Grantee Research Project Results
2002 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, 2001 through September 30, 2002
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:
This research project is focused on biogeochemical transformations of Hg species in Long Island Sound (LIS), a valuable analog for other near shore/urban marine ecosystems. Our specific objectives are related to several major features of the aquatic biogeochemistry of Hg, particularly elemental mercury (Hg0) cycling and emissions, monomethylmercury (MMHg) production, and the role of organic matter in governing the availability of Hg for competing methylation/reduction reactions. During 2002, our research addressed each of these components. We conducted in situ surveys using our Automated Aqueous Gaseous Elemental Mercury Sampling and Analysis System (AGEMS) to examine spring distributions of Hg0 in LIS. In the spring and late summer, we conducted vertical water column sampling for Hg species and Hg0 in central/western LIS to investigate mechanisms responsible for the Hg0 production and its temporal and spatial patterns. We examined sedimentary MMHg production and cycling in sediments from three geochemically distinct LIS locations. Supporting bottom water sampling for Hg species also was conducted during the spring and early summer. A spring survey of the East River focused on wastewater treatment plant (WTP) input, as this appears to be an important avenue for delivery of Hg to LIS. River/seawater mixing zones are important influences affecting the interactions between Hg and organic matter. A variety of comparison gross reduction rate incubation experiments have been conducted to investigate the control of organic matter (ligands) over Hg0 cycling.
Progress Summary:
Hg0 and Hg Speciation in LIS
Surface Sampling. Our continuing examination of LIS during spring and summer conditions has added to our understanding of the seasonally varying Hg0 (dissolved gaseous mercury [DGM]; approximately 99 percent Hg0 in seawater) distribution in LIS. We used the AGEMS aboard the Connecticut Department of Environmental Protection (CT DEP) vessel R/V John Dempsey for direct analysis of Hg0 in surface waters during consecutive weeks in March 2002. The survey during the first week of March extended from a sampling station near the East River to the eastern end of LIS. Hg0 concentrations (90-170 fM) were lower than in previous early spring surveys, but showed slight elevations in the environs of the Connecticut and East River inputs that have been observed in previous surveys (e.g., March 2000). In contrast, the AGEMS survey conducted during the second week of March revealed that Hg0 concentrations were highest and roughly uniform (165-210 fM) in central/western central LIS to the west of the Housatonic River inflow. The location of the elevated Hg0 within the sound is similar to the distribution previously measured in mid-March (1999) and to that usually seen later in the spring (May). AGEMS surveys conducted during 2001 and 2002, have been added to surveys from 1999-2000, resulting in an average calculated flux of 300 p moles m-2 d-1. Annual emissions from LIS are estimated at 85-90 kg, which indicates remobilization of approximately 35 percent of the Hg inputs (230 kg/y) to LIS. Field measurements are supported by laboratory analyses using a semi-automatic Dissolved Elemental Mercury Analyzer (DEMA). We developed this analyzer as part of our U.S. Environmental Protection Agency (EPA) Science to Achieve Results (STAR) program.
Water Column Studies. As a working hypothesis, we posit that 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). Our DGM surveys have elucidated spatial/seasonal patterns in the distribution of Hg0 in LIS and their correlation with hydrographic conditions. Surface maxima have been observed along the central axis of LIS west of the Housatonic River in May and September in the region between two bottom sills that traverse LIS. During these same months in 2002, in the same region, we examined the hydrographically generated supply of labile Hg in greater detail. Inputs of labile Hg from deep water mixing (e.g., degradation of organic matter at depth), as indicated by surface peaks in nitrate, appear also to supply Hg reactant for DGM production at the surface (e.g., August/September breakdown of thermal stratification).
