# Export of organic carbon, nutrients and metals by the mid-sized Pechora River to the Arctic Ocean

Artem Chupakov, Oleg Pokrovsky, Olga Moreva, Ekaterina Kotova, Taissia

Vorobyeva, Liudmila Shirokova

► To cite this version:

Artem Chupakov, Oleg Pokrovsky, Olga Moreva, Ekaterina Kotova, Taissia Vorobyeva, et al. Export of organic carbon, nutrients and metals by the mid-sized Pechora River to the Arctic Ocean. Chemical Geology, 2023, 632, pp.121524. 10.1016/j.chemgeo.2023.121524. hal-04915866

HAL Id: hal-04915866

https://ut3-toulouseinp.hal.science/hal-04915866v1

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Distributed under a Creative Commons Attribution 4.0 International License
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2 Export of organic carbon, nutrients and metals by the mid-sized Pechora

3 River to the Arctic Ocean

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5 Artem V. Chupakov¹, Oleg S. Pokrovsky²,³*, Olga Y. Moreva¹, Ekaterina I. Kotova¹,

6 Taissia Y. Vorobyeva¹, Liudmila S. Shirokova¹,²

7 ¹N. Laverov Federal Center for Integrated Arctic Research, UrB RAS, 23 Nab Severnoi Dviny,

8 Arkhangelsk, 163000, Russia

9 ²Environment Toulouse, UMR 5563 CNRS, 14 Avenue Edouard Belin, 31400, Toulouse,

10 FRANCE

11 ³BIO-GEO-CLIM Laboratory, Tomsk State University, Tomsk, Russia

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13 * email: oleg.pokrovsky@get.omp.eu

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15 Key words: organic carbon, trace element, yield, permafrost, peatland, colloid, Arctic

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17 Submitted to *Chemical Geology*, after revision, April 2023

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**Abstract**

In contrast to good knowledge of export fluxes of carbon and metals from mainland to the Arctic Ocean by large Arctic Rivers, information on mid-sized rivers is limited, which prevents determining current status and foreseeing future changes in riverine export induced by climate change. Here we focused on one of the ‘middle eight’ Arctic rivers. The Pechora River (S<sub>watershed</sub> = 322,000 km²) is the second largest European Arctic river draining through boreal forest and peatlands with partial (~ 40 %) permafrost coverage. Over 4 consecutive years (2015-2019), we measured weekly to monthly concentrations of carbon, major nutrients, and 40 major and trace elements in filtered (< 0.45 µm) river water at the terminal gauging stations for the Pechora River.

The dependences between dissolved element concentration and river discharge over the full period of observation revealed 3 groups of major and trace solutes according to their seasonal behavior.

Group 1 was comprised of Dissolved Inorganic Carbon (DIC), major anions (Cl, SO₄), alkalis (Li, Na, K, Rb), alkaline-earth metals (Mg, Ca, Sr, Ba), elements present in the form of labile anions and neutral molecules (B, Si, Ge, Mo, Sb) and U. It demonstrated minimal concentrations during spring flooding and autumn high flow events, and maximal concentrations during winter base flow. Concentrations of these element negatively correlated with discharge. These elements primarily reflected the dominant sedimentary lithology of the Pechora catchment and were controlled by influx of underground waters hosted in carbonate rocks. Group 2 included DOC and low-mobility trivalent and tetravalent hydrolysates (Be, Al, Ga, Y, REEs, Ti, Zr, Hf, Th) and some trace metals (V, Cr, Cs, Nb). The transport of these elements to the river from the catchment likely occurred via surface and shallow subsurface waters due to leaching from organic-rich litter layer, and their concentrations positively correlated with discharge. Finally, group 3 involved major- (P, N, K) and micro-nutrients (Fe, Mn, Cu, Zn, Mo) and demonstrated features of both the 1st and 2nd groups as these elements originated from surface (organic-rich) and underground (sedimentary rock-hosted) sources and were removed by autochthonous biotic processes in the water column.
As a result, the concentration of these nutrients exhibited a winter maximum and a summer minimum.

Similar to other Arctic and subarctic rivers, the spring flood, which lasted from May to June, accounted for more than 50% of the annual export of DOC and generally insoluble trace metals. However, the winter baseflow provided sizable proportions of mobile elements such as DIC, major anions, cations, alkaline-earth elements, trace oxyanions and uranium. The size distribution of riverine solutes demonstrated 20 - 50% of DOC in the colloidal (3 kDa – 0.45 µm) form, dominance of low molecular forms (< 3 kDa) of soluble elements (alkali, alkaline earths, anions and neutral molecules) and essentially colloidal status of divalent metals (Cu, Zn, Cd, Pb, Ni), trivalent and tetravalent hydrolysates and U (from 30 - 93 %). Seasonal variations in colloidal fractions of riverine solutes were consistent with the main mechanisms of element mobilization from the watershed to the Pechora River, as reflected in the 3 groups of elements according to their concentration – discharge relationship. Based on the present study and results of other mid-sized Arctic rivers (Severnaya Dvina, Taz), we demonstrate that mean discharges and solute concentrations in August can be used to adequately (± 20 %) approximate mean annual values, hence allowing annual flux assessment via single month sampling campaign. A comparison of contemporary yields of Ca, Mg, Cl, SO₄, DIC and DOC against historical data for the Pechora River basin demonstrated reasonable agreement within the inter-annual variations that is consistent with long-term data series on other Arctic rivers.

# 1. Introduction

The current status of carbon and inorganic nutrient export by Arctic rivers to the ocean is vitally important for carbon and element biogeochemical cycle regulation and future responses of Arctic ecosystems to on-going climate warming (Bense et al., 2012; Pokrovsky et al., 2012;
Vonk et al., 2015, 2019; Cochand et al., 2019; O'Donnell et al., 2021). In particular, permafrost degradation and active (unfrozen) layer thickening enhance the riverine export of inorganic and organic C, N and P (Wild et al., 2019; Zhang et al., 2021). These effects are accentuated in regions where permafrost is discontinuous or sporadic such as coastal Arctic zones and peatlands of continental planes (Vasiliev et al., 2020). Despite the importance of high latitude zones for Earth-scale export of carbon, nutrients and metals to the Arctic Ocean, our understanding of riverine fluxes is highly variable; six large Arctic great rivers (the Ob, Yenisei, Lena, Kolyma, Mackenzie and Yukon) have undergone significant catchment studies while smaller and mid-sized rivers flowing to the Arctic Ocean have received very limited attention. In particular, coastal zones of the European Arctic are known to present one of the highest DOC yields at the global scale (i.e., Li et al., 2017); however, these rivers are essentially studied in Scandinavia with minimal information available for northeastern Europe.

For comparison, thanks to thorough work of the American and Russian State Hydrological Survey and multi-annual sampling within the PARTNERS/ARCTIC GRO programs (Raymond et al., 2007; Cooper et al., 2008; Holmes et al., 2013; McClelland et al., 2015; Tank et al., 2016; Drake et al., 2018; Behnke et al., 2021; Zolkos et al., 2022), there exists detailed systematic understanding of riverine export fluxes of dissolved carbon, nutrients and some trace metals from land to the Arctic Ocean for the five large Arctic rivers (the Mackenzie, Kolyma, Lena, Yenisei, and Ob). Further, numerous studies have quantified elementary export fluxes of small rivers draining organic-rich permafrost peatlands in western Siberia (Frey et al., 2007; Pokrovsky et al., 2020; Krickov et al., 2018, 2019, 2020).

Among the so called 'middle eight' Artic rivers (the Severnaya Dvina, Pechora, Pur, Taz, Khatanga, Olenek, Yana and Indigirka), covering a total watershed area of 1.9 million km² with an average population density of 1.13 people km⁻², only the Severnaya Dvina River received sufficient attention by researchers (Pokrovsky et al., 2010; Johnston et al., 2018; Chupakov et al.,
2020; Gordeev et al., 2022). In this study we focused on the Pechora River (S<sub>watershed</sub> = 322,000 km<sup>2</sup>), the second largest European Arctic river, applying comprehensive seasonally-resolved sampling. The main interest for this river is that it might present disproportionally high yields of organic carbon and relevant elements given that it drains through the Bolshezemelskaya Tundra, the largest peat bog territory in Europe. Additionally, permafrost in these northern peatlands is discontinuous to sporadic and is likely to be highly vulnerable to thawing (Vasiliev et al., 2020). Therefore, the first objective of this study was quantification of mean multi-annual and seasonal export fluxes of dissolved (< 0.45 µm) organic carbon, nutrients, and major and trace elements by the Pechora River. Because the Soviet Hydrological Service has been monitoring the Pechora River since 1932, some basic hydrochemical parameters of this river's water have been available since the 1960s. Such information allowed for reconstructing carbon and major elements yields of the Pechora River over several decades during last century (Gordeev et al., 1996). Taken together, these factors render this river as a most suitable test site for assessing the possible evolution of hydrochemical fluxes relevant to fast changing permafrost-affected environments. Therefore, a secondary objective for this study was development of a quantitative comparison of export fluxes for major solutes in the second half of the 20<sup>th</sup> century and at present.

Due to excessive precipitation and high amount of organic carbon in northern soils, rivers draining permafrost peatlands contain sizable proportions of colloidal (1 nm – 1 µm) organic matter, which can act as an important vector for trace metal transfer (Ingri et al., 2000; Anderson et al., 2006; Ilina et al., 2016; Cuss et al., 2018). The source of colloidal matter in river water changes over the course of the year and includes i) surface influx via forest litter leaching (Bagard et al., 2011), ii) mire and peat waters or mineral-draining of Al-enriched suprapermafrost waters (Raudina et al., 2021), iii) thawing soil ice (Lim et al., 2022), iv) underground Fe-enriched waters that discharge within the riparian/hyporheic zone (Pokrovsky et al., 2010, 2016; Vasyukova et al., 2010), v) autochthonous production of plankton and
macrophyte exometabolites in the water column (Krickov et al., 2019) and even vi)
atmospheric precipitates in the form of snow or rain (Krickov et al., 2022a, b). As a results, the
composition of organic and organo-mineral colloids and the degree of their binding to trace
element (TE) are strongly seasonally variable. Therefore, for assessment of colloidal versus
truly dissolved (i.e., low molecular weight or potentially bioavailable) fluxes of solutes,
seasonally resolved measurements of colloidal fractions are needed. Among Arctic rivers, such
information is only available for small Scandinavian and Northern American rivers on a
seasonal scale (Porcelli et al., 1997; Ingri et al., 2000; Stolpe et al., 2013a, b; Lyvén et al.,
2003; Cuss et al., 2018) and the Severnaya Dvina River (Pokrovsky et al., 2010) and small
rivers of the western Siberia (Krickov et al., 2019) on an annual scale. To improve our
understanding of colloidal riverine transport of carbon and trace elements, a third objective of
this study was to assess the colloidal fraction of riverine solutes across several hydrologically
important seasons and quantify total dissolved versus ‘truly dissolved’ low molecular weight
export of major and trace elements in the Pechora River.

We believe achieving these 3 study objectives - the quantification of seasonal and annual
dissolved and colloidal element fluxes - should 1) provide a basis for assessing climate change
effects on hydrological and hydrochemical fluxes of other mid-sized Arctic rivers, 2) improve
understanding of the mechanisms shaping the hydrochemical composition of Arctic rivers, and
3) set up a base line for assessing future changes in riverine export of carbon, nutrient and
metal from land to the Arctic Ocean.

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143 **2. Methods**

2.1. *Study site*

The Pechora basin is located in a region between 62 and 69 °N and 46 and 66 °E and
encompasses an area of 322,000 km² (Fig. 1). The length of the Pechora River is 1810 km and

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its average annual discharge is 4100 m³ s⁻¹. The climate of the basin is temperate to continental, with an average July temperature of 13.6 °C and an average January temperature of -17.5 °C.

The annual precipitation for the region ranges between 400 and 500 mm. The Pechora Basin rests largely on a plain located between the Ural and Timan mountains; the average watershed elevation across the plain is 105 m. Maximal elevations are observed in the Eastern edge of Ural ridge (up to 1000 m above the sea level). The Pechora's southern watershed divide near the Vychegda and Volga Rivers is much lower in elevation and contains wetlands. Sizable portions (40 %) of the northern and north-eastern reaches of the Pechora Basin are located in the permafrost region (12% continuous, 4% discontinuous, and 24 % intermittent and sporadic according to McClelland et al., 2004). Dominant ground temperatures the northern and north-eastern section very between -0.5 and -2.0 °C (Lammers et al., 2007). The main vegetation in the taiga biome is comprised of spruce, pine, and larch. Forests and bogs cover approximately 70 % and 30 % of the area, respectively (Sidorchuk et al., 2000).

The lithology of the Pechora catchment is complex and dominated by Cretaceous and Jurassic deposits in the north (clays, sands, siltstones) and Paleozoic (Permian, Carboniferous) deposits in the south (limestones, dolostones and coals). Soils are formed onto thick layer of glacial deposits (clays and sands) that are underlain by sandstone, marl and limestone. The upper reaches of the Pechora River are characterized by a predominance of Al-Fe-humus and metamorphic soils with gley features developed under the middle taiga fir and spruce forests (Semikolennykh et al., 2013).

## 2.2. Hydrological gauging stations of the Pechora River and water sampling procedure

For the analysis of inter-annual dynamics of the Pechora River discharge, we used mean monthly and annual values at the terminal gauging station (Oxino) that are available from 1932 to 2019. River water samples were collected in the vicinity of this station. During the
period 2016-2019, we collected about 100 samples from the middle of the river at 0.5 m depth.
We sampled 2, 7, 30 and 70 times in 2016, 2017, 2018 and 2019, respectively. At the Oxino
station, we sampled once a month during summer and winter baseflow and twice a month
during spring flood (May-June) and autumn high flow (September-October). In the Iskateli
sampling station (near the town of Narian-Mar), located approximately 40 km downstream
from Oxino station but still in the freshwater zone, we sampled for dissolved (< 0.45 µm)
fraction. Typically, we sampled 3 times per month during the baseflow, 8 times per month
during spring flood, and 4 times per month during autumn. Besides, once per month during
baseflow, 3 times per month during spring flood and 2 times per month during autumn, we
collected river suspended matter using pre-weighted polycarbonate filters (0.45 µm) in order to
measure total suspended sediment (TSS) concentration. Water samples were collected in pre-
cleaned polypropylene bottles and were immediately filtered through sterile single use
Minisart® filter units (Sartorius, acetate cellulose filter) with a pore size of 0.45 µm. Collected
water samples were processed for colloidal size separation within 24 hours (see section 2.4
below).

