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Citation: Yang, Y.; Wang, P.; Deng, C.;
Liu, S.; Chen, D.; Wang, R. Differential
Spatiotemporal Patterns of Major Ions
and Dissolved Organic Carbon
Variations from Non-Permafrost to
Permafrost Arctic Basins: Insights
from the Severnaya Dvina, Pechora
and Taz Rivers. Land 2024,13, 1765.
https://doi.org/10.3390/
land13111765
Academic Editor: Le Yu
Received: 31 August 2024
Revised: 12 October 2024
Accepted: 23 October 2024
Published: 27 October 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
land
Article
Differential Spatiotemporal Patterns of Major Ions and
Dissolved Organic Carbon Variations from Non-Permafrost to
Permafrost Arctic Basins: Insights from the Severnaya Dvina,
Pechora and Taz Rivers
Yuanyuan Yang 1, Ping Wang 2,3 , Chunnuan Deng 1, Shiqi Liu 2 ,* , Dan Chen 1and Ruixin Wang 2,3
1Yunnan Key Laboratory of Plateau Geographical Processes and Environmental Changes,
Department of Geography, Yunnan Normal University, Kunming 650000, China;
yuanyuanyang@ynnu.edu.cn (Y.Y.)
2Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and
Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China;
wangping@igsnrr.ac.cn (P.W.)
3University of Chinese Academy of Sciences, Beijing 100049, China
*Correspondence: liusq@igsnrr.ac.cn
Abstract: The Arctic river basins, among the most sensitive regions to climate warming, are experi-
encing rapid temperature rise and permafrost thawing that profoundly affect their hydrological and
hydrochemical systems. However, our understanding of chemical export from Arctic basins to oceans
remains limited due to scarce data, particularly in permafrost-dominated regions. This study exam-
ines the spatiotemporal variations and seasonal dynamics of major ions (Na
+
, K
+
, Mg
2+
, Ca
2+
, Cl
−
,
SO
42−
) and dissolved organic carbon (DOC) concentrations across three river basins with varying
permafrost extents: the Severnaya Dvina (2006–2008, 2012–2014), the Pechora (2016–2019) and the Taz
Rivers (2016–2020). All the data were sourced from published Chemical Geological researches and
were taken from Mendeley and PANGAEA datasets. Our results showed that DOC concentrations
ranged from 1.75 to 26.40 mg/L, with the Severnaya Dvina River exhibiting the highest levels of
DOC concentrations, alongside significantly elevated ion concentrations compared to the other two
basins. A positive correlation was observed between DOC concentrations and river discharge, with
peaks during the spring flood and summer baseflow due to leaching processes. The Severnaya Dvina
and Pechora Rivers exhibited the highest DOC values during the spring flood, reaching 26.40 mg/L
and 8.07 mg/L, respectively. In contrast, the Taz River had the highest runoff during the spring
flood season, but the DOC concentration reached its highest value of 11.69 mg/L in the summer.
Specifically, a 1% increase in river discharge corresponded to a 1.25% rise in DOC concentrations in
the Severnaya Dvina River and a 1.04% increase in the Pechora River, while there was no significant
correlation between runoff and DOC concentrations in the Taz River. Major ion concentrations
demonstrated a negative correlation with river discharge, remaining relatively high during winter
low-flow period. A robust power-law relationship between river discharge and concentration of
DOC and major ions was observed, with distinct variations across the three river basins depending
on permafrost extent. The Pechora and Taz Rivers, characterized by extensive permafrost, exhibited
increasing trends in river discharge and DOC concentrations, accompanied by decreasing major ion
concentrations, whereas the non-permafrost-dominated Severnaya Dvina River basin showed the
opposite pattern. The Taz River, with the most extensive permafrost, also displayed a delayed DOC
peak and more complex seasonal ion concentration patterns. These findings highlight the importance
of varying permafrost extents and their implications for water quality and environmental protection
in these vulnerable regions.
Keywords: DOC; major ions; runoff; hydrochemistry; Arctic
Land 2024,13, 1765. https://doi.org/10.3390/land13111765 https://www.mdpi.com/journal/land
Land 2024,13, 1765 2 of 22
1. Introduction
Recent global warming has rapidly accelerated temperature increases in the Arc-
tic region [
1
–
7
], leading to the thickening of the active layer and degradation of per-
mafrost [
8
,
9
]. This degradation results in the release of previously frozen materials into
the atmosphere [
6
,
10
,
11
], hydrosphere [
12
,
13
] and biosphere [
14
–
16
], thereby altering
global biogeochemical [
17
,
18
] cycles and triggering various climate and environmental
impacts [19].
