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The mangrove ecosystems are known to efficiently sequester trace metals both in sediments and plant biomass. However, less is known about the chemistry of rare earth elements (REE) in the coastal environments, especially in the world's largest mangrove province, the Sundarban. Here, the concentration of REE in the sediment and plant organs of eight dominant mangrove species (mainly Avicennia sp.) in the Indian Sundarban was measured to assess REE sources, distribution, and bioaccumulation state. Results revealed that light REE (LREE) were more concentrated than the heavy REE (HREE) (128–144 mg kg−1 and 12–15 mg kg−1, respectively) in the mangrove sediments, with a relatively weak positive europium anomaly (Eu/Eu* = 1.03–1.14) with respect to North American shale composite. The primary source of REE was most likely linked to aluminosilicate weathering of crustal materials, and the resultant increase in LREE in the detritus. Vertical distribution of REE in one of the long cores from Lothian Island was altered by mangrove root activity and dependent on various physicochemical properties in the sediment (e.g., Eh, pH, organic carbon, and phosphate). REE uptake by plants was higher in the below-ground parts than in the above-ground plant tissues (root = 3.3 mg kg−1, leaf + wood = 1.7 mg kg−1); however, their total concentration was much lower than in the sediment (149.5 mg kg−1). Species-specific variability in bioaccumulation factor and translocation factor was observed indicating different REE partitioning and varying degree of mangrove uptake efficiency. Total REE stock in plant (above + live below ground) was estimated to be 168 g ha−1 with LREE contributing ~ 90% of the stock. This study highlighted the efficiency of using REE as a biological proxy in determining the degree of bioaccumulation within the mangrove environment. [ABSTRACT FROM AUTHOR]

Midlatitude anthropogenic mercury (Hg) emissions and discharge reach the Arctic Ocean (AO) by atmospheric and oceanic transport. Recent studies suggest that Arctic river Hg inputs have been a potentially overlooked source of Hg to the AO. Observations on Hg in Eurasian rivers, which represent 80% of freshwater inputs to the AO, are quasi-inexistent, however, putting firm understanding of the Arctic Hg cycle on hold. Here, we present comprehensive seasonal observations on dissolved Hg (DHg) and particulate Hg (PHg) concentrations and fluxes for two large Eurasian rivers, the Yenisei and the Severnaya Dvina. We find large DHg and PHg fluxes during the spring flood, followed by a second pulse during the fall flood. We observe well-defined water vs. Hg runoff relationships for Eurasian and North American Hg fluxes to the AO and for Canadian Hg fluxes into the larger Hudson Bay area. Extrapolation to pan-Arctic rivers and watersheds gives a total Hg river flux to the AO of 44 ° 4 Mg per year (1σ), in agreement with the recent model-based estimates of 16 to 46 Mg per year and Hg/dissolved organic carbon (DOC) observation-based estimate of 50 Mg per year. The river Hg budget, together with recent observations on tundra Hg uptake and AO Hg dynamics, provide a consistent view of the Arctic Hg cycle in which continental ecosystems traffic anthropogenic Hg emissions to the AO via rivers, and the AO exports Hg to the atmosphere, to the Atlantic Ocean, and to AO marine sediments. [ABSTRACT FROM AUTHOR]

In contrast to numerous studies on the dynamics of dissolved (<0.45 µ m) elements in permafrost-affected high-latitude rivers, very little is known of the behavior of river suspended (>0.45 µ m) matter (RSM) in these regions. In order to test the effect of climate, permafrost and physio-geographical landscape parameters (bogs, forest and lake coverage of the watershed) on RSM and particulate C, N and P concentrations in river water, we sampled 33 small and medium-sized rivers (10–100 000 km 2 watershed) along a 1700 km N–S transect including both permafrost-affected and permafrost-free zones of the Western Siberian Lowland (WSL). The concentrations of C and N in RSM decreased with the increase in river watershed size, illustrating (i) the importance of organic debris in small rivers which drain peatlands and (ii) the role of mineral matter from bank abrasion in larger rivers. The presence of lakes in the watershed increased C and N but decreased P concentrations in the RSM. The C:N ratio in the RSM reflected the source from the deep soil horizon rather than surface soil horizon, similar to that of other Arctic rivers. This suggests the export of peat and mineral particles through suprapermafrost flow occurring at the base of the active layer. There was a maximum of both particulate C and N concentrations and export fluxes at the beginning of permafrost appearance, in the sporadic and discontinuous zone (62–64 ∘ N). This presumably reflected the organic matter mobilization from newly thawed organic horizons in soils at the active latitudinal thawing front. The results suggest that a northward shift of permafrost boundaries and an increase in active layer thickness may increase particulate C and N export by WSL rivers to the Arctic Ocean by a factor of 2, while P export may remain unchanged. In contrast, within a long-term climate warming scenario, the disappearance of permafrost in the north, the drainage of lakes and transformation of bogs to forest may decrease C and N concentrations in RSM by 2 to 3 times. [ABSTRACT FROM AUTHOR]

