Variability In MeHg Concentrations Research Articles

A Model for Methylmercury Uptake and Trophic Transfer by Marine Plankton.

Methylmercury (MeHg) concentrations can increase by 100 000 times between seawater and marine phytoplankton, but levels vary across sites. To better understand how ecosystem properties affect variability in planktonic MeHg concentrations, we develop a model for MeHg uptake and trophic transfer at the base of marine food webs. The model successfully reproduces measured concentrations in phytoplankton and zooplankton across diverse sites from the Northwest Atlantic Ocean. Highest MeHg concentrations in phytoplankton are simulated under low dissolved organic carbon (DOC) concentrations and ultraoligotrophic conditions typical of open ocean regions. This occurs because large organic complexes bound to MeHg inhibit cellular uptake and cell surface area to volume ratios are greatest under low productivity conditions. Modeled bioaccumulation factors for phytoplankton (102.4-105.9) are more variable than those for zooplankton (104.6-106.2) across ranges in DOC (40-500 μM) and productivities (ultraoligotrophic to hypereutrophic) typically found in marine ecosystems. Zooplankton growth dilutes their MeHg body burden, but they also consume greater quantities of MeHg enriched prey at larger sizes. These competing processes lead to lower variability in MeHg concentrations in zooplankton compared to phytoplankton. Even under hypereutrophic conditions, modeled growth dilution in marine zooplankton is insufficient to lower their MeHg concentrations, contrasting findings from freshwater ecosystems.

Mercury proxies and mercury dynamics in a forested watershed of the US Northeast.

Although many studies focus on mercury (Hg) and methylmercury (MeHg) dynamics in streams, challenges remain in identifying the relative importance of land cover and seasonality at regulating Hg and MeHg dynamics at the watershed scale. Developing robust proxies for Hg and/or MeHg determination also remains a challenge. Our study used Hg, MeHg, and dissolved organic carbon (DOC) concentration measurements and various DOC fluorescence indices to characterize Hg and DOC dynamics in a forested watershed of the US Northeast. Principal component analysis indicated that land cover/landscape position (i.e., headwater vs. wetland-influenced area vs. lake-influenced area) explained 44% of the variance in Hg, MeHg, DOC concentrations, and DOC quality during the snow-free season, while seasonality (i.e., air temperature and discharge) explained only 21% of the variance in the results. Furthermore, finding a good proxy for Hg that is valid across a range of landscape positions remains a challenge; however, regression analysis indicated that the fluorescence peak Humic C (excitation = 350nm; emission = max (420-480)), which corresponds to the presence of melanoidins in water, explained 21% of the variability in MeHg concentrations across both space and time (p = 0.001), and thus appears to be a possible proxy for MeHg determination in our study watershed. From a management perspective, land cover modifications (lake, reservoir, and wetland) are likely to play more important roles at regulating Hg, MeHg, and DOC exports at the watershed scale than long-term changes in the climate of this region.

Methyl mercury production and loss in Arctic soil

Mercury has been found in polar bears and other top predators in the Arctic at concentrations that pose a risk to the indigenous population, however, the means by which this occurs is uncertain. There has been extensive research on the atmospheric cycling of mercury but little is known about mercury cycling in Arctic terrestrial ecosystems. The objective of this study was to determine whether wet sedge meadow soils within the Truelove Lowlands, Devon Island, NT, Canada (75° 33′N, 84° 40′N) were acting as sources or sinks for methylmercury (MeHg). Over the course of an Arctic summer, MeHg concentrations and other biophysical characteristics were measured at four wet sedge meadows over a 19 day study period that commenced approximately 1 month after snowmelt. Soil MeHg concentrations declined during the study period, indicating a net loss of MeHg over the summer. The dominant ligand in solution appeared to be dissolved organic matter, little sulfide was detected, and it would seem that most of the mercury was unavailable for methylation during the summer sampling period. In soil microcosms, spiked with 5.0 nmol g − 1 (1 µg g − 1 ) HgCl 2, the soil did methylate mercury suggesting that there is the potential for mercury methylation. We also noted significant spatial variability in MeHg concentrations between catenas that could not be explained by other biophysical parameters, which are known to affect methylation. Given our data and previous geochemical data collected from suprapermafrost groundwater during snowmelt, it seems likely that methylation may occur during the spring melt period in the arctic. Furthermore the geochemical variability of the melt water may lead to the spatial variability observed in MeHg concentrations in this study.