Previous studies have explored the concentrations and spatiotemporal dynamics of
dissolved organic carbon (DOC) and major ions in Arctic river basins [
20
–
24
], for example,
emerging solute-induced mineralization with significant increases in annual flux of total
dissolved solids from the Ob, Kolyma and Yukon Rivers [
25
–
27
], along with significant
rises in alkalinity fluxes in major Arctic rivers such as the Ob and Yenisei [28]. In contrast,
other research indicates that the total concentration of major ions in certain regions, like
northern West Siberia, might remain relatively stable in the coming decades, despite
seasonal variability [
29
,
30
]. Furthermore, climate models predict a steady rise in riverine
DOC inputs to the Arctic Ocean, potentially doubling over the next century [
31
], with
seasonal variations in DOC export closely linked to changes in runoff under warming
conditions [
25
,
32
]. River export fluxes of dissolved carbon [
33
,
34
], nutrients and some
alkalinity [
35
,
36
] from land to the Arctic Ocean are fairly well quantified thanks to intensive
work by the US and Russian State Hydrological Surveys and multi-year sampling within the
Partners/Arctic GRO program [
3
]. Studies of riverine dissolved carbon, metalsand water
chemistry ion [
31
] contents in six of the larger Arctic rivers have been richly researched
with a large number of measurement stations. However, the hydrochemical changes in
mid-sized Arctic river basins have not been systematically studied. These rivers, despite
limited data availability, exhibit high sensitivity to climate change and permafrost thaw,
serving as crucial indicators of climatic and environmental shifts in the Arctic [
37
]. Thus, a
comprehensive analysis of water chemistry and organic carbon content in these basins is
critical for understanding the broader implications of climate and environmental changes
in the Arctic and beyond.
The hydrological and hydrochemical processes in the permafrost region are com-
plex, governed by a myriad of factors such as precipitation patterns [
38
–
40
], ice and
snowmelt [
6
,
10
,
11
] and seasonal variations [
41
–
43
]. Rivers in these regions act as integra-
tors of weathered materials in the watershed, serving as key corridors for transporting
continental weathering products and terrestrial carbon to the ocean [
31
]. Hydrological
processes, such as glacier melting and seasonal changes in runoff, play an important role in
controlling the transport of solutes and carbon in river systems by modulating biogeochem-
ical reactions [
31
]. These processes significantly impact the geochemical composition and
fluxes of solutes and carbon in riverine environments. The thawing of permafrost further
alters hydrological dynamics by changing surface and subsurface interactions [
31
,
44
], lead-
ing to the release of previously sequestered organic matter, including dissolved organic
carbon (DOC), into the rivers. As the active layer deepens, DOC is transported along more
extended flow paths through mineral soils, where most of the unstable DOC is degraded
and mineralized into greenhouse gases, which are subsequently released into the atmo-
sphere. The remaining more recalcitrant DOC is transported into rivers and streams [
31
].
Given the likelihood of substantial shifts in material fluxes within Arctic rivers as the
climate continues to warm, understanding the changes in surface water quality in these
watersheds is of paramount importance for comprehending broader environmental and
climatic transformations in the Arctic and globally.
However, fieldwork in these Arctic regions is particularly challenging due to extreme
conditions, limited availability of real-time observational data and the intrinsic variability of
hydrological processes. Current knowledge of permafrost hydrology in the Arctic remains
incomplete, underscoring the need for further investigation into the impacts of permafrost
degradation. This study analyzes the concentrations of dissolved organic carbon and
six major ions (Na
+
, K
+
, Mg
2+
, Ca
2+
, Cl
−
, SO
42−
) in the Severnaya Dvina (2006–2008,
Land 2024,13, 1765 3 of 22
2012–2014) [
1
], Pechora (2016–2019) [
2
] and Taz (2016–2020) [
3
] river basins based on multi-
sourced data collection. Through seasonal analysis, curve fitting, correlation analysis and
trend assessment, the hydrochemical status of these three rivers is systematically evaluated.
This research aims to elucidate the temporal and spatial trends in DOC and major ion
concentrations, identify seasonal dynamics, investigate influencing factors and clarify
the underlying mechanisms. This study provides critical insights into the influence of
permafrost degradation on river water quality under the current Arctic warming. Note that
this study aims to conduct a comprehensive comparative analysis of three river basins with
varying permafrost extents, thereby enhancing our understanding of the mechanisms by
which permafrost thaw influences hydrochemical processes in Arctic basins.
2. Materials and Methods
2.1. Study Area
The study focuses on three mid-sized river basins—the Severnaya Dvina, Pechoraand
Taz River basins—located in the northern Eurasian continent, within the mid- to high-latitude
region of the Northern Hemisphere (ranging from 58.5
◦
to 69
◦
N and 38
◦
to
86◦E)
in the north
of Eurasia (Figure 1). The region is characterized by predominantly plain with a dense
fluvial network and a tundra or temperate continental climate. The vegetation primarily
consists of forest-tundra and coniferous forest biomes, dominated by species such as spruce,
pine and larch [
1
–
3
]. The meteoro-hydrological and permafrost characteristics of these
basins are outlined in Table 1.
Land 2024, 13, 1765 3 of 22
multi-sourced data collection. Through seasonal analysis, curve fitting, correlation analy-
sis and trend assessment, the hydrochemical status of these three rivers is systematically
evaluated. This research aims to elucidate the temporal and spatial trends in DOC and
major ion concentrations, identify seasonal dynamics, investigate influencing factors and
clarify the underlying mechanisms. This study provides critical insights into the influence
of permafrost degradation on river water quality under the current Arctic warming. Note
that this study aims to conduct a comprehensive comparative analysis of three river basins
with varying permafrost extents, thereby enhancing our understanding of the mecha-
nisms by which permafrost thaw influences hydrochemical processes in Arctic basins.
