Environmental Engineering Reference
In-Depth Information
on the younger marshes until the entire coastline was
protected by seawalls in the thirteenth century. Ini-
tially arable crops were grown on the levees, but ditch-
ing and the construction of embankments, dated from
the fi rst century
BCE
to the second century
CE
, allowed
crops to be grown on the salt marshes. The majority
of the marshes were exploited for livestock grazing
from the early settlements onwards, and haymaking
from the fi rst-third centuries onwards. The high fre-
quency of subfossils of
Juncus gerardi
and the low
frequency of
Elytrigia
spp. found in artifi cial mounds
led to the conclusion that unexploited salt marshes
were scarce during most of the occupation period
(Esselink 2000). Nowadays salt marshes in the Wadden
Sea that have never been grazed by livestock are of
recent origin, found only at the eastern point of the
Wadden Sea islands; the oldest one, since 1930, is
found on the island of Terschelling, the Netherlands.
Marshes that had accumulated enough sediment
were embanked. The incentives for embankments
have gradually changed during the twentieth century
from land claims for agriculture to coastal protection.
During the early twentieth century, large-scale accre-
tion works with sedimentation fi elds have been started
to create salt marshes that were to be reclaimed for
agriculture, but some decades later this was no longer
economically feasible due to changes in both socio-
economic conditions and agricultural policies. From
about the 1970s, there has been a growing recogni-
tion that the remaining salt marshes, though largely
'developed' as a result of human intervention, have an
important nature conservation interest (Esselink
2000). Increasing areas of seminatural salt marshes
with sedimentation fi elds were designated as nature
reserves in Denmark, Germany and the Netherlands,
and were included in national parks. Erosion is not
allowed because of coastal defence, and new sedimen-
tation fi elds are not promoted, as they reduce the area
of intertidal fl ats. Hence, existing salt marshes are
squeezed between the seawall or artifi cial sand dikes
and the intertidal fl ats. The existing marshes undergo
a process of maturation, and pioneer and young
marshes become lost as a result of decreased dynamics.
This process is still enhanced when livestock grazing
on the marshes ceases. Nowadays only about 40% of
the salt marshes of the Wadden Sea support livestock
grazing (Esselink
et al
. 2009 ).
Long - term ( >25 years) livestock exclosures in semi-
natural back barrier marshes in the Wadden Sea
revealed that the variation in plant communities along
the elevational gradient decreased. Especially at the
mid- and higher marsh, plant species diversity declined
(Bos
et al
. 2002). Similar changes were recorded in
long-term ungrazed seminatural marshes. At sites
with fast colonization of
Elytrigia atherica
in a semi-
natural salt marsh, the typical zonation of entomo-
fauna communities along an elevational gradient
disappeared (Figure 19.3) and characteristic halobion-
tic species were replaced by common inland species of
tall forb communities (Andresen
et al
. 1990 ). In con-
trast, the invasion of
E. atherica
in salt marshes in
western France did reveal an increase in noncoastal
spider species, web-building and cursorial spiders, but
did not interfere with resident species distibutions,
fi nally resulting in higher species densities and species
richness (P é tillon
et al
. 2005 ). The grazing intensity
of winter-staging geese was less in long-term un-
grazed than in grazed salt marshes in the Wadden Sea
(Figure 19.4 ; Bos
et al
. 2005). Although goose numbers
declined, especially in autumn, in the 10-year-
ungrazed part of the Hamburger Hallig, Germany, the
numbers of some breeding birds increased (Stock &
Hofeditz 2002 ).
North American salt marshes have a different
history. Along the eastern coast, tidal marshes formed
within the last 3000-4000 years as sea level rise
slowed to about 1 mm yr
− 1
, favouring the establishment
of the initial colonizer
Spartina alternifl ora
, a 1 - 2 m tall
grass (Niering 1997). The accumulation of organic
matter controls the accumulation of inorganic matter,
not the reverse. Below-ground plant material is very
important in maintaining salt marshes once they are
established. During the seventeenth to nineteenth cen-
turies
CE
, salt marshes were mown, grazed, ditched and
embanked in order to make them more suitable for
agricultural exploitation. In conjunction with these
early activities, some ditching and diking were done to
regulate tidal fl ooding, and increase the profi tability of
cattle grazing and hay production. However, these
impacts were minor compared to those that followed
the Industrial Revolution (1850s) when, with increased
mechanization, marshes were dredged for marinas,
fi lled for development, ditched for mosquito control,
fi lled with dredge spills and tidal-gated in order to
prevent upland fl ooding. The subsequent invasion of
Phragmites australis
in disturbed coastal wetlands
resulted in outcompeting of the native plant species
and desiccation as a result of strong transpiration out
of its habitat (Bertness
et al
. 2009 ). Wetland protection
laws since the 1970s, and no-net-loss policies, have led