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## 2.3. Analyses of river water solutes

The conductivity and pH were measured using a Hanna HI991300 conductivity meter
and a WTW portable pH meter with combined electrode. Filtered water samples for nutrient
(N, P) analyses were frozen (–20 °C) within 1–3 hours of collection and analysed within 1
week after sampling. DOC and DIC concentrations were measured using methods routinely
used in the Géosciences Environnement Toulouse (GET) laboratory to analyze boreal water
samples (Chupakov et al., 2020). Pyrolyzed (2.5 h at 550 °C) glassware was used for sample
collection and storage. In winter, the DIC concentration was obtained from alkalinity following
a standard HCl titration procedure using an automatic Titro-Line alpha TA10plus titrator

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(Schott, Germany) with an uncertainty of ±2% and a detection limit of 5 × 10⁻⁵ M. During spring flood and summer, when DOC is higher than 10 mg L⁻¹, DIC was measured by a total organic carbon analyzer (Shimadzu TOC-VCSN) [acid addition without combustion] with a detection limit of 0.01 mM and an uncertainty of 5%. The agreement between the two methods of DIC analysis was within 10%. Major anion concentrations (Cl and SO₄) were measured by ion chromatography (HPLC, Dionex ICS 2000) with an uncertainty of 2%. Internationally certified water samples (ION-915, MISSISSIPPI-03, RAIN-97, Pérade-20) were used to check the validity and reproducibility of DIC analysis. There was good agreement between our replicated measurements and the certified values obtained (relative difference < 10%).

DOC was also measured using the aforementioned Shimadzu carbon analyzer with an uncertainty of 5% and a detection limit of 0.1 mg L⁻¹. DOC blanks of filtrate never exceeded 0.1 mg/L, which is quite low for the organic-rich river waters sampled in this study (i.e., 5–30 mg/L DOC). The value of specific UV-absorbency (SUVA₂₅₄, L mg⁻¹ m⁻¹) was measured using 1 cm quartz cuvette in a CARY-50 UV-VIS spectrophotometer (Bruker, UK). The SUVA₂₅₄ can be used as proxies for aromatic C and source of DOM (Weishaar et al., 2003; Ilina et al., 2014). Nutrient (N, P) analyses were based on colorimetric assays (Koroleff, 1983a, b). The uncertainty of P-PO₄ analysis was 5% with a detection limit of 2 µg/L. Uncertainties on N-NH₄, N-NO₃ and N-NO₂ analyses were between 5 and 10 % and detections limits were equal to 5, 10 and 1 µg L⁻¹, respectively.

Filtered solutions for cation analyses were acidified (pH ~ 2) with ultrapure double-distilled HNO₃ and were stored in pre-washed HDPE bottles. The preparation of bottles for sample storage was performed in a clean bench room (ISO A 10,000). Blanks were performed to control the level of pollution induced by sampling and filtration. Major cations (Ca, Mg, Na and K), Si, and trace elements were determined with an ICP-MS Agilent ce 7500 with In and Re as internal standards and 3 external in-house standards. These were placed each 10 samples
per series of river water. For all major and most trace elements analyzed by ICP MS, the concentrations in blanks were below analytical detection limits (≤ 0.1-1 ng/L for Cd, Ba, Y, Zr, Nb, rare earth elements (REEs), Hf, Pb, Th, U; 1 ng/L for Ga, Ge, Rb, Sr and Sb; ~ 10 ng/L for Ti, V, total P (Ptot), Cr, Mn, Fe, Co, Ni, Cu, Zn and As). The SLRS-6 (Riverine Water Reference Material for Trace Metals certified by the National Research Council of Canada) was used to check the accuracy and reproducibility of analyses (Yeghicheyan et al., 2019).

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## 2.4. Size separation procedure for assessing colloidal forms of elements

About 1 L of unfiltered river water was collected in pre-cleaned light-protected polypropylene jars for size fractionation and filtered through disposable sterile Nalgene filter unit (0.45 µm). This was followed by centrifugal ultrafiltration through 3 kDa single-used Amicon Ultracell cartridges (see details in Krickov et al., 2019). The disposable 50-mL polypropylene cartridges were fitted with regenerated cellulose filters which were first treated with 1% HNO₃, and rinsed 3 times via passing 50 mL of MilliQ water through the membrane. The DOC and metal blanks were 3 to 10 times lower than the minimal concentrations of these components in experimental fluids. The first portion of sample supernatant that passed through Amicon cartridges was discarded. Each fraction of filtrate was subdivided into a non-acidified and acidified portion, for carbon/UV/pH and trace element measurements, respectively. The centrifugal ultrafiltration was run at 20°C using Eppendorf 5920 centrifuge (4000 rpm, 20 min). Vacuum filtration was performed using a Mityvac MV8255 PVC-made hand pump. The filtration and ultrafiltration allowed for separation of colloids (3 kDa – 0.45 µm) and LMW fraction (LMW< 3 kDa). All filtrations and colloidal separations were run in a protected laboratory environment within 24 hours of collection from the river.

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## 2.5. Data treatment and flux calculation
Element export fluxes were calculated based on measured discharges at the Oxino gauging station and concentrations over the period of observation (August 2015 – December 2019). Based on these data, we first estimated daily load (mass of component day⁻¹) obtained from a time series of paired river flow and constituent concentration data. Annual element fluxes were estimated similar to LOADEST method (https://water.usgs.gov/software/loadest/, Holmes et al., 2012) from calculated daily element loads. The latter were obtained from a calibration regression, applied to daily discharge from RHS. The calibration regression was constructed from a time series of paired streamflow and measured element concentration data for 2015-2019. A similar method was used for flux estimation in other Arctic and subarctic rivers such as Severnaya Dvina (Chupakov et al., 2020) and Taz (Pokrovsky et al., 2022), for which a large number of hydrochemical measurements and daily discharge values were available over a period of several consecutive years.

Power regressions between river discharge and solute concentration, widely used in boreal and Arctic rivers (i.e., Gordeev and Sidorov, 1993; Zakharova et al., 2005, 2007; Chupakov et al., 2020) were used to represent the seasonal variation of major and trace components:

$$C_i = k \cdot Q_i^n \quad (1)$$

where Cᵢ represents measured solute concentration for a given day of the year, Qᵢ is water discharge for this day, k and n are empirical constants for each element.

Based on obtained size fractionation data on river solutes, we also calculated the mean multiannual monthly concentrations in the < 0.45 µm and < 3 kDa fractions. This allowed quantifying the percentage of the LMW< 3kDa element in the total dissolved (< 0.45 µm) form and assessing the share of colloidal or LMW fraction in overall element export by the Pechora River during different seasons of the year. Logarithmic dependences between element colloidal
fraction and river discharge were used to approximate seasonal variations of colloids in the river water.

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## 3. Results

### 3.1. Seasonal behavior of dissolved (< 0.45 µm) elements

Mean multi-annual discharge, concentrations of TSS, major and trace solutes are presented in **Table 1** whereas mean monthly parameters are listed in **Table S1** of the Supplement. The seasonal dynamics of solute concentration allowed us to distinguish 3 groups of elements. The Group 1 was represented by high mobility elements that likely originated from groundwater reservoirs and exhibited maximal concentrations during winter and minimal concentrations during spring flood and autumn high flow: DIC, Cl, SO₄, Li, B, F, Na, Mg, Ca, Rb, Sr, Mo, Ba, W, U(VI) as illustrated in **Fig. 2**. These elements decreased their concentration with increased discharge. Group 2 was represented by elements of transitional hydrological periods and exhibited maximal concentration during the spring flood (May-June) and autumn high flow periods (October-November): DOC, Be, Al, Ti, V, Ni, Ga, Cs, Se, Nb, Y, Zr, REEs, Hf, Th. While the concentration (C) of group 2 elements generally increased with increased discharge (Q), two independent trends of C with Q (winter → spring and summer → autumn) could be often distinguished (**Fig. 3**) as some elements in this group (DOC, Be, V, Ga) demonstrated maximal concentrations immediately after freshet and during autumn floods.

In addition to these 2 main elemental groups, a third group can be formed from other solutes that did not fit in. According to their chemical properties, K and Rb should reside within group 1 as they demonstrated accumulation in winter, during low discharge, but also exhibited peaks during high discharge in autumn (**Fig. 4 A**). It is worth noting that certain
elements (Si, K, Ge) strongly involved in biological processes exhibited minimal concentration
during summer baseflow (July-September), during the maximal terrestrial plant uptake and
activity of plankton, periphyton and aquatic macrophytes. However, typical macronutrients
(NO3 and PO4) did not exhibit a clear minimum during the summer period. Instead, they
demonstrated local maxima during winter but otherwise displayed no clear seasonal or
discharge-related pattern (Fig. 4 B). Redox-sensitive Mn (but also Co and Cd) had maximal
concentrations during the ice-off, just before the spring peak of discharge (Fig. 4 C). Iron
exhibited very specific seasonal dynamics with 3 annual maxima (spring flood, autumn flood
and winter baseflow) as illustrated in Fig. 4 D. Finally, numerous trace elements did not exhibit
any specific seasonal dynamics and their concentrations were independent of river discharge:
Cu, Zn, Cr, Ga, Ge, As, Sn, Tl.

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3.2. *Element pairwise correlations*

Considering the entire dataset for the < 0.45 µm fraction, elemental groups 1 and 2 were
distinguished based on their pairwise (Pearson) correlations (Table 2). Group 1 consists of
labile high solubility elements, strongly correlated (R<sub>Pearson</sub> ≥ 0.57, p < 0.05) with DIC and
included major anions, Si, alkaline (Li, Na, K and Rb) and alkaline-earth metals (Mg, Ca and
Sr), trace oxyanions and neutral molecules (B and Mo), and uranyl, thereby corresponding to
the 1st group of elements identified via their concentration – discharge pattern in section 3.1.
The 2nd group of elements exhibited strong correlations with DOC (R<sub>Pearson</sub> ≥ 0.70, p < 0.05)
and was represented by low mobile trivalent and tetravalent hydrolysates (Al, Ti, Y, Zr, REEs,
Hf and Th) and some trace metals known to be linked to DOM (Be, Ni, V, Nb and Cs). The
majority of these elements demonstrated a strong (R<sub>Pearson</sub> ≥ 0.90, p < 0.05) link to total
dissolved Al, suggesting either a common origin or an Al enriched colloidal carrier (see section
3.3 below) in the river water. Note that correlations of elements with total dissolved Fe—which
is known to be important carrier of numerous trace elements in river waters in the colloidal form—were low and that only chloride, phosphate, nitrate, Li, B, Na, Mg, Si, P<sub>tot</sub>, Ca and As showed significance (p < 0.05). This may indicate a common source (underground water) of Fe and these elements in the river water during ntably winter baseflow.

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326 **3.3. Role of colloidal transport across seasons**

Colloidal forms of DOC, major and trace elements were investigated on several occasions during winter baseflow, spring flood and the summer-autumn period. The mean annual and monthly values of the colloidal fraction (Table 1, Table S2 of the Supplement) allowed us to distinguish several groups of elements according to their colloidal affinity. On the annual scale, low solubility, low mobility trivalent and tetravalent elements, such as Fe, Y, REEs, Zr, Hf, Th and Nb, but also Mn exhibited > 80-90% of colloidal forms. A sizable colloidal proportion (35 to 80 % in average) was observed for DOC, light trivalent and tetravalent hydrolysates (Al, Ga and Ti), divalent transition metals (Co, Ni, Cu and Zn), P<sub>total</sub>, some trace metals (Be, V, Cr, Cd and Pb), As, Ge, Se and U. Weakly (< 20 %) affected by colloidal forms were major anions (DIC, Cl and SO<sub>4</sub>), Si, alkalis (Li, Na, K and Rb), alkaline-earth metals (Mg, Ca, Sr and Ba), nutrients (NO<sub>3</sub>, NH<sub>4</sub> and PO<sub>4</sub>), oxyanions and neutral molecules (B, Mo, Sb and W).

The seasonal distribution of colloidal forms demonstrated that, for all solutes, the highest proportion of colloids is observed during the spring flood or summer period and the lowest proportion occurs during the winter baseflow. Based on the relationship between percentage of colloidal fraction and river discharge, we distinguished 3 elemental groups. Dissolved organic carbon, alkaline-earth metals (Be, Mg, Ca, Sr and Ba), Si, trivalent and tetravalent trace elements (Al, Ti, Ga, Y and Zr), Co, Nb and U exhibited an increase in
colloidal fraction with increased discharge following the general order ‘winter < summer/autumn < spring’ (Fig. 5). Note a huge contrast in U colloidal forms, ranging from < 10% during winter baseflow to > 80% during spring flood. The 2nd group of solutes exhibited rather constant proportion of colloids regardless of the season, and did not show any dependence on discharge: Mn, Fe, Ptot, V, Zn, Cd, Pb, As, Ge, Cs, Hf and Th. Close to this group are organically-bound trace elements (Cr, Ni, Cu, Se and REEs) that demonstrated a summer-time maximum of colloidal forms, pronounced at moderate discharges.