2. Materials and Methods
2.1. Study Area
The study focuses on three mid-sized river basins—the Severnaya Dvina, Pecho-
raand Taz River basins—located in the northern Eurasian continent, within the mid- to
high-latitude region of the Northern Hemisphere (ranging from 58.5° to 69° N and 38° to
86° E) in the north of Eurasia (Figure 1). The region is characterized by predominantly
plain with a dense fluvial network and a tundra or temperate continental climate. The
vegetation primarily consists of forest-tundra and coniferous forest biomes, dominated by
species such as spruce, pine and larch [1–3]. The meteoro-hydrological and permafrost
characteristics of these basins are outlined in Table 1.
Figure 1. Locations and permafrost coverage of the Severnaya Dvina, the Pechora and the Taz River
basins.
Figure 1. Locations and permafrost coverage of the Severnaya Dvina, the Pechora and the Taz
River basins.
Land 2024,13, 1765 4 of 22
Table 1. Meteoro-hydrological and permafrost conditions of the study area (1990–2019).
River Basin
Multi-Year
Average
Temperature, ◦C
Multi-Year
Average
Precipitation,
mm
Multi-Year Mean
Evapotranspiration,
mm
Multi-Year
Average
Runoff, mm
Mean Active
Layer
Thickness, cm
Mean Ground
Ice Content, %
Severnaya Dvina 1.82 721 392 328 /59.2
Pechora −2.29 709 277 441 81.8 71.9
Taz −5.44 597 284 319 84.6 80.7
Note: Active layer thickness data cover over the years 2000–2016.
The Severnaya Dvina River basin (58.5
◦
–65
◦
N, 38
◦
–56
◦
E) spans 0.36 million km
2
and
is formed by the confluence of the Sukhona and Yug Rivers, with the Vychegda River as
its largest tributary. The basin features an extensively developed hydrological network,
consisting of approximately 61,900 rivers and streams with a total length of 206,000 km. The
climate in this region is predominantly temperate continental, characterized by a multi-year
annual mean temperature of around 1.82
◦
C, annual precipitation of 721 mm, average
annual evapotranspiration of 392 mm and average runoff of approximately 328 mm. There
is no permafrost developed in the basin, and the watershed is covered with thick glacial
deposits [1].
The Pechora River basin (62
◦
–69
◦
N, 46
◦
–66
◦
E), spanning an area of about
0.32 million km2
with a river length of about 1810 km, is located on the plains between
the Ural and Timan mountains at an average elevation of 105 m. The basin experiences a
temperate continental climate, with an average annual air temperature of
−
2.29
◦
C, annual
precipitation of 709 mm, average evapotranspiration of 277 mm and average runoff of
441 mm. Approximately 40% of its northern and northeastern parts are located in the
permafrost zone [2].
The Taz River basin (62
◦
–68
◦
N, 78
◦
–86
◦
E), covering 0.15 million km
2
with a river
length of 1400 km, originates from the Siberian Uvaly, a hilly region within the central West
Siberian lowlands. The basin lies entirely within the permafrost zone, which is sporadic
and island in the south and continuous in the north. The average annual temperature
of this basin is
−
5.44
◦
C with an annual precipitation of 597 mm, evapotranspiration of
284 mm and runoff of 319 mm. The main biomes are tundra, forest tundra and taiga, and
the basin’s lithology is composed primarily of clays, silts and sands, overlaid by Quaternary
sediments including loess, fluvial, glacial and lake deposits [3].
2.2. Data and Methodology
The concentration of major ions and DOC data were obtained from the literature and
shared data websites (Table 2). In total, these hydrochemical data were collected from
a total of 17 sampling sites across three river basins. Specifically, the Severnaya Dvina
River basin contributed 128 and 51 datasets from the periods 2006–2008 and 2012–2014,
respectively; the Pechora River basin provided 161 datasets from 2016 to 2019; and the Taz
River basin contributed 278 datasets from 2016 to 2020. Average monthly discharge data
from 2012 to 2014 were available for the Severnaya Dvina River basin, with daily discharge
data collected for the Pechora and Taz River basins.
The study mainly collected the river runoff, DOC and concentration of six ions (Na
+
,
K
+
, Mg
2+
, Ca
2+
, Cl
−
and SO
42−
) in the Severnaya Dvina River, Pechora River and Taz
River. Correlation analysis was used to judge the relationship between river runoff, DOC
and the six major ions in the rivers. Seasonal analysis was conducted according to the
following classifications: spring (May, June and July), summer (August, September and
October) and winter (November to April). This classification was used to observe the
seasonal variations in the concentrations of various substances in the three rivers, in line
with previous studies [
45
,
46
]. Nonlinear fitting was used to show the relationship between
riverine substance content and runoff.
Land 2024,13, 1765 5 of 22
Table 2. Research data statement.
