Methylmercury cycling in the Bohai Sea and Yellow Sea: Reasons for the low system efficiency of methylmercury production.

Coastal seas contribute the majority of human methylmercury (MeHg) exposure via marine fisheries. The terrestrial area surrounding the Bohai Sea and Yellow Sea (BS and YS) is one of the mercury (Hg) emission "hot spots" in the world, resulting in high concentrations of Hg in BS and YS seawater in comparison to other marine systems. However, comparable or even lower Hg levels were detected in seafood from the BS and YS than other coastal regions around the word, suggesting a low system bioaccumulation of Hg. Reasoning a low system efficiency of MeHg production (represented by MeHg/THg (total Hg) in seawater) may be present in these two systems, seven cruises were conducted in the BS and YS to test this hypothesis. MeHg/THg ratios in BS and YS seawater were found to be lower than that in most coastal systems, indicating that the system efficiency of MeHg production is relatively lower in the BS and YS. The low system efficiency of MeHg production reduces the risk of Hg in the BS and YS with high Hg discharge intensity. By measuring in situ production and degradation of MeHg using double stable isotope addition method, and MeHg discharge flux from various sources and its exchange at various interfaces, the budgets of MeHg in the BS and YS were estimated. The results indicate that in situ methylation and demethylation are the major source and sink of MeHg in the BS and YS. By comparing the potential controlling processes and environmental parameters for MeHg/THg in the BS and YS with the other coastal seas, estuaries and bays, lower transport efficiency of inorganic Hg from water column to the sediment, slower methylation of Hg, and rapid demethylation of MeHg were identified to be major reasons for the low system efficiency of MeHg production in the BS and YS. This study highlights the necessity of monitoring the system efficiency of MeHg production, associated processes, and controlling parameters to evaluate the efficiency of reducing Hg emissions in China as well as the other countries.

Mercury (Hg) in the "Skin" of the Ocean: Dissolved Gaseous Hg, Total Hg, and Hg Redox Chemistry in Sea Surface Microlayer and Implication for Air-Sea Hg Exchange.

The sea surface microlayer (SML) is the uppermost ∼1000 µm of the surface of the ocean. With distinct physicochemical properties and position relative to the adjacent subsurface waters (SSWs), the ubiquitous distribution and high dynamics of the SML greatly regulate the global air-sea gas exchange and biogeochemistry. Mercury (Hg) redox chemistry in surface seawaters and air-sea exchange of gaseous Hg (mainly Hg(0)) fundamentally control the global oceanic Hg cycle. However, the occurrence and transformation of Hg in the SML have been poorly quantified. Here we optimize the traditional SML sampling system to make it more suitable for dissolved gaseous Hg (DGM, mainly Hg(0)) sampling. We then assess the temporal and spatial variability of DGM, total Hg, dissolved organic carbon (DOC), and Hg redox chemistry in the SML and SSWs of diverse marine environments. Our data suggest a general DGM, total Hg, and DOC enrichment in the SML relative to the SSWs but with complex variability in time and space. The incubation experiments further reveal the complex characteristics of Hg redox chemistry between the SML and SSWs. We discuss important implications of the SML Hg cycle on air-sea Hg exchange and suggest wider investigations of the SML Hg cycle in the global hydrosphere.

Coastal streams and sewage outfalls: Hot spots of mercury discharge, pollution and cycling in nearshore environments.

The coastal streams (CSs) and sewage outfalls (SOs) are widely distributed and direct anthropogenic stress on global coastal ecosystems. However, the CS/SO-associated mercury (Hg) discharge, pollution and cycle in nearshore environment are less quantified. Here, we report that total Hg (THg) and methylmercury (MMHg) concentrations in waters of CSs (n = 8) and SOs (n = 15) of the northern China were ∼102 to 103 times of coastal surface waters and 10 to 102 times of major rivers in China and other regions. The CS/SO discharges resulted in the increase of total organic carbon (TOC) contents, THg and MMHg concentrations and TOC-normalized THg and MMHg concentrations in sediments of CS/SO-impacted coasts. The laboratory experiments further illustrated that the CS/SO-impacted sediments characterized with high potentials of dissolved THg and MMHg productions and releases. Our findings indicate that the layout optimization of SOs is able to reduce the Hg risk in coastal environment.