We conducted these studies aboard the University of Connecticut vessel R/V Challenger at stations west of the Housatonic River, along the central axis of LIS, in May and September 2002. Three of the stations were between the bottom sills that traverse LIS. Small differences in surface and bottom temperatures in this region in both May (1°C or less) and September (<0.2°C) indicate that mixing of bottom waters to the surface is likely occurring. Mixing may be driven by tidal and residual flow over the bottom sills. In May 2002, surface water reactive Hg concentrations (2 pM; unfiltered) and nitrate concentrations (3-4 µM) were supported by bottom water concentrations at two stations sampled vertically between the sills. Similarly, surface water reactive Hg (6 pM; unfiltered) and nitrate concentrations (4 µM) were supported by bottom water concentrations in September 2002, at a sampling station near the eastern sill. As was the case in May 2000, Hg0 concentrations were elevated between the sills (120-370 fM) in May 2002, as compared to a sampling site to the east (150 fM; near Housatonic River inputs). However, unlike September 2000, Hg0 concentrations were not elevated between the sills or to the east in September 2002 (<50-135 fM; see discussion of Hg0 incubation experiments). Interestingly, elevated Hg0 in the region of the sills correlated with elevated nitrate to the east of the sills. As in May and September 2000, surface water nitrate was elevated to the east of the eastern sill (16 µM) in May 2002. These high nitrate levels seem to point to "upwelling," or mixing of bottom waters that provide inputs to surface waters. However, this process does not seem to directly support Hg0 production (e.g., labile Hg inputs) between the sills.
Bacterial Activity Measurements. In 2002, we began using fluorogenic model substrates to measure bacterial extracellular enzymatic activity. The bacteria utilize a glucose (DiFMUGlu or MUF-beta-D-Glu) or amino acid (Leu-MCA) containing organic substrate, and release a fluorescent molecule (MUF or AMC fluorochrome). The amount of fluorescent molecule is quantified in a solution, and the rate of increase in concentration indicates the rate of bacterial activity. These are generalized activity measurements for processes that utilize the substrates. Among the microbial processes potentially occurring are the production of labile Hg from Hg associated with organic material, and the bacterially mediated production of Hg0. In September 2002, bacterial activity as measured by the glucose substrates (0.68-14.07 µgC L-1 h-1 for MUF-beta-D-Glu, 0.57-12.88 µ/gC L-1 h-1 for DiFMUGlu, 48-hour incubations) was highest at all depths at the sampling site near the eastern sill. Bacterial activity also was high in bottom waters near the western sill. These findings support the hypothesis that reactant Hg is supplied by vertical mixing of bottom waters, although factors such as dissolved organic carbon (DOC) levels may account for the lack of Hg0 production (see discussion of Hg0 incubation experiments).
River-Seawater Mixing Zones
We hypothesize that estuarine reactions (i.e., mixing of river-borne Hg species with seawater high in Cl- and major cations) and direct water treatment facility (WTF) discharges (sewage) increase the labile Hg fraction available for reduction, enhancing localized production of Hg0. This physicochemical transformation results from shifts in speciation associated with the presence of inorganic complexing ligands (i.e., Cl- ions) in coastal seawater, and displacement of sequestered Hg in river water by the increased activity and competition from cations such as Ca2+ and Mg2+. Therefore, the river-coastal water mixing regime is one of the most critical parts of the watershed.
East River and LIS. We previously have sampled the East River during the summer season, and the total Hg concentrations (unfiltered) in August 1999, were very similar to those measured in September 2000 (approximately 40 pM). This indicates that the waters of the East River are similar to Connecticut WTF effluent with respect to total Hg concentrations. Elevated unfiltered reactive Hg and DGM concentrations were measured at Ward's Island WTP inputs in September 2000, and the concentrations of both decreased eastward toward LIS (Throgs Neck), due to mixing.
In May 2002, we sampled the East River during the spring high-flow period using the R/V Challenger. Total (60-160 pM; unfiltered) and reactive Hg (19-85 pM; unfiltered) concentrations were elevated as compared to the summer surveys, and were highest at the input from the Ward's Island WTP. As in September 2000, concentrations of both parameters decreased with increasing salinity towards LIS. Total suspended solids (TSS) levels were high (13-21 ppm) during this high-flow period, and suspended sediment loads likely account for the elevated Hg levels. Dissolved total Hg concentrations (2-6 pM) were similar to those measured in September 2000, and showed no correlation with salinity. Surprisingly, after mixing with LIS water, measured reactive Hg (unfiltered) was high in the East River at the entrance to LIS (20 pM; Throgs Neck) in May 2002. Additionally, there was an increase in reactive bottom water and total Hg from May to September 2002, between the sills in western/central LIS. We hypothesize that the East River is supplying reactant Hg to the waters of LIS that supports the Hg0 production in western/central LIS.
