the Fram Strait as part of the West Spitsbergen Current (WSC) where it forms much of the bottom layer of the West Svalbard fjords in summer. However, starting in 2006, AW has begun occupying much more of the water column (Tverberg et al., Reference Tverberg, Skogseth, Cottier, Sundfjord, Walczowski, Inall, Falck, Pavlova, Nilsen, Hop and Wiencke2019; Skogseth et al., Reference Skogseth, Olivier, Nilsen, Falck, Fraser, Tverberg, Ledang, Vader, Jonassen, Søreide, Cottier, Berge, Ivanov and Falk-Petersen2020), a process referred to as “Atlantification”. This occurs in part due to changes to patterns of the wind stress field in the area (Pavlov et al., Reference Pavlov, Tverberg, Ivanov, Nilsen, Falk-Petersen and Granskog2013) and the wandering of large-scale ocean currents. In addition to increasing temperatures, changes to the inflow of AW are so critical because this water body is the main nutrient contributor (see section “Nutrients”) to the European Arctic (Duarte et al., Reference Duarte, Meyer and Moreau2021).

In contrast to the warming in Western Svalbard, driven largely by increased AW temperature, Hanna and Cappelen (Reference Hanna and Cappelen2003) observed a significant cooling trend in southern Greenland seawater surface temperatures in eight meteorological stations from 1958 to 2001 (−1.22°C in 44 years), while the rest of the world was warming (+0.55 °C in 44 years). They suggested that this cooling could be attributed to a positive phase in the North Atlantic Oscillation (NAO), which leads to northerly winds over Greenland pushing cold air masses down to the south, and was highly positively correlated (r = 0.76) to the historic seawater temperature trend (Hanna and Cappelen, Reference Hanna and Cappelen2003). However, after 2001, southern Greenland air and seawater temperatures began increasing and a more recent study by Jiang et al. (Reference Jiang, Ye and Xiao2020) found that in addition to climate indices, such as the NAO, that greenhouse gas concentrations are key drivers for seawater temperature changes in Greenland.

Salinity

The salinity of seawater creates bounding limits for the presence of many, but not all, marine species found throughout the Arctic (see section “Species richness”; Węsławski et al., Reference Węsławski, Kendall, Włodarska-Kowalczuk, Iken, Kędra, Legezynska and Sejr2011) and salinity changes can have impacts on the trophic structure of local fjord ecosystems (Bridier et al., Reference Bridier, Olivier, Chauvaud, Sejr and Grall2021). Changes in salinity also induce changes in total alkalinity, a key parameter of the carbonate system (see section “Carbonate system”). In general, fjords have three distinct strongly stratified water masses (Stigebrandt, Reference Stigebrandt, Bengtsson, Herschy and Fairbridge2012):

1. 1) Surface water: generally, the lowest salinity due to local freshwater supply.

2. 2) Intermediate water: mirrors the stratification of adjacent coastal waters but with some phase delay.

3. 3) Basin water: rests below the sill level and contains the densest waters, which may enter from outside the fjord (NB: not all fjords have a sill).

The increasing rates of rainfall, glacial melt, and river discharge into fjords (see section “Terrestrial runoff”) may hypothetically impact the thickness and extent of the low-salinity layer in their inner regions so greatly that it slows the rate of the overturning circulation and deep-water renewal (Bianchi et al., Reference Bianchi, Arndt, Austin, Benn, Bertrand, Cui, Faust, Koziorowska-Makuch, Moy, Savage, Smeaton, Smith and Syvitski2020). High precipitation in temperate fjords can create a persistent low-salinity layer in surface waters (Gillibrand et al., Reference Gillibrand, Turrell and Elliott1995; Gibbs, Reference Gibbs2001) that accentuates salinity stratification and limits phytoplankton access to nutrient-rich saline bottom waters (see section “Primary production”), except during wind-induced mixing episodes (Sakshaug and Myklestad, Reference Sakshaug and Myklestad1973; Goebel et al., Reference Goebel, Wing and Boyd2005; Bianchi et al., Reference Bianchi, Arndt, Austin, Benn, Bertrand, Cui, Faust, Koziorowska-Makuch, Moy, Savage, Smeaton, Smith and Syvitski2020). A decrease in salinity of the surface water (0–50 m) in Young Sound (E Greenland) and on the adjacent shelf has been observed (Sejr et al., Reference Sejr, Stedmon, Bendtsen, Abermann, Juul-Pedersen, Mortensen and Rysgaard2017). The lower density of the freshening surface means that bottom water in the deeper part of the fjord is isolated from exchange with shelf water (Boone et al., Reference Boone, Rysgaard, Carlson, Meire, Kirillov, Mortensen, Dmitrenko, Vergeynst and Sejr2018).

Light (PAR and UV)

The light available throughout the water column, here specifically photosynthetically active radiation (PAR), is a key driver of the presence and composition of benthic and pelagic phototrophic communities due to their need to photosynthesise (see section “Biomass”). Assuming the availability of necessary nutrients (see section “Nutrients”), this means that light plays a major role in the global carbon cycle by controlling the geographical and depth distributions of primary producers (see section “Primary production”; Gattuso et al., Reference Gattuso, Gentili, Antoine and Doxaran2020). In the Arctic, three processes linked to climate change that affect the penetration of light into the water column have been well researched:

1. 1) Current and future projected sea ice loss (see section “Sea ice”) creates longer sea ice free periods that allow for greater penetration of light (Pavlov et al., Reference Pavlov, Leu, Hanelt, Bartsch, Karsten, Hudson, Gallet, Cottier, Cohen, Berge, Johnsen, Maturilli, Kowalczuk, Sagan, Meler, Granskog, Hop and Wiencke2019).

2. 2) Projected increases in freshwater input (see section “Terrestrial runoff”) reduce light penetration in the coastal zone by increasing turbidity via the delivery of particulate and dissolved organic matter (DOM; Frigstad et al., Reference Frigstad, Kaste, Deininger, Kvalsund, Christensen, Bellerby, Sørensen, Norli and King2020; Nowak et al., Reference Nowak, Hodgkins, Nikulina, Osuch, Wawrzyniak, Kavan, Łepkowska, Majerska, Romashova, Vasilevich, Sobota and Rachlewicz2021).

3. 3) If summer cloudiness increases as the Arctic warms, it will decrease incident PAR above the sea surface (Bélanger et al., Reference Bélanger, Babin and Tremblay2013).

Largely due to increased freshwater inputs, most fjords in Western Svalbard (1997–2019; Konik et al., Reference Konik, Darecki, Pavlov, Sagan and Kowalczuk2021), and many fjords on mainland Norway (1935–2007; Aksnes et al., Reference Aksnes, Dupont, Staby, Fiksen, Kaartvedt and Aure2009) have experienced a regime shift towards darker water, a phenomenon referred to as “darkening” or “browning”. It is hypothesised that this darkening of water may cause a reduction in primary production (see section “Primary production”; Aksnes et al., Reference Aksnes, Dupont, Staby, Fiksen, Kaartvedt and Aure2009). Areas distant from sources of freshwater input (e.g., glaciers and rivers; section “Terrestrial runoff”) could, however, experience an increase in light penetration as is occurring in the open Arctic Ocean where reduced sea ice leads to increased PAR and thereby primary production (see section “Primary production”; Arrigo and van Dijken, Reference Arrigo and van Dijken2011).

