Ph.D Anaïs Lebrun
email: anais.lebrun@imev-mer.fr

Candidate

Steeve Candidate

Ph.D Comeau

Ph.D Frédéric Gazeau

Pierre Jean-

Ph.D Gattuso

Dorte Krause-Jensen

Paul Renaud

Senio Researcher

Professor, Jan Marcin Weslawski

Maria Wlodarska-Kowalczuk

Steeve Comeau

Frédéric Gazeau

Jean-Pierre Gattuso

Global and Planetary Change Impact of climate change on Arctic macroalgal communities

Keywords: climate change, macroalgae, Arctic, fucoids, kelps, coralline algae climate change -macroalgae -Arctic -fucoids -kelps -coralline algae

The Arctic region faces a warming rate that is more than twice the global average. Seaice loss, increase in precipitation and freshwater discharge, changes in underwater light, and amplification of ocean acidification modify benthic habitats and the communities they host. Here we synthesize existing information on the impacts of climate change on the macroalgal communities of Arctic coasts. We review the shortand long-term changes in environmental characteristics of shallow hard-bottomed Arctic coasts, the floristics of Arctic macroalgae (description, distribution, life-cycle, adaptations), the responses of their biological and ecological processes to climate change, the resulting winning and losing species, and the effects on ecosystem functioning. The focus of this review is on fucoid species, kelps, and coralline algae which are key ecosystem engineers in hard-bottom shallow areas of the Arctic, providing food, substrate, shelter, and nursery ground for many species. Changes in seasonality, benthic functional diversity, food-web structure, and carbon cycle are already occurring and are reshaping Arctic benthic ecosystems. Shallow communities are projected to shift from invertebrate-to algal-dominated communities. Fucoid and several kelp species are expected to largely spread and dominate the area with possible extinctions of native species. A considerable amount of functional diversity could be lost impacting the processing of land-derived nutrients and organic matter and significantly altering trophic structure and energy flow up to the apex consumers. However, many factors are not well understood yet, making it difficult to appreciate the current situation and predict the future coastal Arctic ecosystem. Efforts must be made to improve knowledge in key regions with proper seasonal coverage, taking into account interactions between stressors and across species.

Introduction

The Arctic Ocean and its surrounding waters contain less than 1% of the global ocean volume but comprise 25% of the world's continental shelves [START_REF] Jakobsson | The Arctic Ocean: boundary conditions and background information[END_REF]. It includes the Barents and Bering Seas north of the Arctic circle, the Eurasian shelves of the Kara and Laptev seas, the Siberian coast of the Beaufort Sea, the Canadian Archipelago, Greenland, and the Chukchi Sea [START_REF] Piepenburg | Recent research on Arctic benthos: common notions need to be revised[END_REF]. This ocean receives 11% of the global runoff [START_REF] Shiklomanov | World water resources at the beginning of the 21st century[END_REF] and is responsible for 7-10% of the global burial of organic carbon [START_REF] Stein | The organic carbon cycle in the Arctic Ocean[END_REF].

The Arctic region is warming at a rate that is more than twice that of the global average [START_REF] Richter-Menge | Arctic shows no sign of returning to reliably frozen region of recent past decades[END_REF]. From 1982 to 2017, the sea surface temperature in ice-free regions increased by 0.5°C per decade, and it is expected to rise by 2 to 5°C over the next 80 years, under SSP1-2.6 and SSP5-8.5 respectively [START_REF] Kwiatkowski | Twenty-first century ocean warming, acidification, deoxygenation, and upper-ocean nutrient and primary production decline from CMIP6 model projections[END_REF]. With a current 10% decrease in sea ice extent per decade, the Arctic could face an ice-free summer as early as 2035 (Notz and SIMIP Community, 2020). The decrease in albedo arising from the melting of sea ice and snow leads to an exacerbation of global change in this region, referred to as polar amplification [START_REF] Chapman | Recent variations of sea ice and air temperature in high latitudes[END_REF]. In addition to sea-ice loss, climate change involves increases in precipitation and freshwater discharge, changes in underwater light, and amplification of ocean acidification (pH decrease of 0.18 to 0.45 on the Arctic basin scale by 2100, vs 0.06 to 0.32 units on the global scale; [START_REF] Steinacher | Imminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model[END_REF][START_REF] Terhaar | Arctic Ocean acidification over the 21st century co-driven by anthropogenic carbon increases and freshening in the CMIP6 model ensemble[END_REF]. All these processes modify marine habitats and the communities they host [START_REF] Duarte | Tipping Elements in the Arctic Marine Ecosystem[END_REF][START_REF] Kortsch | Climate-driven regime shifts in Arctic marine benthos[END_REF]. The structure and function of Arctic marine ecosystems are, thus, foreseen to change dramatically [START_REF] Renaud | Arctic Sensitivity? Suitable Habitat for Benthic Taxa Is Surprisingly Robust to Climate Change[END_REF]. Footprints of current global change are already visible in marine Arctic ecosystems [START_REF] Wassmann | Footprints of climate change in the Arctic marine ecosystem: footprints of climate change[END_REF]. The Arctic shelves are likely to be especially vulnerable due to their shallow depth and seasonally shifting ice cover [START_REF] Piepenburg | Recent research on Arctic benthos: common notions need to be revised[END_REF].

Macroalgal benthic communities are an essential component of hard-bottomed shallow water ecosystems in the Arctic. Fucoid species and kelps are large brown seaweeds that can dominate arctic rocky intertidal and subtidal areas and exhibit a high biomass [START_REF] Hop | Species composition, zonation, and biomass of marine benthic macroalgae in Kongsfjorden[END_REF]Iñiguez et al., 2016a;[START_REF] Ronowicz | Glacial and depth influence on sublittoral macroalgal standing stock in a high-Arctic fjord[END_REF]. Providing a food source, substrate, sheltered area, and nursery for many species [START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF], they are hotspots of Arctic marine biodiversity [START_REF] Włodarska-Kowalczuk | Assessing species richness of macrofauna associated with macroalgae in Arctic kelp forests (Hornsund, Svalbard)[END_REF][START_REF] Krause-Jensen | Seasonal sea ice cover as principal driver of spatial and temporal variation in depth extension and annual production of kelp in Greenland[END_REF]. Fucoid and kelps play an important role in the transfer of carbon [START_REF] Filbee-Dexter | Ocean temperature controls kelp decomposition and carbon sink potential[END_REF][START_REF] Lewis | Quantifying intertidal canopy-forming macroalgal production, extent, degradation, and blue carbon potential[END_REF] through high production, export of dissolved organic matter and detritus, as well as carbon sequestration (Krause-Jensen and Duarte 2016).

Coralline algae are other important ecosystem engineers in the Arctic [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF].

These long-lived species (up to 1200-1600 years for individuals found in the North Pacific and the Labrador sea; [START_REF] Adey | Subarctic rhodolith beds promote longevity of crustose coralline algal buildups and their climate archiving potential[END_REF] can form crusts or rhodolith beds that host important biodiversity [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF]. Despite relatively low annual growth rates, coralline algae have a high potential for calcium carbonate accumulation [START_REF] Freiwald | Reefal coralline algal build-ups within the Arctic Circle: morphology and sedimentary dynamics under extreme environmental seasonality[END_REF] providing a three dimensional habitat dominated by vagile invertebrates [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF]. Rhodolith beds are an important part of the prevailing coastal ecosystem and the biofilms on their surface represent a major food source for many grazing organisms [START_REF] Steneck | Herbivory and the Evolution of Nongeniculate Coralline Algae (Rhodophyta, Corallinales) in the North Atlantic and North Pacific[END_REF]).

Fucoid species, kelp forests, rhodolith beds and their associated flora and fauna make up the hard-substrate benthic communities of the Arctic coasts. With global change, a general expansion of both native and non-native kelp species has been documented during the last decades (Węslawski et al., 2010;[START_REF] Kortsch | Climate-driven regime shifts in Arctic marine benthos[END_REF][START_REF] Bartsch | Changes in kelp forest biomass and depth distribution in Kongsfjorden, Svalbard, between 1996-1998 and 2012-2014 reflect Arctic warming[END_REF]. Warmer waters and longer sea-ice-free periods may favour kelps growth, reproduction, and survival (Krause-Jensen and Duarte 2016). Fucoids are also spreading in the Arctic with a coverage that has already doubled and may triple by the end of the century (Jueterbock 2016;[START_REF] Kędra | Decadal change in macrobenthic soft-bottom community structure in a high Arctic fjord (Kongsfjorden, Svalbard)[END_REF]. The current and projected expansion of Fucoid species and kelp forests will exert cascading effects on the composition and functioning of shallow coastal Arctic ecosystems [START_REF] Krause-Jensen | Seasonal sea ice cover as principal driver of spatial and temporal variation in depth extension and annual production of kelp in Greenland[END_REF]. Arctic calcified algae distribution will be constrained by the upper temperature limit in the South and by calcium carbonate saturation state in the North [START_REF] Steinacher | Imminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model[END_REF][START_REF] Chan | Recent density decline in wild-collected subarctic crustose coralline algae reveals climate change signature[END_REF]. Furthermore, warming and increased advection are causing poleward migration, bringing thermally tolerant boreal-subarctic species that compete with local species adapted to cold waters (Węslawski et al., 2010). Shifts in species distribution, abundance and performance are therefore foreseen [START_REF] Wiencke | Seaweeds and their communities in polar regions[END_REF][START_REF] Traiger | Mature and developing kelp bed community composition in a glacial estuary[END_REF]. These changes are expected to alter carbon cycling as well as the timing and magnitude of benthic primary production [START_REF] Grebmeier | A major ecosystem shift in the Northern Bering Sea[END_REF][START_REF] Scherrer | Mechanistic model identifies increasing light availability due to sea ice reductions as cause for increasing macroalgae cover in the Arctic: light causes arctic macroalgal increase[END_REF].

Methods

The objectives of the present study are: (1) to gather and synthesize existing information in order to provide an overview of potential impacts of climate change on the macroalgae communities of Arctic coasts and (2) to highlight knowledge gaps on species, ecological process or subregions of the Arctic. We explored for shallow and hard-bottomed Arctic coasts, the short-and long-term changes in environmental characteristics, species responses through biological processes, the resulting winning and losing taxa and the impacts on ecosystem functioning. Relevant articles were searched on "ISI Web of Science'' and "GoogleScholar'' using a Boolean 2022-09-24). A total of 91 papers were identified. The online tool "connectedpapers" (connectedpapers.com) was used to explore related studies; it returned 28 additional papers. An article was considered as 'relevant' if the study focused on the impact of climate change -using one or more environmental parameters (e.g., temperature, salinity, light) -on Arctic macroalgal communities (up to 77 m depth, which is the depth at which the deepest coralline algae are described in the studies analyzed here) -whether on a single species or a community (fucoids, kelps or coralline algae). Potentially relevant articles not written in English were not considered. Each relevant article was tagged as an "in situ" study (i.e., conducted in the field) or "ex situ" study (i.e., conducted in the laboratory). Both or none of these tags could be assigned as appropriate, for example for broad review articles and modeling exercises. The references are listed in Table S1.