Despite the elevated Hg levels in the East River in May 2002, Hg0 (0-75 fM) was quite low, with only a slight elevation at one sampling station (average of 130 fM) near the Ward's Island effluent input. Mixtures of Ward's Island effluent (collected at the WTP in May 2002, from chlorine contact tank prior to discharge) and LIS water also showed very low Hg0 concentrations (< 50 fM). Bacterial activity measurements with a glucose substrate (0.29-7.99 µgC L-1 h-1 for DiFMUGlu, 48-hour incubations) were elevated at only one sampling site near the Ward's Island effluent inflow, although bacterial activity was lowest in effluent waters entering the East River, and activity decreased in waters entering LIS. These findings indicate little biologically mediated reduction of labile Hg to Hg0 through most of the East River in May 2002. At this time of the year, the intensity and duration of sunlight does not lead to significant abiotic Hg0 formation, and other factors such as DOC levels may limit Hg0 production (see discussion of Hg0 incubation experiments).
Hg-Organic Interactions
The major flux of Hg delivered to the sound from the Connecticut River is the result of watershed leaching. Spring runoff contributes large amounts of Hg to the Connecticut River that is tightly bound to dissolved and colloidal organic ligands and particulate matter, and that are largely unreactive (not reducible with Sn(II)). Complexation of inorganic mercury cations (Hg2+; labile Hg) by natural organic compounds has been posited as an influential and often controlling feature of the aquatic biogeochemical cycling of this toxic metal, and is a working hypothesis for the present study. Ligand characteristics indicate that the majority of Hg2+ dissolved in fresh (>99 percent) and coastal salt waters (>50 percent) is associated with organic complexes. Although many studies suggest that the majority of Hg present in natural waters is complexed with organic ligands, little quantitative information currently exists regarding the abundance and strength of such Hg-complexing agents in natural waters.
Ligand Studies. Using a new wet chemical ligand titration technique, we have found that the concentration of Hg-binding organic ligands in LIS and its environs ranges from <1 to >60 nN, and that the conditional stability constant values (affinities of the organic matter for Hg) are very high (logK' = 22-25). These values indicate that Hg is quantitatively associated with organic complexes in waters of salinity <15 percent, and often can be dominated in saltier waters as well. We have constructed a ligand capacity balance for LIS based on measurements of ligand concentrations, DOC analyses, and estimates of some DOC fluxes that are particularly difficult to measure. The principal sources of ligands to LIS are riverwater (47 percent) and phytoplankton DOC exudation (31 percent), while the only loss term of significance identified so far is tidal exchange with the low DOC/low ligand waters of the continental shelf. The seasonal variations in ligand abundance (lowest during winter and highest during summer and spring) are a reflection of the importance of river flow and primary production as these sources are strongest in the spring and summer in LIS.
Hg0 Incubation Experiments. We also have used the titration data to set up experiments to examine the effect of speciation on the important biogeochemical process of in situ reduction of Hg+2 to Hg0 occurring in LIS. We examined the effect of adding competitive complexing agents (EDTA and chloride) to solutions with and without a common form of DOC (humic acid) on the gross reduction rate. Experimental solutions were spiked with Hg and continuously bubbled with Hg-free air under light and dark conditions, while the Hg0 produced was monitored. The gross rate of reduction was determined from the final steady state rate of Hg0 collection from the sparger. We found that the addition of humic acid to the solution increases the rate of reduction. This finding was not new. However, the effect of adding competitive ligands at high enough concentrations to shift the Hg speciation away from the humic acid was to lower the reduction rate to almost zero. This indicates that Hg must be bound to organic material to be reduced in natural waters by abiological processes. It also suggests an unusual relationship between Hg+2 reduction and DOC. At low DOC concentrations (typical of seawater) when Hg may not be 100 percent organically bound, increasing DOC will result in increased Hg0 production. At the equivalence point, when 100 percent of the Hg present is complexed with organics, further additions of DOC will serve only to dilute the Hg-DOC complexes and attenuate the light, thus lowering the Hg specific reduction rate.
