Grantee Research Project Results
2000 Progress Report: Microbiological and Physicochemical Aspects of Mercury Cycling in the Coastal/Estuarine Waters of Long Island Sound and Its River-Seawater Mixing Zones
EPA Grant Number: R827635Title: Microbiological and Physicochemical Aspects of Mercury Cycling in the Coastal/Estuarine Waters of Long Island Sound and Its River-Seawater Mixing Zones
Investigators: Fitzgerald, William F. , Visscher, Pieter T.
Institution: University of Connecticut
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 1999 through September 30, 2002
Project Period Covered by this Report: October 1, 1999 through September 30, 2000
Project Amount: $592,035
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text | Recipients Lists
Research Category: Watersheds , Heavy Metal Contamination of Soil/Water , Water , Safer Chemicals
Objective:
We have undertaken a comprehensive field and laboratory study to investigate physicochemical/microbiological reactions and processes controlling Hg cycling, speciation and bioavailability in the waters and sediments of Long Island Sound (LIS) and its watershed/coastal water interface (Connecticut River, East River). Our specific objectives are focused on several major features of the aquatic biogeochemistry of Hg, particularly elemental mercury (Hg0), which plays a governing role over the bioavailable Hg species (monomethylmercury or MMHg), and interactions between terrestrial watersheds and nearshore marine waters. We are testing the following hypotheses: (1) The Hg0 distribution in LIS is spatially/temporally variable, related to the distribution of labile Hg (labile inorganic and organically associated Hg species), and the in situ supply of reducing agents (bacterial activity and solar radiation). (2) Estuarine reactions (i.e., mixing of river borne Hg species with seawater high in Cl- and major cations) and direct WTF discharges (sewage) increase the labile Hg fraction available for reduction, enhancing localized production of Hg0. (3) Hg0 is the predominant Hg cycling product of bacterial activity in the oxic zone, while net in situ synthesis of MMHg is most significant in redox transition zones (i.e., shallow sedimentary regimes and water basins that experience seasonal hypoxia). (4) Organic matter-Hg interactions are the major control over the behavior and fate of Hg in aquatic systems.Progress Summary:
Hg0 in Long Island Sound. It has recently been revealed that in situ Hg0 production in the waters of LIS and emissions to the local/regional atmosphere are major processes (Rolfhus, 1998). Extensive investigations of the air-sea partitioning of Hg in LIS are being conducted to complement and extend the preliminary work conducted by this laboratory. We are using an Automated aqueous Gaseous Elemental Mercury sampling and analysis System (AGEMS), designed by our laboratory for ship-board use, which allows direct analysis of Hg0 in surface waters. These Hg0 (dissolved gaseous mercury or DGM; approximately 99 percent Hg0 in seawater) measurements are supported by laboratory analyses using traditional techniques as described by Rolfhus (1998). AGEMS surveys of DGM in LIS were first done in January, March, and May of 1999, and were continued in March, May, and September of 2000 as part of the current study. Samples for Hg speciation were collected during the surveys in 2000, which were done as part of our cooperative sampling arrangement with the Connecticut DEP Water Quality Survey (WQS) using the R/V John Dempsey.
Our DGM surveys have elucidated spatial/seasonal patterns in the distribution of Hg0 in LIS and their correlation with hydrographic conditions. DGM often decreases with decreasing salinity west of the Connecticut River (salinity decreases from east to west in LIS), particularly during the spring. As suggested by hypothesis #1, these spring distributions are attributable to enhanced Hg0 production at a reactive Hg source (e.g., Connecticut River), with the distribution maintained by biological production and simple mixing of DGM in LIS (Rolfhus & Fitzgerald, in press). We have documented the DGM distribution during spring "high flow" conditions in greater detail. For example, the March 2000 distribution shows a DGM maximum at the Connecticut River (eastern central LIS) and decreases linearly with salinity toward western LIS. In contrast, the DGM maximum in May of both 1999 and 2000 was west of the Housatonic River (western central LIS), demonstrating a dramatic change in the DGM distribution from early to late spring. Ancillary data from the CT DEP WQS aids in understanding these patterns. For example, nitrates (NO2- + NO3-) are a tracer of fresh water inputs (reactive Hg supply), and indicate inputs at the Connecticut River and in area of the Housatonic River during each March and May survey.