### 3.4. Seasonal and annual fluxes of elements in the Pechora River.

The four-year mean monthly and annual discharges and fluxes of TSS and all dissolved components are listed in Tables 1 and Table S1. The winter baseflow (6 months, from November to end of April) accounted for less than 10% of DOC and lithogenic elements (Al, Ti, Zr, Hf, Y and REEs) and heavy metals (V, Cr, Ni, Cu and Cd), < 5% of the TSS and 20 to 30% of soluble mobile elements (DIC, Cl, SO4, Na, Mg, Ca, Mn, Sr and U). The spring flood (May-June) provided 40 to 60% of overall river export of DOC, NH4, PO4, Si, K, Ti, Mn, Fe, V, Cr, Ni, Cu, As, Zr, Ba and Pb and dominated (> 60%) the export flux of hydrolysates (Al, Y, REEs, Nb, Ti, Zr, Hf and Th) and Cd. The summer and autumn period (July to October) accounted for 30 to 40% of annual flux of TSS and all elements with the exception of Mn and Al (17 to 21%). The summer baseflow period (July and August) provided 21 ± 4% of annual solute flux. Moreover, considering August as most representative for the open-water baseflow period (given that the August discharge is 5407 m³ s⁻¹ which is similar to the mean annual value of 5131 m³ s⁻¹) we found that, for 65 dissolved major and trace components, August accounted for 9 ± 2% of annual export flux, because mean August concentration were within ±10% similar to those of mean annual values. Indeed, we noted that the average ratio of mean solute concentration in August to that of mean annual was 0.94±0.25 (n = 65). For just a few (5 to 7) trace elements, for which the agreement was between 30 and 40%, this was still within the uncertainty of the mean
annual. This important observation indicates that, for the Pechora River, the annual downstream fluxes of all solutes can be reasonably (± 20 %) approximated via monitoring one single month of the year.

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### 3.5. *Trends in the Pechora discharge based on historical data*

For assessment of inter-annual dynamics of the Pechora River discharge we used mean monthly and annual values for the Oxino gauging station available from 1932 to 2019. It can be seen from the graphs shown in **Fig. S1** and the related **Table S3** of Mann-Kendall parameters for each month of the year that there is significant increase in annual discharge of the river with maximal increase observed during the hottest month of the year – July. This can tentatively be linked to climate warming and degradation of permafrost landscapes. Increase of winter period discharge can be linked to change of groundwater regime and opening of new taliks leading to increased connection between river water and groundwater reservoirs. At the same time, there is no statistically significant change in the Pechora River discharge during spring flood (May-June) and autumn high flow (October). It is possible that during these transitional periods, the discharge is strongly controlled by a number of factors acting in opposite directions such as the rate of snow thaw, atmospheric precipitation, local ice jams, landslides on permafrost-bearing slopes of the river valley and retention of water by mires at the watershed. Note that the amount of hydrochemical measurements from the RHS (TOC, Alk, Cl, SO₄, Ca, Mg, Si and Na + K), which are typically based on monthly to bi-monthly sampling at the terminal gauging station of the Pechora River and available from the mid-1970's, was not sufficient to assess the temporal trend in concentrations and related fluxes.

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## **4. Discussion**

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### *4.1 Mobilization of solutes from the watershed to the river across the seasons.*
Sizable contribution of the winter period to the total annual export of generally mobile elements (DIC, Cl, SO₄, Na, Mg, Ca, Mn, Sr, Mo and U) in the Pechora River is in agreement with general knowledge of Ca, Mg, DIC and other soluble element seasonality in boreal rivers, including the Russian subarctic zone and neighboring Severnaya Dvina River (e.g., Pokrovsky et al., 2012; Chupakov et al., 2020). The main source of these elements to the river water is deep groundwater which is progressively diluted by surface waters during the open-water period, hence the production of typical anti-correlation between concentration and discharge. This is consistent with quite high concentrations of these labile elements in the groundwater of the Pechora basin (Malov et al., 2015, 2022), exceeding those in the river water by at least a factor of 10.

The flux of DOC and related trace metals of the Pechora River is dominated by the spring freshet (~40 to 60% during May-June), which corroborates previous observations in large and small high-latitude watersheds (Finlay et al., 2006; Raymond et al., 2007; Striegl et al., 2007; Griffin et al., 2018; Chupakov et al., 2020; Pokrovsky et al., 2020, 2022). Similar to other subarctic rivers of permafrost peatlands, the SUVA₂₅₄ (from 4 to 5 L mg⁻¹ m⁻¹, see Fig. S2) demonsrated essentially allochthonous nature of DOM across the year, and suggested its delivery from surrounding peatlands and forest litter.

The peak of export flux during the freshet was observed for trivalent and tetravalent hydrolysates and some heavy metals exhibiting strong affinity to DOM (Cu, Cr, Ni, Cu, Cd and Pb). This is also known for Alaskan rivers (e.g., Rember and Trefry, 2004). Note that some elements of this group (DOC, Be, V and Ga) exhibited maximal concentrations immediately after spring flood and during autumn rains, thereby reflecting leaching of these elements from forest litter and upper soil organic layers. Presumably, the input of these elements to the Pechora River is at its maximum during *i*) thawing of upper soil and litter layer in spring, which occurs after the maximum of the freshet discharge, and *ii*) massive leaching of fresh litterfall by
late autumn rains. Another interesting observation is that redox-sensitive Mn (and also Co and
Cd, which are known to be strongly bound to Mn oxy-(hydr)oxides) exhibited maximal
concentrations after ice-off, just before the spring peak discharge (Fig. 4 C). Here we do not
exclude enhanced mobilization of dissolved Mn and related elements due to sunlight-
accelerated reduction of particulate Mn (III, IV) oxides by DOM in shallow river sediments as
is known in other aquatic environments (Bertino and Zepp, 1991).

Dissolved Fe exhibited particular seasonal behavior that was different from the behavior of
typical insoluble trivalent hydrolysates. First, there was a spring freshet maximum, when Fe was
transported in the form of organic colloids. This is also known from other high latitude regions
such as the Severnaya Dvina River (Pokrovsky et al., 2010) and Central Siberian rivers (Bagard et
al., 2011). Another strong maximum of Fe was observed in winter, when the export of this
element approached 20% of the overall annual flux. This strongly contrasted with other trivalent
and tetravalent hydrolysates whose export flux in winter did not exceed 10 % of the annual level.
We believe that the winter maximum of Fe was linked to underground feeding of the river by
anoxic waters containing abundant Fe(II). The underground waters of northeast European Russia
are known to be rich in divalent iron (Malov, 2013; Malov et al., 2015). According to our data of
the Narian-Mar region (Pechora mouth), Fe(II) concentration in shallow groundwater ranges from
1,000 to 2,000 µg L⁻¹, which is sizably higher than the mean Fe concentration in the Pechora River
waters during winter period (400 to 600 µg L⁻¹). However, given that the majority (> 90 %) of
total dissolved Fe in the river water was present in the colloidal (3 kDa – 0.45 µm) form, we
suggest that, in the main stem of the Pechora River, the groundwater-originated divalent Fe was
rapidly oxidized and stabilized in the form of high molecular weight organo-ferric colloids as is
the case in surface waters of both permafrost and non-permafrost regions (Pokrovsky et al., 2010,
2016; Bagard et al., 2011; Krickov et al., 2019; Hirst et al., 2017). Furthermore, in the beginning
of the winter season, when the surrounding peatlands are not entirely frozen, a sizable fraction of
riverine Fe can originate from suprapermafrost influx of shallow subsurface / soil waters flowing to the river along the boundary of the active (unfrozen – frozen) layer. In permafrost peatlands similar to those widely abundant in the low reaches of the Pechora River, such waters are known to be strongly enriched in colloidal Fe (Raudina et al., 2019, 2022). At the redox interfaces in peatlands, a large fraction of DOM is removed via coagulation (coprecipitation) with iron hydroxides (Riedel et al., 2013). Part of these Fe(III)-OC compounds can be represented by colloidal forms as demonstrated above. Moreover, strong removal of DOC together with Fe(III) oxy(hydr)oxide particles (e.g., Orem and Gaudette, 1984; Nierop et al., 2002; Riedel et al., 2013) occurring during baseflow periods may be partially responsible for lowering DOC concentration at low discharge

Between the two maxima (winter baseflow and spring flood), dissolved Fe exhibited a weak summer minimum which could be linked to Fe removal via either biological (bio-degradation, uptake by plankton and periphyton, e.g., Shirokova et al., 2017) or physico-chemical processes of organo-ferric colloids photodegradation (e.g., Oleinikova et al., 2017b; Chupakova et al. 2018). Macro-nutrients (NO₃, NH₄, PO₄ and Si) also exhibited a weak summer minimum, likely to be linked to biological uptake by river plankton and periphyton as is known for other rivers in temperate and boreal zones (Newcomer Johnson et al., 2016; Chupakov et al., 2020) and notably those draining through permafrost peatlands (Vorobyev et al., 2017; Serikova et al., 2018; Pokrovsky et al., 2022).

Although element correlations do not imply causality, the groups of solutes identified by their affinity to either DIC or DOC in section 3.2 are fully consistent with the groups established based on a concentration-discharge relationship and seasonal pattern in concentrations. It is interesting that Al was the best proxy for a number of low solubility, and usually low mobility, hydrolysates that were presumably transported in the river water in the form of organo-aluminum colloids. The governing role of Al enriched organic colloids in
element speciation in permafrost peatland soil porewaters has been evidenced via on-site size
fractionation procedure (e.g., Raudina et al., 2021). In contrast, correlations with total dissolved
Fe, known to be important carrier of a number of trace elements in boreal and subarctic river
waters in the form of colloids (Vasyukova et al., 2010; Krickov et al., 2019) were quite low,
and only some anions (Cl, SO₄, PO₄ and NO₃), Li, B, Na, Mg, Si, Ca and As exhibited sizable
correlations with Fe. While for As, this can be due to its transport in the form of organo-ferric
colloids (e.g., Guo et al., 2011), the correlations between other elements and Fe merely reflect
their common origin during winter baseflow, when the river is fed by deep groundwater, and
becomes strongly enriched in both Fe(II) and these labile elements. The groundwater source is
represented by the products of mother rock (limestone, dolomite, and gypsum) dissolution and
some chloride and sulfate brines, known for the NE European Arctic basin (Malov et al., 2015,
2022). This is further supported by strong (0.99 ≤ R<sub>Pearson</sub> ≤ 0.94, p < 0.01) correlations between
chloride concentration and that of SO₄, Li, B, Na, Mg, Ca and Sr. Altogether, these mark a
strong link between river water and groundwater brines that is mostly pronounced during
winter baseflow. In addition to this groundwater source, some nutrients (P, N and Si) could be
accumulated under ice due to their diffusion from organic-rich sediments in the river bed and in
the riparian zone similar to rivers of other permafrost peatlands (Vorobyev et al., 2017; Krickov
et al., 2018; Pokrovsky et al., 2022). To summarize, the two groups of major and trace elements
revealed in this work reflect both different sources of elements in the river and different forms
of elemental transport in the river water.

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4.2. Colloidal export of DOC and trace elements

Transport of DOC and insoluble trace metals, notably hydrolysates such as Al, Ti, Ga, Y,
REEs, Ti, Zr, Hf and Th and also heavy metals (e.g., V, Cr, Co, Ni, Cu, Cd and Pb) is strongly
linked to DOC as evident from pairwise correlation (see sections 3.2 and 4.1 above) because

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these elements are carried in the form of organo-mineral (Fe and Al) colloids similar to permafrost-free European subarctic (Vasyukova et al., 2010; Ilina et al., 2016), Siberian (Pokrovsky et al., 2006; Bagard et al., 2011), Canadian (Cuss et al., 2018) and Alaskan (Stolpe et al., 2013a, b) watersheds. In this investigation, we provide a new dimension to the documentation of elemental transport in high-latitude permafrost-impacted rivers: the variation of colloidal export of major and trace elements across seasons.

Analysis of the colloidal fraction of solutes as a function of season (Table S2) and river discharge (Fig. 5) demonstrated sizable increase in the relative proportion of colloidal forms with an increase in discharge for DOC and many trace elements. Such a contrast between baseflow (winter, summer) and highflow (freshet, autumn rains) shown in Fig. 5 can be explained by enhanced export of allochthonous large-sized organic and organo-Fe-Al colloids generated during spring flood and autumn storms. This occurs via plant litter leaching and input of surface waters from adjacent peatlands to the main river stem. A prominent example is U, showing a sharp increase in colloidal forms with an increase in discharge between the winter baseflow and the spring flood (from 14 – 16 % in January-February to 92 – 93 % in May-June, Table S2, Fig. 5 F). As is known for other Arctic rivers (Pokrovsky et al., 2010; 2022; Bagard et al., 2011), uranium is present as ionic UO₂²⁺-carbonate/bicarbonate complexes in winter, at high DIC, and as uranyl-organic / organo-ferric colloids during the spring flood and high flow in autumn (e.g., Porcelli et al., 1997). A group of trace elements (Cr, Ni, Cu, Se and REEs) exhibiting a summer-time maximum of the colloidal fraction could mark their mobilization from peatlands. These elements exhibit specific affinity to DOM and demonstrate elevated concentrations in peat (Stepanova et al., 2015) and peat water (Raudina et al., 2021; Pokrovsky et al., 2018) from which they are delivered to the river during the late summer and autumn period, at moderate discharges.
The affinity between major and trace elements and organic and organo-mineral colloids strongly controlled the forms of elements exported by the river. Over the course of the year, many solutes (e.g., total dissolved P, Be, 'heavy' metals (Mn, Co, Zn, Cr, V and Pb), Se, trivalent and tetravalent hydrolysates (Fe, Al, Ga, Y, REEs, Zr, Hf and Th) and Nb) demonstrated a stable and sizable (> 60-80 %) proportion of colloidal forms. Because the maximal fraction of colloids for these elements was observed during the spring flood, at maximum discharge, the overall colloidal export of these elements greatly exceeded their true dissolved (< 3 kDa) yield. As a result, the majority (> 60%) of annual export for these usually insoluble trace elements in the Pechora River occurred in the form of colloids. This result corroborates previous observations for the Severnya Dvina River (Pokrovsky et al., 2010), which drains through boreal taiga forest and peatlands and small rivers across the western Siberian peatlands (Krickov et al., 2019). In particular, the latter authors demonstrated that, in the permafrost zone, the high molecular weight colloidal fraction of Fe, Al and Y, REE, Zr, Hf and Th was high (90 ± 10 %) in spring but decreased to 60 ± 20 % in summer. Similarly, divalent metals (Cu, Co, Cd, Pb and Mn) had the highest colloidal fraction in spring and the lowest in summer and autumn, across all permafrost zones.

