E Lupon

U Turrian

J Malloizel-Delaunay

A Bura-Rivière

J L Grolleau

cicatrisation des plaies : une étude descriptive multicentrique entre février et avril 2018

Keywords: healing, residents, wound, dressing, medical studies

Résumé :

Objectif : En France on recense 2,5 millions de plaies, qui représentent en termes de coût et de qualité de vie du patient, un véritable problème de Santé Publique. C'est au statut d'interne que le médecin peut être amené pour la première fois à prendre en charge la cicatrisation de plaies. Cette enquête réalisée auprès des internes français fait un état des lieux de la formation des internes dans ce domaine.

Matériels et méthodes :

Résultats Données démographiques

Sept-cent-onze questionnaires ont été complétés. Plus de la moitié des internes étaient en médecine générale (57 %), 25 % en spécialité médicale et 18 % en spécialité chirurgicale.

Les internes ayant répondu au questionnaire ont en moyenne effectuée 2,25 années d'internat.

Les subdivisions les plus représentées sont Toulouse (29,7 %), Angers (10 %) et Rennes (8,9 %).

Intérêt et exposition à la discipline

Conclusion

Cette étude montre pour la première fois la perception de manque de formation des internes en médecine dans le domaine de la cicatrisation des plaies en France. Elle permet également de souligner l'intérêt de tous les internes en médecine dans le domaine et leur désir de

  et cicatrisation des plaies : une étude descriptive multicentrique entre Février et Avril 2018 Medical residents and wound healing: A French national survey Titre courant : Internes en médecine et cicatrisation des plaies Auteurs : (Noms, prénoms dans l'ordre, titres et fonctions, affiliations) E Lupon, U Turrian, J Malloizel-Delaunay, A Bura-Rivière, JL Grolleau 5 auteurs : -Elise LUPON : interne en chirurgie plastique -Université Toulouse III Paul Sabatier -Service de chirurgie plastique -1 Avenue Jean Poulhès -31059 Toulouse Cedex 9, France -Ulysse TURRIAN : interne en médecine vasculaire -Université Toulouse III Paul Sabatier Service de médecine vasculaire -1 Avenue Jean Poulhès, 31059 Toulouse Cedex 9, France -Julie MALLOIZEL-DELAUNAY : Praticien hospitalier -Service de médecine vasculaire -1 Avenue Jean Poulhès, 31059 Toulouse Cedex 9, France -Alessandra BURA-RIVIÈRE : Professeur universitaire -Service de médecine vasculaire -1 Avenue Jean Poulhès, 31059 Toulouse Cedex 9, France -Jean Louis GROLLEAU : Professeur universitaire -Service de chirurgie plastique -1 Avenue Jean Poulhès, 31059 Toulouse Cedex 9, France

Figure 1 .Figure 1 :Figure 2 .Figure 2 .

 1122 Figure 1. Intérêt des internes pour la prise en charge des plaies et la cicatrisation Figure 1: Interns' interest in wound management and healing

Figure 3 .Figure 3 :

 33 Figure 3. Opinion sur la formation concernant la cicatrisation des plaies Figure 3: Opinion on training on wound healing

Figure 1 .Figure 2 .Figure 3 .

 123 Figure 1. Intérêt des internes pour la prise en charge des plaies et la cicatrisation

  Un questionnaire standardisé unique a été envoyé aux internes de médecine. Les questions concernaient les caractéristiques de l'interne, son intérêt, sa mise en pratique et sa formation dans la prise en charge des plaies, ainsi que l'opinion des intéressés sur leur formation.IntroductionLe nombre de personnes affectées par des plaies est croissant en France, estimé actuellement à environ 2,5 millions. L'étiologie des plaies est multifactorielle et conséquence du vieillissement de la population et du mode de vie. La prise en charge des plaies représente en termes de coût et de qualité de vie un véritable problème de Santé Publique[START_REF] Meaume | Workload and prevalence of open wounds in the community: French Vulnus initiative[END_REF].

	La HAS préconise que, quel que soit le type de plaie, son traitement soit d'abord celui de son
	étiologie. La prescription sur ordonnance (s'il y a lieu) de chaque type de pansement doit être
	la plus précise possible. [2]
	C'est en tant qu'interne que le médecin est amené pour la première fois à soigner des patients
	atteints de plaies et parfois à en coordonner la prise en charge [3].
	Le soignant accompagnant ces malades se doit de développer des compétences multiples :
	biomédicales et particulièrement thérapeutiques, relationnelles et pédagogiques. [4]. Nous
	nous sommes intéressés à la formation des jeunes internes.
	L'objectif de cette étude est d'évaluer la perception des internes français sur leur formation
	dans le domaine des plaies et de la cicatrisation et l'intérêt qu'ils y portent.
	Matériel et méthode
	Nous avons créé un questionnaire standardisé unique pour cette étude, destiné aux internes en
	médecine exerçant en France.
	Le questionnaire a été diffusé via un courriel envoyé sur les groupes d'internes en médecine
	des 28 CHU français, sur un réseau social entre février et avril 2018.
	Le questionnaire a été diffusé en collaboration avec les présidents des internats de chaque
	subdivision, identifié via un site d'information pour les internes et via le guide des villes et
	des spécialités créé par l'Inter Syndicale Nationale des Internes (ISNI). [5]

Résultats : Sept-cent-onze internes ont répondu. La prise en charge des plaies est considérée un sujet important, souvent considéré comme indispensable pour les sujets interrogés. La majorité des internes (69 %) considèrent que tous les médecins sont concernés. La formation en matière de prise en charge des plaies et de cicatrisation est jugée insuffisante (94 %). En grande majorité (79 %), les internes n'ont jamais suivi de formation sur la prise en charge des plaies et la cicatrisation. Les internes sont en demande d'une formation supplémentaire (98 %).

Conclusion : La prise en charge des plaies et la cicatrisation est un sujet qui intéresse les internes. Les internes sont très favorables à une formation lors de leur cursus pour améliorer la réalisation de cette pratique, à laquelle ils sont confrontés et devant laquelle ils déclarent être souvent en difficulté.

Mots clés : cicatrisation, internes, plaie, pansement, études médicales

Ce questionnaire était auto-déclaratif et anonyme et comportait deux parties (annexe I).

La première partie concernait les caractéristiques de l'interne répondant (spécialité, année d'étude et subdivision d'affectation).La deuxième partie portait sur le sujet d'étude spécifiquement : la proximité des internes avec la prise en charge des plaies et la cicatrisation (intérêt du sujet général et personnel, fréquence d'exposition). Étant donné qu'un médecin, quelle que soit sa spécialisation, est la personne compétente pour donner son avis sur tout type de plaies, l'étude concernait la cicatrisation des plaies de toutes étiologies confondues.

L'analyse descriptive a été faite en indiquant la moyenne et/ou l'écart-type pour les variables quantitatives et le pourcentage pour les variables qualitatives.

Les réponses aux différentes questions ont été comparées selon la catégorie professionnelle de chaque répondants (médecine générale, spécialité médicale, spécialité chirurgicale) à l'aide d'un test de Chi 2 . Le seuil de significativité statistique a été fixé à p≤0,05.

Les analyses ont été réalisées avec le logiciel Excel.

