Mao Tseng - Academia.edu (original) (raw)

Papers by Mao Tseng

Research paper thumbnail of Elevated Phytoplankton Biomass in Marginal Seas in the Low Latitude Ocean: A Case Study of the South China Sea

Advances in Geosciences - A 6-Volume Set

Research paper thumbnail of Tungsten and other heavy metal contamination in aquatic environments receiving wastewater from semiconductor manufacturing

Journal of Hazardous Materials, 2011

Through analyses of water and sediments, we investigate tungsten and 14 other heavy metals in a s... more Through analyses of water and sediments, we investigate tungsten and 14 other heavy metals in a stream receiving treated effluents from a semiconductor manufacturer-clustered science park in Taiwan. Treated effluents account for ∼ 50% of total annual river discharge and <1% of total sediment discharge. Dissolved tungsten concentrations in the effluents abnormally reach 400 μg/L, as compared to the world river average concentration of <0.1 μg/L. Particulate tungsten concentrations are up to 300 μg/g in suspended and deposited sediments, and the corresponding enrichment factors are three orders of magnitude higher than average crust composition. Surprisingly, the estimated amount of tungsten exported to the adjacent ocean is 23.5 t/yr, which can approximate the amount from the Yangtze River should it be unpolluted. This study highlights the urgency of investigating the biological effect of such contamination.

Research paper thumbnail of Field cryofocussing hydride generation applied to the simultaneous multi-elemental determination of alkyl-metal(loid) species in natural waters using ICP-MS detection

Journal of Environmental Monitoring, 2000

Research paper thumbnail of Rapid Determination of Inorganic Mercury and Methylmercury in Biological Reference Materials by Hydride Generation, Cryofocusing, Atomic Absorption Spectrometry After Open Focused Microwave-assisted Alkaline Digestion

Journal of Analytical Atomic Spectrometry, 1997

Research paper thumbnail of Rapid and Quantitative Microwave-assisted Recovery of Methylmercury From Standard Reference Sediments

Journal of Analytical Atomic Spectrometry, 1997

CHUN MAO TSENG, ALBERTO DE DIEGO†, FABIENNE M. MARTIN AND OLIVIER FX DONARD* L aboratoire de Chim... more CHUN MAO TSENG, ALBERTO DE DIEGO†, FABIENNE M. MARTIN AND OLIVIER FX DONARD* L aboratoire de Chimie Bio-Inorganique et Environnement, EP CNRS 132, Université de Pau, Hélioparc, 64000, Pau, France. E-mail: Olivier.Donard@univ-pau.fr

Research paper thumbnail of A unique seasonal pattern in dissolved elemental mercury in the South China Sea, a tropical and monsoon-dominated marginal sea

Geophysical Research Letters, 2013

A unique seasonal pattern in dissolved elemental mercury (DEM) was observed in the tropical monso... more A unique seasonal pattern in dissolved elemental mercury (DEM) was observed in the tropical monsoon-dominated South China Sea (SCS). The DEM concentration varied seasonally, with a high in summer of 160 AE 40 fM (net evasion 580 AE 120 pmol m À2 d À1 , n = 4) and a low in winter of 60 AE 30 fM (net invasion À180 AE 110, n = 4) and showed a positive correlation with sea surface temperature (SST). The elevated DEM concentration in summer appears mainly abiologically driven. In winter, the SCS acts as a sink of atmosphere Hg 0 as a result of low SST and high wind of the year, enhanced vertical mixing, and elevated atmospheric gaseous elemental mercury. Annually, the SCS serves as a source of Hg 0 to the atmosphere of 300 AE 50 pmol m À2 d À1 (385 AE 64 kmol Hg yr À1 ,~2.6% of global emission in~1% of global ocean area), suggesting high regional Hg pollution impacts from the surrounding Mainland (mostly China).

