Recently Completed Projects
Influence of Nitrogen on Methylmercury Accumulation in Shellfish and Planktivorous Fish Along the Long Island Sound Shoreline of Connecticut
This collaboration with Zosia Baumann was funded by Connecticut Sea Grant (NOAA) and began in early 2016 (2/2016- 2018). We began sampling locations along the CT coastline in spring 2016 and there were further collections in 2017. Samples are collected for sediment, water and biota. The hypotheses that were tested with the project are: 1) methylmercury (MeHg) concentrations in sediments and water will be lowest at sites with the highest levels of total nitrogen (N); and 2) High total N concentrations will be associated with an overall decrease in MeHg concentrations in phytoplankton cells, bivalves, and fish. These two hypotheses reflected our overall initial assessment of the complex interactions between nutrient and carbon levels and MeHg production and bioaccumulation. The overall objectives were to examine links between nutrients, including dissolved and particulate species of N and phosphorus (P) and the net Hg methylation rate and the influence of N on Hg and MeHg uptake and consequently bioaccumulation in marine phytoplankton, bivalves and fish. We expected that while more nutrients could stimulate more methylation in sediments, overall the various interactions, such as the effects of nutrients and carbon on MeHg bioavailability and organism growth, will lead to less bioaccumulation at the more eutrophic locations. Gunnar Hansen is the PhD student working on the project and Wes Huffman collected zooplankton as a side project to examine concentrations in these organisms in more detail. One interesting aspect of the study was a collaboration with Robert Cerrato at Stony Brook in aging the clams collected.
We found that in contrast to fish, MeHg in clams does not increase with age – see figure. The results of the study have been presented at various conferences and papers from this work are being prepared for publication. Results for the concentrations of MeHg in silversides collected in various coastal embayments (Newark, Stratford, Jordan Cove and Mumford Cove), including various sites in New Haven Harbor, are shown in the attached figure. Concentrations do not correlate strongly with sediment Hg concentrations, suggesting the role of other factors in influencing MeHg bioaccumulation.
Atmospheric Measurements and Air-Sea Exchange on the US Arctic Geotraces Cruise
A paper published in 2018 (DiMento et al., 2018) highlighted many of the results from our involvement in the Arctic GEOTRACES cruise, a collaborative project with colleagues at University of Tennessee Space Institute (Steve Brooks) and Chris Moore, funded by the NSF Chemical Oceanography program. Additionally, a Scientia article highlighted some of the activities related to the air-sea exchange of mercury in the Arctic and in other locations, focusing on NSF funded studies and those of Swedish collaborators. The Arctic is being substantially altered by climate change and this will have a significant impact on the biogeochemistry of Hg in the region and its bioaccumulation as methylmercury (MeHg) into Arctic marine seafood and marine mammals. The major activity of the NSF grant was the participation in the Artic GEOTRACES research expedition on the research vessel Healy, which occurred from August to November, 2015. The cruise proceeded from Dutch Harbor to the North Pole and back. The UConn research group was responsible the underway sampling equipment for measurement of dissolved gaseous elemental Hg (Hg(0)). The equipment was maintained by Strve Brooks on board during the cruise, and others helped in collection of samples of ice cores, snow and mlt ponds samples that were shipped and analyzed at UConn. The UConn group was responsible for the analysis of atmospheric mercury samples (wet deposition, gas speciation and aerosols) collected on board. Overall, the impact of ice and other factors on the magnitude of the air-sea exchange fluxes is hotly debated but poorly characterized. The underway dissolved gaseous mercury analyzer collected high resolution samples of Hg(0) in conjunction with high resolution air collections (see figure for the water data) that was used to estimate gas-exchange, and examine factors influencing Hg0 concentration and distribution. Overall, while much higher concentrations of Hg(0) were found under the ice, with low values in the ice-free zones, the atmospheric concentration was very consistent throughout the cruise. The differences are much more pronounced than expected based on other studies in the Arctic and elsewhere. These activities and related analyses will allow for a clearer understanding of the magnitude of atmospheric deposition to the Arctic Ocean, and gas evasion to the atmosphere. The results formed part of Brian DiMento’s PhD thesis – he graduated in May 2017.