  La majorité des internes interrogés (98,7 %), toutes spécialités confondues, estiment que la prise en charge des plaies et la cicatrisation est importante ou indispensable. Le détail des réponses en fonction du statut professionnel est présenté sur la figure1. 'autre part, la thématique de la prise en charge des plaies est plus que jamais d'actualité dans la société. Ceci est notamment lié au problème des plaies chroniques qui s'accentue avec le vieillissement de la population et le développement des maladies métaboliques. Si la prise en charge s'améliore sur le territoire en matière de plaies et cicatrisation depuis la création de la Société française et francophone de cicatrisation en 1997, la création du premier diplôme de Plaies et Cicatrisation en 1998 et le développement des réseaux ville-hôpital[START_REF] Guyadec | Histoire du pansement[END_REF], les internes restent les premiers interlocuteurs à prendre en charge ces plaies à l'hôpital public, or ils n'ont pas de véritable formation dans le domaine.La majorité des internes questionnés (96 %) pensent que les médecins sont concernés. À l'analyse des résultats de cette étude, la formation en matière de prise en charge des plaies et de la cicatrisation est insuffisante (94 %). Les internes sont en demande d'une formation supplémentaire (98 %) pour la bonne réalisation de cette pratique, à laquelle ils sont confrontés et devant laquelle ils sont souvent en difficulté. L'analyse étant multicentrique avec un taux de participation élevé et une représentation de toutes les académies françaises, il semblerait que ce constat soit valable dans toute la France, effectivement les résultats sont uniformes dans toutes les villes de France questionnées et concernent autant les futurs médecins généralistes, futurs chirurgiens que les futurs spécialistes.L'analyse spécifique des réponses en fonction de la catégorie des internes révèle que les internes de chirurgie semblent être les plus concernés par le sujet, avec des chiffres significativement plus marqués dans ce sens par rapport aux 2 autres catégories d'internes.Différentes solutions pourraient aider à pallier ces lacunes avec notamment l'instauration de cours sur la cicatrisation des plaies dans le cursus universitaire de l'étudiant, auxquels les internes interrogés sont favorables. On peut également citer la diffusion de l'information sur l'existence des centres de plaies et de cicatrisation (télémédecine, centre de cicatrisation, équipe mobile de cicatrisation, applications mobiles, algorithmes décisionnels). La promotion de supports de formation à la prise en charge des plaies (e-learning, revues dédiées, cours en ligne) et la mise en place d'équipes mobiles de cicatrisation dans les CHU permettraient également d'agir dans ce sens.Si des carences en matière de formation dans la cicatrisation des plaies sont bien présentes, il existe d'autres domaines dans lesquels la formation des internes fait défaut. Ces lacunes ont des conséquences sur la prise en charge des patients dans leur pratique future, notamment dans le domaine des pathologies vasculaires. Ainsi, Niclot et al.[START_REF] Niclot | Identification des freins au dépistage de l'anévrisme de l'aorte abdominale en médecine générale : étude qualitative auprès de 14 médecins généralistes exerçant à Paris[END_REF] ont mis en évidence le retentissement péjoratif lié au manque de connaissance des médecins généralistes sur le dépistage de l'anévrisme de l'aorte abdominale. Une réactualisation continue de la formation des jeunes médecins est donc indispensable pour prendre en charge les patients de façon optimale.

	Opinion sur la formation dans le domaine
	La majorité (97 %) des internes éprouvent des difficultés à choisir le pansement adapté
	Plus de la moitié des internes (51,9 %) caractérisent la pratique d'« indispensable », et 68,5 % au stade de cicatrisation (figure 2).
	de ces internes appartiennent aux spécialités chirurgicales. Cette difficulté est fréquente pour 47 % d'entre eux. L'analyse spécifique montre que 54 %
	La majorité des internes questionnés (96 %) pensent que les médecins sont concernés des spécialités chirurgicales sont « parfois » en difficulté alors que 54,8 % des médecins
	par le sujet. Seulement 4 % des internes interrogés estiment que cette discipline ne concerne généralistes sont « souvent » en difficulté. Plus d'un quart des internes (28 %) sont
	que les infirmières. systématiquement en difficulté devant le choix du pansement adapté à une plaie.
	La fréquence de prescription de pansements est disparate en fonction de la spécialisation de Quatre-vingt-quatorze pour cent des internes déclarent leur formation insuffisante
	l'interne. pendant leur cursus. Ceci est moins marqué pour les spécialités chirurgicales (86 %) que pour
	les spécialités médicales (93 %) et les internes de médecine générale (97 %). Ces données
	sont rapportées sur la figure 3.
	Les internes de spécialité chirurgicale éprouvent significativement moins de difficultés devant
	cette situation que les autres catégories d'internes. Le plus souvent, ils éprouvent « parfois »
	des difficultés (54 % vs 15 % pour les internes d'autres spécialités) alors que les autres
	internes ressentent plus fréquemment « souvent », voire « systématiquement » des difficultés.
	Ils sont ceux ayant le plus souvent déjà suivi une formation (24 %, ce qui est très légèrement
	supérieur aux autres catégories d'internes) et les moins touchés par le sentiment
	Finalement, certains internes ont été formés à la discipline lors de congrès ou lors de la d'insuffisance de formation (avec cependant 86 % des internes percevant leur formation
	réalisation du diplôme universitaire sur la cicatrisation des plaies. comme insuffisante) et 92 % d'entre eux seraient favorables à une formation pendant
	Nous avons recensé quelques items inscrits au programme de l'internat traitant de la prise en l'internat (plus ou moins l'externat). Si la majorité des internes exercent une bonne pratique (74 %), plus du quart des étudiants ne Lorsqu'il s'agit d'initier ou d'adapter un protocole de soins, les internes de spécialités charge de plaies mais ces items sont spécifiques d'étiologies (item 329.8 « plaie des parties suivent pas les recommandations (26 %), dont 19 % avec une attitude à risque, délétère, par la chirurgicales sont les plus autonomes devant cette situation. Effectivement, 33 % d'entre eux
	prescription d'antiseptiques sur la plaie, à chaque réfection du pansement. molles », item 137 « ulcère de jambe », item 116 « complication de l'hypomobilité et du choisissent seuls le pansement le plus adapté. Ce taux est trois fois supérieur à celui rencontré
	décubitus »). chez les internes des autres catégories. Plus de la moitié des internes interrogés (58 %) ne connaissent pas les centres de Paradoxalement, les internes de spécialités chirurgicales sont les moins informés sur
	l'existence des plaies et cicatrisation (69 % contre 54 % des internes de médecine générale).

Ainsi, 35,8 % des internes, toutes spécialités confondues, prescrivent souvent des pansements et 46,6 % de façon plus occasionnelle (« de temps en temps »). Une faible minorité n'y est jamais exposée (6,1 %).

Mise en pratique

Les stratégies adoptées devant une plaie sont rapportées par les internes, toutes catégories confondues, de la manière suivante : choix du pansement laissé à l'infirmière (44,6 %), utilisation d'un référentiel (24,8 % dont 32 % des internes de médecine générale), appel à un spécialiste (18,7 %, dont 31 % d'internes de spécialités médicales), choix seul (11,9 %).

Les recommandations proposent dans le traitement d'une plaie chronique, son nettoyage par du savon et de l'eau, et de ne pas utiliser d'antiseptiques

[START_REF] Barrois | Faut-il utiliser les antiseptiques sur les plaies chroniques ?[END_REF]

.

plaies et de cicatrisation. Les internes de spécialités chirurgicales sont les moins informés (69 %, contre 54 % des internes de médecine générale).

En grande majorité (79 %), les internes n'ont jamais suivi de formation sur la prise en charge des plaies et la cicatrisation.

Les internes sont quasiment unanimes sur leur désir de formation dans le domaine.

Effectivement, 98 % des internes interrogés trouveraient utile d'avoir davantage de formation.

Parmi ces internes, 93 % seraient favorables à une formation pendant l'internat (plus ou moins l'externat). Plus précisément, un peu plus de la moitié des internes (52 %) seraient favorables à une formation lors de l'externat puis lors de l'internat. Quarante-et-un pour cent des internes ne souhaiteraient une formation que pendant l'internat, ceci est surtout marqué pour les internes de spécialités chirurgicales (55,4 %).

Discussion

Au décours de sa carrière, tout médecin peut être amené à donner un avis sur une plaie et il doit être capable de mettre en place les premières mesures du traitement (comme le choix d'un pansement adapté) et savoir adresser le patient au bon interlocuteur, si celle-ci nécessite une expertise particulière. C'est pourquoi l'étude se voulait générale et englobait les plaies de toutes étiologies confondues (brûlures, plaies aiguës, plaies postopératoires, plaies chroniques d'origine vasculaire, escarres..).

D

Seulement 21 % des internes interrogés ont affirmé avoir déjà effectué une formation dans le domaine de la cicatrisation des plaies. Le détail de cette formation était renseigné dans le questionnaire. Ainsi les internes ayant déclaré avoir bénéficié d'une formation l'ont acquise au travers de cours lors de stage (par des infirmières, des médecins référents de différentes spécialités ou des laboratoires), lors de cours de DESC et lors de conférence de préparation à l'ECN pendant l'externat.

Certains internes déclaraient un apprentissage personnel (les exemples cités comprenaient l'EMC, les recommandations de la HAS, des cours en ligne..) et lors du suivi de seniors et infirmières compétents au cours de certaines consultations dédiées.

Cette enquête révèle que la prise en charge des plaies est un sujet important pour les internes, souvent considéré comme indispensable.

Effectivement ce sont eux qui accordent le plus d'importance à la discipline, puisqu'ils sont ceux qui caractérisent le plus la discipline « d'indispensable » (69 %).