Bibliometric analysis

Until 20 years ago, only a limited number of studies have been conducted to assess how marine macroalgae communities respond to global change in the Arctic (Fig. 1). The number of ex situ and in situ studies started rising in the mid 1990s to reach a total of 53 and 54 studies respectively in 2022. Aside from studies conducted along Russian coastlines that have not been translated into English and assessed here, in situ research has been carried out over the whole Arctic region (Fig. 2A). However, only a small portion of the Arctic coastlines has been studied; more than half of the papers report research conducted in Svalbard, with a particular emphasis on Kongsfjorden (Fig. 2B).

Shallow hard-bottomed Arctic characteristics: past, present, and future

Seasonality

Arctic coastal environments are characterized by strong seasonal changes in photosynthetically available radiation (PAR). In spring, daylight lengthens, temperature increases and ice begins to melt. Thawing land and sea ice decreases salinity and affects the light regime afterwards by delivering dissolved and particulate matter which increases turbidity (Fig. 3 and4). In winter, the daylight period shortens, temperature decreases, and sea and land ice forms. These marked seasonal fluctuations strongly influence biological processes and marine ecosystems [START_REF] Kędra | Status and trends in the structure of Arctic benthic food webs[END_REF]. In particular, the vertical zonation and productivity in shallow Arctic coastal areas are controlled by fluctuating salinity, turbidity, sea-ice shading, available organic matter, ice scouring, and sedimentation rate (e.g., [START_REF] Conlan | Ice scour disturbance to benthic communities in the Canadian High Arctic[END_REF][START_REF] Wlodarska-Kowalczuk | Soft-bottom macrobenthic faunal associations and factors affecting species distributions in an Arctic glacial fjord (Kongsfjord, Spitsbergen)[END_REF].

Environmental changes impact seasonality. For all regions of the Arctic Ocean, an increase in the duration of the open water period is underway, ranging from 3.9 to 13.8 days per decade [START_REF] Bliss | Regional variability of Arctic sea ice seasonal change climate indicators from a passive microwave climate data record[END_REF]. Warming induces earlier and more intense ice melt with an increase in meltwater runoff and flux of particles in spring (Fig. 4; [START_REF] Svendsen | The retreat of the barents sea ice sheet on the western svalbard margin[END_REF]. It is projected that this will strongly affect biological diversity in Arctic fjords (e.g., [START_REF] Jørgensen | Environmental influences on benthic fauna associations of the Kara Sea (Arctic Russia)[END_REF][START_REF] Wassmann | Footprints of climate change in the Arctic marine ecosystem: footprints of climate change[END_REF]. On the one hand, a shorter ice season, a smaller ice-covered area and a reduction in ice thickness due to warming reduce ice-related pressures such as ice shading or ice scouring on shallow benthic communities (Węslawski et al., 2010). On the other hand, the associated increase in turbidity will restrict the euphotic layer and will enhance local hyposalinity and coastal siltation (Węslawski et al., 2011;[START_REF] Bartsch | Changes in kelp forest biomass and depth distribution in Kongsfjorden, Svalbard, between 1996-1998 and 2012-2014 reflect Arctic warming[END_REF]. As a result, species expansion and primary production may be more constrained near meltwater outflows, but less further away [START_REF] Hopwood | Review article: How does glacier discharge affect marine biogeochemistry and primary production in the Arctic?[END_REF].

Habitat and pulse disturbances

The habitat, especially the bottom structure and exposure to disturbances, are regulating factors and key elements in the structuring of benthic ecosystems [START_REF] Yesson | Community composition of epibenthic megafauna on the West Greenland Shelf[END_REF]. Erosion, as well as river and glacial runoff, supply large amounts of sediments in coastal areas. Hard-bottom substrates only represent ca. 35% of the Arctic coasts [START_REF] Lantuit | The Arctic coastal dynamics database: a new classification scheme and statistics on Arctic permafrost coastlines[END_REF][START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF].

As ice protects the shoreline from coastal erosion, a longer period of wave exposure is anticipated as a result of the shortening of the ice-covered season. This will lead to a considerable increase in resuspended sediments (Fig. 3a and4; [START_REF] Trefry | Sources, concentrations, composition, partitioning and dispersion pathways for suspended sediments and potential metal contaminants in the Coastal Beaufort Sea[END_REF], a coarser sediment structure, and significant changes in community composition (Węslawski et al., 2010). This is called a pulse disturbance event. Unstable substrate and fine sediment are resulting in a patchy distribution of rhodolith beds in Svalbard [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF].

During storm events, the increase in the frequency and intensity of waves leads to coastal soil erosion. In the fall and winter, seasonal storms produce detritus by detaching living seaweed from the bottom, enabling the long-term survival of benthic organisms without phytoplankton or ice algae supply in polar winter [START_REF] Mincks | Trophic structure on the West Antarctic Peninsula shelf: detritivory and benthic inertia revealed by δ13C and δ15N analysis[END_REF][START_REF] Morata | Climate change and diminishing seasonality in Arctic benthic processes[END_REF]. This accumulation of detritus due to storm events makes the Arctic food web more stable and increases its persistence over the course of the year [START_REF] Kędra | The shallow benthic food web structure in the high Arctic does not follow seasonal changes in the surrounding environment[END_REF]. Storms also induce the accumulation of unattached and uncemented coralline algal banks forming rhodolith beds (see section 5.3, Freiwald and Henrich 1994).

Disturbance is an important driver of spatio-temporal biotic patterns, increasing the patchiness of benthic biomass [START_REF] Gutt | On the direct impact of ice on marine benthic communities, a review[END_REF]. Beyond coastal erosion, ice scouring and high sedimentation rate are major disturbances in glacial bays [START_REF] Clark | Effects of sea-ice cover on marine benthic communities: a natural experiment in Commonwealth Bay, East Antarctica[END_REF]. Ice scouring is caused by the passage of an iceberg in a shallow area scraping the bottom and damaging the habitat and the organisms therein (Fig. 3b). High sedimentation rate is due to a huge discharge of meltwater, diluting organic matter, increasing turbidity, reducing primary production, hindering recruitment, obstructing filtering appendages and even burying bottom dwellers (Hall 1994). Due to repeated exposure to disturbance events, the size of organisms and the complexity of the community structure both decrease when getting closer to the glacier [START_REF] Wlodarska-Kowalczuk | Soft-bottom macrobenthic faunal associations and factors affecting species distributions in an Arctic glacial fjord (Kongsfjord, Spitsbergen)[END_REF]. Additionally, the faunal biomass in the macroalgal belt exhibits greater spatial and temporal variability at shallow depths revealing a fragmentation of the community by patches, such as at Hansnest (Kongsfjorden, Svalbard), down to 5 m, which indicates recurrent disturbance at various spatio-temporal scales. [START_REF] Paar | Temporal shift in biomass and production of macrozoobenthos in the macroalgal belt at Hansneset, Kongsfjorden, after 15 years[END_REF].

The recolonization rate after a pulse disturbance is low in the Arctic [START_REF] Al-Habahbeh | Arctic coastal benthos long-term responses to perturbations under climate warming[END_REF] and the removal of a large portion of the initial biomass typically favors the establishment of early succession stages [START_REF] Conlan | Ice scour disturbance to benthic communities in the Canadian High Arctic[END_REF]. The pulse-disturbed hard bottom area is first colonized by algae and opportunistic taxa such as hydrozoans and small chitons. Then, larger and longer-lived taxa such as barnacles, colonial sponges, or ascidians slowly overtake the area [START_REF] Al-Habahbeh | Arctic coastal benthos long-term responses to perturbations under climate warming[END_REF]. With increasing distance to the disturbed area, trophic structure becomes more complex and biomass and diversity increase [START_REF] Wlodarska-Kowalczuk | Benthic response to chronic natural physical disturbance by glacial sedimentation in an Arctic fjord[END_REF].

As glaciers thin and melt, ice scouring will become more frequent in the short-term, but less in the long-term, then promoting species interactions and leading to more stable and biologically controlled community development (Fig. 4;Węslawski et al., 2010;[START_REF] Bartsch | Changes in kelp forest biomass and depth distribution in Kongsfjorden, Svalbard, between 1996-1998 and 2012-2014 reflect Arctic warming[END_REF].

Reduced ice scouring will favor species expansion at shallow depths [START_REF] Paar | Temporal shift in biomass and production of macrozoobenthos in the macroalgal belt at Hansneset, Kongsfjorden, after 15 years[END_REF][START_REF] Scherrer | Mechanistic model identifies increasing light availability due to sea ice reductions as cause for increasing macroalgae cover in the Arctic: light causes arctic macroalgal increase[END_REF]. However, reducing sea ice cover, sea level rise, and rising surface air and permafrost temperatures are already accelerating erosion of Arctic coastlines [START_REF] Nielsen | Increase in Arctic coastal erosion and its sensitivity to warming in the twenty-first century[END_REF]. Furthermore, the occurrence and severity of extreme events such as sediment slides and storms are projected to rise, increasing both the intensity and frequency of high sedimentation events and coastal erosion [START_REF] Müller | Impact of oceanic warming on the distribution of seaweeds in polar and cold-temperate waters[END_REF][START_REF] Kędra | Decadal change in macrobenthic soft-bottom community structure in a high Arctic fjord (Kongsfjorden, Svalbard)[END_REF]. Deposit of a sediment layer on the algae and its surroundings will degrade settling conditions for the algae and its associated community [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF]. The associated increase in turbidity will negatively impact growth and survival of macroalgae (see section 6.2). This will inevitably have consequences on the distribution of macroalgal benthic communities.

Arctic macroalgae: characteristics, distributions and ecological role

Fucoid species, kelps, and coralline algae are key species of hard-bottomed coastal ecosystems in the Arctic. Fucoids dominate intertidal algal assemblages, while kelps dominate the subtidal domain until they reach their low light threshold. The deepest kelps have been recorded at 61 m depth in the Disko Bay region (Greenland, Krause-Jensen et al., 2019). Coralline algae form the understory of kelp forests or rhodolith beds and are found down to 77 m depth [START_REF] Jørgensbye | Overview of coralline red algal crusts and rhodolith beds (Corallinales, Rhodophyta) and their possible ecological importance in Greenland[END_REF].