Gaseous mercury exchange between air and highly dynamic tidal flats: A laboratory incubation experiment.

Gaseous mercury (mainly elemental mercury, Hg(0)) exchange between air and Earth's surfaces is one of the most critical fluxes governing global Hg cycle. As an important and unique part of intertidal ecosystem, tidal flat is characterized by periodic inundation and exposure due to tidal cycle, generating varying hydrological, photochemical and biogeochemical processes. However, quantitative and mechanistic understanding of Hg(0) dynamics between air and exceptionally dynamic tide flats has remained limited to date. In this study, we select five representative tidal flat sediments from typical coastal habits of Chinese coastlines to perform laboratory incubation experiments for deciphering the effect of the interaction of tidal cycle and solar radiation on Hg(0) dynamics over tidal flats with different sediment compositions. We show that sediment Hg concentration, tidal cycle and solar radiation collectively modulate the air-surface Hg(0) exchange over tidal flats and highlight that the photochemistry dominates the Hg(0) production and emission over tidal flats. We find that the daytime inundation presents highest Hg(0) emission fluxes for Hg-poor sediment, but the daytime exposure is the hot moment of Hg(0) emission from Hg-rich sediments and substantially contributes to daily Hg(0) emission fluxes. In the treatment to mimic semidiurnal tide, the daily Hg(0) fluxes are positively correlated to sediment Hg concentrations. Combining our mechanistic insights on air-surface Hg(0) exchange over tidal flats and related data and knowledge reported by other studies, we discuss the implications of our study for field measurement and model development of Hg(0) dynamics over highly dynamic tidal flats. We conclude that the air-surface Hg(0) dynamics over tidal flats are extremely complex and highly variable, and a greater understanding the interactions between natural processes, human impacts and climate forcings will better constrain current and future Hg biogeochemical cycle in global tidal flats.

Non-conservative mixing behaviors of mercury in subterranean estuary: Coupling effect of hydrological and biogeochemical processes and implications for rapidly changing world.

Coastal ecosystem is an important zone of mercury (Hg) storage and hotspot of neurotoxic methylmercury (MMHg) production and bioaccumulation. The releases of Hg from coastal aquifer or subterranean estuary (STE) via submarine groundwater discharge (SGD) to coastal waters provide an important source of Hg from land to seas. However, the transport and biogeochemical transformation of Hg in STEs are less quantified. In this study, we documented total Hg (THg) and MMHg dynamics in two distinct STEs (a sandflat at an open coast versus a mudflat at a bay) during two different seasons (December versus August) in the temperate coast of northern China, and further applied the salinity-based conservative mixing model (CMM) to quantify the coupling effect of hydrological and biogeochemical processes on STE Hg cycle. Our field data presented large variations of THg and MMHg concentrations and%MMHg/THg of groundwater and sediment in both STEs over time and space. The CMM results clearly displayed substantial divergences of dissolved THg and MMHg from salinity in groundwater between sites and seasons, and the concentration and percent deviations in the Hg-rich mudflat were significantly higher than those in the Hg-poor sandflat. Our findings indicate the non-conservative mixing behaviors of Hg along the groundwater flow paths of both STEs, and the Hg-rich intertidal zone could be hotspot for the production and source of dissolved THg and MMHg to coastal waters via SGD. Our results provide field evidence to highlight that the hydrological shifts and biogeochemical processes collectively drive complex transport and biogeochemical transformation of Hg in STEs. The non-conservative mixing behaviors of Hg in STEs also highlight that, for more accurately calculating SGD-derived Hg fluxes to coastal seas, we need to carefully select the groundwater zonation of STE to better represent the output endmember. Our findings also address that human activities and climate change will profoundly alter the Hg biogeochemical cycle and toxicology in global coastal aquifers.