Ce sont les internes les plus exposés et pour 43 % des internes de spécialité chirurgicale, la prescription des pansements est quotidienne (vs respectivement 1 % et 14 % des internes de médecine générale et de spécialité médicale).

La limite de cette enquête est constituée par une surreprésentation des internes de certaines villes : Toulouse, Rennes, Angers et un défaut de représentation de certains grands centres français. Ce biais est probablement lié à une proximité des investigateurs de l'étude avec les internes de ces villes.

QUESTIONNAIRE PLAIE ET CICATRISATION :

Spécialité (préciser la sous-spécialité) : ……………………………………. Année d'internat : ……………………………………. Ville (lieu de l'internat) : …………………………….


Artem G Lim

Jeroen E Sonke

Ivan V Krickov

Rinat M Manasypov

Sergey V Loiko

Oleg S Pokrovsky
email: oleg.pokrovsky@get.omp.eu

Enhanced particulate Hg export at the permafrost boundary, western Siberia

Keywords: particulate, suspended, landscape, bog, lake, forest, thaw, Siberia

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The high-latitude critical zone is subject to changes in hydrological regime [START_REF] Bring | Arctic terrestrial hydrology: A synthesis of processes, regional effects, and research challenges[END_REF][START_REF] Wrona | Transitions in Arctic ecosystems: Ecological implications of a changing hydrological regime[END_REF] due to widespread permafrost thaw. Climate warming also stimulates organic carbon (OC) release [START_REF] Vonk | Reviews and syntheses: Effects of permafrost thaw on Arctic aquatic ecosystems[END_REF], and many trace compounds including toxic mercury (Hg) are tightly linked to OC, both in soils and in river waters (Regnel and Hammar, 2004;[START_REF] Zhang | Biogeochemical drivers of the fate of riverine mercury discharged to the global and Arctic oceans[END_REF][START_REF] Kohlenberg | Controls on boreal peat combustion and resulting emissions of carbon and mercury[END_REF][START_REF] Olson | Mercury in Active-Layer Tundra Soils of Alaska: Concentrations, Pools, Origins, and Spatial Distribution[END_REF][START_REF] St | Unprecedented increases in total and methyl mercury concentrations downstream of retrogressive thaw slumps in the western Canadian Arctic[END_REF].

Determination of the river Hg flux to the Arctic Ocean has been a topic of active research, since it is of similar magnitude as Hg atmospheric deposition in the Arctic Ocean Hg budget [START_REF] Amos | Global biogeochemical implications of mercury discharges from rivers and sediment burial[END_REF][START_REF] Dastoor | Arctic Ocean: Is it a sink or a source of atmospheric mercury?[END_REF][START_REF] Fisher | Riverine source of Arctic Ocean mercury inferred from atmospheric observations[END_REF]. [START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF] recently constrained a comprehensive pan-arctic river Hg budget that shows equal particulate Hg (PHg) and dissolved (DHg) fluxes to the Arctic Ocean of 22 Mg y -1 each. Particulate organic carbon (POC) contributes substantially to the total OC export from continents to the ocean [START_REF] Schlesinger | Transport of organic carbon in the world's rivers[END_REF][START_REF] Lal | Soil erosion and the global carbon budget[END_REF][START_REF] Lamoureux | Seasonal fluxes and age of particulate organic carbon exported from Arctic catchments impacted by localized permafrost slope disturbances[END_REF][START_REF] Galy | Global carbon export from the terrestrial biosphere controlled by erosion[END_REF][START_REF] Li | The carbon flux of global rivers: a re-evaluation of amount and spatial patterns[END_REF], and a two-fold increase of Arctic river POC flux by 2100 has been predicted [START_REF] Gordeev | River flux of dissolved organic carbon (DOC) and particulate organic carbon (POC) to the Arctic Ocean: what are the consequences of the global changes?[END_REF].

Consequently, the anticipated increase in POC flux implies that the riverine export of PHg to the coastal zone of the Arctic Ocean may also strongly increase. However, the magnitude of this increase is likely region-specific and depends on a number of environmental parameters such as permafrost coverage, lithology, and the presence of forest, wetlands and lakes in the watershed. Understanding how these parameters affect POC and PHg dynamics is critical for predicting of climate change impacts on Hg transport from the land to the Arctic Ocean.

Despite recent efforts in characterizing PHg fluxes in large Arctic Rivers [START_REF] Coquery | The distribution of dissolved and particulate mercury in three Siberian estuaries and adjacent Arctic coastal waters[END_REF][START_REF] Amos | Global biogeochemical implications of mercury discharges from rivers and sediment burial[END_REF][START_REF] Kirk | Multiyear total and methyl mercury exports from two major sub-Arctic rivers draining into Hudson Bay, Canada[END_REF][START_REF] Leitch | The delivery of mercury to the Beaufort Sea of the Arctic Ocean by the Mackenzie River[END_REF][START_REF] Carrie | Determination of mercury biogeochemical fluxes in the remote Mackenzie River Basin, northwest Canada, using speciation of sulfur and organic carbon[END_REF]Quemerais et al., 2009;[START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF], these studies do not identify the factors controlling PHg fluxes in high latitude rivers, which depend on the size of the river watershed and its landscape (physio-geographical) parameters. In this regard, large continental plains such as the western Siberia Lowland (WSL) which contains huge reservoir of frozen and thawed organic carbon and metals such as Hg [START_REF] Sheng | A high-resolution GIS-based inventory of the west Siberian peat carbon pool[END_REF][START_REF] Stepanova | Major and trace elements in peat profiles in Western Siberia: impact of the landscape context, latitude and permafrost coverage[END_REF][START_REF] Schuster | Permafrost stores a globally significant amount of mercury[END_REF] may be especially useful in assessing environmental controls on PHg export to the Arctic Ocean.

Because of the vast amounts of frozen peat with an average thickness of 1 to 3 m, extending over a territory of more than 1 million km², the permafrost thaw in this region can strongly affect the coastal Arctic system via enhanced export of organic-rich river suspended matter (RSM).

Due to the homogeneity of the WSL landscape, lithology, and topography, one can use the natural north-south gradient of permafrost zone distribution to assess the direct impact of permafrost conditions on PHg in river waters. The export of Hg by rivers is known to be strongly controlled by organic carbon [START_REF] Schuster | Mercury export from the Yukon River Basin and potential response to a changing climate[END_REF][START_REF] Kirk | Mercury in Arctic marine ecosystems: Sources, pathways and exposure[END_REF][START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF]. In western Siberia, there is a maximum of organic carbon concentration and export fluxes in the sporadic to isolated permafrost zone [START_REF] Krickov | Riverine particulate C and N generated at the permafrost thaw front: case study of western Siberian rivers across a 1700km latitudinal transect[END_REF]. This zone corresponds to the boundary of permafrost occurrence, and the thawing front of frozen peatlands. We therefore hypothesized a maximum of PHg export by rivers located at the appearance of permafrost (sporadic to isolated zone).

To test this hypothesis and to improve current understanding of magnitude and seasonality of PHg export, we quantified concentrations of PHg across a 1700 km latitudinal gradient with special emphasis on permafrost-bearing zones during three main hydrological periods, the peak of spring flood (early June), the summer base flow (August), and the autumn high flow before the ice on (October). This latitudinal gradient corresponds to that of temperature and permafrost coverage and also reflects the decrease in active layer thickness (ALT) from S to N. We aimed at assessing the effects of latitude (temperature), permafrost coverage and basic landscape features (proportion of bogs, lakes and forest at the watershed) as well as the size of the river watershed on PHg concentration, PHg:POC ratio, and PHg run-off and flux. Particular attention has been paid to the impact of the size of the river watershed, given that the small rivers are strongly understudied in terms of their suspended load (Holmes et al., 2002). While small rivers allow better assessment of landscape parameters impact on export fluxes, such rivers are hardly accessible in pristine regions, and at the same time, small rivers are often anthropogenically contaminated in populated regions. The WSL allows us to overcome these obstacles due to well-developed road infrastructure in an otherwise pristine region. Finally, we will use our observations to predict changes in PHg fluxes under climate warming, landscape changes and progressive permafrost thaw in the largest frozen peatland province in the world.