Data Type Streams Source of Data
Temperatures,
measured precipitation,
evapotranspiration,
runoff,
active layer
thickness
ESSD—the pan-Arctic catchment database (ARCADE)
(copernicus.org) (temperature, precipitation,
evapotranspiration and runoff are averaged over three
decades, 1990–2019, and active layer thickness is averaged
over 2000–2016)
Subterranean ice https://doi.org/10.5194/essd-2022-144, 2022 (accessed on
17 January 2024)
River runoff, river
water chemistry
data
Severnaya Dvina River
https://doi.org/10.1016/j.chemgeo.2010.02.018 (accessed
on 19 January 2024)
https://doi.org/10.1594/PANGAEA.746512
(accessed on 19 January 2024)
https://doi.org/10.1016/j.chemgeo.2020.119491
(accessed on 19 January 2024)
Pechora River
https://doi.org/10.1016/j.marpolbul.2023.115317
(accessed on 21 January 2024)
https://doi.org/10.1016/j.chemgeo.2023.121524
(accessed on 21 January 2024)
Mendeley Data, V1,
https://doi.org/10.17632/4mk7nrrk8d.1
(accessed on 21 January 2024)
Taz River
https://doi.org/10.1016/j.chemgeo.2022.121180
(accessed on 23 January 2024)
Mendeley Data, V1,
https://doi.org/10.17632/gwmyt2kmw3.1
(accessed on 23 January 2024)
https://data.mendeley.com/datasets/rgwzydrg92/1
(accessed on 23 January 2024)
Additionally, the Pearson correlation coefficient, described as follows, was used in
this study:
R=
∑Xi−XYi−Y
r∑Xi−X2∑Yi−Y2
where X
i
and Y
i
represent the measured values for each of the two categories of runoff,
DOC or six ion concentrations and
X
and
Y
are the mean values of the two categories,
respectively. R= 0 indicates that the two variables are not correlated, a positive value
indicates a positive correlation, a negative value indicates a negative correlation, and a
larger absolute value indicates a stronger correlation. R-values are shown in the lower left
section of the correlation analysis chart. In the correlation analysis, the significance level
p< 0.001 is displayed as ***, indicating a strong correlation between the two parameters;
the significance level p< 0.01 is displayed as **, indicating a strong correlation between
the two parameters; the significance level p< 0.05 is displayed as *, indicating a strong
correlation between the two parameters.
3. Results
3.1. Spatiotemporal Variations of Riverine DOC and Major Ion Concentrations
The Severnaya Dvina, Pechora and Taz River basins exhibit notable differences in
discharge, dissolved organic carbon (DOC) concentrations and major ion levels (refer to
Figures 2–5). Specifically, the Pechora Basin has the highest discharge, reaching up to
Land 2024,13, 1765 6 of 22
1.5
×
10
4
m
3
/s during the spring freshet, whereas the Taz Basin shows the lowest discharge,
with spring values around 2818 m
3
/s and a winter minimum of 156 m
3
/s. Conversely, the
Severnaya Dvina River consistently demonstrates significantly higher DOC and major ion
concentrations across all seasons compared to the Pechora and Taz basins.
Land 2024, 13, 1765 6 of 22
× 104 m3/s during the spring freshet, whereas the Taz Basin shows the lowest discharge,
with spring values around 2818 m3/s and a winter minimum of 156 m3/s. Conversely, the
Severnaya Dvina River consistently demonstrates significantly higher DOC and major ion
concentrations across all seasons compared to the Pechora and Taz basins.
Figure 2. Seasonal discharge and hydrochemical characteristics of Severnaya Dvina River (2006–
2008).
Figure 2. Seasonal discharge and hydrochemical characteristics of Severnaya Dvina River (2006–2008).
Land 2024,13, 1765 7 of 22
Land 2024, 13, 1765 7 of 22
Figure 3. Seasonal discharge and hydrochemical characteristics of Severnaya Dvina River (2012–
2014).
Figure 3. Seasonal discharge and hydrochemical characteristics of Severnaya Dvina River (2012–2014).
Land 2024,13, 1765 8 of 22
Land 2024, 13, 1765 8 of 22
Figure 4. Seasonal discharge and hydrochemical characteristics of Pechora River (2016–2019).
Figure 4. Seasonal discharge and hydrochemical characteristics of Pechora River (2016–2019).
Land 2024,13, 1765 9 of 22
Land 2024, 13, 1765 9 of 22
Figure 5. Seasonal discharge and hydrochemical characteristics of Taz River (2016–2020).
The average DOC concentrations in the Severnaya Dvina River were 12.78 and 11.84
mg/L in the periods 2006–2008 and 2012–2014, respectively. The maximum DOC values
recorded were 16.87 and 14.72 mg/L, with peak levels of 26.4 and 16.85 mg/L occurring
during spring, while the lowest values observed were 3.35 and 7.07 mg/L, with corre-
sponding mean minimum values of 10.55 and 10.07 mg/L during the summer and winter.
In the Pechora River, the average DOC concentration from 2016 to 2019 was 6.73 mg/L,
Figure 5. Seasonal discharge and hydrochemical characteristics of Taz River (2016–2020).
The average DOC concentrations in the Severnaya Dvina River were 12.78 and
11.84 mg/L in the periods 2006–2008 and 2012–2014, respectively. The maximum DOC
values recorded were 16.87 and 14.72 mg/L, with peak levels of 26.4 and 16.85 mg/L
Land 2024,13, 1765 10 of 22
occurring during spring, while the lowest values observed were 3.35 and 7.07 mg/L,
with corresponding mean minimum values of 10.55 and 10.07 mg/L during the summer
and winter. In the Pechora River, the average DOC concentration from 2016 to 2019 was
6.73 mg/L, with a maximum DOC concentration of 14.06 mg/L in summer. However, its
DOC concentration was higher in spring (8.07 mg/L) than in summer (7.24 mg/L). The
minimum DOC value was 1.75 mg/L, with a mean minimum value of 4.88 mg/L, both
recorded in winter. The average DOC concentration in the Taz River from 2016 to 2020 was
9.96 mg/L, with maximum values of 18.98 mg/L in spring and 14.06 mg/L in summer and
a minimum value of 7.53 mg/L recorded in winter.
