Environmental Engineering Reference
In-Depth Information
Scandinavia on sloping soils (Moen 1990, Dierssen
1996).
Changes in land use have most affected groundwater-
fed fen systems. In the lowlands in particular, dense
networks of canals and drainage systems were built
to quickly transport water out of the area in winter,
while in summer water was brought into the area from
large rivers. Such practices had a major effect on the
integrity of hydrological systems in the area and con-
sequently also on the nutrient cycling in the wetlands.
Ecohydrological research in the Biebrza catchment in
eastern Poland, for instance, showed that the hydro-
chemical and associated trophic gradients were very
smooth over large distances (Wassen
et al.
1990, 1996).
This spatial uniformity was related to the absence
of drainage works resulting in an almost stagnant
water body in the mire that kept it saturated with
mineral-rich groundwater. Compared to Poland the fen
systems in the Netherlands were much more varied
on the landscape level. The former large hydrological
gradients had been split up in numerous small ones
due to large-scale changes in the regional hydrology.
This resulted in significant hydrochemical differences
even within a single small fen (van Wirdum 1995,
Wassen
et al.
1996). Despite these large differences
in hydrological systems, the species composition and
the site conditions in the fens are very similar. The
groundwater table never dropped more than a few tens
of centimetres below the surface and the water in the
rooting zone was rich in base cations, especially Ca
2+
and Fe
2+
(Wassen
et al.
1996).
In most of north-west Europe natural fens are
almost non-existing. Extensive fens and fen meadow
areas in Europe still occur in the Biebrza Valley,
Poland, in the Baltic states and in Scandinavia. In
other parts the fen meadow reserves are numerous, but
small. Some examples of relatively well-preserved fen
meadow reserves are the Drentsche Aa in the Nether-
lands, the Zwarte Beek in Belgium and the Wümme
and Peene Valleys in Germany.
the landscape but, due to deep drainage of surround-
ing agricultural peat areas, they are now elevated
wetlands that have turned from exfiltration areas into
infiltration areas. Their hydrological function is
mainly to store water, which is pumped into the
wetlands from agricultural areas. Nowadays the later
successional vegetation stages (shrubs of
Betula
,
Alnus
and
Salix
) prevail in most areas. The initial stages
with mesotrophic vegetation have become rare. Those
fens that have not been overgrown by shrubs acidify
rather quickly (Kooijman & Bakker 1995, van Diggelen
et al.
1996). The mires growing in these wetlands are
also threatened by eutrophication, because they are
influenced by surface water from polluted rivers
during most of the summer (Barendregt
et al.
1995).
9.3.4 Spring mires
Most spring mires (Plate 9.2c) in western Europe have
disappeared due to a decreased inflow of ground-
water from surrounding infiltration areas as a result
of changes in land use. Intensive agricultural crop
growing on the former heathlands and forests have
decreased infiltration of rainwater considerably. In
many areas, the spring systems themselves have been
completely removed. From the Netherlands, for
instance, documentation exists that iron-rich spring
cupolas were exploited as a source of iron ore in the
19th century. In north-west Poland many spring
systems are still present and their infiltration areas have
not been hydrologically disturbed. Yet most of them
have been damaged by drainage in downstream lake
areas (Wolejko
et al.
1994). Due to decreasing lake
levels in the 19th century, erosion processes have been
triggered that have washed away most of the peat that
had accumulated around the spring systems.
9.3.5 Floodplain mires
Floodplain mires (Plate 9.2d) usually have very fer-
tile soils and most of them have been reclaimed for
agriculture when financial means and technical skills
were available to drain the areas. Remnants of former
floodplain mires can still be found in very low-lying
areas that could not be drained easily and were left
as nature areas. Examples are sections of the Biebrza
9.3.3 Terrestrialization mires
In flatlands, in particular, the original mires (fens or
bogs) are long gone and terrestrialization has taken
place under quite different hydrological conditions
(Plate 9.2a). Once such areas were the lowest areas in
