Particulate suspended matter (PSM) of rivers is a significant factor for carbon, nutrient, and trace metal transfer from land to ocean. Towards better understanding the role that PSM exerts on major ...and trace elements in riverine systems, here we report the results of an experimental study which utilizes a two-fold approach to assess interaction between PSM and riverine solutes. First, we measured element leaching (via desorption and dissolution in distilled water, simulating snow melt) from PSM of the largest Siberian river, the Ob River. Second, we quantified the capacity of PSM to adsorb dissolved organic carbon (DOC), macro- and micronutrients and trace elements from organic-rich waters of the river floodplain. We documented sizable desorption of organic carbon, some major and trace metals, oxyanions and insoluble elements from PSM; the majority (>50 %) of elements were released over the first hour of reaction. In contrast, PSM of the Ob River was capable of removing 20 to 90 % of dissolved OC, nutrients (Si, P), and trace elements from the tributary and floodplain fen. Our experiments demonstrated preferential adsorption of aromatic compounds large molecular size colloids. Taken together, the adsorption of solutes by PSM can sizably decrease the concentration and modify the molecular size distribution, and therefore the potential bioavailability of major (DOC, P, Si) and trace micronutrients. Overall, the PSM of the Ob River exhibited high reactivity with respect to natural waters and was capable of modifying the elemental composition of the tributary and floodplain fen waters. This transfer of organic carbon and nutrients in the surface-adsorbed (particulate) form is especially important during spring flood and requires specific consideration in short-term biogeochemical cycles of elements in continental waters.
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•High capacity of particulate suspended matter (PSM) to release DOC and nutrients and adsorb riverine solutes•Adsorption of aromatic, high molecular weight organic C and colloids onto PSM•Nutrients (P, Si, Zn, V) and toxicants (As, Cd) from tributary and fen waters are adsorbed onto PSM.•PSM of a large river as a hydrochemical composition regulator for floodplain water bodies
Shallow thermokarst lakes are important sources of greenhouse gases (GHGs) such as methane (CH4) and carbon dioxide (CO2) resulting from continuous permafrost thawing due to global warming. ...Concentrations of GHGs dissolved in water typically increase with decreasing lake size due to coastal abrasion and organic matter delivery. We hypothesized that (i) CH4 oxidation depends on the natural oxygenation gradient in the lake water and sediments and increases with lake size because of stronger wind‐induced water mixing; (ii) CO2 production increases with decreasing lake size, following the dissolved organic matter gradient; and (iii) both processes are more intensive in the upper than deeper sediments due to the in situ gradients of oxygen (O2) and bioavailable carbon. We estimated aerobic CH4 oxidation potentials and CO2 production based on the injection of 13C‐labeled CH4 in the 0–10 cm and 10–20 cm sediment depths of small (~300 m2), medium (~3000 m2), and large (~106 m2) shallow thermokarst lakes in the West Siberian Lowland. The CO2 production was 1.4–3.5 times stronger in the upper sediments than in the 10–20 cm depth and increased from large (158 ± 18 nmol CO2 g−1 sediment d.w. h−1) to medium and small (192 ± 17 nmol CO2 g−1 h−1) lakes. Methane oxidation in the upper sediments was similar in all lakes, while at depth, large lakes had 14‐ and 74‐fold faster oxidation rates (5.1 ± 0.5 nmol CH4‐derived CO2 g−1 h−1) than small and medium lakes, respectively. This was attributed to the higher O2 concentration in large lakes due to the more intense wind‐induced water turbulence and mixing than in smaller lakes. From a global perspective, the CH4 oxidation potential confirms the key role of thermokarst lakes as an important hotspot for GHG emissions, which increase with the decreasing lake size.
Thermokarst lakes play a pivotal role in the modification of the C cycle now and will be crucial under warming climate conditions. Here, based on injection of 13C‐labeled CH4 in sediments of small, medium, and large thermokarst lakes in the West Siberian Lowland, we found that methane oxidation in the upper sediments was similar in all lakes, while below 10 cm depth, large lakes had faster oxidation rates than small lakes due to the intense wind‐induced water turbulence and mixing. This largely explains the increase of GHG emissions with decreasing lake size.
