Aquatic marginal wetlands, being adapted
to nutrient-rich environments, can
respond to eutrophication by increasing their absorption and use of the extra
nutrients. If nutrients are at an artificially high concentration, wetlands can remove
the excess and, as such, are valuable in improving water quality. Some nutrients will
be incorporated into plant tissue as enhanced growth, but this is normally a small
proportion of the total concentrations removed. More important for nitrogen
compounds is the nitrification- denitrification process, in which local oxygenation of
soil adjacent to plant roots facilitates conversion of excess nitrates into gaseous
components which are then lost to the atmosphere, whereas phosphates are
absorbed by microflora or retained in plant litter and sediments.
Reedbeds are particularly effective because
not only can they absorb dissolved
nutrients and enhance oxygen-dependent decomposition of organic matter, but the
vegetation structure acts both as a net, causing particles in suspension to settle into
the sediment, and as an attachment surface for microorganisms. They can, in this
way, efficiently strip water of its nutrient load and improve its quality.
Removal of nutrients by soils in aquatic
marginal wetlands can have a major
influence on water quality. It has been estimated that a 30 m wide strip of riparian
woodland along a tributary of the River Garonne in southwest France would be
enough to remove all of the nitrate entering the groundwater from surrounding
agricultural land.
The same process can work in reverse.
The River Rhine in eastern France
inundates its floodplain with heavily polluted water. As this water infiltrates the soil, it
is cleansed of excess nitrates and phosphates such that the water which enters the
groundwater is significantly less polluted than the river water. This process is
biologically controlled, in that the areas with the most developed wetland vegetation
are most efficient at purifying the water.
The ability of wetland vegetation to clean
water is being exploited commercially,
with beds of emergent reeds (Phragmites) or cattails (Typha) proving most
successful. Water treatment wetlands can be created and maintained with relative
ease; natural wetlands, too, can be effective, but will suffer reduction in species
diversity, as nutrient enrichment favours the most competitive species at the
expense of others. Excess nutrient inputs may eventually destroy macrophytes in
permanently inundated fringe wetlands, effectively destroying the wetland and
creating an open water environment. This process normally proceeds according to
a mechanism in which eutrophication of lakes leads eventually to domination by
phytoplankton, but can be caused by more subtle changes.
In the Norfolk Broads, in eastern England,
there is a clear relationship between
nitrate loading of water and the loss of stands of Phragmites australis. As nitrate
concentration increases, Phragmites grows more vigorously, increasing the
proportion of above-ground to below-ground growth. Most of the losses in the
Norfolk Broads have been from a growth form known as 'hover', which consists of
floating mats of Phragmites; these depend upon their root and rhizome mat for
mechanical strength and become unstable if emergent shoot growth is too high,
making them vulnerable to erosion.