Fucoids

In the Arctic intertidal coasts of the Arctic, seaweed diversity is low, dominated by fucoids that can experience changes in a multitude of abiotic drivers including ice scouring, desiccation, UV radiation, and wave exposure. Fucoids are an order of intertidal brown algae widely distributed along temperate and Arctic coasts [START_REF] Obluchinskaya | The biochemical composition and antioxidant properties of Fucus vesiculosus from the Arctic region[END_REF]. They are native to the North Pacific Ocean and have radiated in the North Atlantic Ocean [START_REF] Coyer | A mtDNA-based phylogeny of the brown algal genus Fucus (Heterokontophyta; Phaeophyta)[END_REF] resulting in a few common species such as Fucus spiralis, F. vesiculosus, F. serratus, and F. distichus. The latter is a homothallic cold-adapted species [START_REF] Coyer | A mtDNA-based phylogeny of the brown algal genus Fucus (Heterokontophyta; Phaeophyta)[END_REF][START_REF] Jueterbock | The fate of the Arctic seaweed Fucus distichusunder climate change: an ecological niche modeling approach[END_REF]. Fucoids inhabit a broad range of habitats from exposed rocky shores to calm bays and estuaries making this order one of the most abundant group of organisms of intertidal rocky shores in the Northern Hemisphere [START_REF] Lüning | Seaweeds: their environment, biogeography, and ecophysiology[END_REF][START_REF] Kucera | Assigning morphological variants of Fucus (Fucales, Phaeophyceae) in Canadian waters to recognized species using DNA barcoding[END_REF]. They are important ecosystem engineers forming wide coastal belts in Greenland, Jan Mayen, Norway, Svalbard, Baffin Island, and Alaska [START_REF] Smolina | Variation in thermal stress response in two populations of the brown seaweed, Fucus distichus, from the Arctic and subarctic intertidal[END_REF]. They are a food source, substratum and shelter for epiphytic and epibenthic communities with high species richness (including crustacean, snails and polychaetes) preventing them from desiccation when the tide is low (Ingolfsoon 2008). Brown (Dictyosiphon foeniculaceus, Stictyosiphon tortilis), red (Pylaiella littoralis) and green algae (Urococcus foslieanus, Ulothrix flacca, Acrosiphonia arcta and Prasiola crispa) constitute the understory of fucales canopy [START_REF] Wulff | Biodiversity, biogeography and zonation of marine benthic micro-and macroalgae in the Arctic and Antarctic[END_REF].

A fucale individual is composed of three parts: (1) a holdfast, a root-like structure that anchors the individual to the substrate, (2) a short stipe, a stem-like structure that allows the algae to sway in currents; and (3) a frond -or blade-leaf structure(s) that actively participates in photosynthesis. The frond is flattened, more or less dichotomously branched, and can contain air bladders and reproductive extremities when fertile. Fucales can reproduce both asexually by fragmentation or sexually. Once fertile, sporophytes release sperm and/or eggs from their conceptacles. Fertilization takes place and results in a zygote which settles on a hard substrate, germinates and forms a new sporophyte. The release usually happens at low tide, when there is little water motion, favouring gamete meeting but limiting dispersal [START_REF] Schoenwaelder | The effect of elevated UV radiation on Fucus spp. (Fucales, Phaeophyta) zygote and embryo development[END_REF]. Effective dispersal is however possible for homothallic fucale since only one rafting individual is enough to establish a new population by self-fertilization [START_REF] Jueterbock | The fate of the Arctic seaweed Fucus distichusunder climate change: an ecological niche modeling approach[END_REF].

Through thalli dislodgement and receptacle release, fucoids are important contributors to carbon production as they can exhibit annual rates of production equal to that of kelps [START_REF] Lewis | Quantifying intertidal canopy-forming macroalgal production, extent, degradation, and blue carbon potential[END_REF].

Kelps

Kelps are large brown algae found in subtidal cool waters, close to the shore and forming dense canopy called kelp beds or kelp forests [START_REF] Bolton | The biogeography of kelps (Laminariales, Phaeophyceae): a global analysis with new insights from recent advances in molecular phylogenetics[END_REF]. Although the term "kelp" frequently refers to large canopy-forming brown algae species (and will be used as-well in that sense in this study), in its strictest sense, the term refers to the order Laminariales [START_REF] Bolton | The biogeography of kelps (Laminariales, Phaeophyceae): a global analysis with new insights from recent advances in molecular phylogenetics[END_REF][START_REF] Teagle | The role of kelp species as biogenic habitat formers in coastal marine ecosystems[END_REF]. Like fucoids, they comprise three parts: a holdfast, a strong and flexible stipe and one or several blades.

Kelps have a biphasic life cycle characterized by the alternation of an asexual sporophyte and a sexual gametophyte stage [START_REF] Roleda | Stress physiology and reproductive phenology of Arctic endemic kelp Laminaria solidungula J. Agardh[END_REF]. A fertile sporophyte produces millions of spores, resulting in the germination of a gametophyte when conditions are suitable. Male and female gametophytes then produce sperm and eggs respectively. After the meeting of the gametes and a potential fertilization, the zygote settles and develops, giving a new generation of sporophyte.

Only Laminaria solidungula is endemic from the Arctic, other species from the common genus Laminaria, Saccharina and Alaria are of Atlantic and Pacific origin and extend into subarctic and cooler temperate waters [START_REF] Wulff | Biodiversity, biogeography and zonation of marine benthic micro-and macroalgae in the Arctic and Antarctic[END_REF]. These kelp species, and more generally polar algae, have adapted to this cold environment [START_REF] Gómez | Light and temperature demands of marine benthic microalgae and seaweeds in polar regions[END_REF]. Metabolic strategies such as enzymes able to work at low temperature, a higher unsaturated fatty acid content and antifreeze proteins allow them to survive to cold temperature [START_REF] Gómez | Light and temperature demands of marine benthic microalgae and seaweeds in polar regions[END_REF].

Kelps are important primary producers able to capture and store large amounts of CO2 as biomass (Krause-Jensen and Duarte, 2016). Kelp forests substantially contribute to the global carbon cycle [START_REF] Wernberg | Grazers extend blue carbon transfer by slowing sinking speeds of kelp detritus[END_REF]Krause-Jensen and Duarte, 2016). The global seaweed productivity (including canopies of brown algae from the orders Laminariales, Fucales, Tilopteridales, and Desmarestiales) has been estimated at 536 ± 31 gC m⁻ ² year⁻ ¹ by [START_REF] Pessarrodona | Global seaweed productivity[END_REF].

Exuded dissolved organic carbon as well as detritus resulting from detachment and fragmentation by waves and shredding by herbivores are important contributions to shallow and deep-water food webs [START_REF] Krumhansl | Production and fate of kelp detritus[END_REF][START_REF] Krause-Jensen | Sequestration of macroalgal carbon: the elephant in the Blue Carbon room[END_REF]. At the global scale, 82% of the kelp primary production is exported to adjacent communities [START_REF] Krumhansl | Production and fate of kelp detritus[END_REF].

Microbial films, sessile colonial filter feeders (bryozoan, hydrozoan, polychaetes) and mobile herbivores (sea urchins, polychaetes, crustaceans, mollusks) characterize their epiphytic fauna.

Kelp forests support a high biodiversity by providing habitat, food, and nursery area for fish and invertebrates [START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF]. By altering light, sedimentation, physical abrasion, and bottom currents they provide optimal conditions for the development of an understory mainly composed of red and some green and brown algae [START_REF] Teagle | The role of kelp species as biogenic habitat formers in coastal marine ecosystems[END_REF][START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF]. Devaleraea ramentacea, Palmaria palmata, Phycodrys rubens, Ptilota gunneri and Chaetomorpha and Desmarestia genus make up the common dense understory of kelp forest and deeper assemblages found in the Arctic [START_REF] Hop | The marine ecosystem of Kongsfjorden, Svalbard[END_REF].

Coralline algae

Coralline algae are red calcareous algae, growing as concentric laminae (i.e., crust) or radial branchs or columns (i.e., nodules; [START_REF] Bosence | The occurrence and ecology of recent rhodolithsa review[END_REF]. The surface of coralline algae can be smooth or matt, from reddish to deep pink with violet tinge [START_REF] Jackson | Lithothamnion glaciale[END_REF]. They are coastal species distributed worldwide from the tropics to the Arctic areas [START_REF] Foster | Rhodolits: between rocks and soft places[END_REF].

Vegetative growth and division of the crust or nodules into two or more individuals is a method of propagation used by coralline algae. To propagate, coralline algae can also reproduce sexually or asexually through the formation of spores [START_REF] Jackson | Lithothamnion glaciale[END_REF].

Coralline algae are long-lived and precipitate calcium carbonate within their cell walls and between cells, leading some to consider them as important long-term carbon burial actors in coastal ecosystems (van der Heijden and Kamenos, 2015) although calcification is a source of CO2.

Temperature, PAR, salinity, and seawater chemistry determine the distribution of coralline algae [START_REF] Adey | Subarctic rhodolith beds promote longevity of crustose coralline algal buildups and their climate archiving potential[END_REF][START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF]. While temperature primarily determines species-specific geographical distribution, PAR limits coralline occurrence at depth even if they can be found at great depth due to highly efficient photosynthetic pigments at low light [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF].

In the Arctic, coralline thrive within extreme seasonal variations of light regime, nutrient supply, and temperature [START_REF] Teichert | Rhodolith beds (Corallinales, Rhodophyta) and their physical and biological environment at 80°31′N in Nordkappbukta (Nordaustlandet, Svalbard Archipelago, Norway)[END_REF]. To survive the polar night, they store carbohydrates produced during the active photosynthetic season, and use them in the winter, when photosynthesis is not possible [START_REF] Freiwald | Reefal coralline algal build-ups within the Arctic Circle: morphology and sedimentary dynamics under extreme environmental seasonality[END_REF]. Corallines are also suspected of storing nitrogen and phosphorus during winter, when the water column is enriched, before the phytoplankton bloom depletes the water column from nutrients [START_REF] Freiwald | Reefal coralline algal build-ups within the Arctic Circle: morphology and sedimentary dynamics under extreme environmental seasonality[END_REF].

Thereby, despite their low growth rate (0.2 mm/month; [START_REF] Adey | Colonization, succession and growth rates of tropical crustose coralline algae (Rhodophyta, Cryptonemiales)[END_REF], Arctic and subarctic coralline algae have a high standing stock that could be similar to tropical species [START_REF] Freiwald | Reefal coralline algal build-ups within the Arctic Circle: morphology and sedimentary dynamics under extreme environmental seasonality[END_REF].

