METALS - Marine Pollution: What Everyone Needs to Know - page 99

their otherwise low availability. By oxidizing the soil right by
the roots, plants can alter the distribution of metals in wet-
land sediments. Concentrations of metals tend to be higher in
vegetated soils than in unvegetated ones, and are particularly
high in soils near plant roots. Molecular tools are being used
to better understand the mechanisms of metal uptake, trans-
location, sequestration, and tolerance in plants.
Salt marsh plants can absorb available metals from the sed-
iments and store them largely in their roots. When wetland
plants move metals up from root tissue they accumulate in
leaves and stems. The degree of upward movement is depen-
dent on the species of plant, the particular metal and various
environmental conditions. Cordgrass transports significant
levels of metals to aboveground tissues, and so plays a role in
the transfer of metals through estuarine food webs. The met-
als that are moved up into stems and leaves become available
to the marsh ecosystem if they are excreted from the leaves,
which is what cordgrass does. Cordgrass has salt glands on
the underside of its leaves for excreting salt; metals can be
excreted from them as well. The release of metals from leaf tis-
sues is a way for the plant to reduce its tissue levels of metals,
but this increases the availability of metals in the ecosystem,
with potential uptake into estuarine food webs. Metals not
excreted from leaves will be in the leaves and stem when the
plant dies, falls to the marsh surface and decays. The detritus
produced as a result of this decay, will have elevated metals
that will then be available to animals such as mollusks and
small crustaceans that eat the detritus. In contrast to cordgrass,
the invasive common reed Phragmites australis is widely used
in constructed wetlands for treatment of wastewaters contain-
ing metals. P. australis concentrates more of its metals below
ground in root and rhizome tissue, moving smaller amounts
to aboveground tissues.
It has been shown that concentrations of metals such as iron,
nickel, and chrome are 10 to 100 times higher than normal in
mangroves downstream from mining sites and that mangrove