These results explain certain aspects of Hg cycling in LIS. First, higher Hg0 concentrations in the summer are supported by the higher concentrations of DOC and ligand during this time, which likely brings Hg speciation and DOC values to the point of optimal reduction rates. This phenomenon, coupled with low wind speeds, leads to a build up of Hg0 during the summer. Second, DOC may at times be high enough to prevent reduction to Hg0 even when there is excess of Hg+2 (labile Hg) available for reduction. Such conditions may have existed in the field during the East River survey in May 2002, and in LIS during September 2002. Third, the results also suggest that reduction below the photic zone can be explained by Hg-organic associations, and that this flux is likely the dominant reduction term on a depth-integrated basis. Our results are from synthetic solutions used over short time periods; therefore, bacterial contributions to "dark" reduction are minimal. The important dark reductions of Hg may be abiological and organically mediated.
Methylmercury Production in Sediments
We continued to examine relationships between microbial activity and sediment geochemistry in affecting Hg methylation in LIS, focusing on complex linkages between sulfur chemistry, Hg speciation, and organic matter. A preliminary mass balance of MMHg in LIS suggests in situ production as the main source, and we hypothesize that most MMHg is synthesized microbially in sediment. We examined factors influencing MMHg in sediment at three representative sites spanning the benthic trophic gradient in LIS, ranging from fine-grain, organic-rich substrate in the west, to sandy, low organic material in the east. Surveys were conducted in August 2001, and March 2002, aboard the CT DEP vessel R/V John Dempsey, and in June 2002, aboard the University of Connecticut vessel R/V Connecticut.
Methylation Rate Measurements. In August 2001, both MMHg and total Hg were related positively with organic matter and acid-volatile sulfide (AVS) in surface sediment; however, rates of Hg methylation were inversely related to both of these parameters. Rates of Hg methylation were assayed by injecting a 200Hg-enriched standard (stable isotope) into sediment cores and incubating them at ambient conditions. We determined methylmercury produced from the isotope (CH3 200Hg+) by inductively coupled plasma/mass spectrometry (ICPMS). Although comparatively more MMHg was present in organic-rich western LIS sediments, the rate of Hg methylation was greater in the eastern sound. The rate of 200Hg methylation in LIS sediments was inversely related to the distribution coefficient (Kd) of substrate inorganic Hg, which was controlled by both AVS and organic matter. We hypothesize that organic matter and AVS affect Hg methylation in LIS by influencing the partitioning of inorganic Hg between dissolved and particulate phases, thereby regulating the availability of dissolved Hg to methylating bacteria. The sediments that were collected in March and June of this year are expected to support this hypothesis and show the seasonal effects of temperature and autochthonous organic matter inputs on Hg methylation in LIS sediments.
Vertical profiles of Hg species and ancillary biogeochemical parameters show the effects of bioturbation on Hg methylation in coastal marine sediments. The profile of 200Hg methylation in the eastern sound varied dramatically with depth, showing marked increases at about 2 and 6 cm depth, unlike typical Hg methylation profiles observed in freshwater sediments and at the western LIS sites. Such excursions of the 200Hg methylation rate corresponded to peaks in the bioturbation index (change in organic carbon [LOI] normalized to change in depth in sediment), suggesting a link between physical sediment disturbance and enhanced Hg methylation in coastal marine sediments. Bioturbation may introduce fresh organic matter to depths where active sulfate reduction occurs, or it may promote chemical conditions that favor dissolution of Hg+2 substrate, both of which may result in increased methylation in sediment.
Implications for LIS. The estimated sediment-water flux of dissolved MMHg (~10 kg y-1) is consistent with the preliminary mass balance for LIS, pointing to sedimentary production and mobilization of MMHg as the principal source. Additionally, bioaccumulation estimates suggest that most of the MMHg in LIS plankton is attributable to sedimentary synthesis and mobilization. Assuming that all the sediment-derived MMHg is accumulated by phytoplankton in LIS, which has 200-400 g C m0-2 y-1 primary production, we predict that they would have 0.7-1.4 ng MMHg g-1 wet weight. Our average measured level of MMHg in suspended particulate matter of LIS, most of which is autochthonous, is 0.9 ng g-1. It is likely that most of the MMHg in higher trophic levels of LIS, which ultimately derive organic matter and MMHg from primary producers, also can be attributed to Hg methylation in the sediments. Work on the bioaccumulation of MMHg in LIS biota is being investigated with zooplankton collected in June 2002, and four species of fish (alewife, bluefish, winter flounder, lobster) collected in June and September 2002, with the assistance of the CT DEP.