Rhodoliths -or maërl -are free-living structures mostly composed of crustose or branching coralline algae (>50% in volume) that settle on a particulate substrate or are detached from existing hard substrates [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF]. Coralline algae are the master builders of such three-dimensional calcified frameworks, with cavities and branches, that provide shelter for a variety of organisms and support a high biodiversity [START_REF] Teichert | Rhodolith beds (Corallinales, Rhodophyta) and their physical and biological environment at 80°31′N in Nordkappbukta (Nordaustlandet, Svalbard Archipelago, Norway)[END_REF][START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF]. Rhodoliths beds support a diverse range of benthic species, including a microalgal epiphyton (comprising foraminifera and diatoms), a vagile epifauna dominated by grazers (crustaceans, gastropods and sea urchins), a sessile epifauna of filters feeders (bivalves, serpulids, spirorbids, bryozoan) and an infauna that includes boring bivalves and polychaetes living within the algal thalli and the sediment deposited around the rhodoliths [START_REF] Bosence | The occurrence and ecology of recent rhodolithsa review[END_REF][START_REF] Freiwald | Reefal coralline algal build-ups within the Arctic Circle: morphology and sedimentary dynamics under extreme environmental seasonality[END_REF]. Rhodoliths act as habitat, feeding, nursery, and spawning ground for various species including fishes (Thormar 2008).

The integrity of coralline algae requires grazers to keep them healthy by removing excessive fast growing fleshy algae and sessile invertebrates [START_REF] Steneck | Herbivory and the Evolution of Nongeniculate Coralline Algae (Rhodophyta, Corallinales) in the North Atlantic and North Pacific[END_REF]. One conspicuous herbivore is the sea urchin Strongylocentrotus droebachiensis whose larvae metamorphose on the surface of living coralline algae [START_REF] Pearce | Induction of metamorphosis of larvae of the green sea urchin, Strongylocentrotus droebachiensis, by coralline red algae[END_REF]. The formation and distribution of coralline algal communities rely heavily on these interactions with herbivores [START_REF] Freiwald | Reefal coralline algal build-ups within the Arctic Circle: morphology and sedimentary dynamics under extreme environmental seasonality[END_REF].

In the Arctic, rhodolith communities have been observed in Greenland (Thormar 2008, Jørgensbye and Halfar 2017), Alaska [START_REF] Konar | Rhodolith bed: a newly discovered habitat in the North Pacific Ocean[END_REF], North Norway [START_REF] Freiwald | Reefal coralline algal build-ups within the Arctic Circle: morphology and sedimentary dynamics under extreme environmental seasonality[END_REF], and Svalbard [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF][START_REF] Sswat | Distribution and composition of the epibenthic megafauna north of Svalbard (Arctic)[END_REF]. Phymatolithon, Clathromorphum, Lithophyllum and Lithothamnion, are widespread genera in shallow shelf areas with Lithothamnion able to colonize a large range of the dysphotic zone (up to 90 m deep for Lithothamnion; [START_REF] Teichert | Rhodolith beds (Corallinales, Rhodophyta) and their physical and biological environment at 80°31′N in Nordkappbukta (Nordaustlandet, Svalbard Archipelago, Norway)[END_REF].

Species responses to global change: effects on biological processes

Most studies on the response of Arctic macroalgae to climate change have been performed ex situ, likely due to the multiple challenges for carrying out in situ experiments (Table 1; Müller et al., 2009;[START_REF] Wilson | Effects of increasing water temperatures on survival and growth of ecologically and economically important seaweeds in Atlantic Canada: implications for climate change[END_REF]. Warming, changes in underwater light regime, local hyposalinity and ocean acidification can all have large effects on biological processes including reproduction, primary production, and respiration [START_REF] Müller | Impact of oceanic warming on the distribution of seaweeds in polar and cold-temperate waters[END_REF][START_REF] Bonsell | Long-term patterns of benthic irradiance and kelp production in the central Beaufort Sea reveal implications of warming for Arctic inner shelves[END_REF].

Warming

Temperature is the main physical driver in most ecosystems and has a major influence on polar benthic species distribution [START_REF] Drewnik | Benthic crustacea and mollusca distribution in Arctic fjordcase study of patterns in Hornsund[END_REF]. Warming will increase suitable habitat for F. distichus, fostering seaweed meadows in the intertidal Arctic [START_REF] Coyer | Trans-Pacific and trans-Arctic pathways of the intertidal macroalga Fucus distichus L. reveal multiple glacial refugia and colonizations from the north Pacific to the North Atlantic: Phylogeography of Fucus distichus in the Northern Hemisphere[END_REF]. Among coldtemperate kelp species, many have growth temperature optima well above the temperature at which they are currently growing [START_REF] Bischoff | Temperature requirements for growth and survival of macroalgae from Disko Island (Greenland)[END_REF][START_REF] Müller | Impact of oceanic warming on the distribution of seaweeds in polar and cold-temperate waters[END_REF][START_REF] Meyer | Recruitment of benthic invertebrates in high Arctic fjords: Relation to temperature, depth, and season: Recruitment in Arctic fjords[END_REF]. Since enzymatic processes are temperature-dependent, it has a strong effect on all physiological processes (e.g., [START_REF] Brown | Toward a metabolic theory of ecology[END_REF][START_REF] Gómez | Light and temperature demands of marine benthic microalgae and seaweeds in polar regions[END_REF]. For most kelp species, growth at 0-5 °C is typically less than one third of the rates reached at their optimal temperature ranging from 10 to 15 °C [START_REF] Wiencke | Seaweeds and their communities in polar regions[END_REF][START_REF] Roleda | Stress physiology and reproductive phenology of Arctic endemic kelp Laminaria solidungula J. Agardh[END_REF]. By promoting photosynthesis, germination, gametogenesis, fertility and survival enhancement, warming is projected to increase the biomass of kelps such as Saccharina latissima, Laminaria hyperborea, Laminaria digitata, and Nereocystis luetkeana and expand their distribution to higher latitudes (Li et al., 2020a;[START_REF] Goldsmit | Kelp in the eastern Canadian Arctic: current and future predictions of habitat suitability and cover[END_REF].

Warming may also have negative impacts on some macroalgal species. Spore settlement and gametophyte growth rate can be lower at higher temperature [START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF]. For Laminaria solidungula, the only truly Arctic endemic kelp species, the recruitment becomes limited when temperature exceeds 10 °C [START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF].

Rising temperature drives the increasing frequency of marine heatwaves [START_REF] Oliver | Longer and more frequent marine heatwaves over the past century[END_REF][START_REF] Golubeva | Marine heatwaves in siberian Arctic seas and adjacent region[END_REF]. Even intertidal species, adapted to extreme environmental changes, may be impacted. Photosynthetic performances of F. distichus and F. serratus decreased when exposed to temperatures higher than 24 °C and 28 °C, respectively [START_REF] Jueterbock | Thermal stress resistance of the brown alga Fucus serratus along the north-Atlantic coast: Acclimatization potential to climate change[END_REF][START_REF] Smolina | Variation in thermal stress response in two populations of the brown seaweed, Fucus distichus, from the Arctic and subarctic intertidal[END_REF]. Except above 32 °C, these species recovered completely within a day after 60 min. of exposure to warm temperature. However, higher durations of treatment have not been tested. In the Kara, Laptev and Siberian seas, the duration of marine heatwaves increased by 10 to 12 days between 2000 and 2020 [START_REF] Golubeva | Marine heatwaves in siberian Arctic seas and adjacent region[END_REF]. Testing longer durations may reveal potential adaptation or, on the contrary, an increasing reduction in photosynthetic performance that may impair fucoids dispersal in the Arctic.

Arctic and subarctic coralline algae are more sensitive to warming, which is consistent with the fact that they live at greater depths and are therefore less exposed to large changes in temperature. Present temperature appears to be warm enough for growth and cold enough to allow conceptacle production in the summer [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF]. For Lithothamnion glaciale, the production of conceptacles, containing the spores, is limited when water temperature is above 9 °C [START_REF] Hall-Spencer | Biological studies on nongeniculate Corallinaceae[END_REF]. Warming might therefore impair its reproductive capacity. Furthermore, its primary production and calcification rates are lower at high temperature with an optimum at ca. 4 °C [START_REF] Schoenrock | Influences of salinity on the physiology and distribution of the Arctic coralline algae, Lithothamnion glaciale (Corallinales, Rhodophyta)[END_REF]. Longer duration of marine heatwaves will increase temperature deeper and may reach coralline distribution depths with significant impact on their development.

Changes in underwater light

By influencing the spatial and depth distributions of primary producers, underwater light is a driver of the composition of macroalgal dominated ecosystems. Light has a significant impact on the global carbon cycle by influencing primary production. Photoperiod also regulates the timing of reproduction of F. distichus [START_REF] Bird | Control of formation of receptacles in Fucus distichus L. subsp. distichus(Phaeophyceae, Fucales)[END_REF], L. solidungula [START_REF] Lüning | Seaweeds: their environment, biogeography, and ecophysiology[END_REF] and A. esculenta [START_REF] Martins | Daylength influences reproductive success and sporophyte growth in the Arctic kelp species Alaria esculenta[END_REF]. Several climate change-related processes affect light penetration in the Arctic Ocean. The underwater light conditions are influenced by sea ice cover and water turbidity which both attenuate PAR and UV-radiation (UVR; [START_REF] Bonsell | Long-term patterns of benthic irradiance and kelp production in the central Beaufort Sea reveal implications of warming for Arctic inner shelves[END_REF][START_REF] Hanelt | Light regime in an Arctic fjord: a study related to stratospheric ozone depletion as a basis for determination of UV effects on algal growth[END_REF]. In general, it can be projected that, with increasing air and water temperature, PAR will decrease during part of the summer, near glaciers and river outflows, due to a higher input of particles and coloured dissolved material (Fig. 3c). Furthermore, increased summer cloudiness with warming reduces incident PAR over most of the Arctic Ocean [START_REF] Bélanger | Light absorption and partitioning in Arctic Ocean surface waters: impact of multiyear ice melting[END_REF]. However, in early spring or further away from the glaciers and rivers, PAR could increase due to the decrease in sea ice cover (Fig. 3d; Arrigo and van Dijken 2011).