Regarding major ions, the Severnaya Dvina River during the years 2006–2008 showed
that Ca
2+
and SO
42−
had higher mean values in summer than in winter, while other ions
had the highest mean value in winter. The results of the integrated seasonal analysis of the
Severnaya Dvina River are consistent with the study published by Pokrovsky and Chu-
pakov et al. [
1
–
4
]. Except for Ca
2+
(546.3 mg/L), Cl
−
(111 mg/L) and SO
42−
(3776 mg/L),
which had their highest concentrations during specific seasons (spring and summer) be-
tween 2006 and 2008, the maximum concentrations of the other ions in the Severnaya Dvina
River basin generally occurred in winter, while the minimum concentrations were mostly
observed in spring. The study of the Taz River conducted by Pokrovsky et al., published
in Chemical Geology, revealed that soluble elements originating from groundwater and
deep soil mineral horizons (Cl
−
, SO
42−
, Na
+
, K
+
, Mg
2+
, Ca
2+
) reached their maximum
concentrations at the end of winter, just before the spring ice-off. In contrast, dissolved
organic carbon (DOC) exhibited peak concentrations during the spring flood and autumn
high flow, with minimal values recorded in winter [
32
]. Based on this article, the current
paper sought to analyze the concentration of elements and daily runoff data of the Taz River
from Mendeley (https://data.mendeley.com), in order to provide a more comprehensive
understanding of the elemental characteristics of the Taz River. The seasonal characteristics
of major ion concentrations in the Pechora and Taz Rivers are similar to those in the Sever-
naya Dvina River, with most ions reaching their maximum and minimum mean values in
winter and spring, respectively, except for a few deviations. For instance, in the Taz basin,
except for the minimum mean concentration of Na
+
(2.37 mg/L) and Mg
2+
(3.22 mg/L) in
spring, the minimum mean values of other ion concentrations occurred in summer, and the
concentrations of the six ions all showed the minimum concentration in spring.
Despite the moderate size of the study area’s watersheds, runoff, the concentration
of DOC and major ions in mid-sized Arctic river basins have responded significantly to
environmental changes under global warming. In recent years, total river runoff in the
Pechora and Taz River basins has increased, in line with other Arctic river trends [
47
].
In the Severnaya Dvina River, the dissolved organic carbon (DOC) content during the
period of 2012–2014 is relatively lower compared to that of 2006–2008. Additionally, the
concentrations of Ca
2+
and SO
42−
recorded in 2012 are notably lower than those observed in
2006–2008. Conversely, the concentrations of Na
+
, K
+
, Mg
2+
and Cl
−
in 2012 are relatively
higher than in 2006–2008. Between 2015 and 2019, the average annual runoff in the Pechora
River increased from 4108 m
3
/s to 6553.5 m
3
/s, while the DOC and major ion content
decreased to some extent. In the Taz River, the mean annual runoff in 2020 is higher than
that of 2015, increasing from 1020.27 m
3
/s to 1232.45 m
3
/s. However, the DOC and major
ion content in 2020 are lower than in 2015 (Figure 6).
The three river basins exhibit pronounced seasonal variability in both runoff and
water quality, with distinct seasonal patterns differing among the basins. Analysis of
water chemistry data from the Severnaya Dvina River reveals that DOC concentrations
peak during the spring at 16.87 mg/L and 14.72 mg/L during the years 2006–2008 and
2012–2014, respectively, with consistently high levels in the spring and summer, while
winter exhibits the lowest concentrations. Conversely, Na
+
, K
+
, Mg
2+
, Ca
2+
, Cl
−
and
SO
42−
concentrations reach their highest levels during winter, with the lowest values
observed in spring (Figures 2and 3). Similarly, in the Pechora River, DOC concentrations
are highest in spring, at 8.07 mg/L, and lowest in winter, at around 4.88 mg/L, during the
Land 2024,13, 1765 11 of 22
years 2016–2019. However, the ion concentrations, with the exception of Na
+
, are highest
in winter and lowest in spring (Figure 4). Notably, the Taz River shows a more varied
seasonal distribution of DOC with peak concentrations in spring at 18.98 mg/L, and the
mean maximum is 11.69 mg/L in summer. Ion concentrations of Na
+
and Cl
−
peak in
spring, while K
+
Mg
2+
, Ca
2+
and SO
42−
reach their maximum levels in winter. Overall, ion
concentrations in the Taz River are predominantly high in winter, followed by spring and
a notable variability in summer, where ion levels exhibit significant fluctuations between
high and low extremes (Figure 5).
Land 2024, 13, 1765 11 of 22
Figure 6. Interannual trends in discharge, DOC and major ions in the Severnaya Dvina (a), Pechora
(b) and Taz (c) River basins.
