2001 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: R827635
Title: 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: Hiscock, Michael
Project Period: October 1, 1999 through September 30, 2002
Project Period Covered by this Report: October 1, 2000 through September 30, 2001
Project Amount: $592,035
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text |  Recipients Lists
Research Category: Water and Watersheds , Mercury , Water , Safer Chemicals


Consumption of marine fish and seafood products is the principal pathway by which humans are exposed to the very toxic organo-mercurial, monomethylmercury (MMHg). Consequently, there is an urgent need for increased knowledge and understanding of the marine biogeochemical cycling of Hg and the impact of anthropogenically related Hg inputs. Biologically productive, nutrient-rich near-shore regions, which support major commercial and recreational fisheries, are of special interest. Accordingly, our EPA-STAR Hg research is focused on Long Island Sound (LIS), its watershed, and river-seawater mixing zones. This major natural resource can provide a valuable analog for other near-shore/urban marine ecosystems. Our reaction-speciation focused investigations are designed to allow the results to be applied in other marine regions. Such an approach is essential given the complexity and variability of fertile estuaries and adjacent coastal waters, which are major repositories for natural and pollutant riverborne/watershed derived substances such as Hg. Our specific objectives are concerned with several major features of the aquatic biogeochemistry of Hg, particularly elemental mercury (Hg0) cycling and emissions, MMHg production, interactions between terrestrial watersheds, rivers and near-shore marine waters, and the role of organic matter in governing the availability of Hg for competing methylation/reduction reactions.

Progress Summary:

Hg0 in Long Island Sound

We have continued our examination of LIS during spring and summer conditions, and have documented the seasonal Hg0 (dissolved gaseous mercury or DGM; approximately 99 percent Hg0 in seawater) distribution in greater detail. We are using an Automated aqueous Gaseous Elemental Mercury sampling and analysis System (AGEMS), designed by our laboratory for shipboard use, which allows for direct analysis of Hg0 in surface waters. Field measurements are supported by laboratory analyses using a semi-automatic Dissolved Elemental Mercury Analyzer (DEMA). AGEMS surveys of DGM in LIS were done in March, May, and September of 2000, and also in April, June, and August of 2001. Samples for Hg speciation measurements were collected at selected surface sites during the 2000 and 2001 surveys, and the Connecticut Department of Environmental Protection (CT DEP) Water Quality Monitoring Program provided nutrient and other water quality measurements for all surveys.

Surface Sampling

Our DGM surveys have elucidated spatial/seasonal patterns in the distribution of Hg0 in LIS and their correlation with hydrographic conditions. For example, a peak in surface water DGM (250 fM) was observed west of the Housatonic River (western central LIS) in May 2000. During the late summer, September 2000, a peak in surface water DGM was found to the west of the Housatonic River (400 fM), and also east of the mouth of the East River (300 fM, western end of LIS). 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). Interestingly, peaks in surface nitrate concentrations are nearly coincident with peaks in surface DGM. Nitrate (NO3-) can be a tracer of fresh water input and/or labile Hg supply, and is typically elevated in the area of inputs from the Connecticut, Housatonic, and East Rivers during the periods sampled. However, based on a simple mixing calculation, the nitrate concentration in the Housatonic River would account for only about one-half of the LIS nitrate concentration near the river in May 2000, and only a fraction of the nitrate present in September 2000.

The surface DGM maxima near the Housatonic River in May and September 2000 were found in the region between two bottom sills that traverse LIS. Small differences in surface and bottom temperatures in this region at those times indicate that mixing of bottom waters to the surface is likely occurring. Inputs of labile Hg from deep water mixing (e.g., degradation of organic matter at depth), as indicated by surface peaks in nitrate, could supply reactant for DGM production at the surface. In May 2000, one-half of the surface water nitrate peak (2.5 µM) near the Housatonic River is supported by the nitrate concentration in the river, and the other half can be accounted for by bottom water nitrate concentrations. In September 2000, elevated surface water nitrate (10 µM) in the same region can be accounted for entirely by nitrate concentrations in bottom water. Conversely, surface and bottom water nitrate (8 µM) near the East River in September appears to result from simple mixing of nitrate from the East River, implying that reactant for elevated DGM in western LIS was supplied by the East River.