Supersaturation of Hg0 in all seasons indicates that Hg is lost from the Sound to the atmosphere via evasion. The average calculated Hg0 flux (Wanninkhof, 1992) agrees well between earlier 1995-1997 (334 p moles m-2 d-1) and later 1998-2000 LIS surveys (316 p moles m-2 d-1). General trends include increased Hg0 flux at increased Hg0 concentrations during warmer months and at higher wind velocities. Annual emissions are estimated at 85-90 kg (Rolfhus & Fitzgerald, in press; Balcom, et al., 2000b), which indicates remobilization of approximately 35 percent of the Hg inputs (230 kg/y) to LIS (Fitzgerald, et al., 2000). Because most Hg entering LIS has an anthropogenic origin (>70 percent), a substantial pollution component is being remobilized and its lifetime extended in active reservoirs. The aqueous production of Hg0 competes for reactant (i.e., labile Hg) with the in situ biological synthesis of MMHg, such that water bodies with a large production of Hg0 may have smaller amounts of MMHg in biota and accumulation in the sediment. Additionally, extensive and careful empirical documentation of the spatial and temporal variability of Hg in coastal waters such as LIS can yield an indicator as to the status and trends of Hg pollution in the system, and provides a means for constraining biogeochemical Hg cycling and mixing models. If as hypothesized, Hg0 production and emissions are related to the supply of reactant Hg (hypothesis #2), then remedial measures should be reflected by a decrease in the Hg0 distribution.
River-Seawater Mixing Zones. The atmospheric and aquatic biogeochemical Hg cycle associated with LIS and its environs is affected not only by localized discharges (e.g., rivers; waste water treatment facilities) and tidal exchange, but from the direct and indirect (via watershed leaching) airborne transport and deposition of Hg from regional and longer range sources. Inputs of particular significance to LIS are the East River with its large sewage loadings, and the Connecticut River which contributes about 70 percent of the annual input of freshwater, and whose extensive watershed extends northward into Canada.
The major flux of Hg delivered to the Sound from the Connecticut River is the result of watershed leaching. Estuarine reactions (i.e., mixing of river borne Hg species with seawater) increase the labile Hg fraction available for reduction (hypothesis #2). This physicochemical transformation results from shifts in speciation, associated with the presence in coastal seawater of inorganic complexing ligands (i.e., Cl- ions) and displacement of sequestered Hg in river water by the increased activity and competition from cations such as Mg++. This hypothesis suggests that direct water treatment facility (WTF) discharges (sewage) into estuarine or coastal saline environments (e.g., East River) lead to an enhanced localized production of Hg0.
Through ligand titrations, ligand activities were found to range from 6 (Connecticut River) to 0.3 (Mid-Atlantic Bight) nM, and followed a mixing line within the Connecticut River estuary. The conditional stability constant (K') for this material appears quite high (logK' = 24) and implies that >90 percent of the dissolved Hg present in seawater is complexed to dissolved organic matter (hypothesis #4). The relatively low abundance of ligand equivalents per mole of dissolved organic carbon, coupled with the high stability constant, suggests that the complexation site is a form of reduced organic sulfur (thiol).
In April 2000, the area around the surface front from the Connecticut River was sampled where it enters LIS. A wide salinity range (0.5-27 ppt) was incorporated in this sampling design, with the highest salinity at depth inside the plume and outside the plume (LIS water). Both the reactive (labile) Hg in unfiltered samples, and the reactive normalized to total Hg, showed a strong positive correlation with salinity. The correlation for the normalized reactive Hg (both filtered and unfiltered samples) indicates that the increase in labile Hg concentrations with salinity was significant and independent of the change in Hg total. These findings support hypothesis #2 above. A wide range of DGM concentrations were measured (25 to 370 fM), but the correlation with salinity was weak as might be expected for a gaseous constituent in this highly turbulent area.