The three river basins exhibit pronounced seasonal variability in both runoff and wa-
ter quality, with distinct seasonal patterns differing among the basins. Analysis of water
chemistry data from the Severnaya Dvina River reveals that DOC concentrations peak
during the spring at 16.87 mg/L and 14.72 mg/L during the years 2006–2008 and 2012–
2014, respectively, with consistently high levels in the spring and summer, while winter
exhibits the lowest concentrations. Conversely, Na+, K+, Mg2+, Ca2+, Cl− and SO42− concen-
trations reach their highest levels during winter, with the lowest values observed in spring
(Figures 2 and 3). Similarly, in the Pechora River, DOC concentrations are highest in
spring, at 8.07 mg/L, and lowest in winter, at around 4.88 mg/L, during the years 2016–
2019. However, the ion concentrations, with the exception of Na+, are highest in winter
and lowest in spring (Figure 4). Notably, the Taz River shows a more varied seasonal dis-
tribution of DOC with peak concentrations in spring at 18.98 mg/L, and the mean maxi-
mum is 11.69 mg/L in summer. Ion concentrations of Na+ and Cl− peak in spring, while K+
Mg2+, Ca2+ and SO42− reach their maximum levels in winter. Overall, ion concentrations in
the Taz River are predominantly high in winter, followed by spring and a notable varia-
bility in summer, where ion levels exhibit significant fluctuations between high and low
extremes (Figure 5).
3.2. Relationship Between Discharge and Hydrochemical Variability
Significant correlations were observed between river discharge, DOC and major ion
(Na+, K+, Mg2+, Ca2+, Cl−, SO42−) concentrations across the Severnaya Dvina, Pechora and
Taz River basins (p < 0.05). In the Severnaya Dvina and Pechora Rivers, DOC concentra-
tions increased significantly with rising discharge (p < 0.001, R > 0). On average, for every
1% increase in discharge, DOC concentrations rose by 1.25% in the Severnaya Dvina River
and by 1.04% in the Pechora River. In contrast, runoff had no statistically significant im-
pact on DOC levels in the Taz River. DOC concentrations in all three basins peaked during
Figure 6. Interannual trends in discharge, DOC and major ions in the Severnaya Dvina (a), Pechora
(b) and Taz (c) River basins.
3.2. Relationship Between Discharge and Hydrochemical Variability
Significant correlations were observed between river discharge, DOC and major ion
(Na
+
, K
+
, Mg
2+
, Ca
2+
, Cl
−
, SO
42−
) concentrations across the Severnaya Dvina, Pechora and
Taz River basins (p< 0.05). In the Severnaya Dvina and Pechora Rivers, DOC concentrations
increased significantly with rising discharge (p< 0.001, R > 0). On average, for every 1%
increase in discharge, DOC concentrations rose by 1.25% in the Severnaya Dvina River
and by 1.04% in the Pechora River. In contrast, runoff had no statistically significant
impact on DOC levels in the Taz River. DOC concentrations in all three basins peaked
during the spring and summer high-flow periods, correlating positively with river runoff.
Higher DOC transport occurred during the spring flood (May–July) and summer high-flow
season (August–October), with concentrations declining during the winter low-flow period
(November–April).
Except for K
+
in the Pechora River and K
+
and SO
42−
in the Taz River basin (p> 0.05),
major ions in all three basins decreased significantly with increasing discharge (p< 0.05,
Land 2024,13, 1765 12 of 22
R < 0) (Figure 7). Major ion concentrations, characterized by their solubility and mobility,
were inversely correlated with runoff, decreasing sharply during high-flow periods and
increasing during winter low flows.
Land 2024, 13, 1765 12 of 22
the spring and summer high-flow periods, correlating positively with river runoff. Higher
DOC transport occurred during the spring flood (May–July) and summer high-flow sea-
son (August–October), with concentrations declining during the winter low-flow period
(November–April).
Except for K+ in the Pechora River and K+ and SO42− in the Taz River basin (p > 0.05),
major ions in all three basins decreased significantly with increasing discharge (p < 0.05,
R < 0) (Figure 7). Major ion concentrations, characterized by their solubility and mobility,
were inversely correlated with runoff, decreasing sharply during high-flow periods and
increasing during winter low flows.
The nonlinear fitting curve between river substance concentrations and runoff across
the Severnaya Dvina, the Pechora and the Taz Rivers revealed a strong power-law rela-
tionship, with DOC and ion concentrations fitting the equation y = axb. Among these ba-
sins, the Severnaya Dvina exhibited the best overall fit between hydrochemical variables
and runoff (Figure 8). In the Pechora basin, all major ions except for K+ and DOC showed
a strong relationship with runoff, which potentially due to the leaching processes of the
exit litter fall of the previous year during freshet of the next year [48,49]. Notably, while
DOC concentrations generally increased with higher runoff, there was substantial varia-
bility in DOC levels across different seasons (Figure 9). In contrast, K+ concentrations re-
mained relatively consistent between spring high-flow and summer seasons, ranging
from 0.3 to 0.9 mg/L. The Taz River basin demonstrated the most complex interactions,
with only Na+, Mg2+, Ca2+ and Cl− showing a significant power-law relationship with run-
off (Figure 10).
Land 2024, 13, 1765 13 of 22
Figure 7. Cont.
Land 2024,13, 1765 13 of 22
Land 2024, 13, 1765 13 of 22
Figure 7. The correlations between element concentrations in the Severnaya Dvina, Pechora
and Taz Rivers: discharge, DOC, Na
+
, K
+
, Mg
2+
, Ca
2+
, Cl
−
and SO
42−
; color represents the
correlation coefficient.