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 watershed = 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, SO 4), 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 >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 to 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, SO 4 , 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. [Display omitted] • High resolution export fluxes of DOC, major and trace elements in the mid-sized Arctic river. • Seasonal variations in colloidal fractions of solutes in the Pechora River. • Minimal concentrations of groundwater-related elements in spring and maximal concentrations in winter. • Mobilization of low-mobile trace metals during spring flood in the form of organo-Al colloids. [ABSTRACT FROM AUTHOR]

[Display omitted] • Si release rate (R Si) from forest and agricultural soils was measured in mixed-flow reactors. • The pool of labile Si in forest soil was determined in column experiments. • Labile Si pool correlated with Si bound to amorphous Fe and Al compounds. • R Si weakly depended on pH and was similar for forest and agricultural soils. • Across the world, soils may regulate Si release at some 'universal' rate which explains narrow range of riverine Si yields. Despite the importance of silicon (Si) as benefecial nutrient for many plants, including economically-important cereals, the reactivity of various Si pools in soils (silicate minerals, amorphous compounds, phytoliths, organic litter) is not fully quantified which does not allow predicting the capacity of agricultural or forested soil to provide soluble Si to soil porewaters where it can be used by plants. Towards better understanding of factors controlling bioavailable Si in soils, here we quantified the release rate of Si from several pairs of French forest and agricultural topsoils, developed on calcareous or loess parent material. We used mixed-flow and column reactors, in acetate and carbonate buffers and distilled water, at various pH (4–8) and time of reaction (day to month). The rate of Si release from soil (R Si) exceeded that of crystalline clay minerals by 1–2 orders of magnitude, being 5–10 times lower than that of various allophanes. The rates were weakly dependent on pH (compared to clays or phytoliths) and varied from 5 × 10−7 to 2 × 10−6 mol/g soil / day at 4 < pH ≤ 8. In terms of Si reactivity, four studied soil groups followed the order: "calcaric cambisols ≈ hypereutric cambisols ≥ luvisols ≥ albeluvisol". Calcaric and hypereutric cambisols as well as luvisols exhibited a weak decrease of R Si with pH increase. The rate of Si release from albeluvisol increased 2 times with a pH increase from 4 to 8. There was no measurable difference in R Si between agricultural and forest soils. The pool of labile Si in forest soils was quantified via soil column flow-through experiments. The breakthrough curves of Si demonstrated high concentrations (1.6–5.7 mg/L) over first several hours of reaction in the soil column. The water leachable Si pool ranged from 0.04 to 0.08 mg Si g soil −1, corresponding to labile Si stock in the 0–20 cm soil of 80–160 kg ha−1. These values can meet the annual requirements of plants in forests and cultivated soils. The pool of labile Si correlated (R Pearson > 0.90; p < 0.05) with Si associated with amorphous Fe and Al compounds and soluble bioavailable Si extracted using CaCl 2 method, but negatively correlated with total Si content in soils and Si of phytoliths. The main pools of soil labile Si could be allophanes, organo-Fe-Al-Si compounds and adsorbed forms of Si onto Fe and Al hydroxides. We hypothesize that, because of low sensibility of R Si to type of soil and fluid pH, the majority of soils regulate Si release at some 'universal' rate which is further reflected in relatively narrow range of riverine Si concentrations and export fluxes across the world. Therefore, the modeling of chemical weathering and element export flux in the watersheds should incorporate the experimentally measured reactivity of the whole soil rather than individual constituting primary or secondary minerals. [ABSTRACT FROM AUTHOR]