For atmospheric radiation conditions, further stratospheric ozone loss will result in a higher UV-B burden in the Arctic (Manney et al., Reference Manney, Santee, Rex, Livesey, Pitts, Veefkind, Nash, Wohltmann, Lehmann, Froidevaux, Poole, Schoeberl, Haffner, Davies, Dorokhov, Gernandt, Johnson, Kivi, Kyrö, Larsen, Levelt, Makshtas, CT, Nakajima, Parrondo, Tarasick, von der Gathen, Walker and Zinoviev2011). The impact of UV-B on benthic communities in Arctic fjords has been extensively studied; however, the results with respect to the ecological implications are still somewhat inconclusive (see Bischof and Steinhoff, Reference Bischof, Steinhoff, Wiencke and Bischof2012, for review). UV-B may negatively affect biological processes in shallow waters, as experimentally tested for the germination of seaweed spores (Wiencke et al., Reference Wiencke, Roleda, Gruber, Clayton and Bischof2006). However, under natural field conditions, kelp spores germinating under parental canopies might not be exposed to harmful UV-B, and it remains questionable to what extent biologically significant UV-B fluxes will propagate into subtidal communities (i.e., deeper than 10 m; Laeseke et al., Reference Laeseke, Bartsch and Bischof2019).

Summary

Models show that a global temperature rise of +2°C will translate to +4°C of warming in the air temperature of the Arctic (Overland et al., Reference Overland, Dunlea, Box, Corell, Forsius, Kattsov, Olsen, Pawlak, Reiersen and Wang2019), with the worst-case scenario showing +15°C of winter air warming by 2100 (Overland et al., Reference Overland, Dunlea, Box, Corell, Forsius, Kattsov, Olsen, Pawlak, Reiersen and Wang2019). One must also consider the disproportionately larger surface heat fluxes into the Arctic (Bischof et al., Reference Bischof, Convey, Duarte, Gattuso, Granberg, Hop, Hoppe, Jiménez, Lisitsyn, Martinez, Roleda, Thor, Wiktor, Gabrielsen, Hop and Wiencke2019) that may inhibit the stabilisation of the global climate even if an effective emissions reduction strategy is implemented (Overland et al., Reference Overland, Dunlea, Box, Corell, Forsius, Kattsov, Olsen, Pawlak, Reiersen and Wang2019). There is therefore a high level of certainty that the rate of increasing seawater temperature will further accelerate in the future (Meredith et al., Reference Meredith, Sommerkorn, Cassotta, Derksen, Ekaykin, Hollowed, Kofinas, Mackintosh, Melbourne-Thomas, Muelbert, Ottersen, Pritchard, Schuur, Pörtner, Roberts, Masson-Delmotte and Zhai2019).

Rapidly increasing seawater temperatures appear to be accelerating the phenomenon of Atlantification, a process that will potentially decrease the density differences between polar surface water and the AW that rest below, which in turn may lead to more mixing and larger ocean heat fluxes towards the surface (Polyakov et al., Reference Polyakov, Alkire, Bluhm, Brown, Carmack, Chierici, Danielson, Ellingsen, Ershova, Gårdfeldt, Ingvaldsen, Pnyushkov, Slagstad and Wassmann2020). The changes to the salinity itself may also cause trophic restructuring of the ecosystems throughout many Arctic fjords (see section “Biomass”), with inherent knock-on effects to the human societies that are structured around present ecosystem services (see section “Fisheries”).

Less clear than the increases in temperature and changes in salinity are the changes to light penetration in Arctic fjords. While it appears evident that light penetration in the open Arctic Ocean will increase over time (Pavlov et al., Reference Pavlov, Leu, Hanelt, Bartsch, Karsten, Hudson, Gallet, Cottier, Cohen, Berge, Johnsen, Maturilli, Kowalczuk, Sagan, Meler, Granskog, Hop and Wiencke2019), it is still unclear whether or not this will hold true within fjords. While sea ice is melting rapidly within most fjords, there is also an increased rate of turbid water runoff. So while there is a longer period in which light may contact the sea surface, it is becoming more difficult for light to penetrate these waters. This is an area of investigation that still requires much research (e.g., Walch et al., Reference Walch, Singh, Søreide, Lantuit and Poste2022).

Carbonate system

Increased atmospheric carbon dioxide (CO₂) globally raises the partial pressure of CO₂ in seawater (pCO₂). The ocean has absorbed >25% of anthropogenic CO₂ emissions since the industrial revolution (Friedlingstein et al., Reference Friedlingstein, Jones, O’Sullivan, Andrew, Bakker, Hauck, Le Quéré, Peters, Peters, Pongratz, Sitch, Canadell, Ciais, Jackson, Alin, Anthoni, Bates, Becker, Bellouin, Bopp, Chau, Chevallier, Chini, Cronin, Currie, Decharme, Djeutchouang, Dou, Evans, Feely, Feng, Gasser, Gilfillan, Gkritzalis, Grassi, Gregor, Gruber, Gürses, Harris, Houghton, Hurtt, Iida, Ilyina, Luijkx, Jain, Jones, Kato, Kennedy, Goldewijk, Knauer, Korsbakken, Körtzinger, Landschützer, Lauvset, Lefèvre, Lienert, Liu, Marland, McGuire, Melton, Munro, Nabel, Nakaoka, Niwa, Ono, Pierrot, Poulter, Rehder, Resplandy, Robertson, Rödenbeck, Rosan, Schwinger, Schwingshackl, Séférian, Sutton, Sweeney, Tanhua, Tans, Tian, Tilbrook, Tubiello, Van Der Werf, Vuichard, Wada, Wanninkhof, Watson, Willis, Wiltshire, Yuan, Yue, Yue, Zaehle and Zeng2022), which moderates climate change at the cost of ocean acidification, a process that describes the increase in dissolved inorganic carbon (DIC), the concomitant decline of pH, and the saturation state of calcium carbonate (CaCO₃; Gattuso and Hansson, Reference Gattuso, Hansson, Gattuso and Hansson2011). The projected decrease in pH and CaCO₃ saturation state will lead to undersaturation of surface waters with respect to aragonite-type CaCO₃ in the entire Arctic Ocean by 2040 (Steinacher et al., Reference Steinacher, Joos, Frölicher, Plattner and Doney2009). This undersaturation has already been observed in situ throughout many Arctic Seas from 2008 onwards (e.g., Zhang et al., Reference Zhang, Yamamoto-Kawai and Williams2020; Fransner et al., Reference Fransner, Fröb, Tjiputra, Goris, Lauvset, Skjelvan, Jeansson, Omar, Chierici, Jones, Fransson, Ólafsdóttir, Johannessen and Olsen2022). This is due in part to the decrease of salinity (see section “Salinity”), which lowers the buffering capacity of these systems (Qi et al., Reference Qi, Ouyang, Chen, Wu, Lei, Chen, Feely, Anderson, Zhong, Lin, Polukhin, Zhang, Zhang, Bi, Lin, Luo, Zhuang, He, Chen and Cai2022). Aragonite undersaturation has negative consequences on ecologically important aragonite-shelled organisms in Arctic fjords (see section “Biomass”; Comeau et al., Reference Comeau, Gattuso, Nisumaa and Orr2012), which may have large knock-on consequences for a number of other taxa (see section “Species richness”; Bednaršek et al., Reference Bednaršek, Naish, Feely, Hauri, Kimoto, Hermann, Michel, Niemi and Pilcher2021; Niemi et al., Reference Niemi, Bednaršek, Michel, Feely, Williams, Azetsu-Scott, Walkusz and Reist2021).