Methylmercury Interactions with Marine Plankton
This project, which finished in 2/2016, was funded through NSF Chemical Oceanography, and was collaboration with the research groups of Nick Fisher at Stony Brook, who focused on examining the impacts of water chemistry and other factors on phytoplankton uptake and trophic transfer, and Elsie Sunderland at Harvard, whose group focused on the modeling. Kati Gosnell, the student who worked on the project, graduated in spring 2016. The overarching objective of the field, lab and modeling studies was to provide insight into the mechanism of entry of methylmercury (MeHg) into marine food webs and how ocean properties
influence its production, uptake and subsequent bioaccumulation. The research focused around three main objectives: 1) Characterize the variability in uptake of MeHg and Hg by diverse marine phytoplankton and trophic transfer to zooplankton as a function of biological attributes and relevant environmental and physical variables; 2) Examine the relative importance of decomposing phytoplankton and fecal pellets as a source of MeHg to marine waters compared to production during carbon degradation by bacteria; and 3) Refine the global oceanic budget for MeHg including the role of primary and secondary producers in MeHg production, transport and accumulation based on lab and field data. The research at UConn mostly focused on the field component, but also measured the variability in MeHg uptake into marine phytoplankton and its trop-hic transfer to zooplankton under controlled lab conditions. Studies in the field looked at the impact of biological differences (cell walls, size) and environmental factors and composition of DOC and specific organic ligands. The studies analyzed correlations between Hg and MeHg measured in size fractionated plankton and over different vertical depths from various ocean cruises and coastal/shelf sites and compared these to ancillary data on water chemistry. Two papers have been produced to date: Gosnell and Mason (2015) reported on samples collected in the tropical Pacific Ocean; Gosnell et al. (2016) was focused on studies within Long Island Sound a
nd adjacent waters
(see attached figures). Both phytoplankton and zooplankton size classes were analyzed for their Hg and MeHg concentrations. In addition, in the LIS study, stable isotopes analyses of C, N and S in plankton were used to further assess sources of MeHg to plankton and the differences in sources for different parts of LIS and offshore waters. Data was also incorporated into Schartup et al. (2015), which focused on the sources of MeHg in Lake Melville.
Overall, the study assisted in identifying the vector for MeHg entry and accumulation in marine food-webs, which is a critical gap in our present knowledge. As shown in the attached figures, the concentrations of MeHg in zooplankton were comparable for the open ocean and coastal waters even though the water MeHg concentrations were higher in the coastal waters. These differences were even more enhanced for the phytoplankton.
Photochemical Redox Chemistry of Mercury and Selenium and the Degradation and Transformations of Their Methylated Compounds
This work was initially funded by a NSF grant but even though the grant has been completed the work continued as the results provided new insights into the air-sea exchange of mercury (Hg) and selenium (Se). The work was completed by Brian DiMento, a PhD student who graduated in May 2017, with model development of a global Se model being done by Rob, in collaboration with Harvard researchers. The results of the photochemical studies are applicable to other studies, such as GEOTRACES, and to the study of the global and ocean cycling of Se. Studies examining the photochemical oxidation of selenite and the reduction of selenite showed that these reactions are very slow under environmental conditions. However, in contrast, methylated Se compounds are rapidly degraded. Results from one experiment examining the decomposition of dimethylselenium in coastal shelf break (SB) seawater is shown in the attached figured. The product of the decomposition, selenite (Se(IV)), is monitored in these experiments and shows the rapid degradation of the methylated compound in the presence of light. Various experiments suggest that the rate constant for degradation is about 45 per day (up to 10-3 per sec). There was some indication that nitrate enhanced the reaction rate. In the dark, the compound is stable under the time constraints of the experiment. The photochemical results, data from the Metzyme cruise and the selenium global model were published in 2018 in Global Biogeochemcial Cycles (Mason et al.).