Towards a better understanding of photosynthetic algae interaction with less common trace elements, we performed adsorption experiments of Al, Ga, Ge, Cr (VI), Mo, and W onto four diatom species typical for marine, estuarine, and freshwater aquatic systems. Using a surface complexation approach, we tentatively identified the nature of functional groups involved in the complexation of trace elements with diatom surface moieties and we quantified the stability of surface complexes formed at the diatom – solution interface. The differences in binding properties between freshwater and marine species are linked to variable proportion of three main binding groups at the diatom surfaces, namely amine, carboxylate and silanol. Under acidic conditions, the oxyanion (MoO 4 2− , WO 4 2− , CrO 4 2− ) sorption occurred electrostatically with a 1:1 ratio to the protonated amino groups. Trivalent cations (Ga 3+ and Al 3+ ) were complexed by deprotonated carboxyl groups, while the anionic hydroxylated forms of these metals (Ga(OH) 4 − and Al(OH) 4 − ) formed mononuclear monodentate complex with positively charged amino groups at higher pH. Finally, Ge(IV) sorption was controlled by interaction with neutral carboxylic, silanol and amino groups. Taken together, the surface complexation model developed in this study allows rigorous description of anions and trivalent hydrolysates adsorption onto diatom surfaces and demonstrates potential importance of diatoms for the control of trace element transfer in aquatic settings. [ABSTRACT FROM AUTHOR]

The rising concern about the potential toxicity of synthetic gold nanoparticles (AuNPs) in aquatic environments requires a rigorous estimation of physico-chemical parameters of reactions between AuNPs and major freshwater microorganisms. This study addresses the interaction of 10-nm size, positively charged AuNPs with periphytic freshwater diatoms (Eolimna minima). The adsorption experiments on viable cells were performed in 10mMNaCl and 5mMNaCl + 5mMNaHCO3 solution at a variable pH (3-10), at an AuNPs concentration from 1 βg/L to 10,000 βg/L, and an exposure time from a few minutes to 55 days. Three types of experiments, adsorption as a function of time (kinetics), pH-dependent adsorption edge, and constant-pH "Langmuirian" type isotherms, were conducted. In addition, long-term interactions (days to weeks) of live diatoms (under light and in the darkness) were performed. The adsorption was maximal at a pH from 3 to 6 and sizably decreased at a pH of 6 to 10. Results of adsorption experiments were modeled using a second order kinetic model, a Linear Programming Model, Freundlich isotherm, and a ligand binding equation for one site competition. The adsorption of AuNPs(+) most likely occurred on negatively-charged surface sites of diatom cell walls such as carboxylates or phosphorylates, similar to previously studied metal cations. Under light exposure, the AuNPs were stabilized in aqueous solution in the presence of live cells, probably due to the production of exometabolites by diatoms. The adsorbed amount of AuNPs decreased after several days of reaction, suggesting some AuNPs desorption. In the darkness, the adsorption and assimilation were stronger than under light. Overall, the behavior of positively charged AuNPs at the diatom-aqueous solution interface is similar to that of metal cations, but the affinity of aqueous AuNPs to cell exometabolites is higher, which leads to the stabilization of nanoparticles in solution in the presence of diatoms and their exudates. During photosynthetic activity and the pH rising above 9 in the vicinity of diatom cells, the adsorption of AuNPs strongly decreases, which indicates a decreasing potential toxicity of AuNPs for photosynthesizing cells. The present study demonstrates the efficiency of a thermodynamic and kinetic approach for understanding gold nanoparticles interaction with aquatic freshwater peryphytic microorganisms. [ABSTRACT FROM AUTHOR]

High-latitude boreal and arctic surface/inland waters contain sizeable reservoirs of dissolved organic matter (DOM) and trace elements (TE), which are subject to seasonal freezing. Specifically, shallow ponds and lakes in the permafrost zone often freeze solid, which can lead to transformations in the colloidal and dissolved fractions of DOM and TE. Here, we present results from experimental freeze-thaw cycles using iron (Fe)- and DOM-rich water from thaw ponds situated in Stordalen and Storflaket palsa mires in northern Sweden. After ten cycles of freezing, 85% of Fe and 25% of dissolved organic carbon (DOC) were removed from solution in circumneutral fen water (pH 6.9) but a much smaller removal of Fe and DOC (< 7%) was found in acidic bog water (pH 3.6). This removal pattern was consistent with initial supersaturation of fen water with respect to Fe hydroxide and a lack of supersaturation with any secondary mineral phase in the bog water. There was a nearly two- to threefold increase in the low-molecular-weight (LMW) fraction of organic carbon (OC) and several TEs caused by the repeated freeze-thaw cycles. Future increases in the freeze-thaw frequency of surface waters with climate warming may remove up to 25% of DOC in circumneutral organic-rich waters. Furthermore, an increase of LMW OC may result in enhanced carbon dioxide losses from aquatic ecosystems since this fraction is potentially more susceptible to biodegradation. [ABSTRACT FROM AUTHOR]