Extensive studies have been performed on wildfire impact on terrestrial and aquatic ecosystems in the taiga biome, however consequences of wildfires in the tundra biome remain poorly understood. In ...such a biome, permafrost peatlands occupy a sizable territory in the Northern Hemisphere and present an extensive and highly vulnerable storage of organic carbon. Here we used an experimental approach to model the impact of ash produced from burning of main tundra organic constituents (i.e., moss, lichen and peat) on surrounding aquatic ecosystems. We studied the chemical composition of aqueous leachates produced during short-term (1 week) interaction of ash with distilled water and organic-rich lake water at 5 gsolid L−1 and 20 °C. The addition of ash enriched the fluid phase in major cations (i.e., Na, Ca, Mg), macro- (i.e., P, K, Si) and micronutrients (i.e., Mn, Fe, Co, Ni, Zn, Mo). This enrichment occurred over <2 days of experiment. Among 3 studied substrates, moss ash released the largest amount of macro- and micro-components into the aqueous solution. To place the obtained results in the environmental context of a peatbog watershed, we assume a fire return interval of 56 years and that the entire 0–10 cm of upper peat is subjected to fire impact. These mass balance calculations demonstrated that maximal possible delivery of elements from ash after soil burning to the hydrological network is negligibly small (<1–2 %) compared to the annual riverine export flux and element storage in thermokarst lakes. As such, even a 5–10 fold increase in tundra wildfire frequency may not sizably modify nutrient and metal fluxes and pools in the surrounding aquatic ecosystems. This result requires revisiting the current paradigm on the importance of wildfire impact on permafrost peatlands and calls a need for experimental work on other ecosystem compartments (litter, shrubs, frozen peat) which are subjected to fire events.
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•Laboratory study of ash from tundra vegetation.•Peat, moss and lichen ashes rapidly released major and trace elements to the fluid.•Moss ash released the highest amount of solutes.•Negligible effect of tundra wildfire on riverine export and lake storage of nutrients and metals.
In order to foresee the impact of permafrost thaw on CO2 emissions by high-latitude rivers, in-situ measurements across a permafrost and climate/vegetation gradient, coupled with assessment of ...possible physico-chemical and landscape controlling factors are necessary. Here we chose 34 catchments of variable stream order (1 to 9) and watershed size (1 to >105 km2) located across a permafrost and biome gradient in the Western Siberian Lowland (WSL), from the permafrost-free southern taiga to the continuous permafrost zone of tundra. Across the south-north transect, maximal CO2 emissions (2.2 ± 1.1 g C-CO2 m−2 d−1) occurred from rivers of the discontinuous/sporadic permafrost zone, i.e., geographical permafrost thawing boundary. In this transitional zone, fluvial C emission to downstream export ratio was as high as 8.0, which greatly (x 10) exceeded the ratio in the permafrost free and continuous permafrost zones. Such a high evasion at the permafrost thawing front can stem from an optimal combination of multiple environmental factors: maximal active layer thickness, sizable C stock in soils, and mobilization of labile organic nutrients from dispersed peat ice that enhanced DOC and POC processing in the water column, likely due to priming effect. Via a substituting space for time approach, we foresee an increase in CO2 and CH4 fluvial evasion in the continuous and discontinuous permafrost zone, which is notably linked to the greening of tundra increases in biomass of the riparian vegetation, river water warming and thermokarst lake formation on the watershed.