The rate of degradation is much faster for methylated Se than that of MeHg under similar conditions. Again, MeHg is stable in the dark. In comparable experiments, with western Long Island Sound (WLIS) and SB water, shown in the figure below, the rate of degradation is about an order of magnitude slower. There was no significant difference in the rate for the different waters. This has generally been found to be so for a variety of coastal waters. Differences in the DOC content or the salinity do not appear to have a large impact on the photochemical demethylation rate.
Interestingly, while the concentrations of MeHg are much higher in coastal waters and marshes, and the degradation rates at the surface are similar, because of differences in light attentuation, the water column integrated degradation rate is higher for open ocean waters than for coastal waters, and this is also enhanced because of the much higher UV penetration in the open ocean. the results of these studies are compiled in a paper in Marine Chemistry (DiMento et al.).
Climate Impact on Coastal Ecosystem Methylmercury: Human Exposure Implications
This project, which finished in summer 2016, was funded through the NSF/NIH Oceans and Human Health Initiative and was a collaboration between Rob Mason and Evan Ward at UConn and Celia Chen and Brian Jackson at Dartmouth College. The goals of the proposed research were to use experimental approaches, field studies, and modeling to investigate the interactive effects of changes in temperature and nutrient loading/carbon content associated with climate on the fate of MeHg in marine ecosystems. The research focused on MeHg production, input to the water column, and bioaccumulation in coastal food webs, and its potential impact on human exposure to MeHg. The specific aims of the project were to: 1) Determine the effects of temperature, nutrient and carbon loading on the net MeHg formation in sediments and the associated net inputs of MeHg to the water column in coastal ecosystems; 2) Determine th
e interactive effects of temperature and nutrient supply/ecosystem eutrophication on bioaccumulation of MeHg by primary producers, and transfer between primary and secondary consumers in estuarine food webs; and 3) Predict how climate-induced changes in MeHg bioaccumulation in coastal food webs will increase exposure and risk to sensitive populations. The attached conceptual model was used to focus the studies completed under this project. The Dartmouth collaborators mostly focused on the bioaccumulation and biotic studies while the UConn group focused on understanding the geochemical
interactions in sediments and the water column and the exchanged between the different reservoirs. The field and laboratory studies (various micro and mesocosm experiments) tested our hypotheses on how temperature and carbon/nutrients impacted net methylation in sediments, its transfer to the water column, and bioaccumulation in the estuarine food webs. These approaches yielded a matrix of data that is being used to parameterize a Hg ecosystem and exposure model. To date, a number of publications have resulted from this work, which have highlighted the major factors influencing bioaccumulation in estuarine ecosystems: Balcom et al. (2015) examined correlations between different fractions and differences across ecosystems as shown, for example in the attached figure. The correlations for MeHg differ for different ecosystems. The figure illustrates that while in some estuaries, such as the Hudson and Delaware Rivers, there is a reasonable correlation between suspended particulate and surface sediment MeHg, this is often not the case, as found in Long Island Sound, Portsmouth Harbor and other locations. These results are consistent with what was reported in Chen et al. (2014) where MeHg concentrations in forage fish did not correlate with sediment MeHg levels but did correlated with suspended particulate MeHg, indicating the importance of the base of the pelagic food chain in transferring MeHg even to forage fish that feed at the sediment interface.
Examining the Biogeochemical Cycling of Mercury and Methylmercury in Lake Melville, Labrador, Canada
This project was completed in collaboration with Elsie Sunderland from Harvard University, at the request of the Nunatsiavut government. Harvard did most of the sample analysis and modeling and Amina Schartup, a former student, was the post-doc overseeing the project and doing most of the work . UConn personnel were involved mainly in the collection of water, sediment and biota samples on Lake Melville, a large saline water body in Labrador, Canada, and in the analysis of plankton samples. While called a “lake” it is actually connected to the ocean via a convoluted passage (see map). Much of the sample analysis was done at UConn. Results from this study were reported in Schartup et al. (2015) (see attached figure which presents a mass balance for the ecosystems for methylmercury (MeHg)). Overall, the study examined the importance of water column methylation versus other sources of MeHg to the ecosystem, and confirmed its importance in this ecosystem. Furthermore, the study illustrated the potential impact of building a reservoir on the Columbia River, which has been proposed, on the future levels of MeHg in the ecosystem, which would likely increase as a result of the reservoir construction.