Vertical Water Column and Diurnal Sampling

Additional evidence of hydrographic control on Hg0 cycling was observed during water column experiments in western/central LIS during August 2001, and in a diurnal study in eastern LIS during October 2001. We conducted vertical water column sampling during the period of hypoxia in LIS, when strong thermoclines and oxyclines were present. Elemental mercury concentrations in LIS are generally highest in the summer (enhanced photoreduction), typically showing concentration maxima above the thermocline (Rolfhus & Fitzgerald, 2001). Surface DGM concentrations at a site west of the Housatonic River in August 2001 (400 fM) were as high as the peak in September 2000. At two stations where the water column was sampled vertically, labile Hg concentrations were elevated in the thermocline (2-4 pM) and declined sharply at the surface, implying that this reactant was used for DGM production. Elevated total suspended solids (TSS) and nitrate (only a fraction supported by riverine inputs) above the thermocline suggest a source of labile Hg at the top of the thermocline (e.g., decaying organic matter) that seems to be supporting DGM production. In October 2001, elemental mercury was sampled over a diurnal cycle, and samples analyzed in the laboratory. Hg0 was uniform (100-160 fM) over the 24-hour period sampled, and concentrations at 3 m agreed with those at the surface. As anticipated, the importance of photoreduction to Hg0 production appears to be seasonally dependent, and deep water mixing may control surface Hg concentrations in the absence of thermal stratification.

River-Seawater Mixing Zones

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 which are largely unreactive (not reducible with Sn(II); Rolfhus, et al., submitted). We hypothesize that 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. 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 Ca++ and Mg++ (Rolfhus, et al., submitted). Therefore, the river-coastal water mixing regime is one of the most critical parts of the watershed.

Connecticut River and LIS

The Connecticut River was sampled where it enters LIS, first in October 1996 (low flow) and May 1997 (high flow), and as part of the current study in April 2000 (high flow; Rolfhus, et al., submitted). Collections were made along a surface front whose salinity gradient was often 20 ppt over just a few meters. Unfiltered labile Hg (reactive Hg or HgR, 1-6 pM) increased nearly linearly with salinity in all three surveys. Additionally, HgR normalized to HgT (5-55 percent HgT as HgR) shows a strong positive correlation with salinity, particularly in the unfiltered fraction (Rolfhus, et al., submitted). Formation of elevated percent HgT as HgR during estuarine mixing is the result of dissolved inorganic species being converted into more labile inorganic associations (Lamborg, et al., submitted). For both dissolved and unfiltered species, this relationship suggests that increasing chloride content is out-competing dissolved organic ligands.

Mercury species (HgT, HgR, and DGM) concentrations were determined for surface points in LIS within several days (October 1996 and May 2000) after the October 1996 and April 2000 Connecticut River estuary surveys (Rolfhus, et al., submitted). It is striking that a station off the mouth of the Connecticut River exhibited the highest concentrations (8.1 pM HgT, 6 pM HgR and 470 fM DGM) for all three species (unfiltered) in October 1996, and that concentrations of all species decreased towards the east and west in the Sound. The same station off the mouth of the river also showed elevated DGM (200 fM), unfiltered HgR (4.6 pM), and unfiltered HgT (12 pM) in May 2000. These distributions support the contention that the river is supplying Hg substrate that promotes the production of DGM. Rolfhus and Fitzgerald (2001) showed that DGM often decreases with decreasing salinity west of the Connecticut River, particularly during spring high flow conditions. These distributions are attributable to enhanced Hg0 production at a labile Hg source (Conn. R.), with the distribution maintained by biological/photochemical production of Hg0 and mixing of labile Hg during transport westward in LIS.