Higher turbidity near glaciers and rivers limits the vertical distribution of kelps and associated fauna, but decreased sea-ice cover promotes the expansion of kelps in new areas [START_REF] Wiencke | Ecosystem Kongsfjorden: new views after more than a decade of research[END_REF]. For coralline algae, [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF] found that irradiance is the most influential parameter on rhodolith CaCO3 production rates in Svalbard. The projected decrease in PAR near glaciers and rivers will reduce the primary production of both kelps and calcareous algae with implications on the benthic fauna inhabiting this ecosystem [START_REF] Hopwood | Review article: How does glacier discharge affect marine biogeochemistry and primary production in the Arctic?[END_REF].

The depletion of stratospheric ozone causes a significant rise in solar UVR [START_REF] Wiencke | Ecosystem Kongsfjorden: new views after more than a decade of research[END_REF]) which will also be more intense over a longer period and will penetrate deeper in the water column [START_REF] Laeseke | Effects of kelp canopy on underwater light climate and viability of brown algal spores in Kongsfjorden (Spitsbergen)[END_REF]. Down to ca. 6 m depth [START_REF] Hanelt | Influence of UV radiation on the photosynthesis of arctic macroalgae in the field[END_REF][START_REF] Wiencke | Impact of UV-radiation on viability, photosynthetic characteristics and DNA of brown algal zoospores:implications for depth zonation[END_REF], UVR are intense enough to negatively affect physiological and metabolic processes such as photosynthesis, respiration, development, and reproduction [START_REF] Roleda | Sensitivity of the early life stages of macroalgae from the northern hemisphere to ultraviolet radiation: photochemistry and photobiology[END_REF]. For Arctic kelps, a stronger exposure to UVA and UVB leads to a decline in photosynthetic efficiency (e.g., [START_REF] Hanelt | Influence of UV radiation on the photosynthesis of arctic macroalgae in the field[END_REF][START_REF] Heinrich | Temperature and light interactively modulate gene expression in Saccharina latissima (Phaeophyceae)[END_REF] and lowers the germination success and recruitment (Roleda et al., 2006a). Early developmental stages, such as spores, are the most vulnerable to UVA and UVB (Roleda et al., 2006a;[START_REF] Wiencke | Life strategy, ecophysiology and ecology of seaweeds in polar waters[END_REF]. [START_REF] Wiencke | Impact of UV-radiation on viability, photosynthetic characteristics and DNA of brown algal zoospores:implications for depth zonation[END_REF], [START_REF] Roleda | Response of Arctic kelp zoospores to ultraviolet and photosynthetically active radiation in relation to growth depth[END_REF] and [START_REF] Fredersdorf | Interactive effects of radiation, temperature and salinity on different life history stages of the Arctic kelp Alaria esculenta (Phaeophyceae)[END_REF], identified harmful effects of increased UVR on the photosynthetic rate and motility of kelp spores.

However, photoprotection strategy and/or repair of DNA damage have been shown in several macroalgal species including at the zoospore stage (van de Poll et al., 2002[START_REF] Roleda | Response of Arctic kelp zoospores to ultraviolet and photosynthetically active radiation in relation to growth depth[END_REF], Roleda et al., 2006b). Furthermore, it should be noted that most studies consider high UV intensities [START_REF] Wiencke | Ecosystem Kongsfjorden: new views after more than a decade of research[END_REF][START_REF] Laeseke | Effects of kelp canopy on underwater light climate and viability of brown algal spores in Kongsfjorden (Spitsbergen)[END_REF] that are not representative of in situ conditions even more for most Arctic kelps are fertile in autumn-to-spring, avoiding exposure of their spores to high levels of PAR and UVR [START_REF] Olischläger | Seasonal fertility and combined effects of temperature and UVradiation on Alaria esculenta and Laminaria digitata (Phaeophyceae) from Spitsbergen[END_REF]. Since ice and snow melt runoff [START_REF] Hop | The marine ecosystem of Kongsfjorden, Svalbard[END_REF], waves [START_REF] Svendsen | The physical environment of Kongsfjorden-Krossfjorden, an Arctic fjord system in Svalbard[END_REF], and weather conditions fluctuate on short time scales and significantly decrease water clarity and light penetration, it is unlikely that spores could undergo elevated UVR exposure for time periods long enough to induce permanent photodamage (≥ 8 h; [START_REF] Laeseke | Effects of kelp canopy on underwater light climate and viability of brown algal spores in Kongsfjorden (Spitsbergen)[END_REF]. However, spores and gametophytes of kelp species fertile in summer may be limited in their vertical distribution to avoid UVR at the surface [START_REF] Wiencke | Ecosystem Kongsfjorden: new views after more than a decade of research[END_REF].

Fucoid are present in the intertidal zone and therefore undergo higher levels of UVR. [START_REF] Schoenwaelder | The effect of elevated UV radiation on Fucus spp. (Fucales, Phaeophyta) zygote and embryo development[END_REF] found harmful effects of UVR on embryonic development of F. distichus and F. serratus individuals. Under high UVR, zygotes are incapable of polarization, germination and division, and finally die. Increasing UVR may restrict F. distichus to the lower intertidal area of Arctic coasts [START_REF] Jueterbock | The fate of the Arctic seaweed Fucus distichusunder climate change: an ecological niche modeling approach[END_REF].

To our knowledge, no studies on the impact of UVR on Arctic coralline algae have been conducted. Coralline algae are usually under the fucoids and kelp canopee or deep enough not to be impacted by UVR.

Salinity decrease

Increase in river runoff, ice melt, and precipitation will accentuate local hyposaline conditions.

Salinity is one of the drivers of the photophysiology and distribution of algal species [START_REF] Hanelt | Light regime in an Arctic fjord: a study related to stratospheric ozone depletion as a basis for determination of UV effects on algal growth[END_REF]Li et al., 2020a;[START_REF] Diehl | Impacts of combined temperature and salinity stress on the endemic Arctic brown seaweed Laminaria solidungula J. Agardh[END_REF]. Although intertidal fucoids have efficient osmoregulation mechanisms, reduced salinity leads to reduced biosynthesis of mannitol, an important source of carbon for heterotrophic bacteria [START_REF] Groisillier | The mannitol utilization system of the marine bacterium Zobellia galactanivorans[END_REF]. Increase in respiratory O2 consumption at lower salinities (< ~ 21) was also found in fucoids [START_REF] Munda | Chemical composition and physiological properties of fucoids under conditions of reduced salinity[END_REF]. Some species, such as Fucus serratus, have a narrower salinity spectrum, impacting their distribution [START_REF] Munda | Chemical composition and physiological properties of fucoids under conditions of reduced salinity[END_REF]. To avoid damage in hyposaline conditions, energy is transferred from biomass production to osmoregulation, affecting the longterm growth rate (Fig. 3e; Li et al., 2020a). Hyposaline conditions can cause the loss of pigments and bleaching and in prolonged low salinity events, death of macroalgae (Li et al., 2020a). [START_REF] Diehl | Impacts of combined temperature and salinity stress on the endemic Arctic brown seaweed Laminaria solidungula J. Agardh[END_REF] found that low salinity (25) combined with elevated temperature (up to 15 °C) causes a significant decline in the maximum quantum yield of photosystem II (Fv/Fm) of the kelp Laminaria solidungula. Variable salinity conditions also reduce photosynthetic efficiency (Fig. 3f; [START_REF] Karsten | Research note: Salinity tolerance of Arctic kelps from Spitsbergen[END_REF][START_REF] Spurkland | Salinity and irradiance effects on growth and maximum photosynthetic quantum yield in subarctic Saccharina latissima (Laminariales, Laminariaceae)[END_REF]. As a result, the projected local decrease in salinity is expected to alter kelp fitness by limiting photosynthesis (e.g., [START_REF] Traiger | Mature and developing kelp bed community composition in a glacial estuary[END_REF][START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF].

Rhodophyta are generally more sensitive to salinity changes than Phaeophyta and Chlorophyta [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF]. [START_REF] Schoenrock | Influences of salinity on the physiology and distribution of the Arctic coralline algae, Lithothamnion glaciale (Corallinales, Rhodophyta)[END_REF] found that, when subjected to low salinity (i.e., 22), Arctic red coralline algae exhibited lower calcification and photosynthetic rates than at current salinity level (i.e., 33). However, not all rhodophytes are highly sensitive to low salinity.

Lithothamnion glaciale can tolerate both low (< 18) and strongly fluctuating salinity, but at a cost for its photosynthetic capacities [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF][START_REF] Schoenrock | Influences of salinity on the physiology and distribution of the Arctic coralline algae, Lithothamnion glaciale (Corallinales, Rhodophyta)[END_REF]. However, the reduced growth associated with low salinity will have a negative impact on rhodolith beds (Teicher et al., 2014). Although certain taxa are more tolerant to a variable and low salinity, freshwater inputs are projected to hinder most Arctic macroalgae with impaired performances.

Ocean acidification

Ocean acidification is magnified in high latitude regions because cold polar waters favor the dissolution of CO2 and low calcium carbonate saturation states [START_REF] Fransner | Acidification of the Nordic Seas[END_REF]. Even if warming limits CO2 absorption, indirect effects of the increase in sea temperature such as ice loss further promote this process by enhancing air-sea gas exchange, including CO2 [START_REF] Yamamoto-Kawai | Aragonite undersaturation in the Arctic Ocean: effects of ocean acidification and sea ice melt[END_REF]. Also, mixing with river runoff can increase or reduce the buffering capacity of Arctic waters depending on the local geology and releases a large amount of dissolved and particulate carbon from thawing permafrost [START_REF] Polukhin | The role of river runoff in the Kara Sea surface layer acidification and carbonate system changes[END_REF]. With a pH decrease of up to 0.45 units projected over the 21st century [START_REF] Terhaar | Arctic Ocean acidification over the 21st century co-driven by anthropogenic carbon increases and freshening in the CMIP6 model ensemble[END_REF], model simulations estimate that the Arctic will undergo the greatest acidification at the global scale [START_REF] Fransner | Acidification of the Nordic Seas[END_REF]. At the current emission rate, surface waters of the Arctic Ocean will be locally undersaturated with respect to aragonite within a decade [START_REF] Yamamoto | Impact of rapid seaice reduction in the Arctic Ocean on the rate of ocean acidification[END_REF][START_REF] Fransner | Acidification of the Nordic Seas[END_REF]. This can be enhanced by local biological activity such as respiration inside kelp forests during periods of darkness or respiration of organic carbon from land.

However, long photoperiods (>21 h) lead to a continuous increase in pH that potentially benefits calcifiers (Krause-Jensen et al., 2016).