Permafrost Thaw Dominates Mercury Emission in Tibetan Thermokarst Ponds.

Increasing evidence shows that warming is driving Hg release from the cryosphere. However, Hg cycling in thawing permafrost is less understood to date. Here we show that permafrost thaw dominantly supplied no-run thermokarst ponds by permafrost melt waters (PMWs) with high concentration of photoreducible Hg (PRHg) and subsequently controlled Hg(0) emissions in the Tibetan Plateau. This study was motivated by field survey suggesting that thermokarst ponds as recipient aquatic systems of PMWs could be an active converter of PRHg to Hg(0). Annual Hg mass balance in three seasonally ice-covered thermokarst ponds suggests that PMWs were the dominant input (81.2% to 91.2%) of PRHg in all three thermokarst ponds, and PRHg input would be a constraint of Hg(0) emission owing to the fast photoreduction of PRHg to Hg(0) in the water column. Annual Hg(0) emission in the thermokarst ponds of study region was conservatively estimated to increase by 15% over the past half century. Our findings highlight that climate-induced landscape disturbances and changes in hydrogeochemical processes in climate-sensitive permafrost will quickly and in situ drive Hg stored in permafrost for a very long time into the modern day Hg cycle, which potentially offsets the anthropogenic Hg mitigation policies.

Temperature sensitivity of gaseous elemental mercury in the active layer of the Qinghai-Tibet Plateau permafrost.

Soils represent the single largest mercury (Hg) reservoir in the global environment, indicating that a tiny change of Hg behavior in soil ecosystem could greatly affect the global Hg cycle. Climate warming is strongly altering the structure and functions of permafrost and then would influence the Hg cycle in permafrost soils. However, Hg biogeochemistry in climate-sensitive permafrost is poorly investigated. Here we report a data set of soil Hg (0) concentrations in four different depths of the active layer in the Qinghai-Tibet Plateau permafrost. We find that soil Hg (0) concentrations exhibited a strongly positive and exponential relationship with temperature and showed different temperature sensitivity under the frozen and unfrozen condition. We conservatively estimate that temperature increases following latest temperature scenarios of the IPCC could result in up to a 54.9% increase in Hg (0) concentrations in surface permafrost soils by 2100. Combining the simultaneous measurement of air-soil Hg (0) exchange, we find that enhanced Hg (0) concentrations in upper soils could favor Hg (0) emissions from surface soil. Our findings indicate that Hg (0) emission could be stimulated by permafrost thawing in a warmer world.

Air-sea exchange of gaseous mercury in the tropical coast (Luhuitou fringing reef) of the South China Sea, the Hainan Island, China.

The air-sea exchange of gaseous mercury (mainly Hg(0)) in the tropical ocean is an important part of the global Hg biogeochemical cycle, but the related investigations are limited. In this study, we simultaneously measured Hg(0) concentrations in surface waters and overlaying air in the tropical coast (Luhuitou fringing reef) of the South China Sea (SCS), Hainan Island, China, for 13 days on January-February 2015. The purpose of this study was to explore the temporal variation of Hg(0) concentrations in air and surface waters, estimate the air-sea Hg(0) flux, and reveal their influencing factors in the tropical coastal environment. The mean concentrations (±SD) of Hg(0) in air and total Hg (THg) in waters were 2.34 ± 0.26 ng m(-3) and 1.40 ± 0.48 ng L(-1), respectively. Both Hg(0) concentrations in waters (53.7 ± 18.8 pg L(-1)) and Hg(0)/THg ratios (3.8 %) in this study were significantly higher than those of the open water of the SCS in winter. Hg(0) in waters usually exhibited a clear diurnal variation with increased concentrations in daytime and decreased concentrations in nighttime, especially in cloudless days with low wind speed. Linear regression analysis suggested that Hg(0) concentrations in waters were positively and significantly correlated to the photosynthetically active radiation (PAR) (R (2) = 0.42, p < 0.001). Surface waters were always supersaturated with Hg(0) compared to air (the degree of saturation, 2.46 to 13.87), indicating that the surface water was one of the atmospheric Hg(0) sources. The air-sea Hg(0) fluxes were estimated to be 1.73 ± 1.25 ng m(-2) h(-1) with a large range between 0.01 and 6.06 ng m(-2) h(-1). The high variation of Hg(0) fluxes was mainly attributed to the greatly temporal variation of wind speed.