Nutrients

Besides light, macronutrients (e.g., nitrate [NO₃], nitrite [NO₂], ammonium [NH₄], phosphate [PO₄], silicate [SiO₄], and iron [Fe]) are the key drivers of primary production (see section “Primary production”). Within the euphotic zone, the shallower depths where light levels are sufficient for photosynthesis, nutrients are typically the limiting factor for primary production (generally used up by algae, depending on the season). Organic matter sinking out of the euphotic zone is slowly degraded and nutrients are regenerated; however, these nutrients stay at depth, unavailable for primary production, unless deep water is mixed up to the surface (see section “Salinity”; Valiela, Reference Valiela2015). The process of deep water mixing is particularly important because nitrogen may enter fjords via organic matter that is not directly available to primary producers (see section “Primary production”) and must be degraded by bacteria and archaea into bioavailable forms while at depth (e.g., NO₃ and/or NH₄; Valiela, Reference Valiela2015).

Four well-studied processes that can bring deep nutrient-rich water masses to the euphotic zone are the following (Cottier et al., Reference Cottier, Nilsen, Skogseth, Tverberg, Skarðhamar and Svendsen2010):

1. 1) melting at the marine-terminating face of glaciers that drives local upwelling (see section “Glacier mass balance”),

2. 2) reduced stratification of the water column in winter, typically weakened by decreased meltwater runoff (see section “Terrestrial runoff”), allows deeper mixing of the water column by physical forces (e.g., winds and tides),

3. 3) surface currents exiting fjords over steep slopes (e.g., shelf breaks), and

4. 4) icebergs melt from below driving local upwelling similar to marine-terminating glacier fronts (Moon et al., Reference Moon, Sutherland, Carroll, Felikson, Kehrl and Straneo2018).

Glacial meltwater is one of the primary sources of nutrient input into fjords and may be rich in SiO₄ and Fe depending on bedrock geochemistry (Halbach et al., Reference Halbach, Vihtakari, Duarte, Everett, Granskog, Hop, Kauko, Kristiansen, Myhre, Pavlov, Pramanik, Tatarek, Torsvik, Wiktor, Wold, Wulff, Steen and Assmy2019; Hopwood et al., Reference Hopwood, Carroll, Dunse, Hodson, Holding, Iriarte, Ribeiro, Achterberg, Cantoni, Carlson, Chierici, Clarke, Cozzi, Fransson, Juul-Pedersen, Winding and Meire2020). PO₄ may also be introduced by meltwater where it is quickly scavenged (Hopwood et al., Reference Hopwood, Carroll, Dunse, Hodson, Holding, Iriarte, Ribeiro, Achterberg, Cantoni, Carlson, Chierici, Clarke, Cozzi, Fransson, Juul-Pedersen, Winding and Meire2020). Land-terminating glaciers may provide even higher levels of nutrients and organic matter in systems with high levels of snowmelt and/or soil/permafrost leaching, which has large implications for local fjord ecology and adjacent coastal communities (Harris et al., Reference Harris, Macmillan-Lawler, Kullerud and Rice2018; Kotwicki et al., Reference Kotwicki, Grzelak, Opaliński and Węsławski2018; McGovern et al., Reference McGovern, Pavlov, Deininger, Granskog, Leu, Søreide and Poste2020; Delpech et al., Reference Delpech, Vonnahme, McGovern, Gradinger, Præbel and Poste2021).

River runoff is another primary input, with nutrient and organic loads that tend to be similar to neighbouring glaciers. A consideration for riverine inputs that differ from glacial is the increased nutrient load attributed to wastewater from human activities (Tuholske et al., Reference Tuholske, Halpern, Blasco, Villasenor, Frazier and Caylor2021). In Isfjorden, for example, where one may find the largest human settlement on Svalbard, nutrient concentrations in river runoff (i.e., NO₂ + NO₃) can be 12-fold higher than in the uninhabited regions of the fjord (McGovern et al., Reference McGovern, Pavlov, Deininger, Granskog, Leu, Søreide and Poste2020). Very rapid and sudden precipitation events may also lead to high-nutrient freshwater plumes in fjords, but whose effects on local ecosystems tend to remain very localised (McGovern et al., Reference McGovern, Pavlov, Deininger, Granskog, Leu, Søreide and Poste2020).

Summary

If the concentration of CO₂ in the atmosphere keeps increasing as it has done in past decades (IPCC, Reference Masson-Delmotte, Zhai, Pirani, Connors, Péan, Berger, Caud, Chen, Goldfarb, Gomis, Huang, Leitzell, Lonnoy, Matthews, Maycock, Waterfield, Yelekçi, Yu and Zhou2021), the impacts of the seawater CO₂ system on shell-forming organisms will almost certainly become more severe. The weakening or possible local extinction of these organisms may lead to an entire trophic restructuring of ecosystems both within and adjacent to fjords due to the trophic importance of these organisms to small pelagic fish and birds (see section “Biomass”; Bednaršek et al., Reference Bednaršek, Naish, Feely, Hauri, Kimoto, Hermann, Michel, Niemi and Pilcher2021).

Nutrient loading of Arctic fjord waters is likely to increase in the future due to higher rates of river runoff, glacial melt (see section “Terrestrial runoff”; Santos-Garcia et al., Reference Santos-Garcia, Ganeshram, Tuerena, Debyser, Husum, Assmy and Hop2022), and precipitation (Frigstad et al., Reference Frigstad, Kaste, Deininger, Kvalsund, Christensen, Bellerby, Sørensen, Norli and King2020), in combination with increased human activities. Therefore, the biogeochemical properties of fjords are projected to change apace with the climate (McGovern et al., Reference McGovern, Pavlov, Deininger, Granskog, Leu, Søreide and Poste2020). As more glaciers transition from marine- to land-terminating, their fjords will have fewer methods through which deep water mixing resupplies nutrients to the surface. The loss of icebergs caused by the change in a glacier’s status may reduce the transport of nutrients further out towards its mouth, resulting in a tighter concentration at the points of entry for freshwater runoff. These reductions to nutrient input may be offset by increased rates of terrestrial runoff, another point of research whose future outcome remains uncertain.