The nonlinear fitting curve between river substance concentrations and runoff across
the Severnaya Dvina, the Pechora and the Taz Rivers revealed a strong power-law relation-
ship, with DOC and ion concentrations fitting the equation y = ax
b
. Among these basins,
the Severnaya Dvina exhibited the best overall fit between hydrochemical variables and
runoff (Figure 8). In the Pechora basin, all major ions except for K
+
and DOC showed a
strong relationship with runoff, which potentially due to the leaching processes of the exit
litter fall of the previous year during freshet of the next year [
48
,
49
]. Notably, while DOC
concentrations generally increased with higher runoff, there was substantial variability in
DOC levels across different seasons (Figure 9). In contrast, K
+
concentrations remained
relatively consistent between spring high-flow and summer seasons, ranging from 0.3 to
0.9 mg/L. The Taz River basin demonstrated the most complex interactions, with only Na
+
,
Mg
2+
, Ca
2+
and Cl
−
showing a significant power-law relationship with runoff (Figure 10).
Land 2024,13, 1765 14 of 22
Land 2024, 13, 1765 14 of 22
Figure 7. The correlations between element concentrations in the Severnaya Dvina, Pechora and Taz
Rivers: discharge, DOC, Na+, K+, Mg2+, Ca2+, Cl− and SO42−; color represents the correlation coefficient.
Figure 8. Nonlinear curve fitting analysis of concentration–discharge relationships for DOC and
major ions in Severnaya Dvina River basin.
Figure 8. Nonlinear curve fitting analysis of concentration–discharge relationships for DOC and
major ions in Severnaya Dvina River basin.
Land 2024,13, 1765 15 of 22
Land 2024, 13, 1765 15 of 22
Figure 9. Seasonal variations and nonlinear curve fitting analysis of concentration–discharge rela-
tionships for DOC and major ions in Pechora River basin.
Figure 9. Seasonal variations and nonlinear curve fitting analysis of concentration–discharge relation-
ships for DOC and major ions in Pechora River basin.
Land 2024,13, 1765 16 of 22
Land 2024, 13, 1765 16 of 22
Figure 10. Seasonal variations and nonlinear curve fitting analysis of concentration–discharge rela-
tionships for DOC and major ions in Taz River basin.
Figure 10. Seasonal variations and nonlinear curve fitting analysis of concentration–discharge
relationships for DOC and major ions in Taz River basin.
Land 2024,13, 1765 17 of 22
4. Discussion
The Severnaya Dvina, Pechora and Taz River basins each display distinct spatiotem-
poral variations in DOC and major ion concentrations, reflective of the complex interplay
between climatic, hydrological and geological factors. These variations highlight the signif-
icance of understanding localized environmental conditions to better manage and protect
water resources in Arctic regions. It is important to note that the data for each basin in
this study are not derived from the same sampling points, and the spatial heterogeneity
of river hydrochemical characteristics must be taken into account. Specific geological
conditions may result in local anomalies in water chemistry. Additionally, the variation
in water sampling points between the two periods [
22
] and the limited number of water
samples may contribute to differences in river water chemistry. The observed seasonal
dilution of major ions during the onset of increased flow, followed by chemical stabilization
towards the end of the high-flow period, highlights the dynamic response of these river
systems to hydrological regime variations. This pattern aligns with previous studies in
Siberian rivers [
29
,
30
], which suggests that increased spring snowmelt floods and summer
precipitation exert a pronounced dilution effect on major ion concentrations [
2
,
50
–
54
]. Data
confirm that the total dissolved solids (TDSs) in all six East Siberian rivers are 32–64% lower
during high-flow seasons compared to low-flow seasons [
22
]. The primary reason for this
difference is that runoff during the low-flow season primarily originates from mineral-rich
groundwater [
55
,
56
]. In contrast, during high-flow seasons, river flow is predominantly
generated by rainfall and the melting of snow and ice. As a result, river water may be
diluted, leading to a lower mineral content [
57
]. During winter baseflow, concentrations
of soluble cations and anions reach their peak, while spring floods lead to a sharp decline
in these elements and a simultaneous increase in DOC. Studies have demonstrated that
the mechanisms driving changes in dissolved organic carbon (DOC) exports in Arctic river
basins are intricate and primarily influenced by permafrost conditions. These conditions
include the extent of permafrost, the age of sediments, the concentration of DOC within
the permafrost and the associated hydrological characteristics of permafrost regions [
25
].
The seasonal variation of dissolved organic carbon (DOC) flux in the six major Arctic rivers
is more pronounced in basins with extensive permafrost coverage. The land source is
relatively new, primarily contributing DOC derived from modern plant litter at the surface.
In contrast, rich peat deposits in areas with discontinuous permafrost are transported by
rivers, which are characterized by the release of older carbon [
58
,
59
]. DOC concentrations
typically rise sharply in May; however, in the Mackenzie and Yenisei River basins, this
increase is delayed until June [
25
]. This observation aligns with the findings of this study,
which indicate that the Severnaya Dvina, Pechora and Taz Rivers exhibit the highest aver-
age DOC levels during the high flow periods of spring and summer. Notably, the extent
of continuous permafrost observed in the six Arctic river studies correlates with greater
restrictions on organic carbon output [
25
]. However, the dissolved organic carbon (DOC)
output from the Taz River, which has a broader coverage of permafrost, is higher than
that of the Pechora River, which has less continuous permafrost coverage. This finding is
inconsistent with the patterns observed in the six major Arctic rivers and may be attributed
to the greater thickness of the active layer of permafrost in the Taz River.
