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Final Progress Reports: Dartmouth College: Methylmercury Production and Fate in Response to Multiple Environmental Factors

Superfund Research Program

Methylmercury Production and Fate in Response to Multiple Environmental Factors

Project Leader: Celia Y. Chen
Grant Number: P42ES007373
Funding Period: 2000-2021

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Final Progress Reports

Year:   2020  2013  2007  2004 

The goal of this project is to understand how toxic metals are transferred from the environment through aquatic food webs to fish resulting in human exposures.  In addition, this project seeks to link population level and genomic responses to metal exposure in the model organism, Daphnia pulex.  In the last year of this study, Drs. Folt and Chen, with their laboratory scientists, have focused on conducting a field sampling program in Lake Champlain, analyzing our existing field data sets, conducting laboratory demographic experiments using Daphnia pulex clones collected from metal contaminated and reference lakes in the Sudbury and Dorset regions of Ontario Canada, and preparing and submitting manuscripts.

In a collaboration with scientists at the US Geological Survey, VT DEC, and St. Lawrence University, scientists have been investigating the relationship of productivity to Hg and MeHg bioaccumulation in zooplankton and fish within a single lake system between basins of contrasting trophic status (oligotrophic vs. eutrophic) in Lake Champlain.  Field work was conducted in the summers of 2005-7 in two sites.  The analysis of zooplankton samples for MeHg required high resolution, low detection level methodologies designed particularly for these studies by the Trace Element Analysis Core facility of the Dartmouth SBRP program.  Results to date show that similar to patterns observed across multiple lakes in the region, the oligotrophic site in L. Champlain has consistently lower MeHg concentrations in zooplankton.  These data are being incorporated with water, fish, and sediment Hg data into an Hg fate model developed by collaborators at St. Lawrence University.

Genomic biomarker development has been conducted collaboratively with Arsenic as an Endocrine Disruptor and Indiana University to identify patterns of gene expression in D. pulex exposed to metals (i.e., As, Cd, Zn). Scientists are using microarrays to link genomic response to demographic responses to metal exposure in natural populations of D. pulex. These include over 200 isolates from seven lakes in the historically polluted Sudbury region of northern Ontario and seven reference lakes in the nearby Dorset area that have received little industrial metal contamination. These isolates, which are now established in the laboratory, have been genotyped and represent over 100 unique clonal populations. Scientists characterized the population-level differences between these two groupings (e.g., metals contaminated lakes vs. reference lakes) when exposed to a suite of metals that will include cadmium, arsenic, copper and nickel. These life-table studies revealed that Sudbury populations have adapted to metal stress, but that these adaptations are metal dependent. For example, greater reproductive success was observed in D. pulex collected from metal contaminated lakes in the presence of Cd compared to daphniids collected from the reference lakes indicating adaptation to Cd; however, in similar tests with arsenic differences between Sudbury area and Dorset area Daphnia were not apparent. Scientists are now investigating these identified phenotypes in relation to microsatellite markers for neutral loci, as well as those linked to metal responsive genes, such as metallothionein. Studies are now underway to investigate variability in adaptive response at both the genomic and population-level within and between lakes for these two regions.

Finally, the lab has been analyzing past field datasets to investigate the temporal pattern of MeHg bioaccumulation in zooplankton and in fish, and the importance of benthic vs. pelagic food webs in the transfer of MeHg and Hg to fish. In the temporal study, seven lakes were studied over the growing season to determine if there were significant differences in MeHg and THg through time and between different size fractions of zooplankton (45-100, 100-202, and >202 µm).  As hypothesized, scientists find that MeHg increases with increasing size of zooplankton whereas THg decreases with increasing size. This confirms that even within the zooplankton food web, MeHg is the Hg species that is biomagnified. Seasonal patterns show that THg and MeHg in zooplankton exhibit similar temporal patterns with the highest concentrations in spring and summer but declining in fall. These findings reinforce the value of the high resolution, high through-put techniques developed with the TEA Core to measure HgI and MeHg in low biomass samples. In another field study, scientists sought to understand the importance of specific prey taxa from littoral (including benthic and shoreline habitats) and pelagic habitats as conduits of metals to fish, they compared Hg, As, Cd, Zn and Se in common littoral invertebrates and pelagic zooplankton prey in five lakes across a range of trophic conditions and over a season in one lake. The lab found that zooplankton consistently had higher concentrations of As, Se, Zn, and Hg than littoral invertebrates. These results support other results they have, indicating that Daphnia are conduit species for Hg trophic transfer.  

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