Lastly, and perhaps most dramatically, future warming may result in a winter melt, thereby preventing the normal seasonal recirculation of nutrients from deep waters and creating a situation where the nutrients in sinking biological matter are no longer resupplied to fjord ecosystems in the euphotic zone. Taken all together, the dramatic warming in the Arctic will likely lead to many fjords losing three of their four primary processes of deep water recirculation. The remaining process, surface currents exiting fjords, may become stronger due to increased river runoff.

Primary production

Primary productivity in Arctic fjord ecosystems is a foundational measure of the trophic energy available in an ecosystem and has extreme interannual variability due to the multitude of non-linear interactions between physicochemical processes in nearshore systems (Hopwood et al., Reference Hopwood, Carroll, Dunse, Hodson, Holding, Iriarte, Ribeiro, Achterberg, Cantoni, Carlson, Chierici, Clarke, Cozzi, Fransson, Juul-Pedersen, Winding and Meire2020). Increasingly frequent warm water intrusions and glacial melt are affecting the inter-annual duration and stability of the pycnocline (i.e., surface salinity; section “Salinity”) and biological pump (i.e., deep water upwelling; section “Nutrients”), thereby modifying phytoplankton bloom periods and their species composition (see section “Species richness”; Piwosz et al., Reference Piwosz, Walkusz, Hapter, Wieczorek, Hop and Wiktor2009; Wiencke and Hop, Reference Wiencke and Hop2016).

Arctic fjord primary production is heavily seasonal, with the highest levels typically reached during phytoplankton bloom events in spring and occasionally autumn. The spring bloom occurs when the nutrients supplied by the deep mixing in winter (see section “Nutrients”) are joined by the sufficient light availability of the spring (see the section “Light (PAR and UV)”). A second bloom may develop in late summer when upwelling driven by marine-terminating glacial melt (see section “Nutrients”) supplies enough additional nutrients to the euphotic zone (Juul-Pedersen et al., Reference Juul-Pedersen, Arendt, Mortensen, Blicher, Søgaard and Rysgaard2015). The separate autumn bloom is driven by the seasonal weakening of water column stratification that leads to an increased deep water mixing while light is still sufficient for photosynthesis (e.g., Eilertsen et al., Reference Eilertsen, Taasen and WesIawski1989).

Even though primary productivity is undoubtedly an important ecological factor in the shallow margins of Arctic fjord systems, with only a few exceptions, it has not been comprehensively quantified. In Kongsfjorden (W Spitsbergen), the loss of sea ice (see section “Sea ice”) has led to changes in spring bloom dynamics, with higher light levels in the water column (see section “Light (PAR and UV)”) earlier in the year driving earlier spring blooms with higher biomass and diversity (see section “Species richness”; Hegseth and Tverberg, Reference Hegseth and Tverberg2013). Pelagic primary productivity in this fjord has been estimated across multiple studies conducted over a 20-year period (1979–1999) and ranges from 4 to 180 mg C m⁻² yr⁻¹ with no clear predictive trend or continuity (Hop et al., Reference Hop, Pearson, Hegseth, Kovacs, Wiencke, Kwasniewski, Eiane, Mehlum, Gulliksen, Wlodarska-Kowalczuk, Lydersen, Weslawski, Cochrane, Gabrielsen, Leakey, Lønne, Zajaczkowski, Falk-Petersen, Kendall, Wängberg, Bischof, Voronkov, Kovaltchouk, Wiktor, Poltermann, di Prisco, Papucci and Gerland2002 and references therein; Duarte et al., Reference Duarte, Weslawski, Hop, Hop and Wiencke2019). Primary production in Nuup Kangerlua (W Greenland) follows a recurring seasonal pattern with the highest production and biomass during the spring bloom or late summer (Juul-Pedersen et al., Reference Juul-Pedersen, Arendt, Mortensen, Blicher, Søgaard and Rysgaard2015; Krawczyk et al., Reference Krawczyk, Meire, Lopes, Juul-Pedersen, Mortensen, Li and Krogh2018). Primary production in Godthåbsfjorden (Nuup Kangerlua, W Greenland) has smaller interannual variability with ranges between 84.6 and 139.1 g C m⁻² yr⁻¹ (Juul-Pedersen et al., Reference Juul-Pedersen, Arendt, Mortensen, Blicher, Søgaard and Rysgaard2015).

Biomass

Phytoplankton biomass is directly related to primary production (see section “Primary production”); however, loss of this biomass can be related to grazing, viral or fungal lysis (e.g., Hassett et al., Reference Hassett, Borrego, Vonnahme, Rämä, Kolomiets and Gradinger2019), or sedimentation. Thus, high primary production does not necessarily lead to high phytoplankton biomass. Due largely to Atlantification (see section “Salinity”), a significant northward advance of temperate phytoplankton and changes of the planktonic organism size distribution towards smaller organisms (i.e., pico- and nanoplankton) have been observed (Oziel et al., Reference Oziel, Neukermans, Ardyna, Lancelot, Tison, Wassmann, Sirven, Ruiz-Pino and Gascard2017; Neukermans et al., Reference Neukermans, Oziel and Babin2018; Konik et al., Reference Konik, Darecki, Pavlov, Sagan and Kowalczuk2021). This means that climate change may be mediating trophic shifts in fjord ecosystems, a process referred to as “borealisation”.

The biomass of zooplankton communities in Arctic fjords relies heavily on the seasonal availability of highly productive phytoplankton (Vereide, Reference Vereide2019), making zooplankton one of the main pathways connecting pelagic primary production (see section “Primary production”) to larger predators. Zooplankton biomass is also affected by local scale perturbations in temperature (see section “Seawater temperature”), salinity (see section “Salinity”), and light availability (see section “Light (PAR and UV)”); all of which are in flux due to the changing climate. The borealisation of West Svalbard fjords, due to Atlantification, is already affecting seabirds via its impacts on zooplankton (Descamps et al., Reference Descamps, Wojczulanis-Jakubas, Jakubas, Vihtakari, Steen, Karnovsky, Welcker, Hovinen, Bertrand, Strzelewicz, Skogseth, Kidawa, Boehnke and Błachowiak-Samołyk2022).

The shifting of the large ocean currents in the Arctic will have widespread effects on pelagic macrozooplankton (i.e., copepods, euphausiids, and amphipods). It was found that the warming occurring in the Kongsfjorden ecosystem (W Svalbard), largely due to increased AW inflow (see section “Seawater temperature”), is having a positive effect on the abundance of euphausiids and amphipods (Dalpadado et al., Reference Dalpadado, Hop, Rønning, Pavlov, Sperfeld, Buchholz, Rey and Wold2016), which are key prey for target fishery species (see section “Fisheries”) such as capelin and polar cod (Dalpadado et al., Reference Dalpadado, Hop, Rønning, Pavlov, Sperfeld, Buchholz, Rey and Wold2016). As the borealisation of the fjords along Western Svalbard continues, it may alter the population dynamics of key prey macrozooplankton species so dramatically that the changes may be tracked by monitoring the diets of local black-legged kittiwakes (Vihtakari et al., Reference Vihtakari, Welcker, Moe, Chastel, Tartu, Hop, Bech, Descamps and Gabrielsen2018).