Research paper thumbnail of Speciation of Mercury in a Fluid Mud Profile of a Highly Turbid Macrotidal Estuary (Gironde, France)

Environmental Science & Technology, 2001

Mercury (Hg) speciation and partitioning have been investigated in a fluid mud profile collected ... more Mercury (Hg) speciation and partitioning have been investigated in a fluid mud profile collected in the high turbidity zone of the Gironde estuary. The formation of the fluid lens generates local and transient oxic-anoxic oscillations following the sedimentation-resuspension tidal cycles under a specific hydrodynamic regime. The total Hg concentration, ranging from 5 to 190 nM, increases with SPM concentration (4-174 g L-1) to a maximum at bottom. Particulate Hg averages 99% of total Hg. Particulate inorganic Hg (IHg(II) P) and monomethyl Hg (MMHg P) exhibit a similar trend: the maximum concentration is observed within the upper layer above the depth of 7 m and the minimum at the bottom layers of the fluid mud. Significant levels of "dissolved" (i.e. filter passing) Hg species (IHg(II) D , Hg°D, MMHg D , DMHg D) are observed within the redox transition interface. In the sub/anoxic fluid mud layer, increasing concentrations of IHg(II) D and MMHg D coincide with decreasing concentrations of IHg-(II) P and MMHg P , respectively. The distribution coefficient (log K d) between the "dissolved" and particulate fraction for IHg(II) averages 4.5 (0.2. A K d minimum for IHg(II) is observed in the surface layer and at the bottom of the fluid mud and coincides with the maximum levels of dissolved Mn and Fe. Log K d for MMHg averages 3.3 (0.9 and presents the highest values (4.3-4.6) in the surface and the lowest (∼2.2) at bottom, corresponding to the particulate carbon profile. These results demonstrate that the fate of IHg(II) and MMHg in the fluid mud system is influenced by the redox cycling of major species such as carbon, Fe, and Mn. It is therefore suggested that the redox oscillations generated by fluid mud formation in the high turbidity zone affect the distribution and transfer of Hg species in macrotidal estuaries.

Research paper thumbnail of Dissolved Elemental Mercury Investigations in Long Island Sound Using On-Line Au Amalgamation-Flow Injection Analysis

Environmental Science & Technology, 2003

A novel semiautomatic dissolved elemental mercury analyzer (DEMA) was developed for investigating... more A novel semiautomatic dissolved elemental mercury analyzer (DEMA) was developed for investigating dissolved elemental Hg (DEM) in natural waters. This on-line setup couples the main analytical steps from sample introduction, gas-liquid separation, and Au amalgamation/separation to final detection/data acquisition using flow injection techniques. This approach provides ease of operation and high analytical performance and is suitable for shipboard use. The analyzer can be fully automated and also be modified to examine other Hg species (e.g., reactive and total Hg and monomethyl-Hg). Here, we present the results of laboratory performance tests and make a comparison with a traditional manual method. DEM measured by both manual and the DEMA show good agreement. Representative field DEM data from spring and summer 1999 in Long Island Sound, U.S.A. (LIS) are presented. Spatial and temporal DEM variations were evident. Rapid and accurate determinations of DEM are necessary to observe its distribution dynamics, evaluate emissions, and assess its role in the aquatic biogeochemical cycling of Hg.

Research paper thumbnail of Modern and Historic Atmospheric Mercury Fluxes in Northern Alaska:  Global Sources and Arctic Depletion

Environmental Science & Technology, 2005

We reconstruct from lake-sediment archives atmospheric Hg deposition to Arctic Alaska over the la... more We reconstruct from lake-sediment archives atmospheric Hg deposition to Arctic Alaska over the last several centuries and constrain a contemporary lake/watershed massbalance with real-time measurement of Hg fluxes in rainfall, runoff, and evasion. Results indicate that (a) anthropogenic Hg impact in the Arctic is of similar magnitude to that at temperate latitudes; (b) whole-lake Hg sedimentation determined from 55 210 Pb-dated cores from the five small lakes demonstrates a 3-fold increase in atmospheric Hg deposition since the advent of the Industrial Revolution; (c) because of high soil Hg concentrations and relatively low atmospheric deposition fluxes, erosional inputs to these lakes are more significant than in similar temperate systems; (d) volatilization accounts for about 20% of the Hg losses (evasion and sedimentation); and (e) another source term is needed to balance the evasional and sedimentation sinks. This additional flux (1.21 (0.74 µg m-2 yr-1) is comparable to direct atmospheric Hg deposition and may be due to some combination of springtime Arctic depletion and more generalized deposition of reactive gaseous Hg species.