The reason for this similarity in colloidal transport in the river water between different regions is most likely similar environmental context of considered watersheds (Severnaya Dvina, Pechora, and small western Siberia Lowland rivers), which includes flat lands covered by coniferous and mixed forest, peatlands and bogs. Mobilization of colloids to the river water occurs either during spring flood, essentially from plant litter when organic topsoil is thawed but the rest of the mineral soil is frozen, or during summer baseflow, when Fe(II)-rich underground waters are released in the riparian/hyporheic zone of the river and subjected to oxidation in the presence of riverine DOM. This massive precipitation of Fe(III) oxy(hydr)oxides produces organo-ferric and organo-aluminum colloids of high molecular
weight (Vasyukova et al., 2010; Krickov et al., 2019) which then persist in the water column of the river.

In addition, the discharge of DOC-rich waters from the peatlands to the river occurs mostly at the end of the active season (end of summer – autumn), as is demonstrated by water isotope tracing in Siberian peatlands (Ala-aho et al., 2018). In permafrost affected territories such as that of the Pechora River, especially within the continuous permafrost zone, soil water is transported along the bottom of the active (thawed) layer in the form of so-called suprapermafrost water, rich in colloidal OC, Al and Fe (Raudina et al., 2021). However, regardless of permafrost coverage of the river watershed, when comparing two neighboring mid-sized Arctic rivers (Severnaya Dvina and Pechora), the similarity of mean summer time concentrations of Fe (220-250 and 230-330 µg/L, in Pechora and Severnaya Dvina, respectively) and Al (20-50 and 60-65 µg/L, respectively) suggests that these are organo-ferric-aluminium colloids that dominate solute transport in both rivers.

### 4.3. Annual export fluxes of river solutes and comparison with other Arctic rivers

Annual DIC yield for the Pechora River (3.4±0.5 t km⁻² y⁻¹) is comparable to that of the Severnaya Dvina River (4.3±0.2 t km⁻² y⁻¹, Chupakov et al., 2020) and the Yukon and Mackenzie rivers (5.4±0.3 and 3.7±0.3 t/km²/y, respectively; see Tank et al., 2012). The DOC yield for the Pechora River (4.0±1.0 t C/ km²/y) is equal to that of the Severnaya River (4.2±0.8 t C/ km²/y, Chupakov et al., 2020), which is the highest among all known rivers with similar runoff, flowing to the Arctic Ocean, given that the Ob, Yukon, Lena, Yenisei, Mackenzie and Taz Rivers exhibit a DOC yield from 0.5 to 2.7 t C km⁻² y⁻¹ (Gordeev et al., 1996; Raymond et al., 2007; Pokrovsky et al., 2022). Equally high annual DOC yields have been reported for small rivers draining peatlands and bogs (i.e., Suzuki et al., 2006; Striegl et al., 2007). Thus, the peatland draining Nadym River (64,000 km² watershed area) of the western Siberia Lowland,
which is located entirely in the discontinuous permafrost zone, yielded 4.4 t C km⁻² y⁻¹ (Gordeev et al., 1996). Also, a small tributary of the Yenisei River draining permafrost peatlands exhibited a DOC yield of > 5 t C km⁻² y⁻¹ (Gandois et al., 2021).

The yields of other macro-solutes such as Cl, Ca, Na, K and Si by the Pechora River measured in the present study with high-resolution frequency are in general accord (± 30 %) with former assessments of export flux by the RHS (1976-1985) as calculated from low-frequency samplings (Gordeev et al., 1996). At the same time, the fluxes measured in this study (Table 1) are also comparable, although somewhat lower (by a factor of 1.3 to 2.3), than more recent estimation of annual yields of macro components from 1985 to 2017, which is based on monthly sampling of Russian Hydrometeorological Survey (3.1±1 t Cl km⁻² y⁻¹; 4.1±2 t SO₄ km⁻² y⁻¹; 5.8±1 t Ca km⁻² y⁻¹; 1.6±0.5 t Mg km⁻² y⁻¹, and 4.7±1.0 t DIC km⁻² y⁻¹; Danilenko et al., 2020). A likely reason for this disagreement is insufficient sampling by RHS during critical periods of the year—spring flood and autumn high flow—which could lead to overestimation of discharge-weighted fluxes calculated based on monthly sampling.

In view of lack of reliable data on multi-annual export flux of trace elements by large or mid-size Arctic rivers, we could compare mean multi-annual concentrations of ~45 trace elements in the Pechora River measured in this work to those reported in most recent studies on rivers of Northern Eurasia (Chupakov et al., 2020; Kolesnichenko et al., 2021; Vorobyev et al., 2021; Pokrovsky et al., 2022; Soromotin et al., 2022) and mean world rivers (Gaillardet et al., 2003) as illustrated in Fig. S3 of the Supplement. The average ratios of trace element concentration (n = 42) in the Pechora River to that in the world rivers, Severnaya Dvina, Taz, Ob, and Lena were equal to 0.95, 0.96, 1.15, 0.73 and 1.04, respectively. Compared to the world average, the Pechora River is rich in Zn, Fe, Ge, Y and Pb, and poor in Cr, Cd, Cs, Th and U. However, for this comparison it is difficult to distinguish i) a bias towards large tropical rivers (Amazon, Congo) in Gaillardet's et al. (2003) compilation; ii) analytical problems on
some trace elements measured in the past, including filtration artifacts, and iii) effect of season and lithology. Note that the difference by a factor of 2 to 3 with the world average cannot be considered as significant, given that this is within the variation among different rivers of the world (Gaillardet et al., 2003). Therefore, we believe that more pertinent for this purpose is a comparison between Pechora and other rivers of Northern Eurasia, recently studied by our group using common filtration and analytical techniques.

Compared to the neighboring Severnaya Dvina River draining permafrost-free taiga forest and peatlands, Pechora is enriched in Zn, Cd, and Pb, most likely due to their release from peatland waters and dispersed ice where the concentration of these elements is exceptionally high (i.e., Lim et al., 2021). Pechora is impoverished in Ca, Mg, Sr, Ba and U due to smaller proportion of carbonate-bearing rocks on its watershed compared to that of Severnaya Dvina. In contrast, compared to the Taz River, Pechora is enriched in U due to lack of carbonate rocks in the former, and it is strongly impoverished in Mn, Fe and Co. These elements are exceptionally high in the Taz River water, where they are accumulated during long winter stagnation and redox processes within the vast floodplain (Pokrovsky et al., 2022). Finally, there is a reasonable agreement in most element concentrations between Pechora, Lena and Ob (given drastic lithological contrast of their watersheds). An impoverishment of Pechora in dissolved Ti, Zr and Hf may reflect higher concentration of silicate suspended matter capable of providing these insoluble elements in organo-colloidal forms (i.e., Krickov et al., 2019, 2020) in a large river compared to a smaller one.

A possibility of using mean August discharge and concentration of solutes to approximate (± 20%) mean annual fluxes and concentrations as presented in section 3.4 is intriguing and thus requires a verification if this finding is not coincidental. To the best of our knowledge, only two other Arctic rivers (Severnaya Dvina and Taz) were sampled with sufficiently high frequency allowing to compare mean multi-annual concentrations and fluxes
of all solutes to those measured during August. For the Taz River, the average ratio of discharge and ~45 solute concentrations in August to those of mean annual is 0.98±0.31. More significant deviations (40-50 %) are observed for a few elements (Al, V, Cd, Cs and Bi) which is still within the range of uncertainties. For the Severnaya Dvina River, most representative period for sampling would be mid-July to mid-August given that the discharge and solute concentration during this time could be approximated within ±10 % to those of the annual values (the ratio is 0.96±0.10) and none of the major or minor solute concentration deviated from mean annual values by more than 15 %. Overall, we argue that, across the Arctic, observations during July-August in the non-permafrost zone and August in the permafrost region can be used for adequate (±20 %) approximation of annual fluxes of all major and trace solutes. Such an uncertainty is fairly comparable with inter-annual variations of riverine export fluxes.

In terms of discharge evolution over past decades, the Pechora River behaves similar to other great and mid-sized Siberian Rivers, with a general increase in discharge during baseflow (especially winter) and less pronounced trends during spring flood (Peterson et al., 2002; McClelland et al., 2006; Smith et al., 2007; Shiklomanov et al., 2006; Bring et al., 2016). At the same time, the Pechora River marks a drastic contrast to the neighboring Severnaya Dvina River which is similar in size and landscape context, but without permafrost in its watershed. Primarily, there is a sizable difference in the water flow, water temperature and heat flux during the open water season in the Northern Dvina and Pechora Rivers at the scale of 9 to 45 years (Georgiadi and Groisman, 2022). In the Severnaya Dvina River, the spring flood and summer baseflow demonstrated statistically significant decrease in discharge (Chupakov et al., 2020). Other seasons did not demonstrate any sizeable trend or exhibited a trend that was within the uncertainty. Most likely the reason for this difference in long-term discharge trend between the Pechora and Severnaya Dvina rivers is due to a lack of permafrost [which regulates the water
supply to the river during baseflow, when trends were most pronounced] in the watershed of
Severnaya Dvina.

5. Conclusions

Several years of monitoring dissolved (< 0.45 µm) major and trace solutes in the second largest European Arctic river, the Pechora River, have demonstrated the existence of typical groups of elements depending on the pattern of 'water discharge – element concentration'. Group 1 is comprised of generally soluble, highly mobile major cations and anions, together with traces oxy-anions and U, and marks the influence of underground water feeding. These elements exhibited sizable export during the winter period under ice. Group 2 included DOC, divalent trace metals, trivalent and tetravalent hydrolysates which were strongly impacted by colloidal transport in the form of organic complexes and organo-Al Fe colloids. Trends were especially pronounced during high discharge periods. Because the maximal fraction of colloids for these elements was observed during spring flood, at maximum discharge, the overall colloidal export of these elements greatly exceeded their true dissolved (< 3 kDa) yield. As a result, the majority (>60%) of the annual export of these metals and insoluble trace elements occurred in the form of colloids. These two groups of elements identified in terms of discharge – concentration dependence, colloidal share and seasonal distribution of element export fluxes were consistent with those described previously in other subarctic and boreal rivers in both permafrost-free and permafrost-bearing forest, wetland and tundra landscapes.

Based on four years of regular observations, we provide mean annual yields of DOC, DIC, major and trace elements of the Pechora River. The yields were comparable to those reported for the neighboring Severnaya Dvina River and small rivers of western Siberian Lowland that also drain through taiga forest and peatland biomes. We demonstrated possibility to adequately approximate the total annual yield (with uncertainty < 20 %) via several samplings in August,
669 when the discharge of the river was very close to the daily annual average and mean monthly
670 concentrations of major and trace solutes were similar to mean multi-annual values. This has
671 potential to allow straightforward assessment of elemental yields in Artic and subarctic rivers via
672 single extensive campaign during one month of summer baseflow. These findings, obtained for the
673 first time on mid-sized Artic River, provide solid basis for possible extrapolation of riverine export
674 fluxes and mechanisms of element migration to much larger territory of permafrost-affected
675 northern territories.

676
677 Acknowledgements : Main funding from RSF grant No 22-17-00253 is acknowledged. OP was
678 partially supported by the TSU Development Programme ‘Priority-2030’. Chris Benker is thanked
679 for English proofread. We thank the Editor Don Porcelli and two anonymous reviewers for
680 constructive and insightful comments and suggestions.
681

682 **Data availability:** All primary data on water discharges and concentrations of major and trace
683 solutes in the Pechora River are available at the Mendeley Data Portal:
684 Pokrovsky, O., Chupakov, A., 2023. Hydrochemistry of the Pechora River water, 2015-2018.
685 Mendeley Data, V1, https://data.mendeley.com/datasets/4mk7nrrk8d.

687
688 References

689 Ala-Aho, P., Soulsby, C., Pokrovsky, O.S., Kirpotin, S.N., Karlsson, J., Serikova, S.,
690 Manasypov, R.M., Krickov, I., Lim, A., Tetzlaff, D., 2018. Permafrost and lakes control
691 river isotope composition across a boreal-arctic transect in the western Siberia lowland.
692 Environmental Research Letters, 13 (3), Art No 034028.

693 Andersson, K., Dahlqvist, R., Turner, D., Stolpe, B., Larsson, T., Ingri, J., Andersson, P., 2006.
694 Colloidal rare earth elements in a boreal river: Changing sources and distributions during the
695 spring flood. Geochim. Cosmochim. Acta, 70(13), 3261-3274.

696 Bagard, M.L., Chabaux, F., Pokrovsky, O.S., Prokushkin, A.S., Viers, J., Dupré, B., Stille, P.,
697 2011. Seasonal variability of element fluxes in two Central Siberian rivers draining high
698 latitude permafrost dominated areas. Geochim. Cosmochim. Acta 75, 3335-3357.

699 Behnke, M.I., McClelland, J.W., Tank, S.E., Kellerman, A.M., Holmes, R.M., Haghipour, N.,
700 Eglinton, T.I., Raymond, P.A., Suslova, A., Zhulidov, A.V., Gurtovaya, T., Zimov, N.,
701 Zimov, S., Mutter, E.A., Amos, E., Spencer, R.G.M., 2021. Pan-Arctic riverine dissolved
702 organic matter: Synchronous Molecular Stability, Shifting Sources and Subsidies. Global
703 Biogeochemical Cycles, 35(4). https://doi.org/10.1029/2020gb006871.

704 Bense, V.F., Kooi, H., Ferguson, G., Read, T., 2012. Permafrost degradation as a control
705 on hydrogeological regime shifts in a warming climate. J. Geophys. Res. 117, F03036.
706 https://doi.org/10.1029/2011jf002143.
707 Bertino, D.J., Zepp, R.G., 1991. Effects of solar radiation on manganese oxide reactions with selected organic compounds. *Environ. Sci. Technol.*, 25 (7), 1267-1273.