Ligand Studies

Complexation of inorganic mercury cations (Hg2+) 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. 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. This is due in large measure to the lack of a suitably sensitive and reliable technique with which to probe Hg speciation. Therefore, Carl Lamborg, a doctoral student in our laboratory, has developed a new method for the determination of activities and conditional stability constants of dissolved organic matter towards Hg using an in vitro reducible-Hg titration approach. This is a wet chemical analog to the electrochemical titrations now in use for ligand studies of many other trace transition metals in natural waters. The new technique is similar to the "reactive Hg" analysis (reduction with Sn(II)), but involves increasing the Hg concentration of samples by ca. 1,000 times. Thus, the organically and inorganically complexed pools are of comparable size, and the uncertainties associated with mass action that confound the "reactive Hg" determination may be addressed. This approach is robust, as demonstrated by analysis using multiple reducing agents of varying strengths and replicate analyses.

Results indicate that complexing agents are present in the dissolved-phase (<0.2 µm) at ca. <1-60 nN concentrations and with log conditional stability constants (logK') in the range of 20-25. Such ligand characteristics indicate that the majority of Hg2+ dissolved in fresh (>99 percent) and coastal salt waters (>50 percent) is associated with organic complexes. In Long Island Sound (LIS), it is clear that the ligands are of terrestrial origin based on analysis of the ligand activity through the salinity gradient of the Connecticut River (conservative type distribution with activity decreasing at higher salinity). Furthermore, the ligand:DOC ratios for a variety of endmember waters for LIS (mole:mole; riverwater ca. 50x10-6, sewage-influenced East River water ca. 10x10-6, offshore water <1x10-6) indicate that shelf waters and sewage possess very ligand poor DOC. Therefore, a substantial percentage of the Hg binding compounds present in the coastal waters of Long Island Sound (ligand:DOC ca. 25x10-6) are allochtonous in origin. Additionally, an inverse relationship between water flow and ligand activity was observed in the Connecticut River. Though data are few, they suggest that the watershed releases a relatively fixed amount of ligand that is diluted to a greater or lesser extent in the river depending on the hydrographic conditions. Therefore, it appears that the carbon turnover rate in watersheds is a very significant process in determining the amount of Hg binding ligands present in lakes, rivers, and the coastal ocean.

Methylmercury Production in Sediments

We have continued to examine the relationships between microbial activity and Hg methylation in Long Island Sound, focusing on complex linkages between sulfur chemistry, Hg speciation, and organic matter. Our hypothesis is that Hg0 is the predominant Hg cycling product of bacterial activity in the oxic zone, although 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). Additionally, we hypothesize that in situ sedimentary production is the primary source of MMHg to the waters and biota of LIS. Findings from November 1999 indicate that LIS sediment MMHg concentrations were dependent on the availability of Hg and were directly related to the organic matter content of the sediment (Hammerschmidt, et al., submitted). However, the percentage of total Hg as MMHg was related inversely to sediment organic content. As a working hypothesis, we suggest that these results are best explained by the availability of Hg to methylating bacteria, which depends on the speciation of Hg in sulfidic porewaters. In low-organic sediments, where (sulfate-reducing) bacterial activity also is low, more of the inorganic Hg is methylated because HgS0 (bioavailable Hg) is the dominant dissolved Hg-sulfide complex as a result of minimal sulfide accumulation. In August 2001, we sampled sediments and overlying water from locations similar to those sampled previously. Direct determinations of Hg methylation rates in sediment are being made using 200Hg isotope. Additional parameters measured include sediment and pore water concentrations of total Hg and MMHg, sediment microprofiles of sulfide and oxygen, and water column Hg species measurements.