Research paper thumbnail of Biogeochemical Cycling of Methylmercury in Lakes and Tundra Watersheds of Arctic Alaska

Environmental Science & Technology, 2006

The fate of atmospherically deposited and environmentally active Hg is uncertain in the Arctic, a... more The fate of atmospherically deposited and environmentally active Hg is uncertain in the Arctic, and of greatest toxicological concern is the transformation to monomethylmercury (MMHg). Lake/watershed mass balances were developed to examine MMHg cycling in four northern Alaska lakes near the ecological research station at Toolik Lake (68°38′ N, 149°36′ W). Primary features of the cycle are watershed runoff, sedimentary production and mobilization, burial, and photodecomposition in the water column. The principal source of MMHg is in situ benthic production with 80-91% of total inputs provided by diffusion from sediments. The production and contribution of MMHg from tundra watersheds is modest. Photodecomposition, though confined to a short ice-free season, provides the primary control for MMHg (66-88% of total inputs) and greatly attenuates bioaccumulation. Solidphase MMHg and gross potential rates of Hg methylation, assayed with an isotopic tracer, vary positively with the level of inorganic Hg in filtered pore water, indicating that MMHg production is Hg-limited in these lakes. Moreover, sediment-water fluxes of MMHg (i.e., net production at steady state) are related to sediment Hg loadings from the atmosphere. These results suggest that loadings of Hg derived from atmospheric deposition are a major factor affecting MMHg cycling in arctic ecosystems. However, environmental changes associated with warming of the Arctic (e.g., increased weathering, temperature, productivity, and organic loadings) may enhance MMHg bioaccumulation by stimulating Hg methylation and inhibiting photodecomposition.

Research paper thumbnail of The SouthEast Asian Time-series Study (SEATS)

Deep Sea Research Part II: Topical Studies in Oceanography, 2007

Research paper thumbnail of Seasonal variability of picoplankton in the Northern South China Sea at the SEATS station

Deep Sea Research Part II: Topical Studies in Oceanography, 2007

Flow cytometric analysis of picoplankton from the Southeast Asia Time-series Station (SEATS) in t... more Flow cytometric analysis of picoplankton from the Southeast Asia Time-series Station (SEATS) in the South China Sea was performed for samples collected every 1-4 months during October 2001 to September 2002 (six cruises) and between November 2004 and December 2005 (four cruises). Prochlorococcus was the most abundant autotrophic picoplankton with the maximum abundance occurring in summer. Synechococcus and picoeukaryotes were 1 or 2 orders of magnitude less abundant during most of the year, but showed a much larger seasonal variation, with maximum abundance occurring in winter to early spring. The winter peak of Synechococcus and picoeukaryotes coincided with the deepening of the mixedlayer depth caused by surface cooling and the strong NE monsoon winds. Maximum bacterial biomass was observed in spring, and generally followed the winter peak of Synechococcus and picoeukaryotes with a time lag. Prochlorococcus contributed up to 80% of the total autotrophic biomass during summer, but Synechococcus and picoeukaryotes accounted for 60-80% of the total autotrophic biomass in winter. Bacterial biomass was less than the autotrophic biomass and the two were only weakly correlated. The interannual variability in the structure and composition of the picoplankton community may be related to the El Nin˜o-Southern Oscillation (ENSO) cycle. During an El Nin˜o year (2001-2002), with higher sea-surface temperature, chlorophyll a was lower and Prochlorococcus and bacterial biomass were higher. In contrast, the higher picoeukaryote biomass during winter was probably responsible for the higher chlorophyll a during the

Research paper thumbnail of Temporal variations in the carbonate system in the upper layer at the SEATS station