709 Bring, A., Fedorova, I., Dibike, Y., Hinzman, L., Mard, J., Mernild, S.H., Prowse, T., Semenova, O., Stuefer, S.L., Woo, M.-K., 2016. Arctic terrestrial hydrology: A synthesis of processes, regional effects, and research challenges. *J. Geophys. Res.: Biogeosci.*, 121, 621-649.

713 Chupakov, A.V., Pokrovsky, O.S., Moreva, O.Y., Shirokova, L.S., Neverova, N.V., Chupakova, A.A., Kotova, E.I., Vorobyeva, T.Y., 2020. High resolution multi-annual riverine fluxes of organic carbon, nutrient and trace element from the largest European Arctic river, *Severnaya Dvina*. *Chem. Geol.* 538, Art No 119491, doi:10.1016/j.chemgeo.2020.119491.

718 Chupakova, A.A., Chupakov, A.V., Neverova, N.V., Shirokova, L.S., Pokrovsky, O.S. (2018) Photodegradation of river dissolved organic matter and trace metals in the largest European Arctic estuary. *Science Total Environment*, 622–623, 1343–1352.

721 Cochand, M., Molson, J., Lemieux, J.M., 2019. Groundwater hydrogeochemistry in permafrost regions. *Permafrost Periglacial Proc.* 30, 90-103.

723 Cooper, L.W., McClelland, J.W., Holmes, R.M., Raymond, P.A., Gibson, J.J., Gua,y C.K., Peterson, B.J., 2008. Flow-weighted values of runoff tracers (δ¹⁸O, DOC, Ba, alkalinity) from the six largest Arctic rivers. *Geophys. Res. Lett.* 35, L18606, doi:10.1029/2008GL035007.

727 Cuss, C.W., Donner, M.W., Grant-Weaver, I., Noernberg, T., Pelletier, R., Sinnatamby, R.N. Shotyk, W., 2018. Measuring the distribution of trace elements amongst dissolved colloidal species as a fingerprint for the contribution of tributaries to large boreal rivers. *Sci Total Environ.*, 642, 1242-1251.

731 Danilenko, A.O., Reshetnyak, O.S., Kosmenko, L.S., Kondakova, M.Yu., 2020. Changes in the chemical denuation intensity in the river Pechora catchment under the influence of non-stationary climate and economic activities. *Water and Ecology* 25(4), 38-49. DOI:10.23968/2305-3488.2020.25.4.38-49.

735 Drake, T.W., Tank, S.E., Zhulidov, A.V., Holmes, R.M., Gurtovaya, T., Spencer, R.G.M., 2018. Increasing alkalinity export from large Russian Arctic River. *Environ. Sci. Technol.* 52, 8302-8308.

738 Finlay, J., Neff, J., Zimov, S., Davydova, A., Davydov, S., 2006. Snow-melt dominance of dissolved organic carbon in high-latitude watersheds: Implications for characterization and flux of river DOC. *Geophys. Res. Lett.*, 33, L10401, doi: 10.1029/2006GL025754

741 Frey, K.E., McClelland, J.W., Holmes, R.M., Smith, L.C., 2007. Impacts of climate warming and permafrost thaw on the riverine transport of nitrogen and phosphorus to the Kara Sea. *J. Geophys. Res.* 112, G04S58, doi: 10.1029/2006JG000369.

744 Gaillardet, J., Viers, J., Dupré, B., 2003. Trace elements in river waters. In: Treatise on Geochemistry, Elsevier, v. 5, pp. 225-272.

746 Gandois, L., Tananaev, N.I., Prokushkin, A., Solnyshkin, I., Teisserenc, R., 2021. Seasonality of DOC export from a Russian subarctic catchment underlain by discontinuous permafrost, highlighted by high-frequency monitoring. *J. Geophys. Res.: Biogeosciences*, 126, e2020JG006152. https://doi.org/10.1029/2020JG006152.

750 Georgiadi, A.G., Groisman, P.Y., 2022. Long-term changes of water flow, water temperature and heat flux of two largest arctic rivers of European Russia, Northern Dvina and Pechora. *Environ. Res. Letters*, 17, Number 8, Art No 085002.

753 Gordeev, V.V., Sidorov, I.S., 1993. Concentration of major elements and their outflow into the Laptev Sea by the Lena River. *Marine Chemistry* 43, 33-45.
755 Gordeev, V.V., Martin, J.M., Sidorov, I.S., Sidorova, M.V., 1996. A reassessment of the
756 Eurasian river input of water, sediment, major elements, and nutrients to the Arctic ocean.
757 Amer. J. Sci. 296, 664-691.

758 Gordeev, V.V., Dara, O.M., Filippov, A.S., Belorukov, S.K., Lokhov, A.S., Kotova, E.I.,
759 Kochenkova, A.I., 2022. Mineralogy of particulate suspended matter of the Severnaya
760 Dvina River (White Sea, Russia). Minerals, 12, Art No 1600.
761 https://doi.org/10.3390/min12121600.

762 Griffin, C.G., McClelland, J.W., Frey, K.E., Holmes, R.M., 2018. Quantifying CDOM and
763 DOC in major Arctic rivers during ice-free conditions using Landsat TM and ETM+ data.
764 Remote Sen. Environ. 209, 395-409.

765 Guo, H., Zhang, B., Zhang, Y., 2011. Control of organic and iron colloids on arsenic partition
766 and transport in high arsenic groundwaters in the Hetao basin, Inner Mongolia. Applied
767 Geochemistry, 26(3), 360-370. doi:10.1016/j.apgeochem.2010.12.009.

768 Hirst, C., Kutscher, L, Murphy, M, Shaw, S., Burke, I.T., Kaulich, B., Maximov, T.,
769 Pokrovsky, O.S., Mörth, C.-M, Andersson, P.S., Porcelli, D., 2017. Characterization and
770 stability of Fe-bearing particles in the Lena River catchment, NE Russia. Geochim.
771 Cosmochim. Acta 213, 553-573.

772 Holmes, R.M., Coe, M.T., Fiske, G.J., Gurtovaya, T., McClelland, J.W., Shiklomanov, A.I.,
773 Spencer, R.G.M., Tank, S.E., Zhulidov, A.V., 2013. Climate Change Impacts on the
774 Hydrology and Biogeochemistry of Arctic Rivers. In: Climatic Change and Global Warming
775 of Inland Waters: Impacts and Mitigation for Ecosystems and Societies, First Edition. Edited
776 by Charles R. Goldman, Michio Kumagai and Richard D. Robarts. John Wiley & Sons, Ltd.

777 Holmes, R.M., McClelland, J.W., Peterson, B.J., Tank, S.E. et al., 2012. Seasonal and annual
778 fluxes of nutrients and organic matter from large rivers to the Arctic Ocean and surrounding
779 seas. Estuaries Coasts 35, 369-382, doi: 10.1007/s12237-011-9386-6.

780 Ilina, S.M., Drozdova, O.Y., Lapitskiy, S.A., Alekhin, Y.V., Demin, V.V., Zavgorodnyaya, Y.A.,
781 Shirokova, L.S., Viers, J., Pokrovsky, O.S., 2014. Size fractionation and optical properties of
782 dissolved organic matter in the continuum soil solution-bog-river and terminal lake of a boreal
783 watershed. Org. Geochem. 66, 14-24.

784 Ilina, S.M., Lapitsky, S.A., Alekhin, Y.V., Viers, J., Benedetti, M., Pokrovsky, O.S., 2016.
785 Speciation, size fractionation and transport of trace element in the continuum soil water – mire
786 – lake – river – large oligotrophic lake of a subarctic watershed. Aquatic Geochem., 22(1), 65-
787 95.

788 Ingri, J., Widerlund, A., Land, M., Gustafsson, O., Andersson, P., Ohlander, B., 2000.
789 Temporal variations in the fractionation of the rare earth elements in a boreal river; the role
790 of colloidal particles. Chem. Geol., 166, 23-45.

791 Johnston, S.E., Shorina, N., Bulygina, E., Vorobjeva, T., Chupakova, A., Klimov, S.I. et al.,
792 2018. Flux and seasonality of dissolved organic matter from the Northern Dvina (Severnaya
793 Dvina) River, Russia. J. Geophys. Research: Biogeosciences, 123,
794 https://doi.org/10.1002/2017JG004337.

795 Kolesnichenko, Y., Kolesnichenko, L.G., Vorobyev, S.N., Shirokova, L.S., Semiletov, I.P.,
796 Dudarev, O.V., Vorobyev, R.S., Shavrina, U., Kirpotin, S.N., Pokrovsky, O.S., 2021.
797 Landscape, soil, lithology, climate and permafrost control on dissolved carbon, major and
798 trace elements in the Ob River, western Siberia. Water (MDPI), 13, Art No 3189.
799 https://doi.org/10.3390/w13223189.

800 Koroleff, F., 1983a. Total and organic nitrogen. In: Grasshoff K., Ehrhardt M. and Kremling K.
801 (eds.), Methods for seawater analysis. Verlag Chemie Weinheim, pp. 162-168.

802 Koroleff, F., 1983b. Determination of phosphorus. In: Grasshoff K., Ehrhardt M. & Kremling
803 K. (eds.), Methods for seawater analysis, Verlag Chemie, Weinheim, pp. 125-136.
804 Krickov, I., Lim, A., Manasypov, R.M., Loiko, S.V., Shirokova, L.S., Kirpotin, S.N., Karlsson, J.,
805 Pokrovsky, O.S., 2018. Riverine particulate C and N generated at the permafrost thaw front:
806 case study of western Siberian rivers across a 1700-km latitudinal transect. Biogeosciences, 15,
807 6867–6884. https://doi.org/10.5194/bg-15-6867-20.

808 Krickov, I.V., Pokrovsky, O.S., Manasypov, R.M., Lim, A.G., Shirokova, L.S., Viers, J., 2019.
809 Colloidal transport of carbon and metals by western Siberian rivers during different seasons
810 across a permafrost gradient. Geochim. Cosmochim. Acta, 265, 221–241,
811 doi:10.1016/j.gca.2019.08.041.

812 Krickov, I.V., Lim, A.G., Manasypov, R.M., Loiko, S.V., Vorobyev, S.N., Shevchenko, V.P.,
813 Dara, O.M., Gordeev, V.V., Pokrovsky, O.S., 2020. Major and trace elements in suspended
814 matter of western Siberian rivers: First assessment across permafrost zones and landscape
815 parameters of watersheds. Geochim. Cosmochim. Acta 269, 429–450,
816 doi:10.1016/j.gca.2019.11.005.

817 Krickov, I.V., Lim, A.G., Shevchenko, V.P., Vorobyev, S.N., Caudaudap, F., Pokrovsky, O.S.,
818 2022a. Dissolved metal (Fe, Mn, Zn, Ni, Cu, Co, Cd, Pb) and metalloid (As, Sb) in snow
819 water across a 2800 km latitudinal profile of Western Siberia: Impact of local pollution and
820 global transfer. Water, 14, Art No 94, https://doi.org/10.3390/w14010094.

821 Krickov, I.V., Lim, A.G., Shevchenko, V.P., Vorobyev, S.N., Pokrovsky, O.S., 2022b. Low
822 molecular weight and colloidal major and trace elements in snow of western Siberia. Chem.
823 Geol. 610, Art No 121090. https://doi.org/10.1016/j.chemgeo.2022.121090.

824 Lammers, R.B., Pundsack, J. W., Shiklomanov, A.I., 2007. Variability in river temperature,
825 discharge, and energy flux from the Russian pan-Arctic landmass. J. Geophys. Res.
826 112(G4), G04S59. DOI:10.1029/2006JG000370.

827 Li, M., Peng, C., Wang, M., Xue, W., Zhang, K., Wang, K., Shi, G., Zhu, Q., 2017. The carbon
828 flux of global rivers: A re-evaluation of amount and spatial patterns. Ecol. Indicators 80, 40-
829 51.

830 Lim, A.G., Loiko, S.V., Kuzmina, D., Krickov, I.V., Shirokova, L.S., Kulizhsky, S.P., Vorobyev,
831 S.N., Pokrovsky, O.S., 2021. Dispersed ground ice of permafrost peatlands: a non-accounted
832 for source of C, nutrients and metals. Chemosphere, 226, Art No 128953.
833 https://doi.org/10.1016/j.chemosphere.2020.128953.

834 Lim, A., Loiko, S., Kuzmina, D., Shirokova, L.S., Pokrovsky, O.S., 2022. Organic carbon, and
835 major and trace elements reside in labile low-molecular form in the ground ice of permafrost
836 peatlands: a case study of colloids in peat ice of Western Siberia. Environmental Science: Proc.
837 Impacts, 24, 1443–1459. DOI: 10.1039/D1EM00547B.

838 Lyvén, B., Hassellöv, M., Turner, D.R., Haraldsson, C., Andersson, K., 2003. Competition
839 between iron- and carbon-based colloidal carriers for trace metals in a freshwater assessed
840 using flow field-flow fractionation coupled to ICPMS. Geochim. Cosmochim. Acta, 67,
841 3791–3802.

842 Malov, A.I., 2013. The use of the geological benchmarks to assess the residence time of
843 groundwater in the aquifer using uranium isotopes on the example of the Northern Dvina
844 basin. Lithol. Mineral Resour., 48, 254–265, https://doi.org/10.1134/S002449021303005X.

845 Malov, A.I., Bolotov, I.N., Pokrovsky, O.S., Zykov, S.B., Tokarev, I.V., et al., 2015. The
846 dating of the thermal water of subarctic hydrothermal system using isotopes of uranium,
847 thorium and carbon. Applied Geochem. 63, 93–104.

848 Malov, A.I., Sidkina, E.S., Mironenko, M.V., Tyshov, A.S., Cherkasova, E.V., 2022. Modeling
849 Changes in the Composition of River Water with Discharged Wastewater: A Case Study in
850 NW Russia. Water 14, Art No 165. https://doi.org/10.3390/w14020165

851 McClelland, J., Déry, S., Peterson, B.J., Holmes, R., Wood, E., 2006. A pan-Arctic evaluation
852 of changes in river discharge during the latter half of the 20th century. Geophys Res. Letters
853 33, L06715.
854 McClelland, J.W., Holmes, R.M. Peterson, B.J., Stieglitz, M., 2004. Increasing river discharge in
855 the Eurasian Arctic: Consideration of dams, permafrost thaw, and fires as potential agents of
856 change. J. Geophys. Res. 109, D18102, doi:10.1029/2004JD004583.