The East River was sampled in late August 1999 and early September 2000. In 1999, we focused on surface sites near the Hunt's Point and Tallman Island WTFs, as well as surface sites to the east toward LIS. In 2000, we sampled surface waters at the site of the Ward's Island WTF effluent discharge, two sites to the west, and sites to the east of Ward's Island as far as the entrance to LIS (Throgs Neck). The latter work was done with the New York City DEP's Harbor Survey aboard the HSV Osprey. Among our sampling sites in 2000, the lowest salinity was measured at the Ward's Island effluent mixing area, and salinity increased to the east toward LIS. According to our hypothesis (#2), the highest reactive Hg and DGM concentrations would be expected at the WTF discharge site where the effluent meets the higher salinity East River water. In fact, the highest reactive Hg and DGM concentrations were measured near Ward's Island, and the concentrations of both decreased toward the east (LIS) due to mixing. The negative correlation between reactive Hg and salinity was very strong, but a mixing line for DGM and salinity indicates additional production of DGM at several sites in the River.
Methylmercury Production in Sediments. Previous marsh and near-shore studies by our group (Langer, et al., in press) suggest that internal (in situ) production is the major source of MMHg in Long Island Sound, and that external sources (e.g., rivers, atmospheric deposition) contribute only a minor fraction. We hypothesize (#3) that as much as 6 to 18 kg y-1 of MMHg are formed each year through the microbial conversion of labile reactive mercury species into MMHg in shallow sedimentary regions of LIS. Given the potential importance and the uncertainty of the production estimate, examination of in situ Hg methylation in LIS is needed.
We conducted a preliminary survey of Hg in sediment of LIS in November 1999. A gradient in sediment type exists longitudinally in LIS, ranging from fine-grain, organic-rich substrate in the western Sound (WLIS) to large grain size (sandy), low organic material in the east (ELIS). We collected surface sediment (top 8 cm) from three representative locations along the sediment-type continuum, examined concentrations of total Hg and MMHg, and evaluated the influence of microbial activity and selected sediment characteristics on Hg levels. Both total Hg and MMHg were highest in WLIS surface sediments. Hammerschmidt, et al. (2000) examined microbial activity, methylation potential and demethylation potential of microbial communities in surface sediment from WLIS and ELIS. Microbial activity was slightly greater in WLIS than ELIS, as expected from the higher organic matter content in WLIS sediment. Methylation and demethylation potentials were estimated indirectly by measuring the rate of ethanethiol (CH3CH2SH) methylation, and the rate of demethylation of ethylmethylsulfide (CH3CH2SCH3) (Hammerschmidt, et al., 2000). Rates of both reactions were higher in WLIS surface sediments, and a larger difference between rates was measured for WLIS sediments, indicating greater potential for accumulation of a methylated Hg species in WLIS.
In contrast to concentrations of total Hg and MMHg, the percentage of total Hg as MMHg in surface sediments increased from west to east in LIS. The percentage of total Hg as MMHg was 0.6 in WLIS and 1.8 in ELIS, indicating greater net methylation of Hg in sandy as compared to organic-rich, silty sediment. This was not predicted by our methylation and demethylation potential study. We suggest that sulfide inhibits bioavailability of Hg for methylation in WLIS sediments. Sulfide affects the chemistry of inorganic Hg in sediments by forming dissolved Hg-sulfide complexes, including HgS0, HgS22-, and HgHS2- (Benoit, et al., 1999a). HgS0 is a major dissolved Hg-sulfide complex when sulfide concentration is low, and charged complexes, mainly HgHS2-, are dominant at higher sulfide levels. The mechanism for uptake of inorganic Hg by methylating bacteria is not known presently, but may be due to diffusion of neutrally charged HgS0 through the cell membrane (Benoit, et al., 1999b). Consequently, maximum rates of Hg methylation in sediment occur where sulfide levels are low, favoring speciation of Hg-sulfide complexes as HgS0, and facilitating uptake of inorganic Hg by methylating bacteria. Sediments in ELIS (lower organic matter content) presumably have lower concentrations of dissolved sulfide, although it was not measured in this preliminary work. As a proxy, we measured acid-volatile sulfide and found concentrations were about 100-fold lower in ELIS than WLIS.