Air-sea exchange of gaseous mercury in the East China Sea.

Two oceanographic cruises were carried out in the East China Sea (ECS) during the summer and fall of 2013. The main objectives of this study are to identify the spatial-temporal distributions of gaseous elemental mercury (GEM) in air and dissolved gaseous mercury (DGM) in surface seawater, and then to estimate the Hg(0) flux. The GEM concentration was lower in summer (1.61 ± 0.32 ng m(-3)) than in fall (2.20 ± 0.58 ng m(-3)). The back-trajectory analysis revealed that the air masses with high GEM levels during fall largely originated from the land, while the air masses with low GEM levels during summer primarily originated from ocean. The spatial distribution patterns of total Hg (THg), fluorescence, and turbidity were consistent with the pattern of DGM with high levels in the nearshore area and low levels in the open sea. Additionally, the levels of percentage of DGM to THg (%DGM) were higher in the open sea than in the nearshore area, which was consistent with the previous studies. The THg concentration in fall was higher (1.47 ± 0.51 ng l(-1)) than those of other open oceans. The DGM concentration (60.1 ± 17.6 pg l(-1)) and Hg(0) flux (4.6 ± 3.6 ng m(-2) h(-1)) in summer were higher than those in fall (DGM: 49.6 ± 12.5 pg l(-1) and Hg(0) flux: 3.6 ± 2.8 ng m(-2) h(-1)). The emission flux of Hg(0) from the ECS was estimated to be 27.6 tons yr(-1), accounting for ∼0.98% of the global Hg oceanic evasion though the ECS only accounts for ∼0.21% of global ocean area, indicating that the ECS plays an important role in the oceanic Hg cycle.

Mercury Redox Chemistry in Waters of the Eastern Asian Seas: From Polluted Coast to Clean Open Ocean.

We performed incubation experiments using seawaters from representative marine environments of the eastern Asian seas to determine the mercury (Hg) available for photoreduction (Hgr(II)), to investigate the Hg redox reaction kinetics, and to explore the effect of environmental factors and water chemistry on the Hg redox chemistry. Results show that Hgr(II) accounted for a considerable fraction of total Hg (THg) (%Hgr(II)/THg: 24.90 ± 10.55%, n = 27) and positively correlated with THg. Filtration decreased the Hgr(II) pool of waters with high suspended particulate matter (SPM). The positive linear relationships were found between pseudo-first order rate constants of gross Hg(II) photoreduction (kr) and gross Hg(0) photo-oxidation (ko) with photosynthetically active radiation (PAR). Under the condition of PAR of 1 m mol m(-2) s(-1), the kr were significantly (p < 0.05) lower than ko (kr/ko: 0.86 ± 0.22). The Hg(0) dark oxidation were significantly higher than the Hg(II) dark reduction. The Hg(II) dark reduction was positively correlated to THg, and the anaerobic condition favored the Hg(II) dark reduction. Filtration significantly influenced the Hg photoredox chemistry of waters with high SPM. UVB radiation was important for both Hg(II) photoreduction and Hg(0) photo-oxidation, and the role of other wavebands in photoinduced transformations of Hg varied with the water chemistry.

Elemental mercury (Hg(0)) in air and surface waters of the Yellow Sea during late spring and late fall 2012: concentration, spatial-temporal distribution and air/sea flux.