Artifact MMHg Formation

Accurate determinations of toxic MMHg in sediment are critical to understanding the biogeochemical cycling of the contaminant and estimating associated potential exposures of aquatic food webs. The formation of MMHg as an artifact is a recently known problem associated with measurement of the chemical in environmental samples, especially aquatic sediments, and can result in significant bias of measurements. Compared to fish tissues that contain more than 90 percent of total Hg as MMHg, natural sediments often contain only 0.5-2.0 percent, and methylation of any ambient inorganic Hg can cause considerable error. Chad Hammerschmidt, a doctoral student in our laboratory, has examined MMHg in surface sediments from LIS and sediment reference material IAEA-405 (Hammerschmidt & Fitzgerald, 2001). The goal of this study was to ascertain the source of artifact MMHg in IAEA-405, and to evaluate if artifact MMHg is formed during analysis of natural estuarine sediments such as those from eastern and central regions of LIS. Measured MMHg concentrations in IAEA-405 were biased considerably by artifact MMHg formed during extraction from the sediment with either distillation or acid-leaching, two of the most commonly used techniques. Artifact MMHg formation is a function of both the availability of inorganic Hg and a methylating potential of an unknown chemical agent. Little or no artifact MMHg was formed during extraction of LIS sediments, yet a potential for abiotic methylation existed. In LIS sediments (and likely those of other natural systems) where levels of inorganic Hg are much lower than those of IAEA-405 sediment, the availability of HgR limits significant synthesis of MMHg during analysis. Overall, these results suggest that abiotic methylation of inorganic Hg can occur whenever both a methylating agent and HgR are present. We hypothesize that an abiotic methylation potential always exists in the environment and conditions that favor formation of dissolved HgR may lead to MMHg synthesis. Future investigations will explore abiotic Hg methylation, a reaction that has been largely discredited as a significant natural source of MMHg, in both the sediments and water column of LIS.


Hammerschmidt CR, Fitzgerald WF. Formation of artifact methylmercury during extraction from a sediment reference material. Analytical Chemistry 2001;73(24):5930-5936.

Rolfhus KR, Fitzgerald WF. The evasion and spatial/temporal distribution of mercury species in Long Island Sound, CT-NY. Geochimica et 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, 2001).

Future Activities:

In the next project year, we will continue our AGEMS/DEMA surveys of the Hg0 distribution in LIS. Particularly in regions of elevated Hg0, vertical water column sampling will be designed to examine the hydrographically generated supply of labile Hg in greater detail. In addition to Hg speciation determinations, vertical water column samples will be collected for the determination of phytoplankton production, generalized bacterial activity, and for laboratory incubations of LIS water to determine rates of net reduction of reactive Hg to Hg0. This year's results have pointed to the importance of microbial activity (and photoreduction) in producing elevated Hg0 in specific areas of LIS. To establish the role of biotic and abiotic factors in Hg0 production, the concentration of Hg0 will be monitored following ionic Hg spike additions to LIS seawater samples. Coupled with the hydrographic surveys, further LIS sediment sampling is planned for the spring and summer of 2002. We will conduct localized sedimentary studies of Hg methylation and its relationship to sulfate-reducing bacterial activity, Hg speciation, redox characteristics, sulfate, and organic matter. The experimental design will focus on gradients in these parameters, and also will include sediment and pore water MMHg and Hg measurements to elucidate seasonal patterns in these parameters.

River-seawater mixing zone sampling will be completed in 2002. The East River has been sampled twice during the summer "low-flow" period, and we plan to do a spring "high-flow" survey to examine labile Hg formation and Hg speciation in the region of sewage effluent/seawater mixing. The Connecticut River has been sampled during the spring (high flow) in the current study. These results, as well as those from previous spring and fall (low flow) surveys, have been summarized by Rolfhus and collaborators (submitted). In 2002, we plan to conduct a variety of mixing experiments using river water and sewage effluent in combination with LIS water, with emphasis on the behavior of reactive Hg in these mixtures.

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
Type Citation Project Document Sources
Journal Article Hammerschmidt CR, Fitzgerald WF. Formation of artifact methylmercury during extraction from a sediment reference material. Analytical Chemistry 2001;73(24):5930-5936. R827635 (2001)
R827635 (Final)
not available
Journal Article Rolfhus KR, Lamborg CH, Fitzgerald WF, Balcom PH. Evidence for enhanced mercury reactivity in response to estuarine mixing. Journal of Geophysical Research-Oceans 2003;108(C11):3353. R827635 (2001)
R827635 (Final)
not available

Supplemental 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 aspects

Relevant 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 Abstract
  • 2000 Progress Report
  • Final Report