Although macrophytobenthos (seaweeds and seagrass) are mostly restricted to a narrow spatial stretch along fjords, being dependent on either rocky substrate (seaweeds) or light-flooded sandy sediments (seagrass), their local biomass can be considerable. The vertical structures they create as ecosystem engineers also translate into a strong bottom-up effect in Arctic fjords. Increases in the biomass of these communities over time have been observed (Kędra et al., Reference Kędra, Włodarska-Kowalczuk and Węsławski2010; Bartsch et al., Reference Bartsch, Paar, Fredriksen, Schwanitz, Daniel, Hop and Wiencke2016), and even though the in situ sampling in the study was spatially limited, the findings were striking enough to conclude that a regime shift of the rocky-bottom community occurred via a sharp increase in macroalgae cover in 1995 (Kortsch et al., Reference Kortsch, Primicerio, Beuchel, Renaud, Rodrigues, Lønne and Gulliksen2012). Indeed, a pan-Arctic study of 38 sites showed a general increase in abundance, productivity, and/or biodiversity, with a poleward migration rate of 18–23 km per decade (Krause-Jensen et al., Reference Krause-Jensen, Archambault, Assis, Bartsch, Bischof, Filbee-Dexter, Dunton, Maximova, Ragnarsdóttir, Sejr, Simakova, Spiridonov, Wegeberg, Winding and Duarte2020). An in situ study in Kongsfjorden (W Svalbard), which compared macroalgae biomass records from 2012 to 2014 against those from 1996 to 1998, found that biomass at the 2.5 m depth had increased by 8.2-fold, and that the community had shifted to shallower waters (Bartsch et al., Reference Bartsch, Paar, Fredriksen, Schwanitz, Daniel, Hop and Wiencke2016). The two forces driving shallower shifts in macrophytobenthos biomass are

1. 1) decreases in sea ice cover (see section “Sea ice”) mean less ice scour and more PAR penetration at the shallow depths macroalgae like to inhabit (Fredriksen et al., Reference Fredriksen, Karsten, Bartsch, Woelfel, Koblowsky, Schumann, Moy, Steneck, Wiktor, Hop, Wiencke, Hop and Wiencke2019 and citations therein) and

2. 2) increased turbidity, which inhibits PAR penetration (see section “Light (PAR and UV)”) to the historically deeper range where macroalgae have been found (Bartsch et al., Reference Bartsch, Paar, Fredriksen, Schwanitz, Daniel, Hop and Wiencke2016).

Demersal fish have a strong top-down effect on fjord ecosystems via predation, and their distribution in fjords is strongly driven by along fjord salinity and temperature gradients (Mérillet et al., Reference Mérillet, Skogen, Vikebø and Jørgensen2022). Fjords with a sill (i.e., physical barrier) that guard the inner waters from external oceanic forces create very cold habitats that harbour specific communities (Kędra et al., Reference Kędra, Włodarska-Kowalczuk and Węsławski2010; Węsławski et al., Reference Węsławski, Kendall, Włodarska-Kowalczuk, Iken, Kędra, Legezynska and Sejr2011). These are hypothesised to offer a refuge for Arctic endemic species against increasing seawater temperatures (see section “Seawater temperature”; Węsławski et al., Reference Węsławski, Kendall, Włodarska-Kowalczuk, Iken, Kędra, Legezynska and Sejr2011; Drewnik et al., Reference Drewnik, Węsławski and Włodarska-Kowalczuk2017). This may be particularly important as continuing poleward expansion of boreal communities and corresponding decreases in dominance of Arctic communities is being observed (see section “Species richness”; Jørgensen et al., Reference Jørgensen, Primicerio, Ingvaldsen, Fossheim, Strelkova, Thangstad, Manushin and Zakharov2019), which will likely have widespread impacts on the established fisheries in the Arctic (see “Summary” of “Social drivers” section).

Species richness

In addition to the primary production of an ecosystem and the biomass therein, the richness of species, their diversity, and evenness are critically important for stable functioning (Dı́az and Cabido, Reference Dı́az and Cabido2001; Gamfeldt et al., Reference Gamfeldt, Lefcheck, Byrnes, Cardinale, Duffy and Griffin2015; Isbell et al., Reference Isbell, Cowles, Dee, Loreau, Reich, Gonzalez, Hector and Schmid2018). Meaning that the more diverse the assemblage of species in an ecosystem is, the more likely that system will be able to withstand a range of external stressors, based on the insurance hypothesis that some species will have redundant characteristics (i.e., biological traits) and that the ones that will survive will be able to maintain the ecosystem functions performed (Yachi and Loreau, Reference Yachi and Loreau1999; Lamy et al., Reference Lamy, Wang, Renard, Lafferty, Reed and Miller2019). A consideration of paramount importance given the massive and rapid impacts that climate change and other human activities are having on Arctic fjords.

The first impacts of climate change on flora or fauna within the European Arctic (i.e., Greenland, Svalbard, and Northern Norway) were noted by Blacker (Reference Blacker1957), followed by a long pause until research on the species richness of rocky shore communities within European Arctic and sub-Arctic fjords showed that they had increased (Hansen and Ingólfsson, Reference Hansen and Ingólfsson1993; Włodarska-Kowalczuk et al., Reference Włodarska-Kowalczuk, Renaud, Węsławski, Cochrane and Denisenko2012; Fredriksen et al., Reference Fredriksen, Karsten, Bartsch, Woelfel, Koblowsky, Schumann, Moy, Steneck, Wiktor, Hop, Wiencke, Hop and Wiencke2019 and references therein). Due primarily to warming seawater (see section “Seawater temperature”), an increase in species richness of rocky littoral microorganisms has also been recorded on Svalbard (Węsławski et al., Reference Węsławski, Wiktor and Kotwicki2010). Similarly, fish species richness has significantly increased in Porsangerfjorden (N Norway) over 2007–2019, facilitated by reductions in sea ice cover (see section “Sea ice”) and the freshening of water (see section “Salinity”; Mérillet et al., Reference Mérillet, Skogen, Vikebø and Jørgensen2022).