This rise in DOC during spring can be attributed to surface runoff traversing organic-
rich soils, which introduces substantial organic matter into the rivers [
33
,
60
–
63
]. Subsurface
water fluxes during summer baseflow are mainly influenced by water–rock interactions,
which depend on the depth and continuity of the permafrost [
29
,
30
]. In areas with con-
tinuous permafrost, these interactions are limited, but in regions with discontinuous and
sporadic permafrost, they have a greater influence on riverine DOC and chemistry [
23
,
64
].
For instance, in the Severnaya Dvina, where permafrost is absent with an annual mean
temperature of 1.82
◦
C, spring snowmelt sharply increases DOC concentrations as runoff
transports organic matter into the river. Similarly, the Pechora River, with 40% discontinu-
ous permafrost coverage and an annual mean temperature of
−
2.29
◦
C, also experiences
a DOC peak in spring due to combined snowmelt and permafrost dynamics. In contrast,
Land 2024,13, 1765 18 of 22
the Taz River, entirely underlain by permafrost with the lowest annual mean tempera-
ture of
−
5.44
◦
C, shows a delayed DOC peak in summer, likely due to slower thawing
of higher ground ice content of 80.70% and the development of a deeper active layer of
~84.60 cm [
65
,
66
], which enhance subsurface water–rock interactions and organic matter
release during high flow.
Warming-induced permafrost degradation may further influence the hydrochemical
dynamics of Arctic rivers [
2
,
67
–
70
]. As the active layer deepens and ground ice melts, the
enhanced chemical weathering of underlying rocks and prolonged water–rock interactions
are expected to alter ion and DOC concentrations [
71
,
72
]. The variable contributions of
rock weathering processes, controlled by hydrological pathways and reaction times, play a
pivotal role in shaping river solute profiles [
73
–
75
]. Additionally, surface organic matter [
76
],
glacial melting [
77
], precipitation [
39
] and anthropogenic
activities [24,78–80]
in Arctic
region are increasingly recognized as critical factors impacting Arctic river water quality.
This study highlights the distinct seasonal and regional characteristics of water quality
and riverine DOC dynamics in mid-sized Arctic basins, distinguishing them from those
observed in larger Arctic rivers. These findings emphasize the importance of considering
localized variations in hydrology and material cycles within Arctic basins. The observed
differences in water quality across basins with varying permafrost development suggest
that, with accelerating climate warming, the hydrological regimes and their impacts on wa-
ter chemistry will undergo significant changes. The comparative analysis of the three basins
indicates that permafrost degradation is likely to result in earlier and more pronounced
changes in water chemistry in response to runoff. This underscores the need to consider
spatial heterogeneity and the delayed effects of differential permafrost development in
future research and management strategies.
5. Conclusions
This study provides a comprehensive analysis of dissolved organic carbon (DOC) and
six major ions (Na
+
, K
+
, Mg
2+
, Ca
2+
, Cl
−
and SO
42−
) in the Severnaya Dvina, Pechora
and Taz Rivers, and it reveals significant differences in multi-year trends and seasonal
hydrochemical characteristics among these river basins. In the Pechora and Taz Rivers,
which are predominantly underlined by permafrost, along with the rising discharge, DOC
concentration exhibits increasing trends, while major ion concentrations exhibit decreasing
trends. In contrast, there is no obvious change trend of DOC content and main ion concen-
tration in the Severnaya Dvina River without permafrost. Seasonally, DOC concentrations
are positively correlated with river discharge, peaking during spring floods and high
summer flows, reflecting the leaching effect of surface waters on soils. Conversely, the con-
centrations of Na
+
, K
+
, Mg
2+
, Ca
2+
, Cl
−
and SO
42−
decrease with increasing river runoff,
indicating a dilution effect. The DOC of the Taz River, with the most developed permafrost,
displays distinct seasonal characteristics compared to the other basins, highlighting the
impact of permafrost extent and the potential impacts of climate warming on permafrost
degradation and associated water chemistry. This study highlights the critical influence of
permafrost degradation under a warming climate on seasonal water quality changes and
the necessity of considering permafrost conditions when assessing future changes in Arctic
river systems.
Author Contributions: Y.Y.: Data curation, Software, Writing—original draft. P.W.: Conceptual-
ization, Writing—review and editing, Project administration. S.L.: Investigation, Data curation,
Software, Supervision, Writing—original draft. D.C. and R.W.: Software, Formal analysis. C.D.:
Conceptualization, Writing—review and editing, Project administration. All authors have read and
agreed to the published version of the manuscript.
Funding: This research was funded by the Science & Technology Fundamental Investigation Program
(Nos. 2022FY101900, 2022FY101901) and the National Natural Science Foundation of China (Nos.
42371033, 42061134017).
Land 2024,13, 1765 19 of 22
Data Availability Statement: The original contributions presented in the study are included in the
article, further inquiries can be directed to the corresponding author.
Acknowledgments: We acknowledge the pan-Arctic catchment database (ARCADE) provided by
Speetjens et al., 2023. We extend our sincere gratitude to all the authors enumerated in Table 2for
their invaluable contributions to the study data. The authors also gratefully acknowledge the editor
and four anonymous reviewers for their valuable comments and suggestions, which have led to
substantial improvements on an earlier version of the manuscript.
Conflicts of Interest: The authors declare no conflicts of interest.
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