857 McClelland, J.W., Tank, S.E., Spencer, R.G.M., Shiklomanov, A.I., 2015. Coordination and
858 sustainability of river observing activities in the Arctic. Arctic, 68, Suppl 1, 59-68.

859 Newcomer Johnson, T.A., Kaushal, S.S., Mayer, P.M., Smith, R.M., Sivirichi, G.M., 2016.
860 Nutrient retention in restored streams and rivers: A global review and synthesis. Water, 8(4),
861 116; https://doi.org/10.3390/w8040116.

862 Nierop, K.G.J., Jansen, B., Verstraten, J.M., 2002. Dissolved organic matter, aluminium and
863 iron interactions: Precipitation induced by metal/cation ratio, pH and competition. Sci. Total
864 Environ. 300 (1-3), 201-211.

865 O'Donnell, J.A., Aiken, G.R., Swanson, D.K., Panda, S., Butler, K.D., Baltensperger, A.P., 2016.
866 Dissolved organic matter composition of Arctic rivers: Linking permafrost and parent material
867 to riverine carbon. Global Biogeochem. Cy. 30, 1811-1826.

868 O'Donnell, J., Douglas, T., Barker, A., Guo, L., 2021. Changing biogeochemical cycles of
869 organic carbon, nitrogen, phosphorus, and trace elements in Arctic rivers. In D. Yang, D.L.
870 Kane, eds. Arctic Hydrology, Permafrost and Ecosystems, p. 315-348.
871 https://doi.org/10.1007/978-3-030-50930-9_11.

872 Oleinikova, O.V., Drozdova, O.Y., Lapitskiy, S.A., Demin, V.V., Bychkov, A.Y., Pokrovsky,
873 O.S., 2017. Dissolved organic matter degradation by sunlight coagulates organo-mineral
874 colloids and produces low-molecular weight fraction of metals in boreal humic waters.
875 Geochim. Cosmochim. Acta 211, 97-114.

876 Orem, W.H., Gaudette, H.E., 1984. Organic matter in anoxic marine pore water: oxidation
877 effects. Org. Geochem. 5(4), 175-181.

878 Peterson, B.J., Holmes, R.M., McClelland, J.W., Vorosmarty, C.J., Lammers, R.B.,
879 Shiklomanov, A.I., Shiklomanov, I.A., Rahmstorf, S., 2002. Increasing river discharge to the
880 Arctic Ocean. Science, 298, 2171-2173.

881 Pokrovsky, O.S., Schott, J., Dupré, B., 2006. Trace element fractionation and transport in
882 boreal rivers and soil porewaters of permafrost-dominated basic terrain in Central Siberia.
883 Geochim. Cosmochim. Acta 70, 3239-3260.

884 Pokrovsky, O.S., Viers, J., Shirokova, L.S., Shevchenko, V.P., Filipov, A.S., Dupré B., 2010.
885 Dissolved, suspended, and colloidal fluxes of organic carbon, major and trace elements in
886 Severnaya Dvina River and its tributary. Chem. Geol. 273, 136-149.

887 Pokrovsky, O.S., Viers, J., Dupré, B., Chabaux, F., Gaillardet, J., Audry, S., Prokushkin, A.S.,
888 Shirokova, L.S., Kirpotin, S.N., Lapitsky, S.A., Shevchenko, V.P., 2012. Biogeochemistry of
889 carbon, major and trace elements in watersheds of Northern Eurasia drained to the Arctic
890 Ocean: The change of fluxes, sources and mechanisms under the climate warming
891 prospective. C.R. Geoscience, 344, 663-677.

892 Pokrovsky, O.S., Manasypov, R.M., Loiko, S.V., Shirokova, L.S., 2016. Organic and organo-
893 mineral colloids in discontinuous permafrost zone. Geochim. Cosmochim. Acta 188, 1-20.

894 Pokrovsky, O.S., Bueno, M., Manasypov, R.M., Shirokova, L.S., Karlsson, J., Amouroux, D.,
895 2018. Dissolved organic matter controls on seasonal and spatial selenium concentration
896 variability in thaw lakes across a permafrost gradient. Environ. Sci. Technol. 52 (18),
897 10254-10262, DOI: 10.1021/acs.est.8b00918.

898 Pokrovsky, O.S., Manasypov, R.M., Kopysov, S., Krickov, I.V., Shirokova, L.S., Loiko, S.V.,
899 Lim, A.G., Kolesnichenko, L.G., Vorobiyev, S.N., Kirpotin, S.N., 2020. Impact of
900 permafrost thaw and climate warming on riverine export fluxes of carbon, nutrients and
901 metals in western Siberia. Water 12, Art No 1817. doi:10.3390/w12061817.
902 Pokrovsky, O.S., Manasypov, R.M., Chupakov, A.V., Kopysov, S., 2022. Element transport in the Taz River, western Siberia. Chem. Geol. 614, Art No 121180.
903 https://doi.org/10.1016/j.chemgeo.2022.121180.
904

905 Porcelli, D., Andersson, P.S., Wasserburg, G.J., Ingri, J., Baskaran, M., 1997. The importance of colloids and mires for the transport of uranium isotopes through the Kalix River watershed and Baltic Sea. Geochim. Cosmochim. Acta 61(19), 4095-4113.

908 Raudina, T.V., Loiko, S.V., Lim, A., Manasypov, R.M., Shirokova, L.S., Istigechev, G.I., Kuzmina, D.M., Kulizhsky, S.P., Vorobyev, S.N., Pokrovsky, O.S., 2018. Permafrost thaw and climate warming may decrease the CO₂, carbon, and metal concentration in peat soil waters of the Western Siberia Lowland. Sci. Total Environ. 634, 1004-1023.
910
911 doi:10.1016/j.scitotenv.2018.04.059.
912

913 Raudina, T.V., Loiko, S., Kuzmina, D.M., Shirokova, L.S., Kulizhsky, S.P., Golovatskaya, E.A., Pokrovsky, O.S., 2021. Colloidal organic carbon and trace elements in peat porewaters across a permafrost gradient in Western Siberia. Geoderma 390, Art No 114971.
915
916 https://doi.org/10.1016/j.geoderma.2021.114971.

917 Raymond, P.A., McClelland, J.W., Holmes, R.M., Zhulidov, A.V., Mull, K., Peterson, B.J., Striegl, R.G., Aiken, G.R., Gurtovaya, T.Y., 2007. Flux and age of dissolved organic carbon exported to the Arctic Ocean: a carbon isotopic study of the five largest arctic rivers. Global Biogeochem. Cy. 21, GB4011, doi: 10.1029/2007GB002934.

921 Rember, R.D., Trefry, J.H., 2004. Increased concentrations of dissolved trace metals and organic carbon during snowmelt in rivers of the Alaskan Arctic. Geochim. Cosmochim. Acta 68(3), 477-489.

924 Riedel, T., Zak, D., Biester, H., Dittmar, T., 2013. Iron traps terrestrially derived dissolved organic matter at redox interfaces. PNAS 110(25), 10101-10105.

926 Semikolennykh, A.A., Bovkunov, A.D., Aleinikov, A.A., 2013. Soils and the soil cover of the taiga zone in the Northern Urals (upper reaches of the Pechora River). Eurasian Soil Sci. 46(8), 821-832.

929 Serikova, S., Pokrovsky, O.S., Ala-Aho, P., Kazantsev, V., Kirpotin, S.N., Kopysov, S.G., Krickov, I.V., Laudon, H., Manasypov, R.M., Shirokova, L.S., Soulsby, D.T., Karlsson J., 2018. High riverine CO₂ emissions at the permafrost boundary of Western Siberia. Nature Geoscience 11, 825-829, doi:10.1038/s41561-018-0218-1.

933 Sidorchuk, A.Y., Panin, A.V., Borisova, O.K., Elias, S.A., Syvistki, J.P., 2000. Channel morphology and river flow in the northern Russian Plain in the Late Glacial and Holocene. Int. J. Earth Sci. 89, 541-549. DOI 10.1007/s005310000132.

936 Shiklomanov, A.I., Yakovleva, T.I., Lammers, R.B., Karasev, I.Ph., Vörösmarty, C. J., Linder, E., 2006. Cold region river discharge uncertainty—estimates from large Russian rivers. J. Hydrol. 326, 231-256.

939 Shirokova, L.S., Labouret, J., Gurge, M., Gerard, E., Zabelina, S.A., Ivanova, I.S., Pokrovsky, O.S., 2017. Impact of cyanobacterial associate and heterotrophic bacteria on dissolved organic carbon and metal in moss and peat leachate: application to permafrost thaw in aquatic environments. Aquatic Geochemistry, 23(5-6), 331-358, https://doi.org/10.1007/s10498-017-9325-7.

944 Smith, L.C., Pavelsky, T.M., MacDonald, G.M., Shiklomanov, A.I., Lammers, R.B., 2007. Rising minimum daily flows in northern Eurasia: A growing influence of groundwater in the high-latitude hydrologic cycle. J. Geophys. Res. 112, G04S47, doi:10.1029/2006JG000327.

947 Soromotin, A.; Moskovchenko, D.; Khoroshavin, V.; Prikhodko, N.; Puzanov, A.; Kirillov, V.; Koveshnikov, M.; Krylova, E.; Krasnenko, A.; Pechkin, A. Major, trace and rare earth element distribution in water, suspended particulate matter and stream sediments of the Ob River mouth. Water 2022, 14, 2442. https://doi.org/10.3390/w14152442.
951 Stepanova, V.M., Pokrovsky, O.S., Viers, J., Mironycheva-Tokareva, N.P. Kosykh, N.P., 952 Vishnyakova, E.K., 2015. Elemental composition of peat profiles in western Siberia: Effect 953 of the micro-landscape, latitude position and permafrost coverage. Applied Geochem. 53, 954 53-70.

955 Stolpe, B., Guo, L., Shiller, A., 2013a. Binding and transport of rare earth elements by organic 956 and iron-rich nanocolloids in Alaskan rivers, as revealed by field-flow fractionation and 957 ICP-MS. Geochim. Cosmochim. Acta 106, 446-462.

958 Stolpe, B., Guo, L., Shiller, A.M., Aiken, G.R., 2013b. Abundance, size distributions and trace-959 element binding of organic and iron-rich nanocolloids in Alaskan rivers, as revealed by 960 field-flow fractionation and ICP-MS. Geochim. Cosmochim. Acta 105, 221-239.

961 Striegl, R.G., Dornblaser, M.M., Aiken, G.R., Wickland, K.P., Raymond, P.A., 2007. Carbon 962 export and cycling by the Yukon, Tanana, and Porcupine rivers, Alaska, 2001-2005. Water 963 Resour. Res., 43, W02411, doi:10.1029/2006WR005201.

964 Suzuki, K., Konohira, E., Yamazaki, Y., Kubota, J., Ohata, T., Vluginsky, V., 2006. Transport 965 of organic carbon from the Mogot experimental watershed in the southern mountainous 966 taiga of eastern Siberia. Nord. Hydrol., 37, 303-312.

967 Tank, S.E., Raymond, P.A., Striegl, R.G., McClelland, J.W., Holmes, R.M., Fiske, G.J., 968 Peterson, B.J., 2012. A land-to-ocean perspective on the magnitude, source and implication 969 of DIC flux from major Arctic rivers to the Arctic Ocean. Global Biogeochem. Cy. 26, 970 GB4018, doi:10.1029/2011GB004192.

971 Tank, S.E., Striegl, R.G., McClelland, J.W., Kokelj, S.V., 2016. Multi-decadal increases in 972 dissolved organic carbon and alkalinity flux from the Mackenzie drainage basin to the Arctic 973 Ocean. Environ. Res. Lett. 11, 054015. doi:10.1088/1748-9326/11/5/054015.

974 Vasiliev, A.A., Drozdov, D.S., Gravis, A.G., Malkova, G.V., Nyland, K.E., Streletskiy, D.A., 975 2020. Permafrost degradation in the Western Russian Arctic. Environ. Res. Lett. 15, Art No 976 045001.

977 Vasyukova, E.V., Pokrovsky, O.S., Viers, J., Oliva, P., Dupre, B., Martin, F., Candaudap, F., 978 2010. Trace elements in organic- and iron-rich surficial fluids of the boreal zone: Assessing 979 colloidal forms via dialysis and ultrafiltration. Geochim. Cosmochim. Acta 74(2), 449-468.

980 Vonk, J.E., Tank, S.E., Mann, P.J., Spencer, R.G.M., Treat, C.C., Striegl, R.G., Abbott, B.W., 981 Wickland, K.P., 2015. Biodegradability of dissolved organic carbon in permafrost soils and 982 aquatic systems: a meta-analysis. Biogeosciences 12, 6915-6930. 983 https://doi.org/10.5194/bg-12-6915-2015.

984 Vonk, J.E., Tank, S.E., Walvoord, M., 2019. Integrating hydrology and biogeochemistry across 985 frozen landscapes. Nat. Commun. 10, 1-4. https://doi.org/10.1038/s41467-019-13361-5.

986 Vorobyev, S.N.; Pokrovsky, O.S.; Serikova, S.; Manasypov, R.M.; Krickov, I.V.; Shirokova, 987 L.S.; Lim, A.; Kolesnichenko, L.G.; Kirpotin, S.N.; Karlsson, J. Permafrost boundary shift 988 in Western Siberia may not modify dissolved nutrient concentrations in rivers. Water 2017, 989 9, 985, doi:10.3390/w9120985.

990 Vorobyev, S.N., Kolesnichenko, Y., Korets, M., Pokrovsky, O.S., 2021. Testing landscape, 991 climate and lithology impact on hydrochemical features of the Lena River and its tributaries. 992 Water (MDPI) 13, Art No 2093. https://doi.org/10.3390/w13152093.