References:
Fitzgerald WF, Vandal GM, Rolfhus KR, Lamborg CH, Langer CS. Mercury emissions and cycling in the coastal zone. Journal of Environmental Science 2002;12(1):92-101.
Hammerschmidt CR, Fitzgerald WF. Formation of artifact methylmercury during extraction from a sediment reference material. Analytical Chemistry 2001;73(24):5930-5936.
Hoppe H-G. Use of fluorescent model substrates for extracellular enzyme activity (EEA) measurement of bacteria. In: Kemp PF, Sherr BF, Sherr EB, Cole JA, eds. Handbook of Methods in Aquatic Microbial Ecology, Boca Raton. Lewis Publishers, 1993, pp. 423-431.
Lamborg CH. Hg speciation and reactivity in the coastal and estuarine waters of Long Island Sound. Ph.D. Dissertation. University of Connecticut, Groton, CT, (in review, 2002).
Langer CS, Fitzgerald WF, Visscher PT, Vandal GM. Biogeochemical cycling of methylmercury at Barn Island Salt Marsh, Stonington, CT. 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, Groton, CT, 1998, 317 pp.
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):407-417.
Rolfhus KR, Lamborg CH, Fitzgerald WF, Balcom PH. Evidence for enhanced mercury reactivity in response to estuarine mixing. Journal of Geophysical Research-Oceans (submitted, 2002).
Wanninkhof R. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research 1992;97(C5):7373-7382.
Vandal GM, Fitzgerald WF, Rolfhus KR, Lamborg CH, Langer CS, Balcom PH. Sources and cycling of mercury and methylmercury in Long Island Sound. Final report to the Connecticut Department of Environmental Protection, Long Island Sound Program, 2002.
Future Activities:
In the next project year, we will conduct surveys of Hg0 distributions in LIS using a two-bubbler, automated AGEMS that will allow for increased spatial and temporal resolution. Surveys extending from the East River to the Block Island Sound will be conducted within a 12-hour period aboard the R/V Connecticut. Additionally, a spring/summer survey of diurnal Hg0 cycling will be conducted at a central/western LIS location as an extension of the fall survey conducted in 2001. In 2003, we will continue additional incubation experiments that are tied in with the AGEMS surveys to examine gross and net reduction of Hg+2 to Hg0. These experiments will be conducted with surface and bottom water at selected stations following the west to east trace constituent gradient in LIS. Incubation experiments will be complemented with measurements of bacterial activity, direct counts of bacteria, and Hg speciation. We will continue to investigate abiotic Hg methylation in sediment pore waters to assess the contribution/significance of such a mechanism relative to microbial processes in affecting levels of sediment MMHg.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 35 publications | 13 publications in selected types | All 11 journal articles |
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Lamborg CH, Tseng C-M, Fitzgerald WF, Balcom PH, Hammerschmidt CR. Determination of the mercury complexation characteristics of dissolved organic matter in natural waters with "reducible Hg" titrations. Environmental Science & Technology 2003;37(15):3316-3322. |
R827635 (2002) R827635 (Final) |
Exit Exit |
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Tseng CM, Balcom PH, Lamborg CH, Fitzgerald WF. Dissolved elemental mercury investigations in Long Island Sound using on-line Au amalgamation-flow injection analysis. Environmental Science & Technology 2003;37(6):1183-1188 |
R827635 (2002) R827635 (Final) |
not available |
Supplemental Keywords:
ecosystem protection, environmental exposure and risk, geographic area, waste, water, bioavailability, chemical mixtures, environmental chemistry, environmental monitoring, hydrology, mercury, Long Island Sound, New York, NY, physicochemical aspects, aquatic, biogeochemical cycling, coastal, emissions, fate and transport, fish consumption, marine environment, mass balance studies, mercury cycling, microbiological aspects, microbiology, river-seawater mixing zones., 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:
http://www.teamHg.uconn.edu Exit Synthesis Report of Research from EPA’s Science to Achieve Results (STAR) Grant Program: Mercury Transport and Fate Through a Watershed (PDF) (42 pp, 760 K)
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.