Environmental Engineering Reference
In-Depth Information
usually found in human-made environments. Relat-
ively natural terrestrialization mires may be found in
eastern Europe and Scandinavia. Most terrestrializ-
ing mires are relatively small. Their size ranges from
less than 1 ha to hundreds of hectares, such as the
Norfolk Broads in England and the Weerribben in the
Netherlands (van Wirdum
et al.
1992). Sasser and
Gosselink (1984) described a very large, floating
freshwater system of
c
.3000 ha in the Mississippi
River delta plain in Louisiana, USA. In Europe the best-
developed terrestrialization mires occur in extensive
peat-cut areas, where regeneration of the fen vegeta-
tion occurs after the original peat layers have been
removed. Terrestrialization mires are ecologically
very diverse. Eutrophic plant communities, such as
reeds, form floating rafts on which in later stages
mesotrophic or even oligotrophic plant communit-
ies can develop. When the floating rafts are large
enough and the peat layer is sufficiently thick, rain-
water lenses are formed which sustain nutrient-poor
acid bog vegetation (van Wirdum 1995). Later in the
succession the terrestrialization mires tend to turn into
willow and alder shrub. In order to keep such mires
open they are mown regularly. This can be done in
summer with small mowing machines or in winter over
the ice.
ticular mosses, stimulate this process by using the high
concentration of CO
2
in the discharging groundwater
as a carbon source. The result is that CaCO
3
precipit-
ates on the leaves and when these leaves die off and
decompose small amounts of CaCO
3
are added to the
soil (van Breemen & Buurman 2002). Under favour-
able climatic conditions peat forming may occur.
Usually peat-forming processes in spring mires do
not last very long. Small changes in the landscape
or in climatological conditions trigger changes in
the groundwater discharge and peat layers start to
decompose. Strongly decomposed peat has a high
resistance to water flow, thus blocking water trans-
port in the spring cupola itself. The discharging
groundwater then will force its way out somewhere
else in the spring system, creating new opportunities
for peat forming or travertine building. A spring
mire, therefore, is a rather dynamic mire system that
can shift rapidly from an accumulating to an erosive
system where sediments are washed away or decom-
posed (Wolejko
et al.
1994). The remaining spring mires
in Europe are very small (often less than 1 ha). Some
of the best-preserved systems can be found at the base
of the Alps and the Slovakian Tatra Mountains.
9.2.5 Floodplain mires
9.2.4 Spring mires
Floodplain mires (Plate 9.1d) are also hydrologically
very dynamic systems. In winter and spring intensive
flooding may occur, which can deposit large amounts
of sand, silt or clay. In summer water tables may
drop to over 1 m below the surface. Under such
conditions the availability of nutrients can be very
high (Loucks 1992, Olde Venterink
et al.
2003). The
productivity of floodplain vegetation consequently can
also be very high. On a geological timescale down-
stream rivers frequently change their course. As a result
flooding frequencies and sedimentation rates also
change in a wide range of mire ecosystems. Eutrophic
floodplains may change into mesotrophic fens and
existing fens or even bogs may become flooded with
surface water. Examples of still existing large flood-
plain mires in Europe are the Danube floodplain in
Romania, the Oder floodplains on the border of
Poland and Germany, the Narev floodplain in Poland
and the Rhone delta in France (see also Chapter 11
in this volume).
Spring mires are almost exclusively fed by discharg-
ing groundwater (Plates 9.1c and 9.2c). They occur
in landscapes with much relief under rather complex
geological and hydrological conditions. Usually they
are situated where large groundwater aquifers are
forced to discharge, due to geological phenomena that
have changed the original stratification of sediments.
At this geological breach a clay layer that has been
pushed upwards may then block a sandy aquifer. Spring
systems can be found in lowlands as elevated spots,
in the middle of a sloping landscape or even close to
a hilltop. In calcareous areas in particular, they can
accumulate large amounts of CaCO
3
(travertine) up to
a height of 30 m, but only under the condition of a
regular discharge of groundwater. When ground-
water, supersaturated for CaCO
3
, comes into contact
with the atmosphere, CO
2
escapes into the atmosphere
and CaCO
3
precipitates. Spring mire species, in par-