Study Site and Methods

Geographical setting

The 32 rivers were sampled in western Siberia Lowland, a large (about 2.6 million km²), peatland and forest zone (Fig. 1). The WSL territory encompasses the taiga, forest-tundra and tundra biomes, the positions of which follow the decrease of mean annual air temperature (MAAT) from -0.5°C in the south to -9.5°C in the north. The permafrost distribution also follows the MAAT and changes from absent, isolated and sporadic in the south to discontinuous and continuous in the north. The active layer thickness (ALT) corresponding to the depth of the seasonal thaw of peat layers is equal to 55±10 cm in continuous permafrost zone, 70±20 cm in discontinuous permafrost zone and 160±50 cm in sporadic to isolated zone [START_REF] Raudina | Dissolved organic carbon and major and trace elements in peat porewater of sporadic, discontinuous, and continuous permafrost zones of western Siberia[END_REF]. Further details of WSL physio-geographical settings, peat and lithological description of the territory are provided elsewhere [START_REF] Kremenetski | Peatlands of the Western Siberian lowlands: current knowledge on zonation, carbon content and Late Quaternary history[END_REF][START_REF] Stepanova | Major and trace elements in peat profiles in Western Siberia: impact of the landscape context, latitude and permafrost coverage[END_REF][START_REF] Pokrovsky | Permafrost coverage, watershed area and season control of dissolved carbon and major elements in western Siberian rivers[END_REF][START_REF] Raudina | Permafrost thaw and climate warming may decrease the CO2, carbon, and metal concentration in peat soil waters of the Western Siberia Lowland[END_REF] and listed in Table S1A of the Supplement. For each biome (taiga, forest-tundra and tundra), several rivers of different watershed size were chosen, following the previous strategy of dissolved WSL river load sampling along the latitudinal transect [START_REF] Pokrovsky | Permafrost coverage, watershed area and season control of dissolved carbon and major elements in western Siberian rivers[END_REF][START_REF] Pokrovsky | Trace element transport in western Siberian rivers across a permafrost gradient[END_REF][START_REF] Vorobyev | Permafrost boundary shift in western Siberia may not modify dissolved nutrient concentrations in rivers[END_REF]. The open water period lasts about 6 months, from May to October. Most small rivers located within the permafrost zone are frozen during winter period. The 6-months winter discharge is ≤ 10% of annual water flow [START_REF] Pokrovsky | Permafrost coverage, watershed area and season control of dissolved carbon and major elements in western Siberian rivers[END_REF].

Sampling

Sampling was performed during three main hydrological seasons: spring flood (17 May -15 June 2016), summer baseflow (1 -29 August 2016) and autumn baseflow before ice on (24 September -13 October 2016). During sampling campaigns, we moved from south to north in spring and from north to south in autumn over 2 to 4-weeks period of time, following the natural change of seasons. This allowed to sample all rivers of the transect at approximately the same hydrological stage, after ice off and before ice on. Compared to a 30-year climate record , the year 2016 was normal for western Siberia in terms of spring, summer and autumn precipitation but the temperature was 4 and 2.7°C higher than normal in spring and summer, respectively, and not different from the average temperature in autumn [START_REF] Rosgidromet | A report on climate features on the territory of the Russian federation in 2016[END_REF].

Ultraclean sampling and handling was used during river suspended matter (RSM) collection [START_REF] Krickov | Major and trace elements in suspended matter of western Siberian rivers: first assessment across permafrost zones and landscape parameters of watersheds[END_REF]. Large volume water samples were taken from the middle of the river at 0.5 m depth in pre-cleaned polypropylene jars (30 to 50 L) from a PVC boat and were decanted during 2 to 3 days to allow the particle settling. The jars were pre-cleaned by detergent, river water, 1 M HCl, MilliQ water, and finally by filling them completely 5 times by river water before collecting the sample for decantation. Some 10% headspace was left in the jars and the lids were not tightened to allow aeration. Note that all river waters sampled in this work were atmospheric oxygen-saturated. As such, the redox conditions in the jars were similar to those of the river water. The jars were protected from daylight to avoid any phytoplankton development and photodegradation of DOM, or photoreduction of Hg. The decantation was done at the river bank, in shadowed areas at the temperature close to that of the river water.

During this decantation, the samples were not subjected to biological transformation as particulate organic C, N and P concentrations remained constant and were found to be identical to those determined after immediate filtration and freezing of filters with particulate organic matter (see Krickov et al., 2018 for details). Further, a number of trace metals also demonstrated reasonable agreement in their concentrations in RSM obtained after 2-3 days of decantation and direct filtration [START_REF] Krickov | Major and trace elements in suspended matter of western Siberian rivers: first assessment across permafrost zones and landscape parameters of watersheds[END_REF]. Nevertheless, some biogeochemical transformation of PHg bioavailability and speciation during sampling could not be excluded.

The water of the bottom layer of the jars (approx. 30% of the initial volume) was centrifuged on-site during 20 min at 3500 rpm using 4 pre-cleaned 50-mL HDPE Nalgene tubes, the particles were recovered on the bottom of these tubes, immediately frozen at -18°C and freeze-dried later in the laboratory. The cleaning of Nalgene tubes for RSM centrifugation, storage and freeze-drying was performed by rinsing with 1N HCl and MilliQ, in a clean room class A 10,000. For determination of RSM bulk concentration, smaller volumes of freshly collected river water (1-2 L) were filtered on-site (at the river bank or in the boat) using preweighed acetate cellulose filters (47 mm, 0.45 µm) and Nalgene 250-mL polystyrene filtration units using a Mityvac® manual vacuum pump. This method provided the RSM concentration (mg L -1 ) which was in reasonable (±10-20%) agreement with the value estimated from total RSM recovery from big jars; see details in [START_REF] Krickov | Riverine particulate C and N generated at the permafrost thaw front: case study of western Siberian rivers across a 1700km latitudinal transect[END_REF].

Analyses

Total Hg concentration in RSM (PHgRSM, ng g -1 ) was determined using a direct mercury analyzer (DMA-80 -Milestone, Italy). Total mercury concentration was measured by combustion of the sample at 750 °C, pre-concentration of the Hg 0 vapor released on a gold trap, and detection by atomic absorption. Analysis of reference materials BCR-482 (lichen, 480 ng g - 89±6 ng g -1 (n=7, 1σ) respectively. The average uncertainty on duplicate sample analysis did not exceed 5% (1σ). Other measured components included total C and N concentration using catalytic combustion with Cu-O at 900°C with an uncertainty of ≤ 0.5% using Thermo Flash 2000 CN Analyzer at Tomsk University and all major and trace elements. For the latter, the RSM samples were subjected to full acid digestion with bi-distilled HNO3 and HF in the clean room following ICP-MS (Agilent 7500 ce) analyses as described previously for OC-rich natural samples [START_REF] Stepanova | Major and trace elements in peat profiles in Western Siberia: impact of the landscape context, latitude and permafrost coverage[END_REF]. The LKSD sediment and 2711a Montana II Soil international standard reference materials (organic-rich sediment and soil) were processed together with RSM samples for quality control (see details in [START_REF] Shevchenko | Impact of snow deposition on major and trace element concentrations and elementary fluxes in surface waters of the Western Siberian Lowland across a 1700 km latitudinal gradient[END_REF]. The HgRSM (ng g -1 ) and total RSM concentration in water (mg L -1 ) allowed calculating PHg concentration in the river water (PHgwater, ng L -1 ).

Data treatment

In order to assess the specific impact of the permafrost on PHg concentration, we separated all the sampled rivers into five categories: permafrost-free (south of 61°N, 6 rivers), isolated (61 to 63.5°N, 4 rivers); sporadic (63.5 to 65°N, 5 rivers); discontinuous (65 to 66°N, 13 rivers), and continuous permafrost zones (north of 66°N, 4 rivers). Statistical analysis of the RSM chemistry and fluxes in WSL rivers followed the methodology elaborated for particulate C, N and P [START_REF] Krickov | Riverine particulate C and N generated at the permafrost thaw front: case study of western Siberian rivers across a 1700km latitudinal transect[END_REF]. Because of the non-normal distribution of the PHgRSM and PHgwater concentration data, we use median ± interquartile range (IQR) when reporting uncertainty. All statistical tests used a significance level of 5% (α = 0.05). The differences in PHg between different seasons and between adjacent permafrost zones were tested using the Mann-Whitney U test for paired data at a significance level of 0.05. For unpaired data, the nonparametric H-criterion Kruskal-Wallis test was performed for all watershed sizes and all permafrost zones. Further, Spearman rank order correlations (significant at p < 0.05) were performed for assessing the effect of latitude, watershed area (Swatershed), percentage of bogs, forest, lakes at the watershed, the permafrost coverage and bulk concentration of RSM on PHgRSM and PHgwater, as well as to characterize the link of PHg with C, N, other major and trace elements. In order to identify the variable(s) responsible for PHg, constrained principal component analysis (PCA) was performed.