While seawater temperatures in Arctic fjords remain below the present mean of 3°C, rising temperatures are projected to decrease species richness; however, upon passing that 3°C threshold, species richness is projected to begin to increase (Benedetti et al., Reference Benedetti, Vogt, Elizondo, Righetti, Zimmermann and Gruber2021). Plankton species richness in particular is expected to see an overall increase with global warming as species shift poleward (see section “Biomass”; Benedetti et al., Reference Benedetti, Vogt, Elizondo, Righetti, Zimmermann and Gruber2021). However, most decreases are expected in East and Southwest Greenland and West Svalbard (Benedetti et al., Reference Benedetti, Vogt, Elizondo, Righetti, Zimmermann and Gruber2021). The temperature of seawater (see section “Seawater temperature”) is described as the primary cause of the overall increase to species richness in the Arctic, with nutrients (see section “Nutrients”) playing an additional role in some areas (Benedetti et al., Reference Benedetti, Vogt, Elizondo, Righetti, Zimmermann and Gruber2021). Due to the almost certain continued increases to both of these drivers, it is likely that while species richness in fjords may decrease in the short term, on a multi-decadal scale it is likely that borealisation of fjord species (see section “Biomass”) will lead to an overall increase in species richness (with the possible exception of plankton). Unfortunately, this will not necessarily equate to a more resilient ecosystem because the incoming boreal species may lack the same diversity of functional traits found in Arctic species (Kędra et al., Reference Kędra, Moritz, Choy, David, Degen, Duerksen, Ellingsen, Górska, Grebmeier, Kirievskaya, van Oevelen, Piwosz, Samuelsen and Węsławski2015; McGovern et al., Reference McGovern, Pavlov, Deininger, Granskog, Leu, Søreide and Poste2020).

Climate change and increased anthropogenic activities are expected to contribute to the potential increases to species richness largely by elevating the potential for the introduction of non-indigenous species (NIS; Chan et al., Reference Chan, Stanislawczyk, Sneekes, Dvoretsky, Gollasch, Minchin, David, Jelmert, Albretsen and Bailey2019), which when established in novel ecosystems are often able to outcompete local species (Wood et al., Reference Wood, Spicer, Kendall, Lowe and Widdicombe2011). There is a particular risk of this along the coasts of Northern Norway and West Svalbard, where warming water masses and high potential for advection via the North Atlantic Current and WSC are good preconditions for the introduction of NIS (Węsławski et al., Reference Węsławski, Kendall, Włodarska-Kowalczuk, Iken, Kędra, Legezynska and Sejr2011; Tarling et al., Reference Tarling, Freer, Banas, Belcher, Blackwell, Castellani, Cook, Cottier, Daase, Johnson, Last, Lindeque, Mayor, Mitchell, Parry, Speirs, Stowasser and Wootton2022). In the Greenland Sea/East Greenland area, three known NIS have already been introduced (among them the Pacific diatom Neodenticula seminae) and five in the Barents Sea/Svalbard area. Among those, the following have become established: the Japanese skeleton shrimp Caprella mutica, the copepod Eurytemora americana, the Chinese mitten crab Eriocheir sinensis, and the red king crab Paralithodes camtschaticus (Chan et al., Reference Chan, Stanislawczyk, Sneekes, Dvoretsky, Gollasch, Minchin, David, Jelmert, Albretsen and Bailey2019). Of these, king crabs were intentionally introduced to the east of Porsangerfjorden (N Norway) in the 1960s to establish a commercial fishery (see “Summary” of “Social drivers” section), and are now spreading west over the north of Norway, causing widespread trophic perturbations (Dvoretsky and Dvoretsky, Reference Dvoretsky and Dvoretsky2015).

Summary

Decreases of sea ice cover (see section “Sea ice”) in the open Arctic Ocean have been associated with increased primary productivity (Ardyna and Arrigo, Reference Ardyna and Arrigo2020), but this relationship has not yet been conclusively measured in fjords. It is, however, hypothesised that this will eventually become a measurable relationship because further warming of seawater (see section “Seawater temperature”) within fjords will almost certainly result in prolonged sea iceice-free periods and larger volumes of meltwater (see section “Terrestrial runoff”), which will provide more nutrients that fuel primary productivity (Piquet et al., Reference Piquet, van de Poll, Visser, Wiencke, Bolhuis and Buma2014).

It is generally agreed that most Arctic fjords ecosystems will experience radical community changes, with many going through stable state shifts from Arctic to boreal (Kortsch et al., Reference Kortsch, Primicerio, Beuchel, Renaud, Rodrigues, Lønne and Gulliksen2012; Fossheim et al., Reference Fossheim, Primicerio, Johannesen, Ingvaldsen, Aschan and Dolgov2015; Pecuchet et al., Reference Pecuchet, Blanchet, Frainer, Husson, Jørgensen, Kortsch and Primicerio2020), though how these changes will look remains unclear. For example, it is known that demersal fish communities have an inherent adaptive capacity to survive long periods of seasonally low food availability (Sun et al., Reference Sun, Clough, Carroll, Dai, Ambrose and Lopez2009), which in combination with their opportunistic feeding strategy (Iken et al., Reference Iken, Bluhm and Dunton2010; Węsławski et al., Reference Węsławski, Kendall, Włodarska-Kowalczuk, Iken, Kędra, Legezynska and Sejr2011) might translate to some degree of stability in the face of the climate-driven changes to fjord ecosystems. Modelling efforts to predict the impact of potential warming and acidification scenarios by 2100 on demersal fish showed that habitat loss would be small (0–11%), with no appreciable difference between losses for Arctic and Arctic-boreal species (Renaud et al., Reference Renaud, Wallhead, Kotta, Włodarska-Kowalczuk, Bellerby, Rätsep, Slagstad and Kukliński2019). The extent of marine forests (macrophytobenthos) within the Arctic basin is also predicted to remain stable (Bringloe et al., Reference Bringloe, Wilkinson, Goldsmit, Savoie, Filbee‐Dexter, Macgregor, Howland, McKindsey and Verbruggen2022), if not increase due to the changing climate (Krause-Jensen et al., Reference Krause-Jensen, Archambault, Assis, Bartsch, Bischof, Filbee-Dexter, Dunton, Maximova, Ragnarsdóttir, Sejr, Simakova, Spiridonov, Wegeberg, Winding and Duarte2020). The depth structure of these forests, however, is likely to shift to shallower waters (Bartsch et al., Reference Bartsch, Paar, Fredriksen, Schwanitz, Daniel, Hop and Wiencke2016).

Until recently, the climatic conditions around Svalbard acted as a barrier to the spread of NIS, but the Atlantification (see section “Salinity”) of the marine environment has partly removed this (Øian and Kaltenborn, Reference Øian and Kaltenborn2020). The encroachment of NIS, due to borealisation, is currently squeezing Arctic species further northward (Fossheim et al., Reference Fossheim, Primicerio, Johannesen, Ingvaldsen, Aschan and Dolgov2015; Kortsch et al., Reference Kortsch, Primicerio, Fossheim, Dolgov and Aschan2015). A process that will almost certainly continue into the future (Filbee-Dexter et al., Reference Filbee-Dexter, Wernberg, Fredriksen, Norderhaug and Pedersen2019). It has been noted, however, that assemblages in Svalbard will likely remain different from those in Northern Norway due to the greater direct human influence on the continent (Kujawa et al., Reference Kujawa, Łącka, Szymańska, Pawłowska, Telesiński and Zajączkowski2021).