Figure: Mass balance for total mercury (red) and methylmercury (black) for lake Melville illustrating the importance of water column net methylation as a source relative to other inputs. Taken from Schartup et al. (2015); and used with permission.
Mercury Cycling in the Delaware River
This two year study examined the distribution and fate of mercury (Hg) and methylmercury (MeHg) in the Delaware River estuary through a project funded by the Delaware Department of Natural Resources and Environmental Control and the Delaware River Commission. The Mason group made seasonal field trips to the estuarine section of the river and examined both water and sediments in late fall, spring and summer. The main aims of the project were to: 1) characterize the concentrations and distributions of Hg and MeHg in the water column, at the surface and at depth, in the turbidity maximum, and sub-tidal region of the Delaware estuary, and in the associated sediments; 2) examine the rates of interconversion of Hg and MeHg in the upper sediments (top 12 cm) at three locations and relate this to sediment characteristics; and 3) analyze surface water samples collected over a year at a subset of stations along a longer reach of the river. The information gained was compared to data from other east coast US estuaries and the importance of sources of Hg and MeHg to the river and the aquatic food chain were examined. The Hg and MeHg information was used to derive an initial conceptual model of the factors important in Hg and MeHg cycling, fate and bioaccumulation within this region of the Delaware, and lead to the publication of Gosnell et al. (2015). One outcome of the project was a comparison of the relative importance of sediment MeHg inputs versus external sources. The paper concluded that there were times when sediment MeHg sources could be important, as summarized in the attached table.
Table: Estimated fluxes of methylmercury to the water column at different times of the year, and overall. Data from Gosnell et al. (2015).
The Role of Marine Aggregates in Enhancing the Uptake of Nanomaterials and Mercury/Methylmercury by Bivalves
This work which has been completed over a number of years has been funded through Sea Grant and was done in collaboration with Evan Ward’s group, mostly by his students on the uptake mechanisms of various potential contaminants into bivalves, and the uptake of nanomaterials into bivalves. Two Sea Grant/NOAA projects were associated with this initiative: Emerging
contaminants in Long Island Sound: Effects of nanoparticles on suspension feeding bivalve molluscs (02/2012-02/2014) and Uptake and accumulation of nanoparticles of bivalves: implications of shellfish safety (02/2010-02/2013). Manufactured nanomaterials are used in a wide variety of ways and are released into the environment where they are also potentially toxic causing mitochondrial and DNA damage, and cell death. The study focus was how feeding of bivalve species affect their uptake of nanoparticles, and how formation of marine flocs enhances assimilation. Two papers were published as a result of this work: Doyle et al. (2015a) and (2015b). Link
Additionally, Roni Ortiz, a MS student who graduated in 2013, performed experiments examining the incorporation of mercury (Hg) and methylmercury (MeHg) into aggregates (“marine snow”) and how this accumulation may enhance the uptake of the Hg species into bivalves and their fate after ingestion. She completed an experiment using stable isotopes to track the incorporation, uptake and tissue redistribution of the mercury and methylmercury after feeding. One additional aspect of this study was that because the stable isotope spikes were used it was possible to examine the net transformation of inorganic Hg into MeHg during the experiments, and the converse, demethylation of MeHg. These was followed in the different size fractions and it was found that net methylation was enhanced in the larger particulate (aggregates), suggesting the formation of microzones where anaerobic bacteria, the principal methylators, were active due to oxygen depletion within the aggregates. The results were published in 2015 (Ortiz et al. 2015 ) and a figure showing the methylation and demethylation in the different size fractions is attached.