993 Weishaar, J.L., Aiken, G.R., Bergamaschi, B.A., Fram, M.S., Fujii, R., Mopper, K. 2003. 994 Evaluation of specific ultraviolet absorbance as an indicator of the chemical composition 995 and reactivity of dissolved organic carbon. Env. Sci. Technol. 37, 4702-4708.

996 Wild, B., Andersson, A., Bröder, L., Vonk, J., Hugelius, G., McClelland, J.W., 2019. Rivers 997 across the Siberian Arctic unearth the patterns of carbon release from thawing permafrost. 998 Proc. Natl. Acad. Sci. USA 116, 10280-10285.

999 Yeghicheyan, D., Aubert, D., Bouhnik-Le Coz, M., Chmeleff, J., Delpoux, Cloquet, C., 1000 Marquet, A., Menniti, C., Pradoux, C., Freydier, R., Vieira da Silva-Filho, E., Suchorski, K.,
1001 2019. A new interlaboratory characterisation of silicon, rare earth elements and twenty two
1002 other trace element mass fractions in the natural river water Certified Reference Material
1003 SLRS-6 (NRC - CNRC). Geostandards Geoanalytical Res. 43 (3), 475-496.

1004 Zakharova, E.A., Pokrovsky, O.S., Dupré, B., Zaslavskaya, M.B., 2005. Chemical weathering
1005 of silicate rocks in Aldan Shield and Baikal Uplift: insights from long-term seasonal
1006 measurements of solute fluxes in rivers. Chem. Geol. 214, 223-248.

1007 Zakharova, E.A., Pokrovsky, O.S., Dupré, B., Gaillardet, J., Efimova, L., 2007. Chemical
1008 weathering of silicate rocks in Karelia region and Kola peninsula, NW Russia: Assessing the
1009 effect of rock composition, wetlands and vegetation. Chem. Geol. 242, 255-277.

1010 Zhang, S.M., Mu, C.C., Li, Z.I., Dong, W.W., Wang, X.Y., Streletskaya, I., Grebenets, V.,
1011 Sokratov, S., Kizyakov, A., Wu, X.D., 2021. Export of nutrients and suspended solids from
1012 major Arctic rivers and their response to permafrost degradation. Adv. Clim. Change Res.
1013 12(4), 466-474.

1014 Zolkos, S., Zhulidov, A. V., Gurtovaya, T. Yu., Gordeev, V. V., Berdnikov, S., Pavlova, N.,
1015 Kalko, E. A., Kuklina, Y. A., Zhulidov, D. A., Kosmenko, L. S., Shiklomanov, A. I.,
1016 Suslova, A., Geyman, B. M., Thackray, C. P., Sunderland, E. M., Tank, S. E., McClelland,
1017 J. W., Spencer, R. G. M., Krabbenhoft, D. P., ... Holmes, R. M., 2022. Multidecadal
1018 declines in particulate mercury and sediment export from Russian rivers in the pan-Arctic
1019 basin. Proc. Nat. Acad. Sci. 119(14), Art No e2119857119.
1020 https://doi.org/10.1073/pnas.2119857119.

1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035 **Table 1.** Mean annual concentrations (C), fluxes (F) and annual yields of solutes (< 0.45 µm) of the Pechora River. SD is standard deviation. TSS is for total suspended solid.
1036
1037

| Index | C ± SD, µg/L | Flux ± SD, tonn·10³/year | Yield ± SD, t km⁻² y⁻¹ | % colloid (3 kDa – 0.45 µm) ±SD |
|---|---|---|---|---|
| TSS | 11000±5500 | 2900±1500 | 9.1±4.5 | n.d. |
| DIC | 13000±2400 | 1100±150 | 3.4±0.47 | n.d. |
| DOC | 6300±410 | 1300±380 | 4.1±1.2 | 36±11 |
| Cl | 4900±860 | 410±40 | 1.3±0.12 | n.d. |
| SO₄ | 7700±1400 | 680±68 | 2.1±0.21 | n.d. |
| P-PO₄ | 14±1.7 | 2±0.18 | 0.0062±0.00056 | n.d. |
| N-NO₂ | 2.5±0.49 | 0.54±0.0053 | 0.0017±0.000016 | n.d. |
| N-NO₃ | 260±43 | 39±3.4 | 0.12±0.011 | n.d. |
| N-NH₄ | 23±5.7 | 3.4±0.32 | 0.011±0.001 | n.d. |
| Ntot | 360±49 | 65±5.6 | 0.2±0.017 | n.d. |

<table><thead><tr><td>Index</td><td>C ± SD, µg/L</td><td>Flux ± SD, tonn·10<sup>3</sup>/year</td><td>Yield ± SD, t km<sup>-2</sup> y<sup>-1</sup></td><td>% colloid (3 kDa – 0.45 µm) ±SD</td></tr></thead><tbody><tr><td>TSS</td><td>11000±5500</td><td>2900±1500</td><td>9.1±4.5</td><td>n.d.</td></tr><tr><td>DIC</td><td>13000±2400</td><td>1100±150</td><td>3.4±0.47</td><td>n.d.</td></tr><tr><td>DOC</td><td>6300±410</td><td>1300±380</td><td>4.1±1.2</td><td>36±11</td></tr><tr><td>Cl</td><td>4900±860</td><td>410±40</td><td>1.3±0.12</td><td>n.d.</td></tr><tr><td>SO<sub>4</sub></td><td>7700±1400</td><td>680±68</td><td>2.1±0.21</td><td>n.d.</td></tr><tr><td>P-PO<sub>4</sub></td><td>14±1.7</td><td>2±0.18</td><td>0.0062±0.00056</td><td>n.d.</td></tr><tr><td>N-NO<sub>2</sub></td><td>2.5±0.49</td><td>0.54±0.0053</td><td>0.0017±0.000016</td><td>n.d.</td></tr><tr><td>N-NO<sub>3</sub></td><td>260±43</td><td>39±3.4</td><td>0.12±0.011</td><td>n.d.</td></tr><tr><td>N-NH<sub>4</sub></td><td>23±5.7</td><td>3.4±0.32</td><td>0.011±0.001</td><td>n.d.</td></tr><tr><td>Ntot</td><td>360±49</td><td>65±5.6</td><td>0.2±0.017</td><td>n.d.</td></tr></tbody></table>
<table>
  <tr>
    <td>Li</td>
    <td>1.9±0.3</td>
    <td>0.19±0.029</td>
    <td>(59±9.1)·10<sup>-5</sup></td>
    <td>2.1±2.4</td>
  </tr>
  <tr>
    <td>Be</td>
    <td>0.0075±0.00026</td>
    <td>0.0021±0.00038</td>
    <td>(6.5±1.2)·10<sup>-6</sup></td>
    <td>67±8.8</td>
  </tr>
  <tr>
    <td>B</td>
    <td>19±3.6</td>
    <td>1.9±0.29</td>
    <td>0.0058±0.00089</td>
    <td>5.2±4.5</td>
  </tr>
  <tr>
    <td>Na</td>
    <td>5900±1400</td>
    <td>520±61</td>
    <td>1.6±0.19</td>
    <td>2.4±1.6</td>
  </tr>
  <tr>
    <td>Mg</td>
    <td>3900±790</td>
    <td>350±42</td>
    <td>1.1±0.13</td>
    <td>5.3±3.8</td>
  </tr>
  <tr>
    <td>Al</td>
    <td>22±8.6</td>
    <td>6.5±2.5</td>
    <td>0.02±0.0076</td>
    <td>61±10</td>
  </tr>
  <tr>
    <td>Si</td>
    <td>3400±330</td>
    <td>380±88</td>
    <td>1.2±0.27</td>
    <td>3.5±2.9</td>
  </tr>
  <tr>
    <td>P</td>
    <td>18±7</td>
    <td>2.8±1.8</td>
    <td>0.0087±0.0056</td>
    <td>80±7.7</td>
  </tr>
  <tr>
    <td>K</td>
    <td>660±73</td>
    <td>83±26</td>
    <td>0.26±0.081</td>
    <td>2.9±1.6</td>
  </tr>
  <tr>
    <td>Ca</td>
    <td>16000±2900</td>
    <td>1500±180</td>
    <td>4.7±0.55</td>
    <td>7.8±6</td>
  </tr>
  <tr>
    <td>Ti</td>
    <td>0.44±0.046</td>
    <td>0.098±0.031</td>
    <td>0.0003±0.000095</td>
    <td>51±27</td>
  </tr>
  <tr>
    <td>V</td>
    <td>0.22±0.026</td>
    <td>0.047±0.014</td>
    <td>0.00015±0.000043</td>
    <td>59±8.3</td>
  </tr>
  <tr>
    <td>Cr</td>
    <td>0.17±0.018</td>
    <td>0.031±0.013</td>
    <td>(9.5±4)·10<sup>-5</sup></td>
    <td>55±13</td>
  </tr>
  <tr>
    <td>Mn</td>
    <td>29±12</td>
    <td>3.5±1</td>
    <td>0.011±0.0032</td>
    <td>82±9.2</td>
  </tr>
  <tr>
    <td>Fe</td>
    <td>300±61</td>
    <td>45±17</td>
    <td>0.14±0.053</td>
    <td>98±0.9</td>
  </tr>
  <tr>
    <td>Co</td>
    <td>0.057±0.013</td>
    <td>0.0099±0.0027</td>
    <td>(3.1±0.83)·10<sup>-5</sup></td>
    <td>67±6</td>
  </tr>
  <tr>
    <td>Ni</td>
    <td>0.94±0.042</td>
    <td>0.2±0.062</td>
    <td>0.00061±0.00019</td>
    <td>45±10</td>
  </tr>
  <tr>
    <td>Cu</td>
    <td>1.4±0.54</td>
    <td>0.28±0.15</td>
    <td>0.00086±0.00047</td>
    <td>56±11</td>
  </tr>
  <tr>
    <td>Zn</td>
    <td>32.6±33.1</td>
    <td>5.5±6.5</td>
    <td>0.017±0.02</td>
    <td>68±22</td>
  </tr>
  <tr>
    <td>Zn*</td>
    <td>11.5±8.0</td>
    <td>2.1±2.3</td>
    <td>0.0064±0.0072</td>
    <td>68±22</td>
  </tr>
  <tr>
    <td>Ga</td>
    <td>0.011±0.0055</td>
    <td>0.0028±0.0023</td>
    <td>(8.7±7.1)·10<sup>-6</sup></td>
    <td>78±10</td>
  </tr>
  <tr>
    <td>Ge</td>
    <td>0.018±0.013</td>
    <td>0.0036±0.0048</td>
    <td>(1.1±1.5)·10<sup>-5</sup></td>
    <td>56±16</td>
  </tr>
  <tr>
    <td>As</td>
    <td>0.57±0.1</td>
    <td>0.094±0.043</td>
    <td>(29±13)·10<sup>-5</sup></td>
    <td>49±9.5</td>
  </tr>
  <tr>
    <td>Se</td>
    <td>0.045±n.d.</td>
    <td>0.011±n.d.</td>
    <td>0.000033±n.d.</td>
    <td>61±11</td>
  </tr>
  <tr>
    <td>Rb</td>
    <td>0.59±0.065</td>
    <td>0.085±0.024</td>
    <td>(26±7.4)·10<sup>-5</sup></td>
    <td>4.8±3.4</td>
  </tr>
  <tr>
    <td>Sr</td>
    <td>85±16</td>
    <td>7.3±0.79</td>
    <td>0.023±0.0025</td>
    <td>7.7±5.8</td>
  </tr>
  <tr>
    <td>Y</td>
    <td>0.15±0.04</td>
    <td>0.042±0.016</td>
    <td>(13±5)·10<sup>-5</sup></td>
    <td>87±5.4</td>
  </tr>
  <tr>
    <td>Zr</td>
    <td>0.075±0.02</td>
    <td>0.019±0.0076</td>
    <td>(5.9±2.3)·10<sup>-5</sup></td>
    <td>91±4.6</td>
  </tr>
  <tr>
    <td>Nb</td>
    <td>0.002±0.00035</td>
    <td>0.00059±0.000027</td>
    <td>(1.8±0.08)·10<sup>-6</sup></td>
    <td>85±9</td>
  </tr>
  <tr>
    <td>Mo</td>
    <td>0.17±0.028</td>
    <td>0.022±0.0067</td>
    <td>(6.9±2.1)·10<sup>-5</sup></td>
    <td>12±7.8</td>
  </tr>
  <tr>
    <td>Cd</td>
    <td>0.015±0.0068</td>
    <td>0.0034±0.0029</td>
    <td>(1.1±0.9)·10<sup>-5</sup></td>
    <td>58±7.7</td>
  </tr>
  <tr>
    <td>Sn</td>
    <td>0.051±n.d.</td>
    <td>0.00036±n.d.</td>
    <td>1.1·10<sup>-6</sup>±n.d.</td>
    <td>n.d.</td>
  </tr>
  <tr>
    <td>Sb</td>
    <td>0.028±0.0044</td>
    <td>0.0052±0.002</td>
    <td>(1.6±0.6)·10<sup>-5</sup></td>
    <td>15±7</td>
  </tr>
  <tr>
    <td>Cs</td>
    <td>0.0011±0.00031</td>
    <td>0.00022±0.00011</td>
    <td>(6.8±3.3)·10<sup>-7</sup></td>
    <td>n.d.</td>
  </tr>
  <tr>
    <td>Ba</td>
    <td>8.6±1</td>
    <td>1.2±0.39</td>
    <td>0.0037±0.0012</td>
    <td>15±8</td>
  </tr>
  <tr>
    <td>La</td>
    <td>0.11±0.036</td>
    <td>0.031±0.012</td>
    <td>(9.8±3.8)·10<sup>-5</sup></td>
    <td>93±6</td>
  </tr>
  <tr>
    <td>Ce</td>
    <td>0.17±0.057</td>
    <td>0.051±0.02</td>
    <td>(16±6.2)·10<sup>-5</sup></td>
    <td>95±1.8</td>
  </tr>
  <tr>
    <td>Pr</td>
    <td>0.031±0.0093</td>
    <td>0.0089±0.0035</td>
    <td>(2.8±1.1)·10<sup>-5</sup></td>
    <td>95±1.8</td>
  </tr>
  <tr>
    <td>Nd</td>
    <td>0.13±0.037</td>
    <td>0.037±0.014</td>
    <td>(12±4.4)·10<sup>-5</sup></td>
    <td>95±2.7</td>
  </tr>
  <tr>
    <td>Sm</td>
    <td>0.028±0.0088</td>
    <td>0.0083±0.0033</td>
    <td>(2.6±1.0)·10<sup>-5</sup></td>
    <td>94±3.9</td>
  </tr>
  <tr>
    <td>Eu</td>
    <td>0.0076±0.002</td>
    <td>0.0021±0.00082</td>
    <td>(6.5±2.6)·10<sup>-6</sup></td>
    <td>77±16</td>
  </tr>
  <tr>
    <td>Gd</td>
    <td>0.031±0.0088</td>
    <td>0.0089±0.0034</td>
    <td>(2.8±1.1)·10<sup>-5</sup></td>
    <td>93±3.8</td>
  </tr>
  <tr>
    <td>Tb</td>
    <td>0.0046±0.0012</td>
    <td>0.0013±0.00048</td>
    <td>(4.0±1.5)·10<sup>-6</sup></td>
    <td>91±5</td>
  </tr>
  <tr>
    <td>Dy</td>
    <td>0.025±0.0069</td>
    <td>0.0073±0.0028</td>
    <td>(2.3±0.87)·10<sup>-5</sup></td>
    <td>91±4</td>
  </tr>
  <tr>
    <td>Ho</td>
    <td>0.005±0.0014</td>
    <td>0.0014±0.00056</td>
    <td>(4.4±1.7)·10<sup>-6</sup></td>
    <td>90±4.2</td>
  </tr>
  <tr>
    <td>Er</td>
    <td>0.014±0.0038</td>
    <td>0.0039±0.0016</td>
    <td>(1.2±0.48)·10<sup>-5</sup></td>
    <td>89±5.4</td>
  </tr>
</table>
<table><tr><td>Tm</td><td>0.0019±0.00055</td><td>0.00051±0.00021</td><td>(1.6±0.67)·10<sup>-6</sup></td><td>85±11</td></tr><tr><td>Yb</td><td>0.012±0.0031</td><td>0.0032±0.0013</td><td>(1.0±0.39)·10<sup>-5</sup></td><td>86±8.4</td></tr><tr><td>Lu</td><td>0.0018±0.00043</td><td>0.00047±0.00019</td><td>(1.5±0.58)·10<sup>-6</sup></td><td>80±9.8</td></tr><tr><td>Hf</td><td>0.0034±0.00044</td><td>0.00083±0.00014</td><td>(2.6±0.44)·10<sup>-6</sup></td><td>80±13</td></tr><tr><td>W</td><td>0.0012±n.d.</td><td>8.2·10<sup>-6</sup>±n.d.</td><td>2.6·10<sup>-8</sup>±n.d.</td><td>16±24</td></tr><tr><td>Tl</td><td>0.0015±0.00054</td><td>0.00028±0.00016</td><td>(8.7±4.8)·10<sup>-7</sup></td><td>n.d.</td></tr><tr><td>Pb</td><td>0.15±0.075</td><td>0.027±0.018</td><td>(8.5±5.5)·10<sup>-6</sup></td><td>68±18</td></tr><tr><td>Th</td><td>0.0084±0.003</td><td>0.0025±0.0011</td><td>(7.7±3.4)·10<sup>-6</sup></td><td>93±3.1</td></tr><tr><td>U</td><td>0.084±0.015</td><td>0.0077±0.0012</td><td>(2.4±0.38)·10<sup>-5</sup></td><td>53±32</td></tr></table>