The Yellow Sea in East Asia receives great Hg input from regional emissions. However, Hg cycling in this marine system is poorly investigated. In late spring and late fall 2012, we determined gaseous elemental Hg (GEM or Hg(0)) in air and dissolved gaseous Hg (DGM, mainly Hg(0)) in surface waters to explore the spatial-temporal variations of Hg(0) and further to estimate the air/sea Hg(0) flux in the Yellow Sea. The results showed that the GEM concentrations in the two cruises were similar (spring: 1.86±0.40 ng m(-3); fall: 1.84±0.50 ng m(-3)) and presented similar spatial variation pattern with elevated concentrations along the coast of China and lower concentrations in the open ocean. The DGM concentrations of the two cruises were also similar with 27.0±6.8 pg L(-1) in the spring cruise and 28.2±9.0 pg L(-1) in the fall cruise and showed substantial spatial variation. The air/sea Hg(0) fluxes in the spring cruise and fall cruise were estimated to be 1.06±0.86 ng m(-2) h(-1) and 2.53±2.12 ng m(-2) h(-1), respectively. The combination of this study and our previous summer cruise showed that the summer cruise presented enhanced values of GEM, DGM and air/sea Hg(0) flux. The possible reason for this trend was that high solar radiation in summer promoted Hg(0) formation in seawater, and the high wind speed during the summer cruise significantly increased Hg(0) emission from sea surface to atmosphere and subsequently enhanced the GEM levels.

Field controlled experiments on the physiological responses of maize (Zea mays L.) leaves to low-level air and soil mercury exposures.

Thousands of tons of mercury (Hg) are released from anthropogenic and natural sources to the atmosphere in a gaseous elemental form per year, yet little is known regarding the influence of airborne Hg on the physiological activities of plant leaves. In the present study, the effects of low-level air and soil Hg exposures on the gas exchange parameters of maize (Zea mays L.) leaves and their accumulation of Hg, proline, and malondialdehyde (MDA) were examined via field open-top chamber and Hg-enriched soil experiments, respectively. Low-level air Hg exposures (<50 ng m(-3)) had little effects on the gas exchange parameters of maize leaves during most of the daytime (p > 0.05). However, both the net photosynthesis rate and carboxylation efficiency of maize leaves exposed to 50 ng m(-3) air Hg were significantly lower than those exposed to 2 ng m(-3) air Hg in late morning (p < 0.05). Additionally, the Hg, proline, and MDA concentrations in maize leaves exposed to 20 and 50 ng m(-3) air Hg were significantly higher than those exposed to 2 ng m(-3) air Hg (p < 0.05). These results indicated that the increase in airborne Hg potentially damaged functional photosynthetic apparatus in plant leaves, inducing free proline accumulation and membrane lipid peroxidation. Due to minor translocation of soil Hg to the leaves, low-level soil Hg exposures (<1,000 ng g(-1)) had no significant influences on the gas exchange parameters, or the Hg, proline, and MDA concentrations in maize leaves (p > 0.05). Compared to soil Hg, airborne Hg easily caused physiological stress to plant leaves. The effects of increasing atmospheric Hg concentration on plant physiology should be of concern.

The linear accumulation of atmospheric mercury by vegetable and grass leaves: Potential biomonitors for atmospheric mercury pollution.

One question in the use of plants as biomonitors for atmospheric mercury (Hg) is to confirm the linear relationships of Hg concentrations between air and leaves. To explore the origin of Hg in the vegetable and grass leaves, open top chambers (OTCs) experiment was conducted to study the relationships of Hg concentrations between air and leaves of lettuce (Lactuca sativa L.), radish (Raphanus sativus L.), alfalfa (Medicago sativa L.) and ryegrass (Lolium perenne L.). The influence of Hg in soil on Hg accumulation in leaves was studied simultaneously by soil Hg-enriched experiment. Hg concentrations in grass and vegetable leaves and roots were measured in both experiments. Results from OTCs experiment showed that Hg concentrations in leaves of the four species were significantly positively correlated with those in air during the growth time (p < 0.05), while results from soil Hg-enriched experiment indicated that soil-borne Hg had significant influence on Hg accumulation in the roots of each plant (p < 0.05), and some influence on vegetable leaves (p < 0.05), but no significant influence on Hg accumulation in grass leaves (p > 0.05). Thus, Hg in grass leaves is mainly originated from the atmosphere, and grass leaves are more suitable as potential biomonitors for atmospheric Hg pollution. The effect detection limits (EDLs) for the leaves of alfalfa and ryegrass were 15.1 and 22.2 ng g(-1), respectively, and the biological detection limit (BDL) for alfalfa and ryegrass was 3.4 ng m(-3).