Examination of the Uptake of Mercury and Methylmercury using Bioreporters
This work was accomplished by Udonna Ndu while a PhD student within the department and also during his post-doctoral research at Rutgers University. The work used various “bioreporters” – bacteria that have been genetically modified so that the produce light in response to the uptake of mercury (Hg) and methylmercury (MeHg) compounds – and also through the examination of the production of elemental Hg. The focus was on the effects of large molecular weight or
ganic complexes as well as smaller thiols such as cysteine and glutathione. Results from these studies were published in a number of papers: a 2016 paper titled “The effect of aqueous speciation and cellular ligand binding on the biotransformation and bioavailability of methylmercury in mercury resistant bacteria” focused on Hg resistant bacteria while an earlier paper (The use of a mercury biosensor to evaluate the bioavailability of mercury-thiol complexes and mechanisms of mercury uptake by bacteria) focused on various aspects of the bioavailability of Hg and MeHg to E. coli and Bacillus strains. Finally, an earlier paper (set the groundwork for these more recent studies
Gas Exchange and Global Mercury Cycling
A long-term interaction with Elsie Sunderland at Harvard and her prior students/post-docs (Anne Soerensen, now at Stockholm University More) led to a number of studies related to air-sea exchange of elemental mercury and its role in the global Hg cycle. A paper by Amos et al. (2015) compared and contrasted model predictions to examine the impacts of legacy mercury, in particular early mining inputs prior to the industrial revolution. The ocean-atmosphere model (Soerensen et al., 2010) built on earlier models and papers, provided the basis for the examination of the factors controlling the flux of elemental Hg to the atmosphere. One of these studies was focused on the potential role of high concentrations of Hg from legacy
anthropogenic inputs on the flux (Soerensen et al., 2012).
A paper discussing the results from a cruise in the equatorial Pacific (Soerensen et al., 2014) highlighted the importance of atmospheric deposition in the high rainfall Inter-Tropical Convergence Zone in driving enhanced evasion in this region, and compared the results from this study with others in the Atlantic that found similar results. Modeling confirmed the importance of atmospheric deposition in driving the enhanced flux. This paper also provided a contrast to the studies completed in the North Atlantic where concentrations of elemental Hg, while overall higher, were much more consistent in concentration in the waters around Bermuda studied over multiple years (Soerensen et al., 2013).
Samples were also collected on a GEOTRACES cruise in the tropical South Pacific Ocean in 2014 in collaboration with Carl Lamborg and Chad Hammerschmidt’s research groups and others. The cruise was from
Ecuador to Tahiti, and underway samples were collected for dissolved gaseous Hg and atmospheric Hg, along with detailed water column sampling for mercury speciation (Bowman et al., paper in review). Estimated gas exchange rates are shown in the figure, which is part of a paper currently in preparation.
Studies were completed at a number of locations examining the wet deposition flux of Hg, and the importance of dry deposition of aerosols and of reactive ionic Hg (RGHg). Studies were completed in South Africa, and these were coordinated by the CSIR and the Weather Service, and an outgrowth of research completed in South Africa examining various aspects of mercury inputs and cycling. A paper examining deposition at Cape Point and in Pretoria was published in 2013. Another study was also completed on Bermuda which included measurements of atmospheric Hg and on aerosols (Gichuki et al., 2014). This study allowed for the examination of the importance of deposition in contributing Hg to this region of the ocean, and linked with the results of the Soerensen et al. (2013) study mentioned above.
Gulf of Mexico
A project completed with collaborators while they were at Harvard (David Senn, Bian Liu and others) examined the impact of hurricanes in the Gulf of Mexico and the role of bottom oxygen water depletion on the biogeochemistry of Hg and MeHg. Two papers were published from this work: a 2009 paper focused on the impact of the hurricanes while the 2016 paper was more focused on the potential impact of low oxygen conditions on Hg methylation. Methylation rates in the gulf of mexico were higher than previously reported for more temperate locations examined within the US and elsewhere.
A NSF-funded study focused on the distribution and formation of MeHg in the sediments of the Chesapeake Bay and the adjacent shelf and slope, examining the factors controlling the net production of MeHg in sediments and the flux of MeHg from sediments to the overlying waters. The project was a collaboration with Cindy Gilmour from the Smithsonian Environmental Research Center. more Two papers were published from this study: Hollweg et al. (2009) and (2010) – see publication list for details. The studies showed that the highest methylation rates were not in regions of the highest Hg concentration and provided further insights into the role of organic matter and sulfide in modulating both the rate of methylation and also the flux of MeHg from sediments. The examination of sediment fluxes took into account the role of speciation in modifying the overall flux.