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•Maximal CO2 emissions from rivers of the discontinuous/sporadic permafrost zone•Peak of C emission: export ratio at the permafrost thawing front•Dissolved and particulate organic carbon as potential controllers of CO2 emissions•Possible priming effect from DOM of peat ice in C evasion from rivers•Future two-fold increase in C emissions from rivers of continuous permafrost zone
Despite the importance of small and medium size rivers of Siberian boreal zone in greenhouse gases (GHG) emission, major knowledge gaps exist regarding its temporal variability and controlling ...mechanisms. Here we sampled 11 pristine rivers of the southern taiga biome (western Siberia Lowland, WSL), ranging in watershed area from 0.8 to 119,000 km2, to reveal temporal pattern and examine main environmental controllers of GHG emissions from the river water surfaces. Floating chamber measurements demonstrated that CO2 emissions from water surface decreased by 2 to 4-folds from spring to summer and autumn, were independent of the size of the watershed and stream order and did not exhibit sizable (>30 %, regardless of season) variations between day and night. The CH4 concentrations and fluxes increased in the order “spring ≤ summer < autumn” and ranged from 1 to 15 μmol L−1 and 5 to 100 mmol m−2 d−1, respectively. The CO2 concentrations and fluxes (range from 100 to 400 μmol L−1 and 1 to 4 g C m−2 d−1, respectively) were positively correlated with dissolved and particulate organic carbon, total nitrogen and bacterial number of the water column. The CH4 concentrations and fluxes were positively correlated with phosphate and ammonia concentrations. Of the landscape parameters, positive correlations were detected between riparian vegetation biomass and CO2 and CH4 concentrations. Over the six-month open-water period, areal emissions of C (>99.5 % CO2; <0.5 % CH4) from the watersheds of 11 rivers were equal to the total downstream C export in this part of the WSL. Based on correlations between environmental controllers (watershed land cover and the water column parameters), we hypothesize that the fluxes are largely driven by riverine mineralization of terrestrial dissolved and particulate OC, coupled with respiration at the river bottom and riparian sediments. It follows that, under climate warming scenario, most significant changes in GHG regimes of western Siberian rivers located in permafrost-free zone may occur due to changes in the riparian zone vegetation and water coverage of the floodplains.
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•The river size (6 order of magnitude in watershed area) had weak impact on CO2 fluxes.•CO2 concentration and emission decreased from spring to summer and autumn.•Lack of diel CO2 variation in rivers•Terrestrial DOC and POC may control C emission from small rivers.•C emission from river surfaces was comparable to dissolved and particulate C export by rivers.
Arctic permafrost soils contain large amounts of organic carbon and the pollutant mercury (Hg). Arctic warming and associated changes in hydrology, biogeochemistry and ecology risk mobilizing soil Hg ...to rivers and to the Arctic Ocean, yet little is known about the quantity, timing and mechanisms involved. Here we investigate seasonal particulate Hg (PHg) and organic carbon (POC) export in 32 small and medium rivers across a 1700 km latitudinal permafrost transect of the western Siberian Lowland. The PHg concentrations in suspended matter increased with decreasing watershed size. This underlines the significance of POC-rich small streams and wetlands in PHg export from watersheds. Maximum PHg concentrations and export fluxes were located in rivers at the beginning of permafrost zone (sporadic permafrost). We suggest this reflects enhanced Hg mobilization at the permafrost boundary, due to maximal depth of the thawed peat layer. Both the thickness of the active (unfrozen) peat layer and PHg run-off progressively move to the north during the summer and fall seasons, thus leading to maximal PHg export at the sporadic to discontinuous permafrost zone. The discharge-weighed PHg:POC ratio in western Siberian rivers (2.7 ± 0.5 μg Hg: g C) extrapolated to the whole Ob River basin yields a PHg flux of 1.5 ± 0.3 Mg y−1, consistent with previous estimates. For current climate warming and permafrost thaw scenarios in western Siberia, we predict that a northward shift of permafrost boundaries and increase of active layer depth may enhance the PHg export by small rivers to the Arctic Ocean by a factor of two over the next 10–50 years.
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•Particulate Hg (PHg) concentration in western Siberia rivers decreases with watershed size.•PHg correlates with organic carbon and anti-correlates with lithogenic elements.•Hg mobilization from organic (peat) rather than mineral soil layers to the rivers.•Maximum PHg concentration and export fluxes in the sporadic permafrost zone.•Climate warming and permafrost thaw may double PHg export to the Arctic Ocean.
Enhanced Hg mobilization from peat soil to the rivers at the permafrost boundary, due to maximal depth of the thawed layer.