The seasonal PHgwater export by rivers was calculated separately for spring (May and June), summer (July and August) and autumn (September and October) and integrated for each 2° -wide latitudinal belt of the WSL territory, following the hydrological approach developed for C, major and trace elements [START_REF] Pokrovsky | Permafrost coverage, watershed area and season control of dissolved carbon and major elements in western Siberian rivers[END_REF][START_REF] Pokrovsky | Trace element transport in western Siberian rivers across a permafrost gradient[END_REF]. This approach was also used in the recent estimate of particulate C, N and RSM fluxes of WSL rivers [START_REF] Krickov | Riverine particulate C and N generated at the permafrost thaw front: case study of western Siberian rivers across a 1700km latitudinal transect[END_REF].

Given the small number of measurements for each individual river, the combined uncertainties on these fluxes are estimated as high as ±30%, which stems mainly from uncertainties on water discharges [START_REF] Pokrovsky | Permafrost coverage, watershed area and season control of dissolved carbon and major elements in western Siberian rivers[END_REF][START_REF] Krickov | Riverine particulate C and N generated at the permafrost thaw front: case study of western Siberian rivers across a 1700km latitudinal transect[END_REF]. However, the integral assessment of riverine export from the WSL territory is robust due to large number of sampled rivers.

Note that there is no direct Hg emission by anthropogenic activity in the studied S-N latitudinal transect of the WSL because there are no industrial centers or coal-fired power plants in the vicinity (50-100 km) of sampling sites. Low Hg contributions from combustion to atmospheric particles is consistent with low concentrations of black carbon in snow collected across the WSL, despite detectable contributions of gas flaring in the northern part of the WSL [START_REF] Evangeliou | Origin of elemental carbon in snow from western Siberia and northwestern European Russia during winter-spring 2014, 2015 and 2016[END_REF].

Results

PHg seasonality and watershed size

Median PHgwater concentration in the WSL river waters progressively decreased from the beginning to the end of open water seasons and were equal to 1.7±2.2, 0.9±1.5 and 0.7±1.4 ng L - concentrations were equal to 237, 124, and 96 ng g -1 . Particulate Hg concentrations in individual rivers (Table S2) demonstrated that, regardless of watershed area, PHg followed the order spring > summer > autumn. The effect of watershed size on both PHgwater and PHgRSM was most pronounced in spring (Fig. 2A andB). Although we collected one sample for each river, the sampling was representative of the riverine particles because: 1) we visited all rivers 3 times per year; 2) the sample volume was large (30 to 50 L), and 3) the river water chemical composition in western Siberia is highly stable over full transects of the river channel and on a time scale of 2 to 3 days [START_REF] Krickov | Major and trace elements in suspended matter of western Siberian rivers: first assessment across permafrost zones and landscape parameters of watersheds[END_REF]. The typical RSM concentrations in WSL rivers range from 3 to 10 mg L -1 with a median value of 7.5 mg L -1 [START_REF] Krickov | Riverine particulate C and N generated at the permafrost thaw front: case study of western Siberian rivers across a 1700km latitudinal transect[END_REF].

Statistical analysis confirmed that PHgRSM depended on watershed area (H= 7.72, p = 0.021) in spring whereas both PHgRSM and PHgwater depended on watershed area in autumn (H=7.13, p = 0.028; H=8.05, p=0.018, respectively) as shown by the H-criterion of the Kruskal-Wallis test and U-test of Mann-Whitney (Table S3). This was also reflected by significant (p < 0.05) RSpearman rank order correlations of HgRSM and size of the watershed (-0.49, -0.39 and -0.45 in spring, summer and autumn, respectively, Table S4).

3.2.Role of landscape parameters and permafrost distribution on PHg

The latitude and permafrost coverage positively correlated with PHgRSM in summer (RSpearman = 0.48 and 0.51, respectively, Table S4). The bog coverage exhibited significant (p < 0.05) control on PHgRSM which was detectable in spring (RSpearman = 0.56). The effect of forest and lake coverage of watersheds was not significant ( p < 0.05). There was a peak of both PHgwater and PHgRSM in rivers of sporadic permafrost zone in spring (Fig. 3 A,B) and a less pronounced maximum of PHgwater in rivers of discontinuous permafrost zone in summer (Fig. 3 B). The spring PHgRSM concentrations in absent, isolated and sporadic permafrost zones were ca.

3-5 times higher compared to those in the discontinuous and continuous permafrost zone in spring, whereas the highest PHgRSM were observed in discontinuous and continuous permafrost zones in summer, when PHgRSM was a factor of 2 to 4 higher than in absent, isolated and sporadic permafrost (Table 1, Fig. 3 B). Consistent with that, a non-parametric Kruskal-Wallis H-test yielded significant differences in PHgwater and PHgRSM among permafrost-free and 4 permafrost-bearing zones in spring (H=15.8, p = 0.0032; H=12.3, p = 0.016, respectively) and autumn (H = 10.2, p = 0.038 for PHgwater, Table S5 A). The U-test of Mann-Whitney for PHgwater and PHgRSM between all adjacent permafrost zones yielded significant differences in spring and for PHgwater in autumn (p < 0.04, Table S5 B). Finally, the Principal Component Analysis (PCA) demonstrated the presence of 3 factors acting on PHgwater, PHgRSM and the PHgflux (Fig. S1 of Supplement). The F1 (30% variance) included latitude, MAAT and permafrost coverage. The F2 (15% variance) comprised the proportion of lakes and forest at the watersheds, C and N. Finally, the F3 (13%) included discharge and the watershed area.

3.3.Correlations of Hg with C, N and other major and trace elements in RSM

There were significant (p < 0.05) positive relationships of PHgRSM with OC and N (RSpearman = 0.74 and 0.71, respectively) during spring period. In contrast, negative correlations between PHgRSM and mineral-originated insoluble elements (Al, Ti, Zr, Th, Nb, Ga, Rare Earth Elements (RSpearman = -0.60 to -0.74) were observed (Table S6). There was no correlation between PHgRSM and other elements in RSM in summer, whereas in autumn, a number of heavy metals and aerosol-deposited elements demonstrated significant (p < 0.05) positive correlation with PHgRSM (RSpearman = 0.41, 0.37, 0.47, 0.43, and 0.59 and 0.42 for Pb, Zn, Cu, Sb, and Mo, respectively).

The Hg:C ratio (µg Hg : g C) in the particulate load of WSL rivers (Table 1) was independent of watershed area and generally 3 times higher in spring compared to summer and autumn (Fig. 4 A). The Hg:C ratio was similar across all permafrost zones in spring (Fig. 4

B).

In summer and autumn, the Hg:C ratio was significantly (p < 0.05) higher in discontinuous and continuous permafrost zone compared to permafrost-free, isolated and sporadic zones (Fig. 4 B).

The average discharge-weighted Hg:C ratio in particles of WSL rivers is equal to 2.7±0.5 µg g -1 which integrates both permafrost-free and permafrost-bearing zones.

PHg export flux from the WSL to the Arctic Ocean

An open-water period (May to October) export flux of PHgwater across the WSL territory demonstrated a significant (p < 0.01, non-parametric Mann-Witney U-test and H-criterion of Kruskal-Wallis) maximum at 61-62°N, in the sporadic permafrost zone (Fig. 5 A). In this latitudinal belt, the riverine PHg fluxes were 2.3 and 1.9 times higher than those in permafrostfree and discontinuous/continuous permafrost zones, respectively. The 2-month spring period provided the maximal contribution to open water PHg fluxes in sporadic permafrost zone, whereas the contribution of summer and autumn was the highest in discontinuous and continuous permafrost zone (Fig. 5 A). The average PHg export yield from WSL river watersheds during open water period of the year (6 months) ranged from 0.2 to 0.5 g PHg km -² and exhibited a maximum at 61-62°N, within the sporadic permafrost zone (Table S2, Fig. 5 B).

This zone provided the highest contribution to the overall PHg export flux from permafrostaffected territory of WSL (56 %, Fig. 5 C). Note that the winter period contributes less than 10% of RSM, POC and PHg fluxes in Arctic Rivers [START_REF] Mcclelland | Particulate organic carbon and nitrogen export from major Arctic rivers[END_REF][START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF]. The mean annual air temperature (MAAT), which could be constrained for each permafrost zone but not for individual rivers, shows a maximum in PHg export yield by WSL rivers that corresponds to the sporadic permafrost zone (Fig. 5 D). Note that the observed maximum in PHg export was not linked to latitudinal variation in river runoff. The latter remained fairly constant over full range of studied sites and independent on latitude (R² = 0.08, Fig. 5 E).