Deep Sea Research Part II: Topical Studies in Oceanography, 2007

The distributions of total carbon dioxide (TCO 2) and alkalinity (TA) in the upper layer at the S... more The distributions of total carbon dioxide (TCO 2) and alkalinity (TA) in the upper layer at the SouthEast Asian Timeseries Study (SEATS) station at 181N, and 1161E in the northern South China Sea (SCS) were determined on 19 cruises between September 1999 and October 2003. The variations in the concentrations of TCO 2 and TA in the mixed layer, which ranged between 1860 and 1920 mmol kg À1 , and 2170 and 2230 mmol kg À1 , respectively, followed a distinct intraannual pattern like that of salinity. The maximum concentrations were found in the winter as enhanced vertical mixing brought the subsurface Tropical Water, which was more saline and elevated in TA and TCO 2 , to the mixed layer. There was an even more well-defined and consistent intra-annual pattern in the variations in the associated fugacity of CO 2 , fCO 2 , that fluctuated between 340 and 400 matm. However, the variations followed the temporal pattern in temperature more closely than that in salinity as fCO 2 rose systematically towards a maximum in the summer and then fell progressively to a minimum in the winter. The intra-annual variations in TA could be accounted for largely by the variations in salinity. Once TA was normalized to the average salinity of 33.5 in the mixed layer, the variations in the resulting NTA were only slightly larger than the analytical uncertainty and they did not follow a consistent intra-annual pattern. On the other hand, consistent intra-annual variations remained evident in NTCO 2 , TCO 2 normalized to a salinity of 33.5, and NfCO 2 , fCO 2 normalized to the average temperature of 27.6 1C in the mixed layer. In fact, the patterns in the intra-annual variations in NTCO 2 and NfCO 2 mimicked each other closely. From the late winter through the summer (February-August), the uptake of carbon in primary production and the evasion of CO 2 to the atmosphere led to a drawdown in NTCO 2 and a decrease in NfCO 2. From the late summer to the early winter (August-December), variations in NTCO 2 and NfCO 2 were small. The variations in TCO 2 and fCO 2 could be explained largely by changes in salinity and temperature, respectively. In the winter (December-February), both NTCO 2 and NfCO 2 were at a maximum, indicating that the effects of the net invasion of atmospheric CO 2 to the SCS and the enhanced vertical mixing of the surface waters with the subsurface Tropical Water dominated over the effect from the higher primary production during this season. Atmospheric fCO 2 was less than fCO 2 in the mixed layer from April through October and exceeded the latter from November through March. For the year as a whole, there was a net invasion of CO 2 of 0.02 mol C m À2 yr À1 , a value that was indistinguishable from zero,

Research paper thumbnail of Biogeochemical Cycling and Contamination of Mercury in Arctic Alaska: Modern and Historic Atmospheric Fluxes

Our research in the U.S. Arctic is focused on (a) atmospheric Hg deposition and contamination of ... more Our research in the U.S. Arctic is focused on (a) atmospheric Hg deposition and contamination of lakes and watersheds, (b) in-lake cycling of Hg, especially methylHg, and (c) the behavior and fate of elemental Hg, whose production and mobilization via water-air exchange are quite significant. These studies are being conducted in the lacustrine and tundra wetland environs near the Arctic LTER Site at the Toolik Field Station (68\deg 38' N, 149\deg 38' W). Atmospheric Hg deposition over the last several centuries has been established with dated sediment archives from five carefully selected (i.e., headwater; small watershed) remote arctic lakes. Mercury budgets for the study lakes have been constructed from atmospheric, water column, watershed, and sedimentary investigations. Results indicate that (1) impact from anthropogenic Hg in the Arctic is of similar magnitude to that at temperate latitudes; (2) whole-lake Hg sedimentation determined using 55 210Pb-dated cores from the five small lakes demonstrates a 3-fold increase in atmospheric Hg deposition since the advent of the Industrial Revolution, (3) the linear correlation between Hg and 210Pb found in rainwater from other locations is observed in the Arctic, and this behavior can be used to constrain the wet atmospheric flux of Hg to lakes and watersheds, (4) volatilization accounts for about 20% of the Hg losses (evasion and sedimentation) from lakes, and (5) another source term is needed to balance the evasional and sedimentation sinks. This additional flux, though small, is comparable to direct atmospheric Hg deposition. It may be due to some combination of Springtime Hg Depletion Events and more generalized deposition of reactive gaseous Hg species in this tundra region, which is about 150 km from the Arctic Ocean.

Research paper thumbnail of Anomalous biogeochemical conditions in the northern South China Sea during the El-Niño events between 1997 and 2003

Geophysical Research Letters, 2009

Anomalous biogeochemical conditions were observed at the SouthEast Asian Time-series Study (SEATS... more Anomalous biogeochemical conditions were observed at the SouthEast Asian Time-series Study (SEATS) station in the northern South China Sea (SCS) during the 1997-98 and 2002-03 El Niño events. The time-series records showed decreases of monthly mean sea surface chlorophylla (S-chl) (and integrated primary production, IPP) by 42% (and 42%) and 13% (and 10%), respectively, below the climatological mean in the winter months (DJF) of the two events. The negative anomalies in S-chl and IPP corresponded to elevated sea surface temperature by 1.2°C and 0.4°C, respectively, above the climatological mean, while the mean wind speed was reduced by about 20% and 11%, respectively. Statistical analysis demonstrated the reduction in S-chl and IPP during El Niño events was caused by the diminished vertical mixing and strengthened stratification. Regional anomalies in hydrographic and biological conditions in the northern SCS (15-21°N and 112-119°E) were consistent with those found at the SEATS site.