Thawing of frozen peat in discontinuous permafrost zones may significantly modify the environment at local (slumps and engineering damages) and global (greenhouse gases regime) scales. We studied the aquatic geochemistry of CO 2 , CH 4 , dissolved organic carbon (DOC), P, Si, and colloidal trace metal from hollows, depressions, permafrost subsidences and soil waters in the actively thawing discontinuous permafrost zone of Western Siberia Lowland (WSL). This site of abrupt permafrost collapse is dominated by minerotrophic fens located within the flat mound peat bog. The CO 2 , DOC, major and trace metal concentrations decreased with the increase of the surface area of the water body, along the hydrological continuum (soil water → hollows → depressions and permafrost subsidences → thaw ponds → thermokarst lakes). Aqueous concentrations of CO 2 , CH 4 , Ca, Si, P, Al, Fe, Nd, and U were a factor of 4 to 10 higher in the site of catastrophic thaw compared to the steady thawing of a palsa peat bog that was previously studied in the same region. The colloids (1 kDa–0.45 μm) formed in hot spots were strongly enriched in Fe, Al, and trivalent and tetravalent hydrolysates relative to organic carbon. Because the increase in the thickness of the thawing depth intensifies the input of inorganic components from deep mineral horizons, abrupt permafrost thaw enriches the surface waters in Al-rich colloids and low molecular weight organic complexes. As a result, the WSL's surface water colloidal composition may shift from DOM-rich and DOM-Fe-rich to DOM-Al-rich, and the release of low-soluble trivalent and tetravalent hydrolysates from the soil to the river will increase. We hypothesize that in sites of abrupt permafrost thaw, there is direct mobilization of soil waters to a hydrological network (rivers and lakes) and there is minimal transformation by autochthonous processes, which is unlike the case of steady permafrost thawing. Therefore, the change in physical factors, such as water pathways and the water residence time, within a given elementary landscape will likely control the overall impact of on-going permafrost thaw on both the surface water chemistry and dissolved greenhouse gas pattern of the territory. For this, high-resolution (< 2 m) remote sensing analysis of water dynamics in the permafrost landscape is necessary. [ABSTRACT FROM AUTHOR]

In order to better understand the chemical composition of snow and its impact on surface water hydrochemistry in the poorly studied Western Siberia Lowland (WSL), the surface layer of snow was sampled in February 2014 across a 1700 km latitudinal gradient (ca. 56.5 to 68° N). We aimed at assessing the latitudinal effect on both dissolved and particulate forms of elements in snow and quantifying the impact of atmospheric input to element storage and export fluxes in inland waters of the WSL. The concentration of dissolvedCcolloidal (< 0.45 µm) Fe, Co, Cu, As and La increased by a factor of 2 to 5 north of 63° N compared to southern regions. The pH and dissolved Ca, Mg, Sr, Mo and U in snow water increased with the rise in concentrations of particulate fraction (PF). Principal component analyses of major and trace element concentrations in both dissolved and particulate fractions revealed two factors not linked to the latitude. A hierarchical cluster analysis yielded several groups of elements that originated from alumino-silicate mineral matrix, carbonate minerals and marine aerosols or belonging to volatile atmospheric heavy metals, labile elements from weatherable minerals and nutrients. The main sources of mineral components in PF are desert and semi-desert regions of central Asia. The snow water concentrations of DIC, Cl, SO4, Mg, Ca, Cr, Co, Ni, Cu, Mo, Cd, Sb, Cs, W, Pb and U exceeded or were comparable with springtime concentrations in thermokarst lakes of the permafrost-affected WSL zone. The springtime river fluxes of DIC, Cl, SO4, Na, Mg, Ca, Rb, Cs, metals (Cr, Co, Ni, Cu, Zn, Cd, Pb), metalloids (As, Sb), Mo and U in the discontinuous to continuous permafrost zone (64-68° N) can be explained solely by melting of accumulated snow. The impact of snow deposition on riverine fluxes of elements strongly increased northward, in discontinuous and continuous permafrost zones of frozen peat bogs. This was consistent with the decrease in the impact of rock lithology on river chemical composition in the permafrost zone of the WSL, relative to the permafrost-free regions. Therefore, the present study demonstrates significant and previously underestimated atmospheric input of many major and trace elements to their riverine fluxes during spring floods. A broader impact of this result is that current estimations of river water fluxes response to climate warming in high latitudes may be unwarranted without detailed analysis of winter precipitation. [ABSTRACT FROM AUTHOR]