Enhancing atmospheric mercury research in China to improve the current understanding of the global mercury cycle: the need for urgent and closely coordinated efforts.

The current understanding of the global mercury (Hg) cycle remains uncertain because Hg behavior in the environment is very complicated. The special property of Hg causes the atmosphere to be the most important medium for worldwide dispersion and transformation. The source and fate of atmospheric Hg and its interaction with the surface environment are the essential topics in the global Hg cycle. Recent declining measurement trends of Hg in the atmosphere are in apparent conflict with the increasing trends in global anthropogenic Hg emissions. As the single largest country contributor of anthropogenic Hg emission, China's role in the global Hg cycle will become more and more important in the context of the decreasing man-made Hg emission from developed regions. However, much less Hg information in China is available. As a global pollutant which undergoes long-range transport and is persistence in the environment, increasing Hg knowledge in China could not only promote the Hg regulation in this country but also improve the understanding of the fundamental of the global Hg cycle and further push the abatement of this toxin on a global scale. Then the atmospheric Hg research in China may be a breakthrough for improving the current understanding of the global Hg cycle. However, due to the complex behavior of Hg in the atmosphere, a deeper understanding of the atmospheric Hg cycle in China needs greater cooperation across fields.

Field controlled experiments of mercury accumulation in crops from air and soil.

Field open top chambers (OTCs) and soil mercury (Hg) enriched experiments were employed to study the influence of Hg concentrations in air and soil on the Hg accumulation in the organs of maize (Zea mays L.) and wheat (Triticum aestivum L.). Results showed that Hg concentrations in foliages were correlated significantly (p < 0.05) with air Hg concentrations but insignificantly correlated with soil Hg concentrations, indicating that Hg in crop foliages was mainly from air. Hg concentrations in roots were generally correlated with soil Hg concentrations (p < 0.05) but insignificantly correlated with air Hg concentrations, indicating that Hg in crop roots was mainly from soil. No significant correlations were found between Hg concentrations in stems and those in air and soil. However, Hg concentrations in upper stems were usually higher than those in bottom stems, implying air Hg might have stronger influence than soil Hg on stem Hg accumulation.

Mercury in leaf litter in typical suburban and urban broadleaf forests in China.

To study the role of leaf litter in the mercury (Hg) cycle in suburban broadleaf forests and the distribution of Hg in urban forests, we collected leaf litter and soil from suburban evergreen and deciduous broadleaf forests and from urban forests in Beijing. The Hg concentrations in leaf litter from the suburban forests varied from 8.3 to 205.0 ng/g, with an average (avg) of (49.7 +/- 36.9) ng/g. The average Hg concentration in evergreen broadleaf forest leaf litter (50.8 + 39.4) ng/g was higher than that in deciduous broadleaf forest leaf litter (25.8 +/- 10.1) ng/g. The estimated Hg fluxes of leaf litter in suburban evergreen and deciduous broadleaf forests were 179.0 and 83.7 mg/(ha x yr), respectively. The Hg concentration in organic horizons (O horizons) ((263.1 +/- 237.2) ng/g) was higher than that in eluvial horizons (A horizons) ((83.9 +/- 52.0) ng/g). These results indicated that leaf litterfall plays an important role in transporting atmospheric mercury to soil in suburban forests. For urban forests in Beijing, the Hg concentrations in leaf litter ranged from 8.8-119.0 (avg 28.1 +/- 16.6) ng/g, with higher concentrations at urban sites than at suburban sites for each tree. The Hg concentrations in surface soil in Beijing were 32.0-25300.0 ng/g and increased from suburban sites to urban sites, with the highest value from Jingshan (JS) Park at the centre of Beijing. Therefore, the distribution of Hg in Beijing urban forests appeared to be strongly influenced by anthropogenic activities.