Future Activities:
In the next project year, we will continue to focus on the spring "high flow" conditions for Hg0 analyses. Particularly in early spring (February and March), the LIS DGM distribution west of the Connecticut River inflow has shown considerable annual variation. These surveys will continue to include Hg speciation analyses (particulate and dissolved analyses for Hg total and MMHg, as well as Hg reactive and total analyses on filtered and unfiltered samples) to put Hg0 determinations in their proper biogeochemical context. Additionally, we will focus efforts more intensively on the summer months when hypoxia is at its maximum in western LIS. The highest average DGM concentrations have been measured during August and September in the Sound (420-510 fM), but the greatest annual variability in the calculated DGM flux has also been found during September in 1998 and 1999 (150 and
820 p moles m-2 d-1). The determinations of DGM in LIS waters, including speciation analyses, will also be extended vertically in the water column. Initial vertical water column sampling (two western and one central LIS site) and analyses were done in August of 1999 to collect baseline Hg speciation information. These profiles will be used to search for production of Hg0 at depth and changes in its distribution across redox transitions (oxyclines) and thermoclines, when and where these phenomena are indicated by the ancillary hydrographic information.
We have now sampled the East River twice in August/September (1999 and 2000), and the Connecticut River/LIS interface once in the spring (April 2000). As proposed, we plan to sample both the high flow (major spring floods) and low flow conditions (late summer) in each of these river-seawater mixing zones. We will likely focus on the same sampling sites described above, with particular interest in the seasonal variability in reactive Hg and Hg0 production/distribution in these areas. An important priority will be to examine the relation between Hg sequestering by organics and river/seawater mixing, since organic-Hg interactions are the key to the availability of Hg.
Future work will include more detailed examination of Hg methylation and MMHg demethylation in Long Island Sound water and sediments. In spring 2001, we plan to sample surface sediments and overlying water from the three locations previously sampled in November 1999. Direct determinations of Hg methylation rates in sediment and water will be made using enriched stable isotopes of Hg in the laboratory of Dr. Cynthia Gilmour at the Benedict Laboratory, Academy of Natural Sciences, MD. Additional parameters to be measured include interstitial water total Hg and MMHg concentrations, pH and sediment microprofiles of sulfide and oxygen. We will continue to examine the relationship between microbial activity and Hg methylation in LIS, focusing on the complex linkage between sulfide chemistry, Hg speciation, and MMHg production.
References:
Benoit JM, Gilmour CC, Mason RP, Heyes A. Sulfide controls on mercury speciation and bioavailability to methylating bacteria in sediment pore waters. Environmental Science & Technology 1999a;33:951-957.
Benoit JM, Mason RP, Gilmour CC. Estimation of mercury-sulfide speciation in sediment pore waters using octanol-water partitioning and implications for availability to methylating bacteria. Environmental Toxicology and Chemistry 1999b;18:2138-2141.
Fitzgerald WF, Vandal GM, Rolfhus KR, Lamborg CH, Langer CS. Mercury emissions and cycling in the coastal zone. Journal of Environmental Science 2000;12(1):92-101.
Langer CS, Fitzgerald WF, Visscher PT, Vandal GM. Biogeochemical cycling of methylmercury at Barn Island Salt Marsh, Stonington, CT, USA. Wetlands Ecology and Management 2001;9(4):295-310.
Rolfhus KR. The production and distribution of elemental mercury in a coastal marine environment. Ph.D. dissertation, University of Connecticut, 1998, pp. 317.
Rolfhus KR, Fitzgerald WF. The evasion and spatial/temporal distribution of mercury species in Long Island Sound, CT-NY. Geochimica Cosmochimica Acta 2001;65(3):407-418.
Wanninkhof R. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research 1992;97(C5):7373-7382.
Journal Articles:
No journal articles submitted with this report: View all 35 publications for this projectSupplemental Keywords:
environmental chemistry, heavy metals, chemical transport, northeast., RFA, Scientific Discipline, Waste, Water, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Hydrology, Bioavailability, Environmental Chemistry, Ecosystem/Assessment/Indicators, Ecosystem Protection, State, Fate & Transport, Ecological Effects - Environmental Exposure & Risk, Environmental Monitoring, Ecological Risk Assessment, Ecology and Ecosystems, Mercury, fate and transport, microbiology, risk assessment, aquatic, Long Island Sound, river-seawater mixing zones, mass balance studies, emissions, fish consumption, mercury cycling, biogeochemical cycling, water quality, Physicochemical aspects, marine environment, solar radiation, coastal, microbiological aspectsRelevant Websites:
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.