Discussion

The influence of watershed size on PHg concentration

The WSL river PHgwater concentrations (typically 0.5 to 5.0 ng L -1 ) are similar to other Arctic rivers of low runoff which drain through boreal zone such as Ob (7.5 ng L -1 , [START_REF] Coquery | The distribution of dissolved and particulate mercury in three Siberian estuaries and adjacent Arctic coastal waters[END_REF], Severnaya Dvina (4 to 11 ng L -1 , [START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF], Yenisey (0.8 to 20 ng L -1 , [START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF] and N. American rivers (0.4 to 3.7 ng L -1 , [START_REF] Schuster | Mercury export from the Yukon River Basin and potential response to a changing climate[END_REF]. The PHgRSM values of the WSL rivers (100 to 1000 ng g -1 ) are in agreement with observations on the Yenisey (60 to 600 ng g -1 , Sonke et al., 2018) but higher than N. American rivers (10 to 40 ng g -1 , [START_REF] Schuster | Mercury export from the Yukon River Basin and potential response to a changing climate[END_REF]. A possible reason for high PHgRSM in WSL rivers is high POC concentration, reflecting the dominance of peatlands in Siberian watersheds. Indeed, small WSL streams (Swatershed < 1,000 km²) exhibited significantly higher concentrations of PHgRSM compared to larger rivers (Fig. 2). The small streams of WSL have the highest POC concentration in the RSM [START_REF] Krickov | Riverine particulate C and N generated at the permafrost thaw front: case study of western Siberian rivers across a 1700km latitudinal transect[END_REF]. This suggests enhanced mobilization of PHg in the form of peat debris. Consistently, histosols, the dominant (≥ 50%) soil group of WSL, can provide large amounts of organic particles, given the high susceptibility of peat to physical disintegration compared to mineral sediments [START_REF] Rezanezhad | Structure of peat soils and implications for water storage, flow and solute transport: A review update for geochemists[END_REF]. The enrichment of the river water in OC-rich particles may occur both at the river banks (especially in small rivers flowing through the wetlands) as well as within extensive floodplains via remobilization of organic-rich sediments at high flow [START_REF] Krickov | Riverine particulate C and N generated at the permafrost thaw front: case study of western Siberian rivers across a 1700km latitudinal transect[END_REF][START_REF] Krickov | Major and trace elements in suspended matter of western Siberian rivers: first assessment across permafrost zones and landscape parameters of watersheds[END_REF]. The floodplains of the Ob river and other rivers of the WSL extend more than 10 times the width of the main channel [START_REF] Vorobyev | Flood zone biogeochemistry of the Ob River middle course[END_REF]. Small rivers also exhibited the largest scatter in PHgRSM concentrations, probably due to the very short transit time in the watershed, leading to a fast response of river particulate load to minor variations in surface hydrology such as local storm events (e.g., [START_REF] Jeong | Differential storm responses of dissolved and particulate organic carbon in a mountainous headwater stream, investigated by high-frequency, in situ optical measurements[END_REF][START_REF] Lee | Variability in runoff fluxes of dissolved and particulate carbon and nitrogen from two watersheds of different tree species during intense storm events[END_REF].

It is also possible that autochthonous processes of suspended matter transformation in the river channel such as photo- [START_REF] Mayer | Photodissolution of particulate organic matter from sediments[END_REF][START_REF] Riggsbee | Suspended sediments in river ecosystems: Photochemical sources of dissolved organic carbon, dissolved organic nitrogen, and adsorptive removal of dissolved iron[END_REF] or bio-degradation [START_REF] Richardson | Measuring heterotrophic respiration rates of suspended particulate organic carbon from stream ecosystems[END_REF][START_REF] Gladyshev | Differences in organic matter and bacterioplankton between sections of the largest Arctic river: Mosaic or continuum?[END_REF][START_REF] Attermeyer | Organic carbon processing during transport through boreal inland waters: Particles as important sites[END_REF] of particulate organic matter (POM) are capable of modifying PHgRSM to a greater extend in large rivers relative to small streams. The large rivers of WSL have longer water residence times compared to small rivers (Alaaho et al., 2018a, b). We speculate that, similar to WSL river CO2 emission [START_REF] Serikova | High riverine CO2 emissions at the permafrost boundary of Western Siberia[END_REF], the longer travel time of RSM in large rivers may decrease the PHg concentration by biotic or abiotic PHg reduction and evasion of gaseous Hg 0 loss [START_REF] Gårdfeldt | Total gaseous mercury exchange between air and water at river and sea surfaces in Swedish coastal regions[END_REF][START_REF] Poissant | Mercury gas exchanges over selected bare soil and flooded sites in the bay St. Francois wetlands (Quebec, Canada)[END_REF][START_REF] Osterwalder | Mercury evasion from a boreal peatland shortens the timeline for recovery from legacy pollution[END_REF]. Overall, the WSL river transport of PHg may be strongly controlled by POM quality and origin, which emphasizes the need to provide molecular-level characterization of POM across permafrost and river-size gradients [START_REF] Mcclelland | Particulate organic carbon and nitrogen export from major Arctic rivers[END_REF][START_REF] Kutscher | Spatial variation in concentration and sources of organic carbon in the Lena River, Siberia[END_REF].

The overwhelming role of POM from peat soils as the main factor supplying PHg to rivers is further confirmed by geochemical analyses of RSM and the links of Hg with other elements in the suspended load. Observed negative correlations of PHgRSM with inorganic components of RSM reflect that Hg is bound to organic matter (N, C, RSpearman = 0.79 and 0.76,)

and not to the mineral fraction of RSM bearing insoluble (Ti, Al, Zr, REE) silicates, nor to partially soluble carbonate particles containing B, Mg, Ca, Sr, B and U. Therefore, during spring, the mineral fraction of RSM decreases PHgRSM by mere dilution with solid mineral matter. This was especially visible in large rivers, having a higher proportion of silicate minerals in their RSM compared to OC-dominated RSM of small rivers. In autumn, PHgRSM was positively correlated with Pb, Zn, Cu, Sb, and Mo which could reflect co-deposition from the atmospheric dust to vegetation and soils [START_REF] Shevchenko | Impact of snow deposition on major and trace element concentrations and elementary fluxes in surface waters of the Western Siberian Lowland across a 1700 km latitudinal gradient[END_REF].

Impact of permafrost coverage on PHg

Climate and permafrost coverage are considered as main drivers of dissolved and particulate river load in the WSL [START_REF] Frey | Geochemistry of west Siberian streams and their potential response to permafrost degradation[END_REF][START_REF] Frey | Amplified carbon release from vast West Siberian peatlands by 2100[END_REF][START_REF] Pokrovsky | Permafrost coverage, watershed area and season control of dissolved carbon and major elements in western Siberian rivers[END_REF]. Due to very flat topography of this region, the permafrost distribution in western Siberia roughly follows the latitude and MAAT so that there is a progressive change of the permafrost zones ("absent → isolated → sporadic → discontinuous → continuous") in the south → north direction. Both PHgwater and PHgRSM demonstrated a maximum around 60-62°N, in the sporadic and discontinuous permafrost zones (Fig. 3). The May-October PHg export fluxes were also maximal in this part of the WSL (Fig. 5 B,C). Note that spatial patterns of PHg fluxes in western Siberia cannot be explained by latitudinal variation of precipitation or runoff, because the water runoff remains constant or gradually increases northward [START_REF] Pokrovsky | Permafrost coverage, watershed area and season control of dissolved carbon and major elements in western Siberian rivers[END_REF]

Fig. 5 E).

The discontinuous permafrost zone can be considered as a large-scale thawing front of the frozen peat, and corresponds to the southern boundary of permafrost persistence. Here, an enhanced mobilization of C-and N-rich particles from soils to rivers [START_REF] Krickov | Riverine particulate C and N generated at the permafrost thaw front: case study of western Siberian rivers across a 1700km latitudinal transect[END_REF] and maximal CO2 emission from rivers [START_REF] Serikova | High riverine CO2 emissions at the permafrost boundary of Western Siberia[END_REF] occur. Further, a maximum of PHgRSM and export fluxes occurred in isolated and sporadic permafrost in spring. This maximum shifted to discontinuous permafrost zone in summer and moved northward, to the discontinuous permafrost zone in autumn (Fig. 3 B and5 A, respectively).