Footnote: * without data on anomalously high Zn (> 50 µg L⁻¹), potentially caused by contamination.

1038

1039

1040

1041

1042

1043

1044

1045

1046

1047

1048

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1055

<table><caption>**Table 2.** Pairwise Pearson correlation coefficients of dissolved (< 0.45 µm) major and trace elements concentration with DOC, DIC, Cl, Fe and Al. Significant correlations (p < 0.05) are indicated by asterisk.</caption><thead><tr><th>Index</th><th>DIC</th><th>DOC</th><th>Cl</th><th>Al</th><th>Fe</th><th>Index</th><th>DIC</th><th>DOC</th><th>Cl</th><th>Al</th><th>Fe</th></tr></thead><tbody><tr><td>DIC</td><td>1</td><td>0.05</td><td>0.73*</td><td>-0.24</td><td>0.27</td><td>As</td><td>-0.17</td><td>-0.68*</td><td>0.46</td><td>-0.47</td><td>0.79*</td></tr><tr><td>DOC</td><td>0.05</td><td>1</td><td>-0.41</td><td>0.89*</td><td>-0.27</td><td>Rb</td><td>0.59*</td><td>0.37</td><td>0.51</td><td>0.34</td><td>0.34</td></tr><tr><td>Cl</td><td>0.73*</td><td>-0.41</td><td>1</td><td>-0.52</td><td>0.67*</td><td>Sr</td><td>0.87*</td><td>-0.32</td><td>0.94*</td><td>-0.47</td><td>0.53</td></tr><tr><td>SO<sub>4</sub></td><td>0.82*</td><td>-0.39</td><td>0.94*</td><td>-0.52</td><td>0.51</td><td>Y</td><td>-0.46</td><td>0.81*</td><td>-0.63*</td><td>0.95*</td><td>-0.17</td></tr><tr><td>P-PO<sub>4</sub></td><td>0.31</td><td>-0.30</td><td>0.55</td><td>-0.32</td><td>0.85*</td><td>Zr</td><td>-0.31</td><td>0.83*</td><td>-0.56*</td><td>0.97*</td><td>-0.21</td></tr><tr><td>N-NO<sub>2</sub></td><td>0.72*</td><td>0.53</td><td>0.44</td><td>0.39</td><td>0.32</td><td>Nb</td><td>-0.10</td><td>0.89*</td><td>-0.45</td><td>0.97*</td><td>-0.27</td></tr><tr><td>N-NO<sub>3</sub></td><td>0.23</td><td>-0.30</td><td>0.61*</td><td>-0.22</td><td>0.57*</td><td>Mo</td><td>0.70*</td><td>0.48</td><td>0.30</td><td>0.39</td><td>0.09</td></tr><tr><td>N-NH<sub>4</sub></td><td>0.34</td><td>-0.06</td><td>0.54</td><td>-0.13</td><td>0.29</td><td>Cd</td><td>0.12</td><td>0.17</td><td>0.27</td><td>0.23</td><td>0.08</td></tr><tr><td>N<sub>tot</sub></td><td>0.24</td><td>-0.01</td><td>0.53</td><td>-0.08</td><td>0.27</td><td>Sb</td><td>-0.04</td><td>-0.25</td><td>0.21</td><td>-0.18</td><td>0.27</td></tr><tr><td>Li</td><td>0.77*</td><td>-0.31</td><td>0.97*</td><td>-0.42</td><td>0.64*</td><td>Cs</td><td>0.04</td><td>0.70*</td><td>-0.21</td><td>0.84*</td><td>-0.14</td></tr><tr><td>Be</td><td>-0.56*</td><td>0.71*</td><td>-0.70*</td><td>0.91*</td><td>-0.29</td><td>Ba</td><td>0.31</td><td>0.12</td><td>0.47</td><td>0.24</td><td>0.37</td></tr><tr><td>B</td><td>0.75*</td><td>-0.34</td><td>0.98*</td><td>-0.45</td><td>0.71*</td><td>La</td><td>-0.40</td><td>0.79*</td><td>-0.57*</td><td>0.95*</td><td>-0.14</td></tr></tbody></table>
<table><tr><td>Na</td><td>0.81*</td><td>-0.31</td><td>0.99*</td><td>-0.44</td><td>0.66*</td><td>Ce</td><td>-0.38</td><td>0.81*</td><td>-0.56*</td><td>0.96*</td><td>-0.16</td></tr><tr><td>Mg</td><td>0.79*</td><td>-0.44</td><td>0.98*</td><td>-0.56*</td><td>0.63*</td><td>Pr</td><td>-0.41</td><td>0.82*</td><td>-0.61*</td><td>0.96*</td><td>-0.18</td></tr><tr><td>Al</td><td>-0.24</td><td>0.89*</td><td>-0.52</td><td>1</td><td>-0.20</td><td>Nd</td><td>-0.40</td><td>0.84*</td><td>-0.62*</td><td>0.96*</td><td>-0.20</td></tr><tr><td>Si</td><td>0.57*</td><td>-0.40</td><td>0.89*</td><td>-0.44</td><td>0.85*</td><td>Sm</td><td>-0.45</td><td>0.81*</td><td>-0.65*</td><td>0.96*</td><td>-0.22</td></tr><tr><td>P</td><td>0.19</td><td>-0.56*</td><td>0.71*</td><td>-0.46</td><td>0.86*</td><td>Eu</td><td>-0.45</td><td>0.82*</td><td>-0.64*</td><td>0.96*</td><td>-0.21</td></tr><tr><td>K</td><td>0.61*</td><td>0.52</td><td>0.37</td><td>0.44</td><td>0.08</td><td>Gd</td><td>-0.43</td><td>0.81*</td><td>-0.63*</td><td>0.96*</td><td>-0.20</td></tr><tr><td>Ca</td><td>0.84*</td><td>-0.37</td><td>0.97*</td><td>-0.51</td><td>0.60*</td><td>Tb</td><td>-0.42</td><td>0.85*</td><td>-0.63*</td><td>0.96*</td><td>-0.21</td></tr><tr><td>Ti</td><td>-0.18</td><td>0.84*</td><td>-0.51</td><td>0.96*</td><td>-0.25</td><td>Dy</td><td>-0.46</td><td>0.80*</td><td>-0.65*</td><td>0.95*</td><td>-0.20</td></tr><tr><td>V</td><td>-0.36</td><td>0.78*</td><td>-0.62*</td><td>0.93*</td><td>-0.20</td><td>Ho</td><td>-0.50</td><td>0.78*</td><td>-0.65*</td><td>0.94*</td><td>-0.20</td></tr><tr><td>Cr</td><td>0.14</td><td>0.09</td><td>0.27</td><td>0.17</td><td>0.41</td><td>Er</td><td>-0.52</td><td>0.77*</td><td>-0.67*</td><td>0.94*</td><td>-0.20</td></tr><tr><td>Mn</td><td>0.26</td><td>0.44</td><td>0.30</td><td>0.45</td><td>0.32</td><td>Tm</td><td>-0.54</td><td>0.77*</td><td>-0.68*</td><td>0.91*</td><td>-0.18</td></tr><tr><td>Fe</td><td>0.27</td><td>-0.27</td><td>0.67*</td><td>-0.20</td><td>1</td><td>Yb</td><td>-0.47</td><td>0.80*</td><td>-0.65*</td><td>0.94*</td><td>-0.17</td></tr><tr><td>Co</td><td>0.18</td><td>0.54</td><td>0.19</td><td>0.58*</td><td>0.31</td><td>Lu</td><td>-0.52</td><td>0.79*</td><td>-0.65*</td><td>0.91*</td><td>-0.18</td></tr><tr><td>Ni</td><td>-0.35</td><td>0.77*</td><td>-0.48</td><td>0.92*</td><td>-0.11</td><td>Hf</td><td>-0.44</td><td>0.78*</td><td>-0.60*</td><td>0.95*</td><td>-0.25</td></tr><tr><td>Cu</td><td>0.57*</td><td>0.24</td><td>0.21</td><td>0.05</td><td>-0.15</td><td>W</td><td>-0.19</td><td>-0.48</td><td>0.28</td><td>-0.24</td><td>0.41</td></tr><tr><td>Zn</td><td>0.54</td><td>-0.30</td><td>0.59*</td><td>-0.32</td><td>0.07</td><td>Pb</td><td>0.38</td><td>0.08</td><td>0.09</td><td>0.05</td><td>-0.03</td></tr><tr><td>Ga</td><td>-0.58*</td><td>0.17</td><td>-0.26</td><td>0.54</td><td>0.31</td><td>Th</td><td>-0.52</td><td>0.73*</td><td>-0.71*</td><td>0.91*</td><td>-0.40</td></tr><tr><td>Ge</td><td>-0.28</td><td>-0.78*</td><td>0.33</td><td>-0.56*</td><td>0.49</td><td>U</td><td>0.91*</td><td>-0.04</td><td>0.84*</td><td>-0.22</td><td>0.40</td></tr></table>

1056

1057

<figure><img src="image_2.png" /><figcaption>Fig. 1. Map of the study site including the watershed of the Pechora River and the monitoring sites.</figcaption></figure>

1058

1059

1060

1061
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>
**Figure 2.** Examples of first group element concentration plotted as a function of the period of the year (left columns) and the discharge (right columns) over 4 year observation period (2015-2019) for DIC (A), Cl (B), Li (C), Mg (D) and U (E).

1067

1068

1069

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

1070

1071

1072 **Figure 2, continued.**

1073

1074

1075
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>
1078 **Figure 3.** Second group element concentration versus discharge over 4 year observation period

1079 (2015-2019) for DOC (A), Ni (B), Zr (C), Yb (D) and Th (E). The trends are power

1080 regressions, separately during winter → spring flood (solid black line) and summer → autumn

1081 (dashed line) periods.
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 3, continued.**</figcaption></figure>

1082

1083
<figure><img src="image_13.png" /><figcaption>**Figure 4.** Potassium (A), PO₄ (B), Mn (C) and Fe (D) concentration plotted as a function of the period of the year (left columns) and the discharge (right columns) over 4 year observation period (2015-2019) in the Pechora River.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure 4**, continued.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. 5.** Proportions of colloidal (3 kDa – 0.45 µm) fractions of DOC (A), Al (B), Mn (C), Fe (D), Ni (E) and U (F) as a function of river discharge during four main seasons: winter (blue diamonds), spring flood (red circles), summer (green squares), and autumn (brown triangles). The solid line represents a logarithmic fit to the data over the course of the year (% colloid = a × ln Q ± b, where Q is discharge, a and b are empirical constants specific to each element).</figcaption></figure>
Pechora River

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>