Increased CO2 in marine systems has consequences on biological processes such as photosynthesis, respiration, calcification, dissolved organic carbon (DOC) release and carbon accumulation in biomolecules (Fig. 3g; Iñiguez et al., 2016b, Wiencke andHop 2016). The decrease in pH as well as calcite saturation state may cause a rapid reduction in coralline algae cover and diversity within 10 to 30 years and could yield to their possible disappearance during this century in the Arctic (SRES A2; IPCC 2007; [START_REF] Büdenbender | Calcification of the Arctic coralline red algae Lithothamnion glaciale in response to elevated CO2[END_REF]. In tropical waters, coralline algae with quick generation time (6 to 8 weeks) can develop resistance to ocean acidification over multiple generations [START_REF] Cornwall | A coralline alga gains tolerance to ocean acidification over multiple generations of exposure[END_REF]. However, it is likely that Arctic species may not have enough time or the capacity to acclimatize due to their longer generation time.

For kelps, a higher CO2 concentration could lead to higher photosynthetic rates. However, Iñiguez et al. (2016a) found that Saccharina latissima and Laminaria solidungula were largely unaffected by elevated pCO2 (carbon dioxide partial pressure, 1272 ± 4 µatm and 1308 ± 31 µatm respectively at 4°C). This suggests that photosynthesis is already C-saturated at 368 ± 15 µatm for Saccharina latissima and 428 ± 22 µatm for Laminaria solidungula. In another study, Iñiguez et al. (2016b) showed a positive and a negative effect of ocean acidification on the growth rate of Alaria esculenta and Desmarestia aculeata, respectively at 1300 ± 36 µatm and 1252 ± 30 µatm. This suggests that Arctic seaweed will be affected by ocean acidification in species-specific ways which may shift their relative biomass dominance with implications for the rest of the trophic web (Iñiguez et al., 2016b).

The impact of ocean acidification on Arctic benthic species is also determined by its interaction with other environmental constraints, such as warming, hyposalinity and PAR decrease [START_REF] Olischläger | Increased pCO2 and temperature reveal ecotypic differences in growth and photosynthetic performance of temperate and Arctic populations of Saccharina latissima[END_REF]. The combination of several environmental stressors may result in a retreat or even extinction of some populations and the expansion of others.

Invasive, resilient and vulnerable species: winners and losers

Global environmental change alters the spatial distribution of species by expanding or contracting suitable habitats [START_REF] Lenoir | Climate-related range shifts -a global multidimensional synthesis and new research directions[END_REF]. Species invasions, declines or extinctions are projected to increase in the future [START_REF] Wiencke | Ecosystem Kongsfjorden: new views after more than a decade of research[END_REF].

Half of the benthic taxa are projected to experience significant changes in the extent of their suitable habitat (> 10% increase or decrease) including dominant and/or endemic species of Arctic shelves [START_REF] Renaud | Arctic Sensitivity? Suitable Habitat for Benthic Taxa Is Surprisingly Robust to Climate Change[END_REF]. It could lead to a high risk of species extinction in polar and cold-temperate regions [START_REF] Müller | Impact of oceanic warming on the distribution of seaweeds in polar and cold-temperate waters[END_REF]. Sea ice loss is thought to drive a profound regime shift in shallow polar benthos from invertebrate-to algal-dominated communities [START_REF] Clark | Light-driven tipping points in polar ecosystems[END_REF]. For example, in Kongsfjorden (Svalbard), the abundance of the originally dominant sea anemones decreased by 80% between the 1990s and the 2000s, and were replaced by filamentous brown algae [START_REF] Kortsch | Climate-driven regime shifts in Arctic marine benthos[END_REF]. The combination of the projected changes in sea ice and species dominance will likely induce fundamental changes in community composition with a loss of biodiversity, functions and altered ecosystem stability [START_REF] Clark | Light-driven tipping points in polar ecosystems[END_REF].

Fucoids

Between 1988 and 2008, Svalbard coasts experienced an increase in the cover and biomass of intertidal marine algae, including Fucus distichus, whose population nearly doubled [START_REF] Kędra | Decadal change in macrobenthic soft-bottom community structure in a high Arctic fjord (Kongsfjorden, Svalbard)[END_REF]. Because ice melt opens new areas for colonization, the habitat suitability of this species in the Arctic would triple by 2100 [START_REF] Jueterbock | The fate of the Arctic seaweed Fucus distichusunder climate change: an ecological niche modeling approach[END_REF]. F. distichus is cold-adapted, surviving for months under ice and frequently exposed to cold temperatures during low tide, giving it an advantage over temperate species (see section 7.4; [START_REF] Svendsen | The physical environment of Kongsfjorden-Krossfjorden, an Arctic fjord system in Svalbard[END_REF]. However, increases in UVR may restrict its distribution to the lower intertidal zone giving ground to other species (Fig. 4; [START_REF] Jueterbock | The fate of the Arctic seaweed Fucus distichusunder climate change: an ecological niche modeling approach[END_REF][START_REF] Schoenwaelder | The effect of elevated UV radiation on Fucus spp. (Fucales, Phaeophyta) zygote and embryo development[END_REF]. [START_REF] Beuchel | Long-term patterns of rocky bottom macrobenthic community structure in an Arctic fjord (Kongsfjorden, Svalbard) in relation to climate variability (1980-2003)[END_REF], [START_REF] Kortsch | Climate-driven regime shifts in Arctic marine benthos[END_REF] and [START_REF] Bartsch | Changes in kelp forest biomass and depth distribution in Kongsfjorden, Svalbard, between 1996-1998 and 2012-2014 reflect Arctic warming[END_REF] reported an increase in macroalgal biomass in Svalbard fjords since the 1980s. Between 1996 and 2014, kelp biomass, mainly Laminaria digitata, has increased 2-4 times most likely because of reduced sea ice cover [START_REF] Hop | Species composition, zonation, and biomass of marine benthic macroalgae in Kongsfjorden[END_REF], Bartsch et al., 2016). [START_REF] Krause-Jensen | Seasonal sea ice cover as principal driver of spatial and temporal variation in depth extension and annual production of kelp in Greenland[END_REF] identified the duration of the open-water period, via changes in light and temperature, as the best predictor of vertical distribution and production of kelps along a latitudinal gradient. These variables explained up to 92% of the depth expansion and 80% of the variation in kelp biomass. These responses are likely to increase in the future. The loss of light-attenuating sea ice on the rocky coastlines of Russian islands, the Kola Peninsula, Spitsbergen, Greenland (Baffin Bay and Greenland Sea), and of the Canadian Archipelago, will provide new habitats for seaweed colonization [START_REF] Müller | Impact of oceanic warming on the distribution of seaweeds in polar and cold-temperate waters[END_REF]. Although concomitant increases in freshwater input, turbidity, and sedimentation will restrict kelp distribution and primary production in some locations, the kelp biomass could double in Arctic regions within less than 30 years [START_REF] Bonsell | Long-term patterns of benthic irradiance and kelp production in the central Beaufort Sea reveal implications of warming for Arctic inner shelves[END_REF][START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF].

Kelps

A few kelp species may gain advantage over the others by better acclimatizing to environmental stresses such as hyposalinity or increased turbidity in the summer. For example, while a poleward retreat of the endemic Laminaria solidungula, a stenothermic species that cannot live outside of a narrow range of cold temperature, is expected (Fig. 4; [START_REF] Müller | Impact of oceanic warming on the distribution of seaweeds in polar and cold-temperate waters[END_REF], Saccharina latissima could benefit from these new conditions, especially from warming, thanks to a high degree of polymorphism, acclimatation and genetic diversity across populations [START_REF] Bartsch | The genus Laminaria sensu lato: recent insights and developments[END_REF][START_REF] Guzinski | Characterization of newly developed expressed sequence tag-derived microsatellite markers revealed low genetic diversity within and low connectivity between European Saccharina latissima populations[END_REF]. While kelp abundance and biomass may largely increase, the species composition will change, most likely impacting their associated fauna.

By gaining ground and increasing in biomass and productivity, kelps promote their associated fauna. For example, the annual growth rate of the sea urchin Strongylocentrotus droebachiensis is expected to increase with the increase in kelps [START_REF] Blicher | Growth and production of sea urchin Strongylocentrotus droebachiensis in a high-Arctic fjord, and growth along a climatic gradient (64 to 77°N)[END_REF]. However, distinction must be made between kelp species because the thallus morphology and species-specific interactions between macroalgae and associated epifaunal species largely affect the epifaunal composition [START_REF] Lippert | Macrofauna associated with macroalgae in the Kongsfjord (Spitsbergen)[END_REF]. For example, Laminaria digitata is not a preferred substrate for epiphytes due to its smooth surface; it therefore harbors a lower epiphytic diversity than Alaria esculenta [START_REF] Lippert | Macrofauna associated with macroalgae in the Kongsfjord (Spitsbergen)[END_REF]. By gaining ground, Kelps also decrease the abundance of taxa that are not associated with them such as sessile invertebrates and suspension feeders like ascidians, barnacles, and sea anemones [START_REF] Beuchel | Temporal patterns of benthic community development in an Arctic fjord (Kongsfjorden, Svalbard): results of a 24-year manipulation study[END_REF][START_REF] Paar | Temporal shift in biomass and production of macrozoobenthos in the macroalgal belt at Hansneset, Kongsfjorden, after 15 years[END_REF]. By promoting only certain species while excluding others, the expansion of kelp species will most likely cause an homogenisation of Arctic coastal benthic communities (Węslawski et al., 2010;[START_REF] Csapó | Coming home -Boreal ecosystem claims Atlantic sector of the Arctic[END_REF].

Coralline algae

Few studies have been carried out on the impacts of climate change on coralline algae. PAR decrease, as well as local hyposalinity, reduce the photosynthetic efficiency, reproduction and carbonate accumulation capacities of coralline algae [START_REF] Schoenrock | Influences of salinity on the physiology and distribution of the Arctic coralline algae, Lithothamnion glaciale (Corallinales, Rhodophyta)[END_REF][START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF]. Ocean acidification and warming may result in a fast decrease in calcification rate and percent cover which could impact the associated fauna (Fig. 4; [START_REF] Teichert | Arctic rhodolith beds and their environmental controls (Spitsbergen, Norway)[END_REF][START_REF] Chan | Recent density decline in wild-collected subarctic crustose coralline algae reveals climate change signature[END_REF]. The distribution of Arctic calcified algae will be constrained by the upper temperature limit of the species in the South and by calcium carbonate saturation state in the North [START_REF] Steinacher | Imminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model[END_REF][START_REF] Chan | Recent density decline in wild-collected subarctic crustose coralline algae reveals climate change signature[END_REF]. Also, competition for space with kelps may hasten their demise. This will lead to ecosystem restructuring and loss of functions resulting from the possible disappearance of current subarctic coralline algae and their associated communities before the end of the century [START_REF] Brodie | The future of the Northeast Atlantic benthic flora in a high CO2 world[END_REF].

