Mercury net methylation
in five tropical flood plain regions of
The Science of the Total Environment,
Volume 261, Issue 1-3,
Guimarães, J R; Meili, M; Hylander, L D; de Castro e Silva, E; Roulet,
M; Mauro, J B; de Lemos, R
Summary of
Points:
·
In
aquatic systems, bottom sediments have often been considered as the main methylmercury (MeHg) production
site. In tropical floodplain areas, however, floating meadows and flooded
forests extend over large areas and can be important Hg methylating
sites.
·
The
study presents a cross-system comparison of the Hg net methylation
capacity in surface sediments, flooded soils and roots of floating aquatic macrophytes, assayed by in situ
incubation with 203Hg and extraction of formed Me203 Hg by acid leaching and
toluene.
·
The
presence of mono-MeHg was confirmed by thin layer
chromatography and other techniques. Study areas included floodplain lakes in
the Amazon basin (Tapajós, Negro and
Amazon Rivers, the Pantanal floodplain (Paraguay
river basin), freshwater coastal lagoons in Rio de Janeiro and oxbow lakes in
the Mogi-Guaçú river, São
Paulo state. Different Hg levels were
added in assays performed in 1994-1998, but great care was taken to standardise all other test parameters, to allow data
comparisons.
·
Net MeHg production was one order of magnitude higher (mean
13.8%, range 0.28-35) in the living or decomposing roots of floating or rooted macrophyte mats (Eichhornia azurea, E. crassipes, Paspalum sp., Eleocharis sellowiana, Salvinia sp., S. rotundifolia and Scirpus cubensis) than in the surface layer of underlying lake
sediments (mean 0.6%, range 0.022-2.5).
·
Methylation
in flooded soils presented a wide range and was in some cases similar to the
one found in macrophyte roots but usually much lower.
·
In a Tapajós floodplain lake, natural concentrations of MeHg in soil and sediment cores taken along a lake-forest
transect agreed well with data on net methylation
potentials in the same samples. E. azurea, E. crassipes and Salvinia presented
the highest methylation potentials, up to 113 times
higher than in sediments.
·
Methylation
in E. azurea from six lakes of the
·
Floating
meadows are sites of intense production of biomass and of highly bioavailable MeHg and appear to
be an essential link of the MeHg cycle in tropical
aquatic systems. [Journal Article; In English;