The assessment of riverine fluxes of carbon, nutrients, and metals in surface waters of permafrost-affected regions is crucially important for constraining adequate models of ecosystem functioning ...under various climate change scenarios. In this regard, the largest permafrost peatland territory on the Earth, the Western Siberian Lowland (WSL) presents a unique opportunity of studying possible future changes in biogeochemical cycles because it lies within a south–north gradient of climate, vegetation, and permafrost that ranges from the permafrost-free boreal to the Arctic tundra with continuous permafrost at otherwise similar relief and bedrocks. By applying a “substituting space for time” scenario, the WSL south-north gradient may serve as a model for future changes due to permafrost boundary shift and climate warming. Here we measured export fluxes (yields) of dissolved organic carbon (DOC), major cations, macro- and micro- nutrients, and trace elements in 32 rivers, draining the WSL across a latitudinal transect from the permafrost-free to the continuous permafrost zone. We aimed at quantifying the impact of climate warming (water temperature rise and permafrost boundary shift) on DOC, nutrient and metal in rivers using a “substituting space for time” approach. We demonstrate that, contrary to common expectations, the climate warming and permafrost thaw in the WSL will likely decrease the riverine export of organic C and many elements. Based on the latitudinal pattern of riverine export, in the case of a northward shift in the permafrost zones, the DOC, P, N, Si, Fe, divalent heavy metals, trivalent and tetravalent hydrolysates are likely to decrease the yields by a factor of 2–5. The DIC, Ca, SO4, Sr, Ba, Mo, and U are likely to increase their yields by a factor of 2–3. Moreover, B, Li, K, Rb, Cs, N-NO3, Mg, Zn, As, Sb, Rb, and Cs may be weakly affected by the permafrost boundary migration (change of yield by a factor of 1.5 to 2.0). We conclude that modeling of C and element cycle in the Arctic and subarctic should be region-specific and that neglecting huge areas of permafrost peatlands might produce sizeable bias in our predictions of climate change impact.
Thermokarst (thaw) lakes of permafrost peatlands are among the most important sentinels of climate change and sizable contributors of greenhouse gas emissions (GHG) in high latitudes. These lakes are ...humic, often acidic and exhibit fast growing/drainage depending on the local environmental and permafrost thaw. In contrast to good knowledge of the thermokarst lake water hydrochemistry and GHG fluxes, the sediments pore waters remain virtually unknown, despite the fact that these are hot spots of biogeochemical processes including GHG generation. Towards better understating of dissolved organic matter (DOM) quality at the lake water – sediment interface and in the sediments pore waters, here we studied concentration and optical (UV, visual) properties of DOM of 11 thermokarst lakes located in four permafrost zones of Western Siberia Lowland. We found systematic evaluation of DOM concentration, SUVA and various optical parameters along the vertical profile of lake sediments. The lake size and hence, the stage of lake development, had generally weak control on DOM quality.
The permafrost zone exhibited clear impact on DOM porewater concentration, optical characteristics, aromaticity and weight average molecular weight (WAMW). The lowest quality of DOM, reflected in highest SUVA and WAMW, corresponding to the dominance of terrestrial sources, was observed at the southern boundary of the permafrost, in the sporadic/discontinuous zone. This suggests active mobilization of organic matter leachates from the interstitial peat and soil porewaters to the lake, presumably via subsurface or suprapermafrost influx. Applying a substitute space for time scenario for future evolution of OM characteristics in thermokarst lake sediments of Western Siberia, we foresee a decrease of DOM quality, molecular weight and potential bioavailability in lakes of continuous permafrost zone, and an increase in these parameters in the sporadic/discontinuous permafrost zone.
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•DOM characteristics in porewaters of thermokarst lake sediments depend on the permafrost zone•Weak effect of lake size on DOM concentration and quality in porewaters of thermokarst lake sediments•Maximal DOC concentration, SUVA254 and aromaticity in the sporadic/discontinuous permafrost•Strong link between peat porewaters and lake sediments via subsurface flow•Possible decrease of DOM quality in lakes of continuous permafrost zone upon climate warming