The active (unfrozen) layer thickness increases from spring to autumn and more importantly it moves northward during the open -water period from May to October [START_REF] Trofimova | Classification of climates and climatic regionalization of the West-Siberian plain[END_REF][START_REF] Raudina | Dissolved organic carbon and major and trace elements in peat porewater of sporadic, discontinuous, and continuous permafrost zones of western Siberia[END_REF]. This increase controls the degree of peat and mineral particles eroding from the soil profile to the rivers (e.g., [START_REF] Krickov | Riverine particulate C and N generated at the permafrost thaw front: case study of western Siberian rivers across a 1700km latitudinal transect[END_REF]. Thus, enhanced mobilization and subsequent river inputs of C, nutrients and metals at the "hot spot" of the permafrost thawing front was reported at the local scale of frozen peat landscapes of western Siberia [START_REF] Loiko | Abrupt permafrost collapse enhances organic carbon, CO2, nutrient and metal release into surface waters[END_REF]. The 2 to 3 times higher Hg:C ratio in spring compared to summer and autumn likely reflects the run-off of snow Hg that accumulated by atmospheric deposition over the previous 6 months cold season. The Hg:C ratio in RSM during summer and autumn was 2 to 3 times higher in northern rivers of discontinuous and continuous permafrost zone compared to southern rivers (Fig.

B).

Note that during this period, the water travel times in the WSL rivers and from the soil profile to the river are much shorter in the north than in the south (Ala-aho et al., 2018b).

We hypothesize that the breakdown of river POM to CO2 [START_REF] Serikova | High riverine CO2 emissions at the permafrost boundary of Western Siberia[END_REF]Krickov et al., 2018) leads to concomitant reduction and volatilization of PHg. This process is then more pronounced in the southern rivers with longer water residence time, while in the north, more refractory organic matter of RSM [START_REF] Gladyshev | Differences in organic matter and bacterioplankton between sections of the largest Arctic river: Mosaic or continuum?[END_REF] and lower biological activity [START_REF] Kawahigashi | Dissolved organic matter in small streams along a gradient from discontinuous to continuous permafrost[END_REF] lead to better preservation of POM-bound Hg in riverine particles. This may produce the observed Hg to C enrichment in the RSM of northern rivers during the summer and fall base flow period.

Overall, PHg generation in WSL rivers across the permafrost gradient may depend on both the PHg supply and losses in the catchments. Atmospheric Hg retention in continental ecosystems is thought to be dominated by the plant Hg pump during the vegetation growth season, where vegetation directly sequesters gaseous elemental Hg 0 from the atmosphere [START_REF] Obrist | Tundra uptake of atmospheric elemental mercury drives Arctic mercury pollution[END_REF][START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF]. During the winter season Hg deposition is likely dominated by wet deposition in the form of snowfall [START_REF] Nelson | A review of mercury concentration and deposition in snow in eastern temperate North America[END_REF]. The main sources of suspended matter in WSL rivers are 1) vegetation litter such as plant leaves in the south and mosses, lichens and dwarf shrub leafs in the permafrost-bearing zone; these organic debris are easily mobilized by surface run-off to the river, notably in spring during snow melt; 2) surface and deep peat and mineral horizons, providing the particles via bank abrasion in spring and via supra-permafrost flow in summer and autumn, and 3) lake and bog open water sediments as well as flood plain sediments. In the north, soil erosion is limited by the low thickness (30-40 cm) of the active (unfrozen) layer and the low net primary productivity, providing POM, whereas in the south, the soil is stabilized by abundant forest vegetation. We suggest that in the north, shallow unfrozen peat depth and lower biomass accumulation cannot supply sufficiently high PHg and the fluxes remain relatively low in spring. In contrast, the PHg concentration and fluxes greatly increase in summer, when the thickness of active (unfrozen) peat layer increases in the north. As such, physical erosion of peat soils in the WSL drives maximum RSM levels at the sporadic permafrost zone due to maximal thickness of unfrozen histosols, notably in the beginning of the snow-free season.

PHg flux and climate warming in western Siberia

The Taz and Pur Rivers (Swatershed = 150,000 and 112,000 km²) integrate a significant part of permafrost-affected WSL territory, and export ca. 0.036 and 0.0067 Mg PHg y -1 (Table 1).

The PHg flux of these two rivers represents less than 0.4 % of the overall Eurasian River (Stotal = 10,962,000 km²) flux (11.3 Mg PHg y -1 , [START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF] which is disproportionally (6.5 times) low compared to their total watershed area and respective discharge. The most likely cause for this is low concentration and low annual yield of RSM in WSL rivers due to low water runoff (100-200 mm y -1 ) and limited river slopes. Measured open-water period PHg export yields of Ob, Pur and Taz (0.20, 0.06, and 0.24 g km -2 y -1 , respectively, Table S2) are lower than the modeled yields by [START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF] (0.63, 1.5 and 1.7 g km -2 y -1 , respectively).

It is possible that the individual PHg fluxes of WSL rivers, assessed in this study, are biased by small number of sampling days, and poor constraining of water discharges for these essentially ungauged rivers. However, the Hg:C ratio, which is based on a large number of measurements, may become an efficient proxy for reconstructing overall Hg export based on known POC export. Indeed, the measured PHg and POC fluxes of Arctic Rivers [START_REF] Leitch | The delivery of mercury to the Beaufort Sea of the Arctic Ocean by the Mackenzie River[END_REF][START_REF] Kirk | Multiyear total and methyl mercury exports from two major sub-Arctic rivers draining into Hudson Bay, Canada[END_REF][START_REF] Carrie | Determination of mercury biogeochemical fluxes in the remote Mackenzie River Basin, northwest Canada, using speciation of sulfur and organic carbon[END_REF][START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF][START_REF] Mcclelland | Particulate organic carbon and nitrogen export from major Arctic rivers[END_REF] yield ranges of Hg:C (Mg:Tg) ratios equal to 2.0-4.8 (MacKenzie), . This is consistent with the discharge-weighted Hg:C ratio of 2.7±0.5 recommended for WSL rivers in this study. Further, using the mean ratio Hg:C of open water time period in rivers of WSL, and overall POC flux of the pan-Arctic rivers (5.77 Tg y -1 , [START_REF] Mcclelland | Particulate organic carbon and nitrogen export from major Arctic rivers[END_REF], the pan-Arctic flux of PHg is equal to 15.6±3.0 Mg y -1 which is comparable to the value 21.9 Mg y -1 , assessed by [START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF]. For the Ob River with its 0.57 Tg POC y -1 flux [START_REF] Mcclelland | Particulate organic carbon and nitrogen export from major Arctic rivers[END_REF], the observation-based PHg flux is 1.5±0.3 Mg PHg y -1 in this study, in agreement with 1.4 Mg PHg y -1 modeled by [START_REF] Jiskra | A vegetation control on seasonal variations in global atmospheric mercury concentrations[END_REF] based on Hg-DOC and Hgdischarge relationships.

Our results allow a first order assessment of the consequences of climate warming and permafrost thaw in western Siberia on riverine transport of PHg from the land to the Arctic Ocean. For this, we can employ a "substituting space for time" approach which was successfully used in western Siberia for the dissolved (< 0.45 µm) fraction of inorganic river loads [START_REF] Frey | Geochemistry of west Siberian streams and their potential response to permafrost degradation[END_REF], DOC [START_REF] Frey | Amplified carbon release from vast West Siberian peatlands by 2100[END_REF][START_REF] Pokrovsky | Permafrost coverage, watershed area and season control of dissolved carbon and major elements in western Siberian rivers[END_REF], trace metals [START_REF] Pokrovsky | Trace element transport in western Siberian rivers across a permafrost gradient[END_REF], nutrients [START_REF] Vorobyev | Permafrost boundary shift in western Siberia may not modify dissolved nutrient concentrations in rivers[END_REF], as well as CO2 emission scenarios [START_REF] Serikova | High riverine CO2 emissions at the permafrost boundary of Western Siberia[END_REF]. In a broader context, the "space-for-time" substitution approach postulates that contemporary spatial phenomena can be used to understand and model temporal processes that are otherwise unobservable, such as past and future events [START_REF] Blois | Space can substitute for time in predicting climate-change effects on biodiversity[END_REF].