Research paper thumbnail of Elevated Phytoplankton Biomass in Marginal Seas in the Low Latitude Ocean: A Case Study of the South China Sea

Advances in Geosciences - A 6-Volume Set

Research paper thumbnail of Tungsten and other heavy metal contamination in aquatic environments receiving wastewater from semiconductor manufacturing

Journal of Hazardous Materials, 2011

Through analyses of water and sediments, we investigate tungsten and 14 other heavy metals in a s... more Through analyses of water and sediments, we investigate tungsten and 14 other heavy metals in a stream receiving treated effluents from a semiconductor manufacturer-clustered science park in Taiwan. Treated effluents account for ∼ 50% of total annual river discharge and <1% of total sediment discharge. Dissolved tungsten concentrations in the effluents abnormally reach 400 μg/L, as compared to the world river average concentration of <0.1 μg/L. Particulate tungsten concentrations are up to 300 μg/g in suspended and deposited sediments, and the corresponding enrichment factors are three orders of magnitude higher than average crust composition. Surprisingly, the estimated amount of tungsten exported to the adjacent ocean is 23.5 t/yr, which can approximate the amount from the Yangtze River should it be unpolluted. This study highlights the urgency of investigating the biological effect of such contamination.

Research paper thumbnail of Field cryofocussing hydride generation applied to the simultaneous multi-elemental determination of alkyl-metal(loid) species in natural waters using ICP-MS detection

Journal of Environmental Monitoring, 2000

Research paper thumbnail of Rapid Determination of Inorganic Mercury and Methylmercury in Biological Reference Materials by Hydride Generation, Cryofocusing, Atomic Absorption Spectrometry After Open Focused Microwave-assisted Alkaline Digestion

Journal of Analytical Atomic Spectrometry, 1997

Research paper thumbnail of Rapid and Quantitative Microwave-assisted Recovery of Methylmercury From Standard Reference Sediments

Journal of Analytical Atomic Spectrometry, 1997

CHUN MAO TSENG, ALBERTO DE DIEGO†, FABIENNE M. MARTIN AND OLIVIER FX DONARD* L aboratoire de Chim... more CHUN MAO TSENG, ALBERTO DE DIEGO†, FABIENNE M. MARTIN AND OLIVIER FX DONARD* L aboratoire de Chimie Bio-Inorganique et Environnement, EP CNRS 132, Université de Pau, Hélioparc, 64000, Pau, France. E-mail: Olivier.Donard@univ-pau.fr

Research paper thumbnail of A unique seasonal pattern in dissolved elemental mercury in the South China Sea, a tropical and monsoon-dominated marginal sea

Geophysical Research Letters, 2013

A unique seasonal pattern in dissolved elemental mercury (DEM) was observed in the tropical monso... more A unique seasonal pattern in dissolved elemental mercury (DEM) was observed in the tropical monsoon-dominated South China Sea (SCS). The DEM concentration varied seasonally, with a high in summer of 160 AE 40 fM (net evasion 580 AE 120 pmol m À2 d À1 , n = 4) and a low in winter of 60 AE 30 fM (net invasion À180 AE 110, n = 4) and showed a positive correlation with sea surface temperature (SST). The elevated DEM concentration in summer appears mainly abiologically driven. In winter, the SCS acts as a sink of atmosphere Hg 0 as a result of low SST and high wind of the year, enhanced vertical mixing, and elevated atmospheric gaseous elemental mercury. Annually, the SCS serves as a source of Hg 0 to the atmosphere of 300 AE 50 pmol m À2 d À1 (385 AE 64 kmol Hg yr À1 ,~2.6% of global emission in~1% of global ocean area), suggesting high regional Hg pollution impacts from the surrounding Mainland (mostly China).

