2 research outputs found
Eight Boreal Wetlands as Sources and Sinks for Methyl Mercury in Relation to Soil Acidity, C/N Ratio, and Small-Scale Flooding
Four years of catchment export and wetland input–output
mass balances are reported for inorganic Hg (Hg<sub>inorg</sub>),
methyl mercury (MeHg), dissolved organic carbon (DOC), and sulfate
in eight Swedish boreal wetlands. All wetlands had a history of artificial
drainage and seven were subjected to small-scale flooding during the
complete study period (two sites) or the two last years (five sites).
We used an approach in which specific runoff data determined at hydrological
stations situated at a distance from the studied sites were used in
the calculation of water and element budgets. All wetlands except
one were significant sinks for Hg<sub>inorg</sub>. Seven wetlands
were consistent sources of MeHg and one (an <i>Alnus glutinosa</i> swamp) was a significant sink. The pattern of MeHg yields was in
good agreement with previously determined methylation and demethylation
rates in the wetland soils of this study, with a maximum MeHg yield
obtained in wetlands with an intermediate soil acidity (pH ∼5.0)
and C/N ratio (∼20). We hypothesize that an increased nutrient
status from poor to intermediate conditions promotes methylation over
demethylation, whereas a further increase in nutrient status and trophy
to meso- and eutrophic conditions promotes demethylation over methylation.
Small-scale flooding showed no or moderate changes in MeHg yield,
maintaining differences among wetlands related to nutrient status
Net Degradation of Methyl Mercury in Alder Swamps
Wetlands are generally considered to be sources of methyl
mercury
(MeHg) in northern temperate landscapes. However, a recent input-output
mass balance study during 2007–2010 revealed a black alder
(<i>Alnus glutinosa</i>) swamp in southern Sweden to be
a consistent and significant MeHg sink, with a 30–60% loss
of MeHg. The soil pool of MeHg varied substantially between years,
but it always decreased with distance from the stream inlet to the
swamp. The soil MeHg pool was significantly lower in the downstream
as compared to the upstream half of the swamp (0.66 and 1.34 ng MeHg
g<sup>–1</sup> SOC<sup>–1</sup> annual average<sup>–1</sup>, respectively, one-way ANOVA, <i>p</i> = 0.0006). In 2008
a significant decrease of %MeHg in soil was paralleled by a significant
increase in potential demethylation rate constant (<i>k</i><sub>d</sub>, <i>p</i> < 0.02 and <i>p</i> < 0.004, respectively). In contrast, the potential methylation
rate constant (<i>k</i><sub>m</sub>) was unrelated to distance
(<i>p</i> = 0.3). Our results suggest that MeHg was net
degraded in the <i>Alnus</i> swamp, and that it had a rapid
and dynamic internal turnover of MeHg. Snapshot stream input-output
measurements at eight additional <i>Alnus glutinosa</i> swamps
in southern Sweden indicate that <i>Alnus</i> swamps in
general are sinks for MeHg. Our findings have implications for forestry
practices and landscape planning, and suggest that restored or preserved <i>Alnus</i> swamps may be used to mitigate MeHg produced in northern
temperate landscapes