Boreal species

The North Atlantic West Spitsbergen Current provides ca. 70% of water inputs into the Arctic Basin [START_REF] Dickson | Possible predictability in overflow from the Denmark Strait[END_REF]. The Bering Sea is also an input source to the Beaufort and Chukchi Seas (Węslawski et al., 2011). In recent decades, warming combined with increased advection induced poleward migrations bringing thermal tolerant boreal-subarctic species in the Arctic Ocean (Fig. 3h; Węslawski et al., 2010). Furthermore, in response to the loss of Arctic sea ice, shipping transport along ice-free routes in the Canadian and Russian Arctic regions has increased [START_REF] Lasserre | Polar super seaways? Maritime transport in the Arctic: an analysis of shipowners' intentions[END_REF]. This will likely have a major role in the introduction of marine species in the Arctic [START_REF] Jueterbock | The fate of the Arctic seaweed Fucus distichusunder climate change: an ecological niche modeling approach[END_REF].

Migration may have caused the reappearance of the bivalve Mytilus edulis in Isfjorden (Svalbard) after more than 1,000 years of absence (Berge andJohnsen 2005, Węslawski et al., 2010). The boreal kelp Laminaria hyperborea has also been recorded in the southern fjords of Spitsbergen, revealing its poleward migration progress [START_REF] Peltikhina | Distribution peculiarities and stocks of Laminaria algae in the area of Isfjord of the Spitsbergen Archipelago[END_REF]. Furthermore, over the last 20 years, at least 20 decapod species have expanded northward [START_REF] Brattegard | Distribution of marine, benthic macro-organisms in Norway: a tabulated catalogue[END_REF]. Migration has also been described in the Pacific Arctic [START_REF] Grebmeier | A major ecosystem shift in the Northern Bering Sea[END_REF][START_REF] Sirenko | Unusual abundance of macrobenthos and biological invasions in the Chukchi Sea[END_REF], in Kongsfjorden [START_REF] Beuchel | Long-term patterns of rocky bottom macrobenthic community structure in an Arctic fjord (Kongsfjorden, Svalbard) in relation to climate variability (1980-2003)[END_REF]Kędra et al., 2009) andin Hornsund (Węslawski et al., 2017).

New dominant species with a different ecosystem engineering role can alter local functional diversity [START_REF] Zacher | Temperature mediates the outcome of species interactions in early life-history stages of two sympatric kelp species[END_REF]. Moreover, new species combinations and interactions modify species distributions [START_REF] Pecl | Biodiversity redistribution under climate change: Impacts on ecosystems and human well-being[END_REF]. Nevertheless, according to [START_REF] Renaud | Arctic Sensitivity? Suitable Habitat for Benthic Taxa Is Surprisingly Robust to Climate Change[END_REF], the Arctic benthic species are unlikely to be fully replaced by boreal species because endemic species are not more vulnerable than boreal species to warming and acidification. Jueterbock (2016) predicted that habitat suitability for Fucus serratus, Fucus vesiculosus, and Ascophyllum nodosum would increase northward by 2100, particularly in Canada, Greenland, and Spitsbergen. They could be present at F. distichus' upper and lower zonation limits in the Arctic but these temperate species would not be able to compete with it because of F. distichus has competitive advantage such has being adaptated to cold conditions and long dark periods [START_REF] Jueterbock | The fate of the Arctic seaweed Fucus distichusunder climate change: an ecological niche modeling approach[END_REF]. Benthic taxa have restricted mobility and range changes are more likely to differ between taxon due to life-span, and dispersal capacity [START_REF] Renaud | Arctic Sensitivity? Suitable Habitat for Benthic Taxa Is Surprisingly Robust to Climate Change[END_REF].

As early as the 1930s, a documented warming of North Atlantic waters induced a major northward migration of boreal species [START_REF] Drinkwater | The regime shift of the 1920s and 1930s in the north Atlantic[END_REF]). [START_REF] Blacker | Benthic Animals as Indicators of Hydrographic Conditions and Climatic Change in Svalbard Waters[END_REF] comprehensive shelf and coastal waters survey revealed changes in species distribution within the Arctic, with the northward migration of boreal echinoderms and decapods. However, only northward changes and increases in the abundance of southerly species were reported. In this early study, no new species was found in the Arctic region. [START_REF] Merzouk | Kelp distribution in the Northwest Atlantic Ocean under a changing climate[END_REF] were unable to document any significant change in dominant kelp species composition or abundance since the 1950s along the northwest Atlantic coast. Similarly, [START_REF] Adey | Elucidating marine biogeography with macrophytes: quantitative analysis of the North Atlantic supports the thermogeographic model and demonstrates a distinct subarctic region in the Northwestern Atlantic[END_REF] found no major changes in the distribution of subtidal algal species in Canada's eastern subarctic or boreal regions over the last 40 years. These observations suggest northward expansions are occurring slower than predicted or are being buffered by other factors [START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF]. Extensive spatial gaps between suitable substrate, slow dispersal of propagules, water temperature, changes in turbidity, sea ice cover, and light penetration all influence the northward range expansion of kelps [START_REF] Wernberg | Status and trends for the world's kelp forests[END_REF]. At least in the short-term, the northern range expansions of boreal kelps across the Arctic may be overestimated [START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF].

Endemic Arctic species cope with environmental changes while also facing increasing competition pressure from invasive species, risk of infection by diseases, and predator-prey interactions with higher herbivore pressure (Węslawski et al., 2010). Borealization is expected to increase biodiversity in the short-term with coexisting inhabitants and competitors [START_REF] Renaud | Arctic Sensitivity? Suitable Habitat for Benthic Taxa Is Surprisingly Robust to Climate Change[END_REF]. Some species may decline, other species or ecosystems may flourish to the benefit of coastal zones [START_REF] Bartsch | Changes in kelp forest biomass and depth distribution in Kongsfjorden, Svalbard, between 1996-1998 and 2012-2014 reflect Arctic warming[END_REF]. New species interactions such as competition and preypredator interactions are occurring and are yielding to new ecosystem functioning. solidungula is present at the lower sub-littoral zone [START_REF] Hop | The marine ecosystem of Kongsfjorden, Svalbard[END_REF][START_REF] Wiencke | Sensitivity and acclimation to UV radiation of zoospores from five species of Laminariales from the Arctic[END_REF]).

Coralline algae are at the lower limit of kelp forest and deeper. Ice melt and increased river runoff increase turbidity. Future: increase in precipitation, wind-speed, river runoff and sediment resuspension are increasing the turbidity locally. The surface cover of F. distichus doubled while UVR rates limit it to the lower intertidal zone giving ground to other fucoid species. Longer icefree periods allow kelp expansion in new areas except for L. solidungula who retreated with warming. The coralline algae cover is reduced due to ocean acidification and competition with other macroalgae.

Conclusion

Coastal macroalgal communities in the Arctic are profoundly affected by increasing temperature, coastal erosion, freshwater input, coastal turbidity and sedimentation as well as decreasing ice cover and seawater pH. Changes in seasonality and food-web structure will likely result in major ecosystem restructuring [START_REF] Morata | Climate change and diminishing seasonality in Arctic benthic processes[END_REF][START_REF] Bonsell | Slow community development enhances abiotic limitation of benthic community structure in a high Arctic kelp bed[END_REF]. In colder coastal Arctic regions, kelp forests may become more productive (Krause-Jensen and [START_REF] Krause-Jensen | Expansion of vegetated coastal ecosystems in the future Arctic[END_REF][START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF]. Also, fucoids may largely extend in the Arctic whereas coralline algae may be more restricted under future conditions. The amount of detritus produced should rise which would affect the ecosystem functioning by damping fluctuations in seasonal carbon supply [START_REF] Norkko | Trophic structure of coastal antarctic food webs associated with changes in sea ice and food supply[END_REF]. This could result in greater resilience of the benthos to variation of interannual pelagic primary production and more stable food-chains [START_REF] Norkko | Trophic structure of coastal antarctic food webs associated with changes in sea ice and food supply[END_REF][START_REF] Renaud | The future of Arctic benthos: Expansion, invasion, and biodiversity[END_REF].

With global change, trawling and overfishing, sea urchins have become very abundant in some areas overgrazing the kelp forest [START_REF] Konar | Biological interactions maintain the boundaries between kelp forests and urchin barrens in the Aleutian Archipelago[END_REF]. The disappearance of kelp forests impacts the whole ecosystem including species that feed on kelps (bristle worms, amphipods, prawns, snails, and brittle stars), species that feed on these herbivores (including species important to commercial fisheries: fishes, lobsters) -and/or species using kelp forest as a refuge area (sea lions, seals, gray whales, sea otters and even seabirds such as gulls or terns; [START_REF] Watson | Stability, resilience, and phase shifts in rocky subtidal communities along the west coast of Vancouver Island, Canada[END_REF]. In some areas, like around Tromsø, the ecosystem has collapsed into a bare and depauperate new state (Comeau, unpublished observations).

Many factors, including their interactions, are not yet understood, making it difficult to predict future coastal Arctic macroalgal communities and the whole ecosystem. It is likely that considerable ecological changes will occur with both positive and negative consequences on associated benthic species [START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF]. Although large-scale warming, acidification and ice melting are on the way, changes at the local scale are also critical for benthic communities [START_REF] Bonsell | Slow community development enhances abiotic limitation of benthic community structure in a high Arctic kelp bed[END_REF]. For example, in sheltered coastal areas, long-term ice changes could be more moderate than in exposed coastal areas [START_REF] Bonsell | Long-term patterns of benthic irradiance and kelp production in the central Beaufort Sea reveal implications of warming for Arctic inner shelves[END_REF].

The extent to which global change will affect benthic communities may differ depending on depth and regions (Węslawski et al., 2010).

Predicting the future of Arctic ecosystems is imperative to anticipate and to adapt to the coming changes [START_REF] Krause-Jensen | Expansion of vegetated coastal ecosystems in the future Arctic[END_REF][START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF]. Nonetheless, the baseline knowledge on ecosystem structure and functioning in the Arctic is limited on both geographical and temporal scales [START_REF] Wassmann | Footprints of climate change in the Arctic marine ecosystem: footprints of climate change[END_REF]. Long-term time series combining environmental conditions and community composition and distribution are essential but particularly scarce in the Arctic, limiting projections of future benthic communities [START_REF] Piepenburg | Recent research on Arctic benthos: common notions need to be revised[END_REF][START_REF] Müller | Impact of oceanic warming on the distribution of seaweeds in polar and cold-temperate waters[END_REF]. Efforts must be made to improve knowledge in key regions with proper seasonal coverage to provide sound scientific advice on ecosystem management [START_REF] Wassmann | Footprints of climate change in the Arctic marine ecosystem: footprints of climate change[END_REF][START_REF] Degen | Patterns and drivers of megabenthic secondary production on the Barents Sea shelf[END_REF][START_REF] Filbee-Dexter | Arctic kelp forests: diversity, resilience and future[END_REF].