Research paper thumbnail of Speciation of Mercury in a Fluid Mud Profile of a Highly Turbid Macrotidal Estuary (Gironde, France)

Environmental Science & Technology, 2001

Mercury (Hg) speciation and partitioning have been investigated in a fluid mud profile collected ... more Mercury (Hg) speciation and partitioning have been investigated in a fluid mud profile collected in the high turbidity zone of the Gironde estuary. The formation of the fluid lens generates local and transient oxic-anoxic oscillations following the sedimentation-resuspension tidal cycles under a specific hydrodynamic regime. The total Hg concentration, ranging from 5 to 190 nM, increases with SPM concentration (4-174 g L-1) to a maximum at bottom. Particulate Hg averages 99% of total Hg. Particulate inorganic Hg (IHg(II) P) and monomethyl Hg (MMHg P) exhibit a similar trend: the maximum concentration is observed within the upper layer above the depth of 7 m and the minimum at the bottom layers of the fluid mud. Significant levels of "dissolved" (i.e. filter passing) Hg species (IHg(II) D , Hg°D, MMHg D , DMHg D) are observed within the redox transition interface. In the sub/anoxic fluid mud layer, increasing concentrations of IHg(II) D and MMHg D coincide with decreasing concentrations of IHg-(II) P and MMHg P , respectively. The distribution coefficient (log K d) between the "dissolved" and particulate fraction for IHg(II) averages 4.5 (0.2. A K d minimum for IHg(II) is observed in the surface layer and at the bottom of the fluid mud and coincides with the maximum levels of dissolved Mn and Fe. Log K d for MMHg averages 3.3 (0.9 and presents the highest values (4.3-4.6) in the surface and the lowest (∼2.2) at bottom, corresponding to the particulate carbon profile. These results demonstrate that the fate of IHg(II) and MMHg in the fluid mud system is influenced by the redox cycling of major species such as carbon, Fe, and Mn. It is therefore suggested that the redox oscillations generated by fluid mud formation in the high turbidity zone affect the distribution and transfer of Hg species in macrotidal estuaries.

Research paper thumbnail of Dissolved Elemental Mercury Investigations in Long Island Sound Using On-Line Au Amalgamation-Flow Injection Analysis

Environmental Science & Technology, 2003

A novel semiautomatic dissolved elemental mercury analyzer (DEMA) was developed for investigating... more A novel semiautomatic dissolved elemental mercury analyzer (DEMA) was developed for investigating dissolved elemental Hg (DEM) in natural waters. This on-line setup couples the main analytical steps from sample introduction, gas-liquid separation, and Au amalgamation/separation to final detection/data acquisition using flow injection techniques. This approach provides ease of operation and high analytical performance and is suitable for shipboard use. The analyzer can be fully automated and also be modified to examine other Hg species (e.g., reactive and total Hg and monomethyl-Hg). Here, we present the results of laboratory performance tests and make a comparison with a traditional manual method. DEM measured by both manual and the DEMA show good agreement. Representative field DEM data from spring and summer 1999 in Long Island Sound, U.S.A. (LIS) are presented. Spatial and temporal DEM variations were evident. Rapid and accurate determinations of DEM are necessary to observe its distribution dynamics, evaluate emissions, and assess its role in the aquatic biogeochemical cycling of Hg.

Research paper thumbnail of Modern and Historic Atmospheric Mercury Fluxes in Northern Alaska:  Global Sources and Arctic Depletion

Environmental Science & Technology, 2005

We reconstruct from lake-sediment archives atmospheric Hg deposition to Arctic Alaska over the la... more We reconstruct from lake-sediment archives atmospheric Hg deposition to Arctic Alaska over the last several centuries and constrain a contemporary lake/watershed massbalance with real-time measurement of Hg fluxes in rainfall, runoff, and evasion. Results indicate that (a) anthropogenic Hg impact in the Arctic is of similar magnitude to that at temperate latitudes; (b) whole-lake Hg sedimentation determined from 55 210 Pb-dated cores from the five small lakes demonstrates a 3-fold increase in atmospheric Hg deposition since the advent of the Industrial Revolution; (c) because of high soil Hg concentrations and relatively low atmospheric deposition fluxes, erosional inputs to these lakes are more significant than in similar temperate systems; (d) volatilization accounts for about 20% of the Hg losses (evasion and sedimentation); and (e) another source term is needed to balance the evasional and sedimentation sinks. This additional flux (1.21 (0.74 µg m-2 yr-1) is comparable to direct atmospheric Hg deposition and may be due to some combination of springtime Arctic depletion and more generalized deposition of reactive gaseous Hg species.

