Environmental Engineering Reference
In-Depth Information
but a continuous process of sedimentation combined
with resuspension. Biofilms of microscopic algae and
bacteria (which produce polymeric substances) may trap
and bind sediments that render the sedimentary sur-
face more resistant to erosive forces and help to retain
particles (Paterson 1997, Austen
et al.
1999). In both
wave-tank (Gleason
et al.
1979) and flume (Fonseca
& Fisher 1986) experiments, sandy bed material was
increasingly retained within seagrass (
Zostera
spp.)
stands with increasing density of stems. In shallow
waters, seagrass meadows not only suppress bed
erosion, but also increase the accretion rate relative
to other similar unvegetated areas (Ward
et al.
1984).
Of course, the causal arrow can also point in the
other direction. For example, in a study on the effects
of storms on the distribution of mussel banks in the
Wadden Sea of Schleswig-Holstein, Germany, Nehls
and Thiel (1993) concluded that the longest-living
mussel banks occurred in areas where there is some
degree of shelter, where the banks get some degree
of protection from westerly storms. In more general
terms, wind and tidal stress factors seem to influence
benthic community structures quite strongly (e.g.
Warwick & Uncles 1980, Thistle 1981, Emerson 1989).
Thus, one of the most interesting phenomena affect-
ing the appearance and biodiversity of intertidal flats
is the mutual interaction between abiotic factors and
the biota present (Verwey 1952, Bruno & Bertness
2001). The establishment on bare intertidal flats of
species that influence the complexity of the habitat
(e.g. seagrasses, oysters, mussels or tubeworms) gen-
erally generates even greater habitat complexity,
more variations in sediment structure and greater
biodiversity (Table 13.1). When the complex intertidal
structures that provide shelter, nutrition and other
favours to various species disappear, for example due
to the scouring of the flats by ice or dredging equip-
ment, local biodiversity and the generative processes
of this biodiversity are greatly reduced.
Table 13.1
List of environmental factors responsible
for the high species diversity in physically complex
habitats such as oyster and mussel banks,
Zostera
beds and
Sabellaria
reefs, relative to the low species
diversity of the bare sand flats that are left after their
destruction. Based on Boström and Bonsdorff (1997);
after Piersma and Koolhaas (1997).
Factor
Bare
Complex
sand flat
habitat type
Habitat complexity
Low
High
Shelter
Low
High
Flow velocity
High
Low
Deposition
Reduced
Enhanced
Sediment
Coarse
Fine
Sediment stability
Low
High
Organic content
Low
High
Food availability
Low
High
Wadden Sea (Allen 2000), to nearly zero in the Baltic
Sea. The pioneer zone of salt marshes consists of annual
plant species, and they do not trap sediment. The peren-
nial grass
Puccinellia maritima
at the low salt marsh
catches sediment, whereas erosion can take place of
unvegetated soil (Langlois
et al.
2001). In the
Festuca
rubra
zone, higher up the salt marsh with less inunda-
tion, the rate of sedimentation is lower than in the
P. maritima
zone (Andresen
et al.
1990). Sedimentation
patterns show spatial variation. Over comparatively
wide marshes a landward decrease of sedimentation
was found in a natural mainland marsh in Sussex,
UK (Reed 1988), and along the Westerschelde, the
Netherlands (Temmerman 2003), a back-barrier marsh
at Skallingen, Denmark (Bartholdy 1997), and an
artificial marsh in the Dollard, the Netherlands
(Esselink
et al.
1998). Superimposed on the large-scale
differences, the rate of sedimentation also declines away
from creeks and ditches (Fig. 13.1). Moreover, higher
rates are found on ungrazed than on heavily grazed
artificial marshes in the Dollard (Esselink
et al.
1998).
Dense, tall vegetation positively affects the rate of
sedimentation (Leonard
et al.
1995). An experiment
with different cattle-grazing regimes over 18 years
revealed a higher position with respect to mean high
tide on ungrazed sections than on heavily grazed
sections of the artificial Leybucht Marsh, Germany
(van Wijnen 1999).
13.2.2 Patterns and processes in
salt marshes
Elevation and sedimentation
The driving force in salt-marsh development is the
tidal amplitude, causing inundation and subsequent
sedimentation of silt. The mean spring-tidal range in
Europe varies from 12.3 m in estuaries to 1.6 m in the