The physical and chemical consequences of massive ground ice (wedges) melt upon permafrost thaw is one of the central issues of environmental research linked to climate warming in the Arctic. Little ...is known about the chemical properties of dispersed ground ice abundant throughout permafrost peatlands that can easily melt with increasing active layer thickness (ALT). This is especially pertinent in continental lowlands, that account for sizeable areas of the Arctic, and contain high amount of organic carbon in both solid (peat) and liquid (porewater) phases. Here we studied 8 peat cores (0–130 cm depth)—comprised of porewater from the active layer (0–45 cm) as well as ice dispersed in frozen peat (40–130 cm)—across a latitudinal profile of Western Siberia Lowland (WSL) extending from discontinuous into continuous permafrost zones. Dissolved Organic Carbon (DOC), alkali and alkaline-earth metals (Ca, Mg, Sr, Ba, Li, Rb, Cs), sulfate, phosphorus, some trace elements (Al, Fe, Mn, Zn, Ni, Co, V, As, Y, REE, Zr, Hf, U) were sizably more than 3 times enriched in peat ice compared to peat porewaters from the active layer. In most sampled cores, there was a local maximum of strong enrichment (up to factors between 14 and 58) in DOC, P, Ca, Mg, Mn, Fe, Sr, As located 30–50 cm below the active layer. This maximum likely occurred due to solute concentration during full freezing of the soil column during winter. There was a sizable correlation between DOC, Al, Fe and other major and trace element concentrations that suggests strong control of organic complexes and organo-mineral (Al, Fe) colloids on element migration throughout the peat profile. The pool of C, major cations and trace metals in peat ice (40–130 cm) was approximately 3–55 times higher than the pool of these elements in porewaters from the active layer (0–40 cm). A 1-m increase of the ALT over the next 100 years is capable of mobilizing 58 ± 38 Tg of DOC from soil ice into the rivers and lakes of the WSL latitudinal belt (63–67 °N). This fast lateral export of C (3.7 ± 2.7 t C km−2 y−1) may double current C yields in WSL rivers (3.4 ± 1.3 t C km−2 y−1). A strong increase (150–200%) in riverine export of Zn, P and Cs may also occur while other micronutrients (Fe, Ni, Co, Ba, Mo, Rb) and toxicants (Cd, As, Al) may be affected to a lesser degree (20–30% increase). We propose a global peat ice inventory in permafrost regions is essential for assessing the consequences of permafrost thaw on surface aquatic systems.
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•Dispersed ground ice in permafrost peatlands contains sizable amount of labile C, metal and nutrients.•We discovered a local maximum of strong enrichment in DOC, P, Ca, Mg, Mn, Fe, Sr, As 30–50 cm below the active layer.•This maximum likely occurred due to solute concentration during full freezing of the soil column in winter.•A correlation between DOC, Al, Fe and trace elements element suggests control of organic complexes and colloids.•Peat ice thaw can double the annual riverine export fluxes of DOC, P, and Zn in western Siberia.
Natural and anthropogenic mercury (Hg) emissions are sequestered in
terrestrial soils over short, annual to long, millennial timescales
before Hg mobilization and run-off impact wetland and coastal ...ocean
ecosystems. Recent studies have used Hg-to-carbon (C) ratios
(RHgC's) measured in Alaskan permafrost mineral and peat
soils together with a northern circumpolar permafrost soil carbon
inventory to estimate that these soils contain large amounts of Hg (between 184 and
755 Gg) in the upper 1 m. However, measurements
of RHgC on Siberian permafrost peatlands are largely
missing, leaving the size of the estimated northern soil Hg budget and
its fate under Arctic warming scenarios uncertain. Here we present Hg
and carbon data for six peat cores down to mineral horizons at
1.5–4 m depth, across a 1700 km latitudinal (56 to
67∘ N) permafrost gradient in the Western Siberian Lowland
(WSL). Mercury concentrations increase from south to north in all soil
horizons, reflecting a higher stability of sequestered Hg with respect
to re-emission. The RHgC in the WSL peat horizons
decreases with depth, from 0.38 Gg Pg−1 in the active layer
to 0.23 Gg Pg−1 in continuously frozen peat of the WSL. We
estimate the Hg pool (0–1 m) in the permafrost-affected part
of the WSL peatlands to be 9.3±2.7 Gg. We review and
estimate pan-Arctic organic and mineral soil RHgC to be
0.19 and 0.63 Gg Pg−1, respectively, and use a soil carbon budget to
revise the pan-Arctic permafrost soil Hg pool to be 72 Gg
(39–91 Gg; interquartile range, IQR) in the upper
30 cm, 240 Gg (110–336 Gg) in the upper
1 m, and 597 Gg (384–750 Gg) in the upper
3 m. Using the same RHgC approach, we revise the
upper 30 cm of the global soil Hg pool to contain 1086 Gg of
Hg (852–1265 Gg, IQR), of which 7 % (72 Gg)
resides in northern permafrost soils. Additional soil and river
studies in eastern and northern Siberia are needed to lower the
uncertainty on these estimates and assess the timing of Hg release to
the atmosphere and rivers.
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