Governance

There are many ways that changes to the drivers detailed above may affect Arctic livelihoods, culture, identity, economy, health, and security, especially for Indigenous Peoples (IPCC, Reference Masson-Delmotte, Zhai, Pirani, Connors, Péan, Berger, Caud, Chen, Goldfarb, Gomis, Huang, Leitzell, Lonnoy, Matthews, Maycock, Waterfield, Yelekçi, Yu and Zhou2021); however, these are not the only drivers of change in the Arctic. Through its top-down control of human societies, governance may have broader impacts on Arctic fjord socio-ecological systems than nearly all other aspects of climate change by controlling the rapid and dramatic direct local impacts that human actions may have on the natural world (Tyler et al., Reference Tyler, Turi, Sundset, Strøm Bull, Sara, Reinert, Oskal, Nellemann, McCarthy, Mathiesen, Martello, Magga, Hovelsrud, Hanssen-Bauer, Eira, Eira and Corell2007; Hovelsrud and Smit, Reference Hovelsrud and Smit2010).

Self-determination in managing climate change impacts has inspired Greenlandic politicians to contemplate joining the Paris Agreement and to look for investors to expand the hydro-power resource enabling the storage and export of green energy within a decade (Bjørst, Reference Bjørst and Heininen2022). In parallel, the national strategy for oil and gas exploration has been abandoned. As another way to grow and diversify its economy, Greenland is in the process of building two international airports to improve transport and connectivity, specifically around tourism (see section “Tourism”). These two examples showcase how the Government of Greenland is managing the right to resources, subsurface and hydropower installation, and how regional governments are becoming key players for domestic development that are increasingly empowered to act on negative trends affecting the regional population, but in ways that may have negative ecological consequences.

In 2018, the Norwegian government decided to close most of its coal mines on Svalbard (the primary original reason for human settlements there) and identified tourism (see section “Tourism”) as a new cornerstone industry (NMJ, 2016). Concurrent with this recent shift is the goal for Svalbard to ensure the best wilderness management in the world (MoCE, 2020). Strict regulations have been followed, and currently underway is a major overhaul and tightening of the environmental protections and tourism management for the archipelago (Granberg et al., Reference Granberg, Ask and Gabrielsen2017; NEA, 2022).

Tourism

In recent decades (until the onset of COVID-19 countermeasures in early 2020), there has been an increasing global interest in the Arctic as a tourist destination, particularly fjords. Promoting this increase in tourism has been an intentional governance choice (see section “Governance”), with the stated goal being the development and diversification of the economies of the sparsely populated peripheral regions of Nordic countries (Ren et al., Reference Ren, James, Pashkevich and Hoarau-Heemstra2021a). This has, however, led to growing human impacts on small and remote destinations where signs of human activities had yet been scarce, and where these anthropogenic disturbances may have wide-ranging consequences.

Ironically, the changing climate is currently serving as a net benefit to Arctic tourism, with tourist arrivals via cruise ship in Longyearbyen (W Svalbard) doubling from 2010 to 2018 (Port of Longyearbyen, 2018; Epinion, 2019). This has led to calls for opportunity-based adaptations to the cruise tourism influx (Dawson et al., Reference Dawson, Stewart, Johnston and Lemieux2016) because the warming Arctic and its melting sea ice (see section “Sea ice”) will ensure that coastal destinations remain the most accessible. This is an important consideration because in addition to the impacts of the humans themselves, the ships they use for transport to and from the Arctic may drive changes in a number of different ways. Some of these may be more apparent, like the introduction of nutrients (i.e., via human waste; section “Nutrients”) and pollutants (Øian and Kaltenborn, Reference Øian and Kaltenborn2020), but some less so, like the introduction of NIS (see section “Species richness”; Hellmann et al., Reference Hellmann, Byers, Bierwagen and Dukes2008; Goldsmit et al., Reference Goldsmit, Archambault, Chust, Villarino, Liu, Lukovich, Barber and Howland2018). These are transported on the hulls of ships, via the emptying of ballast water (Chan et al., Reference Chan, Bailey, Wiley and MacIsaac2013), or by the tourists themselves. Weaver and Lawton (Reference Weaver and Lawton2017) argue that the potential economic benefits of cruise tourism in small coastal communities may be outweighed by their social and environmental stressors, and Ren et al. (Reference Ren, Jóhannesson, Kramvig, Pashkevich and Höckert2021b) stress the need for more locally based management of Arctic cruise tourism.

While human activities in the permanent settlements of Svalbard do have an environmental footprint, this is easily rivalled by that of tourism, where residents are outnumbered by tourists during the high season (Hovelsrud et al., Reference Hovelsrud, Veland, Kaltenborn, Olsen and Dannevig2021). Tourists arriving in Isfjorden (W Svalbard) tend to spend less than 3 days on the archipelago (Hovelsrud et al., Reference Hovelsrud, Veland, Kaltenborn, Olsen and Dannevig2021), but the increase in tourist arrivals has meant a doubling of total tourist nights per year (Visit Svalbard, 2020). Management decisions (see section “Governance”) to deal with this issue are ongoing (Hovelsrud et al., Reference Hovelsrud, Kaltenborn and Olsen2020). Of the cruise ships arriving on the archipelago, the average number of overseas arrivals per year has decreased (Stocker et al., Reference Stocker, Renner and Knol-Kauffman2020), most likely due to a ban on heavy oil fuel in most of the coastal waters of Svalbard. This is endemic to a shift towards smaller expedition cruises and pleasure craft vessels, which have increased by 42% from 2008 to 2018 (NEA, 2022). These smaller vessels benefit more from the retreating sea ice edge (Palma et al., Reference Palma, Varnajot, Dalen, Basaran, Brunette, Bystrowska, Korablina, Nowicki and Ronge2019; Hovelsrud et al., Reference Hovelsrud, Kaltenborn and Olsen2020) due to their ability to sail closer to the ice-edge and glaciers, a demand for which has become a recent market trend (Hovelsrud et al., Reference Hovelsrud, Veland, Kaltenborn, Olsen and Dannevig2021).

Accounting for about a third of all foreign visitors, cruises have for many years been a central part of tourism in Greenland. The country has previously set annual growth targets for cruises as a whole. However, Visit Greenland announced in late 2022 that it will abstain from marketing to conventional cruises after a summer with cruise tourism numbers matching the record year of 2019 (Visit Greenland, 2022). Whether this may actually enable a move from conventional cruises to cleaner and socially less impactful expedition cruise tourism remains to be seen but will have crucial implications for the fjord systems of Greenland as a return to mass tourism will mean greater anthropogenic impact in the future.