Research paper thumbnail of Biogeochemical Cycling of Methylmercury in Lakes and Tundra Watersheds of Arctic Alaska

Environmental Science & Technology, 2006

The fate of atmospherically deposited and environmentally active Hg is uncertain in the Arctic, a... more The fate of atmospherically deposited and environmentally active Hg is uncertain in the Arctic, and of greatest toxicological concern is the transformation to monomethylmercury (MMHg). Lake/watershed mass balances were developed to examine MMHg cycling in four northern Alaska lakes near the ecological research station at Toolik Lake (68°38′ N, 149°36′ W). Primary features of the cycle are watershed runoff, sedimentary production and mobilization, burial, and photodecomposition in the water column. The principal source of MMHg is in situ benthic production with 80-91% of total inputs provided by diffusion from sediments. The production and contribution of MMHg from tundra watersheds is modest. Photodecomposition, though confined to a short ice-free season, provides the primary control for MMHg (66-88% of total inputs) and greatly attenuates bioaccumulation. Solidphase MMHg and gross potential rates of Hg methylation, assayed with an isotopic tracer, vary positively with the level of inorganic Hg in filtered pore water, indicating that MMHg production is Hg-limited in these lakes. Moreover, sediment-water fluxes of MMHg (i.e., net production at steady state) are related to sediment Hg loadings from the atmosphere. These results suggest that loadings of Hg derived from atmospheric deposition are a major factor affecting MMHg cycling in arctic ecosystems. However, environmental changes associated with warming of the Arctic (e.g., increased weathering, temperature, productivity, and organic loadings) may enhance MMHg bioaccumulation by stimulating Hg methylation and inhibiting photodecomposition.

Research paper thumbnail of The SouthEast Asian Time-series Study (SEATS)

Deep Sea Research Part II: Topical Studies in Oceanography, 2007

Research paper thumbnail of Seasonal variability of picoplankton in the Northern South China Sea at the SEATS station

Deep Sea Research Part II: Topical Studies in Oceanography, 2007

Flow cytometric analysis of picoplankton from the Southeast Asia Time-series Station (SEATS) in t... more Flow cytometric analysis of picoplankton from the Southeast Asia Time-series Station (SEATS) in the South China Sea was performed for samples collected every 1-4 months during October 2001 to September 2002 (six cruises) and between November 2004 and December 2005 (four cruises). Prochlorococcus was the most abundant autotrophic picoplankton with the maximum abundance occurring in summer. Synechococcus and picoeukaryotes were 1 or 2 orders of magnitude less abundant during most of the year, but showed a much larger seasonal variation, with maximum abundance occurring in winter to early spring. The winter peak of Synechococcus and picoeukaryotes coincided with the deepening of the mixedlayer depth caused by surface cooling and the strong NE monsoon winds. Maximum bacterial biomass was observed in spring, and generally followed the winter peak of Synechococcus and picoeukaryotes with a time lag. Prochlorococcus contributed up to 80% of the total autotrophic biomass during summer, but Synechococcus and picoeukaryotes accounted for 60-80% of the total autotrophic biomass in winter. Bacterial biomass was less than the autotrophic biomass and the two were only weakly correlated. The interannual variability in the structure and composition of the picoplankton community may be related to the El Nin˜o-Southern Oscillation (ENSO) cycle. During an El Nin˜o year (2001-2002), with higher sea-surface temperature, chlorophyll a was lower and Prochlorococcus and bacterial biomass were higher. In contrast, the higher picoeukaryote biomass during winter was probably responsible for the higher chlorophyll a during the

Research paper thumbnail of Temporal variations in the carbonate system in the upper layer at the SEATS station

