Strengthening the research of riverine mercury (Hg) export is of great significance for understanding the regional and global Hg cycle, especially for the data lacking trans-Himalayan rivers. In this study, three systematic sampling campaigns were conducted in the Koshi River Basin (KRB) during the post-monsoon, pre-monsoon and monsoon seasons. Hg speciation and distribution of river water were analyzed among the different seasons for a total of 88 water samples. The total Hg (THg) concentration of surface water in the KRB ranged from 0.64 to 32.96 ng.L-1 with an average of 5.83 +/- 6.19 ng.L-1 and decreased in the order of post-monsoon (8.79 +/- 7.32 ng.L-1) > monsoon (6.68 +/- 6.12 ng.L-1) > pre-monsoon (2.18 +/- 1.29 ng.L-1). Particulate Hg (PHg) accounted for 63% of THg on average and had a positive correlation with THg among all the three sampling seasons, indicating that the differences in PHg concentration were likely one of the main factors leading to the seasonal and spatial variations in THg in the KRB surface water. The annual Hg exports and fluxes were estimated to be 339.04 kg and 3.88 mu g.m(-2).yr(-1), respectively. Furthermore, Hg export from the KRB had significant seasonal variation and decreased in the order of monsoon (259.47 kg) > post-monsoon (61.18 kg) > winter (9.31 kg) > pre-monsoon (9.08 kg), and this pattern was mainly related to seasonal changes in river runoff. The annual Hg export is projected to increase in the future, especially in the post-monsoon season. Therefore, more attention should be paid to river runoff observations and riverine Hg research for water resources management in the Himalaya. (C) 2020 Elsevier B.V. All rights reserved.

Mercury (Hg), a global contaminant, can sink into cryosphere and be released into runoff through meltwater. The Tibetan Plateau (TP) has been witnessing ongoing shrinkage of alpine glaciers. However, the export of Hg from melting glacier is still sparsely reported. From October 16, 2020 to October 15, 2021, we conducted daily observations to study the variation in total Hg concentrations and its export to the Mingyong River, a glacier-fed river in southeastern TP. Results showed that the Hg concentrations were high during the monsoon season but low during the non-monsoon period. The Hg in runoff correlated with the concentrations of total suspended particulates (TSP) and dissolved inorganic carbon (DIC) during both monsoon and non-monsoon seasons (p < 0.01), and the correlation of Hg with other parameters showed seasonal variations. The input from meltwater, precipitation, and groundwater to riverine Hg were 8.3 g, 264.4 g, and 71.0 g, respectively, and the total export was 211.0 g (yield: 4.3 g/km(2)/year) in the hydrological year, indicating that Mingyong catchment act as a sink for Hg. For the entire TP, the annual export of Hg from glacier runoff was estimated to be 947.7 kg/year. Our study highlights the necessity for further investigations on Hg dynamics to understand the changes in the Hg cycle within glaciated aquatic ecosystems.

Manifestations of climate change in the Arctic are numerous and include hydrological cycle intensification and permafrost thaw, both expected as a result of atmospheric and surface warming. Across the terrestrial Arctic dissolved organic carbon (DOC) entrained in arctic rivers may be providing a carbon subsidy to coastal food webs. Yet, data from field sampling is too often of limited duration to confidently ascertain impacts of climate change on freshwater and DOC flows to coastal waters. This study applies numerical modeling to investigate trends in freshwater and DOC exports from land to Elson Lagoon in Northwest Alaska over the period 1981-2020. While the modeling approach has limitations, the results point to significant increases in freshwater and DOC exports to the lagoon over the past four decades. The model simulation reveals significant increases in surface, subsurface (suprapermafrost), and total freshwater exports. Significant increases are also noted in surface and subsurface DOC production and export, influenced by warming soils and associated active-layer thickening. The largest changes in subsurface components are noted in September, which has experienced a similar to 50% increase in DOC export emanating from suprapermafrost flow. Direct coastal suprapermafrost freshwater and DOC exports in late summer more than doubled between the first and last five years of the simulation period, with a large anomaly in September 2019 representing a more than fourfold increase over September direct coastal export during the early 1980s. These trends highlight the need for dedicated measurement programs that will enable improved understanding of climate change impacts on coastal zone processes in this data sparse region of Northwest Alaska.

Glaciers in the Himalayan region have been receding rapidly in recent decades, drawing increasing concerns about the release of legacy pollutants (e.g., mercury (Hg)). To investigate the distribution, transport and controlling factors of Hg in glacier-fed runoff, from June 2019 to July 2020, a continuous monitoring and an intensive sampling campaign were conducted in the Rongbuk Glacier-fed basin (RGB) on the north slope of Mt. Everest in the middle Himalayas. The total Hg (THg) and methyl Hg (MeHg) concentrations were 1.56 +/- 0.85 and 0.057 +/- 0.025 ng/L, respectively, which were comparable to the global background levels and were mainly affected by the total suspended particulate matter (TSP). In addition, THg and MeHg showed significant diurnal variations, with peak values appearing at approximately 17:00 (upstream) and 19:00 (downstream). Based on the annual runoff and average Hg concentration, the annual export fluxes of THg and MeHg were estimated to be 441 g and 16 g, respectively. The yields of THg and MeHg in the RGB were 1.6 and 0.06 mu g/m(2)/year, respectively. Currently, the annual Hg export of meltwater runoff in the Himalayan region is approximately 337 kg/year. When flowing through the proglacial lake, the THg concentrations decreased by 32% and 15% in the proglacial lake water and in the outlet, respectively, indicating that proglacial lakes had a sedimentation effect on the Hg transport. The Hg export from meltwater runoff in the Himalayas will likely increase considering the meltwater runoff has been projected to increase in the future. Nonetheless, emerging proglacial lakes may exert ambiguous effects on the glacier exported Hg under changing climate. Proglacial lakes could lower the levels and amounts of Hg in the glacier runoff, whereas the outburst of proglacial lakes could lead to an instantaneous release of Hg stored in lake waters and sediments. Our analysis shed light on the environmental impact of glacier retreat in the Himalayas and highlighted the need for integrated monitoring and study of Hg in glacier runoff and glacial lakes.

Thawing permafrost supplies dissolved organic carbon (DOC) to aquatic systems; however, the magnitude, variability and fate of this DOC is not well constrained. Our objective was to examine DOC respiration from seasonally thawed and near-surface (<1.5 m) permafrost soils collected from five locations in the Kolyma River Basin, north-east Russia. We measured soil organic carbon (OC) content, water-soluble macronutrients (DOC, NH4, PO4) and the heterotrophic respiration potentials of soil extract DOC in five-day laboratory incubations. DOC concentrations ranged from 2.8 to 27.9 mg L-1 (n = 14). Carbon respiration was 0.03-0.47 mg C (n = 16) and 8.7-31.4%, total DOC (n = 14). While DOC concentration was a function of soil OC concentration, we did not find a relationship between C respiration and soil OC or DOC concentrations. Respiration was highest in the top active layer, but varied widely among sites, and lowest at the bottom of the active layer. Respiration from yedoma varied across sites (0.04-0.47 mg C respired, 8.7-31.4% total DOC). Despite the small sample size, our study indicates near-surface soils and permafrost are spatially variable in terms of both soil OC content and C respiration rates, and also that OC contents do not predict C respiration rates. While a larger sample size would be useful to confirm these results at broader geographic scales, these initial results suggest that soil OC heterogeneity should be considered in efforts to determine the fate of soil OC released from permafrost-dominated terrestrial ecosystems to aquatic ecosystems following permafrost thaw.