Fisheries

Besides adding nutrients (see section “Nutrients”) and pollutants, humans also engage in extractive behaviours that can upset natural trophic balance. These disturbances are generally monitored via target species and regulated by the management of fisheries (see section “Governance”). However, fishing also affects non-targeted species as well as the structure of the habitats, such as the use of bottom trawls (Gray et al., Reference Gray, Dayton, Thrush and Kaiser2006; Kaiser et al., Reference Kaiser, Clarke, Hinz, Austen, Somerfield and Karakassis2006). While the impacts of tourists are generally inferred via head counts at ports of call, the proxy for tracking the impacts of fishing vessels in the Arctic is by monitoring ship mileage. This value has been increasing in the waters around Svalbard as the ice edge steadily retreats (see section “Sea ice”; Stocker et al., Reference Stocker, Renner and Knol-Kauffman2020), and the duration of the operational season extends (i.e., longer sea iceice-free period per year). Unsurprisingly then the overall number of ships in the Arctic increased by 25% from just 2013 to 2019 (Stocker et al., Reference Stocker, Renner and Knol-Kauffman2020).

In Porsangerfjorden (N Norway), the shrimp fishery, which used the ecologically damaging method of bottom trawling, was closed in the early 1970s after intensive fishing caused the overexploitation of cod as well as small and young fishes (Søvik et al., Reference Søvik, Nedreaas, Zimmermann, Husson, Strand, Jørgensen, Strand, Thangstad, Hansen, Båtevik, Albretsen and Staby2020). This fishery was, however, opened again in 2021 for trial with only a few boats allowed to fish in the outer part of the fjord (G. Søvik, pers. comm.), a demonstration of the direct impact that governance (see section “Governance”) can have on a local ecosystem. Fishing for cod (Gadus morhua), saithe (Pollachius virens), haddock (Melanogrammus aeglefinus), and red king crab (P. camtschaticus) had always been allowed in the fjord with other less damaging gear. Red king crab in particular has become an important commercial fishery with 921 t landed in 2018 (Søvik et al., Reference Søvik, Nedreaas, Zimmermann, Husson, Strand, Jørgensen, Strand, Thangstad, Hansen, Båtevik, Albretsen and Staby2020). Originally a NIS (see section “Species richness”), the adaptation of a fishery for red king crab (Sundet and Hoel, Reference Sundet and Hoel2016), has potentially aided the recovery of kelp forests in Northern Norway by reducing sea urchin grazing pressure (Christie et al., Reference Christie, Gundersen, Rinde, Filbee-Dexter, Norderhaug, Pedersen, Bekkby, Gitmark and Fagerli2019), and is a good example of how governance can help to adapt to the inevitable changes that Arctic fjord ecosystems will experience.

The largest city of Greenland lies at the mouth of Nuup Kangerlua, where hunting for seals and seabirds, as well as fishing for cod, halibut, and redfish is common. Humpback whales have been protected inside the fjord since 2021, while other species remain open for hunting. As of today, fishing is the main economic sector for the country (Grønlands Økonomiske Råd, 2021). And while fisheries are affected by the changing climate, government regulations (see section “Governance”) and changes to the international prices on fish and shrimp likely have a greater impact. In 2021, for example, (because of COVID-19) the prices for cod dropped suddenly compared to previous years, leading to widely felt economic hardships (Andersen, Reference Andersen2022). To limit this reoccurrence, development in the formal economy is seen as important by decision-makers and business owners. However, fishing, hunting, and gathering activities remain a key part of the region’s mixed economy and hold great cultural and social value. This means that it is particularly difficult for the government of Greenland to tightly regulate the extractive behaviour of its citizens, and thereby the ecological impacts they may have. It is in part to address issues like this that many governments of Arctic nations have been leaning away from extractive economic strategies in favour of tourism (see section “Tourism”).

The northernmost fisheries on the planet, found in the fjords and waters around Svalbard, have been strictly regulated since 1977 when Norway claimed the right to regulate fishing 200 nautical miles around Svalbard under the Norwegian Economic Zone Act (reduced slightly in 2010 when a final dividing line agreement with Russia was made). Since 1980, the Directorate of Fisheries has collected detailed information on landings from Norwegian fishers in the Svalbard zone (Misund et al., Reference Misund, Heggland, Skogseth, Falck, Gjøsæter, Sundet, Watne and Lønne2016), and the main fisheries are Atlantic cod G. morhua with close to 75 million tonnes fished in 2021 with an estimated value of 1.2 billions NOK (or 114.2 million €), followed by shrimp (Pandalus borealis; 27.3 million tonnes; 550 million NOK or 52.3 million €), haddock (M. aeglefinus; 22.1 million tonnes; 331 million NOK or 31.5 million €) and snow crab (Chionoecetes opilio; 6.3 million tonnes; 586 million NOK or 55.7 million €; Fiskeridirektoratet, 2022). Within the coastal zone/fjords of Svalbard, the core areas for fishing (mostly for shrimp) are Isfjorden, Krossfjorden, and Hinlopen. At present, it is not possible to deliver landings directly to local communities on Svalbard, with most going to mainland Norway. While there is interest to develop the necessary local infrastructure, it has been inhibited by strict environmental regulations (see section “Governance”). A few local hunters provide seal meat and Atlantic cod to restaurants in Longyearbyen, and it is popular for the locals to fish cod and hunt seals for their own use.

Summary

It is very difficult to predict what the future social structure of Arctic communities will look like. One can, however, seek to understand how and why these societies have changed in the past and present (AMAP, 2017). Future policies that may be developed in order to adapt to the changing personal decisions of the inhabitants of the Arctic will in turn have top-down impacts on many of the drivers detailed in previous sections. One must also remember that the results of climate change research do not automatically translate into adaptive human behaviour (Hovelsrud et al., Reference Hovelsrud, West and Dannevig2015). Indeed, the many international climate meetings (e.g., Conference of the Parties [COPs]) and IPCC projections on the changing climate have had seemingly little impact when introduced into national politics and everyday lives. The need for economic growth and the development of new infrastructure in Arctic communities may very well lead to an increase in CO₂ emissions, rather than a reduction, meaning that social drivers may negatively impact the Arctic climate system even more in years to come. For example, the increase in local pollution in the form of CO₂, sulphur, black carbon emissions, and nutrient runoff are directly affected by how northern communities decide to manage the tourism industry (see section “Tourism”). The failure (or success) of local ecosystems and key taxa are also directly influenced by choices in how to manage northern fisheries (see section “Fisheries”) and the potential expansion of aquaculture endeavours (Heath et al., Reference Heath, Benkort, Brierley, Daewel, Laverick, Proud and Speirs2022), such as the farming of kelp forests.

Social drivers of change are generally perceived first and foremost to have local impacts, but they too are capable of having widespread feedback on the other categories of drivers. For example, while various aspects of climate change will likely have the largest impact on ecosystems and species in the future (Thierry et al., Reference Thierry, Bullock and Gardner2022), the greatest impacts historically have come from human overexploitation of species and destruction of their habitats (Caro et al., Reference Caro, Rowe, Berger, Wholey and Dobson2022). The melting Arctic will allow for even greater exploitation of the resources therein, which will have entirely new impacts that until present had not been possible.