Deep Sea Research Part II: Topical Studies in Oceanography, 2007

The distributions of total carbon dioxide (TCO 2) and alkalinity (TA) in the upper layer at the S... more The distributions of total carbon dioxide (TCO 2) and alkalinity (TA) in the upper layer at the SouthEast Asian Timeseries Study (SEATS) station at 181N, and 1161E in the northern South China Sea (SCS) were determined on 19 cruises between September 1999 and October 2003. The variations in the concentrations of TCO 2 and TA in the mixed layer, which ranged between 1860 and 1920 mmol kg À1 , and 2170 and 2230 mmol kg À1 , respectively, followed a distinct intraannual pattern like that of salinity. The maximum concentrations were found in the winter as enhanced vertical mixing brought the subsurface Tropical Water, which was more saline and elevated in TA and TCO 2 , to the mixed layer. There was an even more well-defined and consistent intra-annual pattern in the variations in the associated fugacity of CO 2 , fCO 2 , that fluctuated between 340 and 400 matm. However, the variations followed the temporal pattern in temperature more closely than that in salinity as fCO 2 rose systematically towards a maximum in the summer and then fell progressively to a minimum in the winter. The intra-annual variations in TA could be accounted for largely by the variations in salinity. Once TA was normalized to the average salinity of 33.5 in the mixed layer, the variations in the resulting NTA were only slightly larger than the analytical uncertainty and they did not follow a consistent intra-annual pattern. On the other hand, consistent intra-annual variations remained evident in NTCO 2 , TCO 2 normalized to a salinity of 33.5, and NfCO 2 , fCO 2 normalized to the average temperature of 27.6 1C in the mixed layer. In fact, the patterns in the intra-annual variations in NTCO 2 and NfCO 2 mimicked each other closely. From the late winter through the summer (February-August), the uptake of carbon in primary production and the evasion of CO 2 to the atmosphere led to a drawdown in NTCO 2 and a decrease in NfCO 2. From the late summer to the early winter (August-December), variations in NTCO 2 and NfCO 2 were small. The variations in TCO 2 and fCO 2 could be explained largely by changes in salinity and temperature, respectively. In the winter (December-February), both NTCO 2 and NfCO 2 were at a maximum, indicating that the effects of the net invasion of atmospheric CO 2 to the SCS and the enhanced vertical mixing of the surface waters with the subsurface Tropical Water dominated over the effect from the higher primary production during this season. Atmospheric fCO 2 was less than fCO 2 in the mixed layer from April through October and exceeded the latter from November through March. For the year as a whole, there was a net invasion of CO 2 of 0.02 mol C m À2 yr À1 , a value that was indistinguishable from zero,

Research paper thumbnail of Biogeochemical Cycling and Contamination of Mercury in Arctic Alaska: Modern and Historic Atmospheric Fluxes

Our research in the U.S. Arctic is focused on (a) atmospheric Hg deposition and contamination of ... more Our research in the U.S. Arctic is focused on (a) atmospheric Hg deposition and contamination of lakes and watersheds, (b) in-lake cycling of Hg, especially methylHg, and (c) the behavior and fate of elemental Hg, whose production and mobilization via water-air exchange are quite significant. These studies are being conducted in the lacustrine and tundra wetland environs near the Arctic LTER Site at the Toolik Field Station (68\deg 38' N, 149\deg 38' W). Atmospheric Hg deposition over the last several centuries has been established with dated sediment archives from five carefully selected (i.e., headwater; small watershed) remote arctic lakes. Mercury budgets for the study lakes have been constructed from atmospheric, water column, watershed, and sedimentary investigations. Results indicate that (1) impact from anthropogenic Hg in the Arctic is of similar magnitude to that at temperate latitudes; (2) whole-lake Hg sedimentation determined using 55 210Pb-dated cores from the five small lakes demonstrates a 3-fold increase in atmospheric Hg deposition since the advent of the Industrial Revolution, (3) the linear correlation between Hg and 210Pb found in rainwater from other locations is observed in the Arctic, and this behavior can be used to constrain the wet atmospheric flux of Hg to lakes and watersheds, (4) volatilization accounts for about 20% of the Hg losses (evasion and sedimentation) from lakes, and (5) another source term is needed to balance the evasional and sedimentation sinks. This additional flux, though small, is comparable to direct atmospheric Hg deposition. It may be due to some combination of Springtime Hg Depletion Events and more generalized deposition of reactive gaseous Hg species in this tundra region, which is about 150 km from the Arctic Ocean.

Research paper thumbnail of Anomalous biogeochemical conditions in the northern South China Sea during the El-Niño events between 1997 and 2003

Geophysical Research Letters, 2009

Anomalous biogeochemical conditions were observed at the SouthEast Asian Time-series Study (SEATS... more Anomalous biogeochemical conditions were observed at the SouthEast Asian Time-series Study (SEATS) station in the northern South China Sea (SCS) during the 1997-98 and 2002-03 El Niño events. The time-series records showed decreases of monthly mean sea surface chlorophylla (S-chl) (and integrated primary production, IPP) by 42% (and 42%) and 13% (and 10%), respectively, below the climatological mean in the winter months (DJF) of the two events. The negative anomalies in S-chl and IPP corresponded to elevated sea surface temperature by 1.2°C and 0.4°C, respectively, above the climatological mean, while the mean wind speed was reduced by about 20% and 11%, respectively. Statistical analysis demonstrated the reduction in S-chl and IPP during El Niño events was caused by the diminished vertical mixing and strengthened stratification. Regional anomalies in hydrographic and biological conditions in the northern SCS (15-21°N and 112-119°E) were consistent with those found at the SEATS site.