In the northern part of the WSL, soil temperature may rise by 0.15 to 0.3 degree per 10 years over next 50 to 100 years [START_REF] Pavlov | Small-scale mapping of trends of the contemporary ground temperature changes in the Russian North[END_REF][START_REF] Anisimov | Continental multiyear permafrost[END_REF]. On a short-term prospective (10-50 years), this will transform the discontinuous and continuous permafrost zones into sporadic and isolated permafrost [START_REF] Anisimov | Permafrost and changing climate: the Russian perspective[END_REF]. Given the current maximum of PHg at the sporadic PF zone, this may enhance both concentrations and fluxes of PHg in the northern rivers, roughly by a factor of 2 (see Figs 3 and5B), which will also enhance the export of PHg to the Arctic Ocean. On a longer prospective (50-150 years), the discontinuous and continuous permafrost may disappear [START_REF] Romanovsky | Thermal state and fate of permafrost in Russia: first results of IPY[END_REF][START_REF] Nadyozhina | Permafrost response to the climate warming as simulated by the regional climate model of the main geophysical observatory[END_REF] and the border of isolated to sporadic permafrost development will follow the Arctic circle [START_REF] Bedritsky | National report "Global climate and soil cover f Russia: estimation of risks and environmental and econoical consequences of land degradation[END_REF]. Moreover, if the current rate of soil warming persists, by 2100, the active layer thickness may reach 4 m in the tundra of Yamal Peninsula which is currently continuous permafrost zone [START_REF] Sudakov | Modeling of thermal regime of permafrost during contemporary climate change[END_REF]. These changes will certainly produce much deeper water paths in organic and mineral soils. Progressive moving of the permafrost thawing front to the north will therefore locally and temporarily increase PHg concentration and fluxes at the sites of maximal ALT. However, complete disappearance of permafrost in the north of WSL may bring about a decrease in PHg export by northern rivers to the values which are currently observed in rivers south of 60°N. Furthermore, if the permafrost-free zone expands northward into the current sporadic zone, this could produce a concomitant reduction in PHg fluxes thus compensating for the shift northwards of the sporadic zone.

Conclusions

The concentration of particulate Hg (PHg) in the WSL rivers is controlled by the organic nature of abundant peat soils that dominate river watersheds. The overwhelming role of deep and intermediate peat horizons rather than mineral layers, surface peat and plant litter in providing Hg to riverine suspended load is confirmed by i) strong positive correlation between C, N and Hg in RSM, and ii) much higher PHgRSM and PHgwater in discontinuous and sporadic permafrost rivers compared to southern, permafrost-free or northern, continuous permafrostbearing rivers.

The concentration of Hg in RSM decreased with the increase in river watershed size, illustrating the importance of organic particles in small rivers which drain peatlands and the abundance of mineral matter from bank abrasion in larger rivers. This was most visible in spring, when the bank abrasion and sediment transport and deposition within the floodplains were pronounced. Overall, the spring flood provided the largest contribution to overall PHg export by WSL rivers to the Arctic Ocean, despite its relatively short duration (2 of 6 months of open-water period).

At the sporadic permafrost zone, we observed maximum concentration and export fluxes of PHg, which corresponded to peak of particulate C and N concentration and fluxes across the latitudinal profile. This suggests the enhanced generation of Hg, C-rich RSM at the thawing front of permafrost, where the thickness of the active layer is maximal.

To assess a northward permafrost boundary shifts with increases in air and soil temperature we used a substituting space for time scenario of climate warming in the WSL, well developed for the dissolved fraction of C and metals. From a short-term (10-50 years) climate warming perspective, the effect of a simple northward shift of permafrost boundary may produce a two-fold increase in PHg concentration and fluxes in rivers of the discontinuous and continuous permafrost zone. On longer time scales (50-100 years), full disappearance of permafrost in the northern part of WSL may decrease the concentrations of PHg in the river water and its export fluxes back to their contemporary level. Table 1. Median (± IQR) and mean (± SD) values of PHg concentration in water (PHgwater, ng L -1 ) and in RSM (PHgRSM, ng g -1 ) as well as the ratio of Hg:C in RSM (µg g -1 ) for 5 permafrost zones and 3 seasons across the WSL transect. For the permafrost-affected zone of WSL, we recommend annual mean Hg concentration equals to 1.8 (median 1.2) ng Hg L -1 in water and 246 (median 139) ng Hg g -1 in RSM. The discharge-weighed Hg:C ratio is estimated as 2.7±0.5 µg g -1 .

Fig. 1 .

 1 Fig. 1. Sampling sites and physico-geographical context of WSL territory investigated in this work.

Fig. 2 .

 2 Fig. 2. Particulate (> 0.45 µm) Hg concentration (median ± IQR) in the river suspended matter (PHgRSM, A) and riverwater (PHgwater, B) as a function of river watershed size.

Fig. 3 .

 3 Fig. 3. Box plot of median and first and third quartiles (25 and 75%) of PHgRSM (A) and PHgwater (B) in five permafrost zones over three seasons. A disconnect between PHgRSM and PHgwater in discontinuous and continuous permafrost regions in summer is due to lower bulk RSM concentration in northern rivers during this period of the year (C).

Fig. 4 .

 4 Fig. 4. The mass ratio Hg:C (µg g -1 ) for rivers and streams of different size (A) and type of permafrost distribution (B).

Fig. 5 .

 5 Fig. 5. Watershed surface-normalized PHgwater export yields during high flow (spring, May and June) and low flow (4 months, July to October) in 5 permafrost zones (A) the total open-water period PHg flux (6 months, May to October) across a 2-degree latitude grid (B). C, The partial contribution and PHg export in Mg of three main permafrost zones of the WSL during 6 months of open water period. D, Relationship between the yield of PHg by WSL rivers and the MAAT (mean annual air temperature) of the watershed (binned to 5 permafrost zones). The error bars in A, B and D represent ±1 S.D. Note that during open water period (May to October), the river runoff from the WSL territory is maximal in spring and independent on latitude (E).

  Footnote : the median values are given in the numerator and the mean values are given in the denominator.

	Season	Variable	Permafrost free		Permafrost
			Absent	Isolated	Sporadic	Discontinuous	Continuous
	Spring	PHgwater, ng L -1	1.7±1.0 1.6±0.7	1.9±0.7 1.8±0.9	3.8±7.6 5.6±4.6	0.8±0.4 0.7±0.2	0.8±0.3 1.3±1.0
	Spring	PHgRSM, ng g -1	211±257 304±204	299±290 430±308	509±617 588±400	86±134 110±70	97±42 155±161
	Spring	Hg:C, µg g -1	4.1±1.5 4.0±1.5	2.7±5.2 3.3±3.2	2.5±5.0 4.6±3.9	2.5±2.4 2.4±1.2	4.4±1.1 4.1±0.7
	Summer	PHgwater, ng L -1	0.8±0.4 1.0±0.8	0.7±0.8 1.0±0.6	1.7±1.4 2.0±1.6	1.2±3.7 2.3±2.8	0.7±0.8 0.9±0.7
	Summer	PHgRSM, ng g -1	94±11 99±28	65±79 137±110	159±178 197±142	288±626 394±405	372±323 385±216
	Summer	Hg:C, µg g -1	1.1±1.0 1.1±0.6	0.6±0.2 0.6±0.3	0.9±0.7 1.1±1.0	2.9±4.3 3.0±2.5	2.6±2.0 2.7±1.6
	Autumn	PHgwater, ng L -1	0.3±0.5 0.5±1.3	0.6±0.4 0.7±0.4	0.8±0.9 1.4±1.1	1.4±1.4 1.6±1.1	1.4±0.3 0.2±0.4
	Autumn	PHgRSM, ng g -1	151±118 151±157	93±33 152±144	142±124 148±89	151±91 141±65	154±101 151±67
	Autumn	Hg:C, µg g -1	1.3±1.3 1.3±0.7	0.5±0.2 0.6±0.5	0.5±0.4 0.9±0.7	1.3±1.0 1.2±0.6	2.4±0.9 2.4±1.1

) and MESS-3 (sediment, 91 ng g -1 ) showed good recoveries of 450±23 ng g -1 (n=6, 1σ) and

in spring, summer and autumn, respectively (Table1). The corresponding HgRSM

Acknowledgements: This work was supported by RSCF No 18-17-00237 "Mechanisms of hydrochemical runoff of the Ob river flood zone" (analyses, modeling); RFBR projects № 18-35-00563\18 and 19-55-15002; the TSU competitiveness improvement programme Project No 8.1.04.2018, the Ministry of Education and Science of the Russian Federation No. 1.8195.2017/P220, the CNRS Chantier Arctique, via the PARCS project, and the H2020 ERA-PLANET (689443) iGOSP and iCUPE programmes.