The poor knowledge about the interactions between multiple abiotic and biotic stressors, changes in climate-sensitive populations over time (including processes of acclimation and adaptation) and the general paucity of baseline species diversity and biogeographic distributions data are a significant stumbling block for understanding the current situation as well as to foresee future changes (Muth et al., 2009;[START_REF] Harley | Effects of climate change on global seaweed communities[END_REF]. Multifactorial studies at the community level that take into account interactions between stressors and between species are desperately lacking [START_REF] Scherrer | Mechanistic model identifies increasing light availability due to sea ice reductions as cause for increasing macroalgae cover in the Arctic: light causes arctic macroalgal increase[END_REF][START_REF] Renaud | Arctic Sensitivity? Suitable Habitat for Benthic Taxa Is Surprisingly Robust to Climate Change[END_REF]. Only 52% of ex situ studies on Arctic kelps, 25% on coralline algae and 0% on fucoids focus on the impact of at least 2 parameters involved in global change. Beyond 2 parameters, the percentages drop to 2.4 and 0% for kelps and coralline algae respectively (Table 1).

Through changes in weather and seasonal conditions and changes in biodiversity, global environmental change affects the local cultural, social, and economic life of indigenous people [START_REF] Hovelsrud | Arctic societies, cultures, and peoples in a changing cryosphere[END_REF]. It has also a critical impact on global markets through enhanced access to mineral resources, fisheries, and industrialization (e.g., [START_REF] Ahdr | Arctic Human Development Report[END_REF]IPCC 2007). Predicting and anticipating the future Arctic ecosystem is now more than ever a priority to adapt to fundamental changes in all sectors of the economy and society. quantum yield: significant interaction between radiation and exposure time, as well as significant radiation effects Germination success moderately positively correlated with initial Fv/Fm Laeseke et al., 2019 temp: 4, 8, 9, 13, 15, 17, 21 ± 1°C salinity: 20, 28, 34 light: PAR (46 ± 8 and 80 ± 10 µmol m-2.s-1), PAR + UVA (7 ± 1 W m-2), PAR + UVA + UVB (0. 5,10,15,20,25,34,45,55,60 quantum yield: -at salinity= 5 after 2 and 5 days of treatment

  search string 'Benthos AND Arctic AND change', 'Benthos AND Arctic AND climate', 'Benthic AND Arctic AND change', 'Benthic AND Arctic AND climate', 'Kelp AND Arctic AND change', 'Kelp AND Arctic AND climate, 'Fucoid AND Arctic AND change', 'Fucoid AND Arctic AND climate, 'Coralline AND Arctic AND change', 'Coralline AND Arctic AND climate' (database searches completed on

Figure 1 :

 1 Figure 1: Cumulative number of scientific papers focusing on the impact of global change on benthic communities through both ex situ and in situ studies in the Arctic since 1957.

Figure 2 :

 2 Figure 2: Number of in situ and ex situ studies per location. A) whole Arctic region, B) focus on the Svalbard coast.

Figure 3 :

 3 Figure 3: diagram of cascading effects of global change in the benthic coastal Arctic environment. The impact of processes framed in blue on benthic organisms are developed in section 5. A plus (+) and a red arrow mean a positive effect, a minus (-) and a blue arrow mean a negative effect, and a (+/-) and a green arrow mean an effect that could be both positive or negative.

Figure 4 :

 4 Figure 4: Schematic representation of the present and expected future Arctic coastal macroalgae communities. Present: fucoids dominate intertidal algae assemblages, while kelps dominate subtidal algae assemblages until they reach their low light threshold. Laminaria

  (7 W.m-2), PAR + UVA + UVB(0.7 W.m-2)

  (21.8 µmol m-2s-1), PAR + UVA (5.65 W.m-2), PAR + UVA + UVB (0.47 W.m-2) quantum yield: -under light exposure, even more under UVA & UVB Roleda et al., 2006b light: PAR (5.5±1.2, 20.6±3.0 µmol m-2s-1), PAR + UVA (10.9±0.4, 11.6±1.9 W.m-2), PAR + UVA + UVB (0.45±0.0, 0.48±0.0 W.m-2) phlorotannin content: higher within the low PAR+UV-A treatment than under high PAR+UV-A exposure Steinhoff et al., 2012 light: PAR (from 0.23 to 61.8 W.m-2) + UVA (from 0 to 1.9 W.m-2) + UVB (from 0 to 0.018 W.m-2)

  (60 µmol m-2s-1), PAR + UVA (7 W.m-2), PAR + UVA + UVB (0.7 W.m-2) quantum yield: specimens from shallow water recover to a greater extent than those from deeper waters[START_REF] Bischof | Acclimation of brown algal photosynthesis to ultraviolet radiation in Arctic coastal waters (Spitsbergen, Norway)[END_REF] light: PAR (20, 60, 800 µmol m-2s-1), PAR + UVA (8 W.m-2), PAR + UVA + UVB (0.8 W.m-2) quantum yield: -to 25% after 4 h of exposure to low light and UV during the first 2

Table 1 :

 1 Conditions of temperature, light, salinity and pCO2 tested on major fucoid, kelp and coralline algae species found in the Arctic

  PAR (4.62 ± 0.4 and 86.7 ± 19.9 µmol m-2s-1), PAR + UVA (5.31 ± 0.58 and 4.76 ± 2.9 W.m-2), PAR + UVA + UVB (0.41 ± 0.01 and 0.12 ± 0.0 W.m-2) PAR (28.8 ±5.05 µmol m-2s-1), PAR + UVA (8.22 ± 0.64 W.m-2), PAR + UVA + UVB (1.27 ± 0.12 W.m-2) germination: -3 after 16h under UVA & UVB, recovery after 6 days[START_REF] Wiencke | Sensitivity and acclimation to UV radiation of zoospores from five species of Laminariales from the Arctic[END_REF] light: PAR (21.8 µmol m-2s-1), PAR + UVA (5.65 W.m-2), PAR + UVA + UVB(0.47 W.m-2)

		Saccorhiza dermatodea	gametophyte
			sporophyte	light light		light: quantum yield: -with increasing total fluence quantum yield: specimens	germination rate: no significant effect of germination: strong dose-dependent inhibitory effect of UVA & UVB temperature and seasonality	UVAB-susceptibility: highest at 2 °C and the beginning of July, not affected by seasonality at 7 °C	Olischläger and Wiencke, 2013 Roleda et al., 2006a,b Bischof et al., 1998
				temperature		light: PAR (60 µmol m-2s-1),	from shallow water recover to
					PAR + UVA (7 W.m-2), PAR + UVA + UVB (0.7 W.m-2)	a greater extent than those
							from deeper waters
						light: PAR (30 µmol m-2s-1),	photosynthesis: -after 6h of	growth rate: -under UV	Aguilera et al., 1999
					PAR + UVA (7 W.m-2), PAR + UVA + UVB (0.7 W.m-2)	exposure to UV radiations	radiations
				light				growth rate: increased with	Marakov et al., 1999
								increasing photoperiod (from
					light: 12:12 h L:D (PAR : 5-10 W.m-2), continuous daylight		12:12 to 24:0, L:D) and decreased with shortening
								the light phase (from 24 to 12
								hours).
				salinity		salinity: 5, 10, 15, 20, 25, 34, 45, 55, 60	10 after 2 days of treatment quantum yield: -at salinity= 5,		Karsten et al., 2007
		Hedophyllum nigripes	gametophyte	temperature		temp: 0, 5, 10, 15, 18, 19, 21, 22 ± 0.5 °C		days gametophytes died after 14 At 21 and 22 °C, all	0.24±0.03, day 14) temperatures overtime (ratio: decreased with increasing abundance of females	Franke et al., 2021
			sporophyte	temperature		temp: 0, 5, 10, 15, 18, 19, 21, 22 ± 0.5 °C		15°C growth rate: + at 10°C, -at	Franke et al., 2021
	Coralline algae	Lithothamnion glaciale	sporophyte	temperature, salinity		temp: 4, 7°C		calcification: reducted at low	metabolic processes:	Schoenrock et al., 2018
						salinity: 22, 33		salinity and temperature treatments	negatively influenced by low salinity
				pCO2	CO2 conditions: 390, 815, 975 and 1570 ppm		high CO2 treatments calcification: reduced under	Büdenbender et al., 2011
		Leptophytum foecundum	sporophyte	salinity		salinity: 10, 20, 30		salinity=10 calcification: higher at	salinity=10 higher pigment change at	Muth et al., 2020
		Phymatolithon tenue	sporophyte sporophyte	temperature, salinity, light light		light: PAR (100 µmol m-2 s-1), dark	temperature range of 13-17°C. photosynthesis: optimal		temperatures under the lowest applied UV inhibition observed only net dissolution in the dark active calcification and no	Fredersdorf et al., 2009 Hofmann et al., 2018
		Saccorhiza dermatodea	gametophyte	pCO2 light temperature, light light: salinity, light light, pCO2	CO2 conditions: 390, 1300 ppm temp: 2, 8, 11, 17°C	gross photosynthesis: no effect of CO2 conditions quantum yield: drop after 2 after 4 more days days under UVR, recovery	growth rate: positively affected by CO2 enrichment germination rate: reduced under PAR+UV-A+UV-B only treatment (-21.4% and 9.6% at 17°C and 7°C outdoor UVR and CO2 enrichment growth rate: + 57% under	decreased respiration and lower accumulation of biostorage molecule under CO2 enrichment condition specimens previously treated with hyposalinity are significantly less susceptible to UV after 8h of exposure phlorotannin content: no effect of light treatments	Karsten et al., 2007 Iñiguez et al., 2016b Steinhoff et al., 2011b Springer et al., 2017 Gordillo et al., 2015

Funding

This study was conducted in the frame of the project FACE-IT (The Future of Arctic Coastal Ecosystems -Identifying Transitions in Fjord Systems and Adjacent Coastal Areas). FACE-IT has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 869154.

Table S1: articles focusing on the responses of shallow Arctic macroalgal communities to global change. The article types in situ and ex situ respectively correspond to studies conducted in the field and in the laboratory. The "none" type includes review articles and modeling exercises.