Environmental Engineering Reference
In-Depth Information
more vulnerable to erosion. In artificial salt marshes
the abandonment of accretion works results in retreat-
ing of the marsh edge (Esselink 2000).
Salt-marsh erosion is predicted as an effect of
increased sea-level rise. An experiment was carried out
at the Wadden Sea island of Ameland, the Nether-
lands. As a result of gas extraction, soil subsidence
of 10 cm had taken place within 15 years, affecting
both the low and high salt marsh. The net elevation
of low-marsh plots did not change, indicating that
subsidence kept pace with sedimentation. In contrast,
the net elevation of the high-marsh plots decreased
by 10 cm, indicating that no extra sedimentation
took place. The vegetation changed in neither the lower
nor the upper plots of the grazed salt marsh (Dijkema
1997, Eysink
et al.
2000).
Most losses in salt-marsh area are caused by
embankments. The area of mainland salt marshes has
greatly declined during recent centuries because the
accretion of new marshes has not kept pace with the
rate of embankment in the northern Netherlands
(Dijkema 1987).
schelde estuary, the Netherlands. It can be closed
during storm surges. Although the barrier allows tidal
exchange, the tidal flow has been restricted. This
caused a 26-cm decrease of the mean high tide and
hence a decreased inundation frequency of the
marsh. The effect on the vegetation was recorded in
permanent plots in the period 1982-90. The initial high
rate of vegetation change slowed down in 1989 and
1990, suggesting that a new equilibrium had estab-
lished. Most plant species had moved down along the
elevational gradient (de Leeuw
et al.
1994).
More small-scale processes took place in New
England, USA. During the past century, about 2000
ha (30%) of Connecticut's tidal marshes have been
degraded or lost through coastal development. Tidal
flow to many marshes was restricted by the con-
struction of impoundments, producing microtidal
environments in which
Phragmites australis
or less
frequently
Typha angustifolia
became established at
the expense of typical tidal marsh communities. In
addition to these human influences,
P. australis
has
also invaded brackish tidal marshes in the lower
Connecticut River system where salinity levels are often
reduced by freshwater inputs (Fell
et al.
2000).
13.4.3 Reduction of tidal amplitude and
salinity
13.4.4 Cessation of livestock grazing
Another important cause of losses of salt marshes
is coastal protection by shortening the coastline of
estuarine coasts. This often coincides with the creation
of storage basins. Discharge of runoff water from the
hinterland can be hindered by prolonged high water
during storm periods.
Desalinization causes the transformation of salt-
marsh communities into communities adapted to
freshwater conditions. The changes are faster in
former salt marshes than in former intertidal flats
(van Rooij & Groen 1996). Continued grazing by
livestock retards the losses of halophytic plant species
(Westhoff & Sykora 1979, van Rooij & Drost 1996).
Moreover, grazing benefits short turfs that are
favoured by winterstaging geese, as in the former salt
marshes. Undisturbed succession results in scrub and
forest with characteristic bird species (van Rooij & Drost
1996, van Wieren 1998). Erosion of the coast by wave
action may result from fixed water levels.
Not all estuaries are dammed. In 1986 a sluice-gate
barrier was completed in the mouth of the Ooster-
As a result of decreased agricultural exploitation and
a non-interference policy in several national parks, large
areas of salt marshes are no longer grazed by live-
stock. Long-term (> 25 years) exclosures in back-
barrier marshes in the Wadden Sea revealed that the
variation in plant communities along the elevational
gradient decreased, and the community of
Atriplex
portulacoides
at the lower marsh and the communit-
ies of
Artemisia maritima
and
E. athericus
at the higher
marsh took over. Especially at the mid- and higher
marsh, the plant species diversity declined (Bos
et al.
2002). Similar changes were recorded in long-term
ungrazed artificial marshes. Exclosures at further dis-
tances from the intertidal flats experienced lower
rates of sedimentation and less spreading of
E. ather-
icus
(Bakker
et al.
2002b). At sites with fast colonization
of
E. athericus
in an artificial salt marsh, the typical
zonation of entomofauna communities along an eleva-
tional gradient disappeared (Fig. 13.4) and charac-
teristic halobiontic species were replaced by common
