Geology Reference
In-Depth Information
of salt marsh environments, and interested readers are
referred to botanical literature for further information.
The general effects of plants on salt marsh deposition,
however, is of great importance, and details in these
mechanisms need to be understood in order to be able to
assess the complexity of salt marsh formation. The ability
of plants to form the necessary dynamic conditions for
deposition and their role in counteracting resuspension, is
of vital importance for salt marsh deposition and repre-
sents an important example in sedimentology, where the
interplay between physical and organic processes is of
fundamental importance for the fi nal product. This was
recognized early in the history of salt marsh studies, and
attempts to quantify the infl uence of vegetation were
made by pioneers of salt marsh accretion studies like
Richards (
1934
) and Nielsen (
1935
) . Stumpf (
1983
) is
most likely the fi rst to attempt to measure the fl ow through
salt marsh vegetation. He injected dyed water with needle
and syringe and timed the movement of the dye in a
densely vegetated salt marsh in Delaware, USA. By
means of the measurements and traditional turbulent
boundary layer theory, he concluded that both the turbu-
lence and fl ow velocities are one order of magnitude less
in the salt marsh canopy than in the adjacent creek envi-
ronment. Furthermore, by injecting dye close to the bot-
tom, he was able to show that the fl ow here was smooth
with the presence of a laminar sublayer which kept the
dye at the bottom with only little vertical exchange.
Technological progress allows still more detailed
measurements of water fl ow even in environments like
inside a salt marsh canopy. Wang et al. (
1993
) mea-
sured current velocity in a salt marsh on the Mississippi
River deltaic plain with an electromagnetic current
meter and concluded that tidal currents in the marshes
were about 10 to 20% of the current in the adjacent
bayou (creek). Christensen et al. (
2000
) used an acous-
tic doppler velocimeter (ADV) in a creek/marsh envi-
ronment on the eastern shore of Virginia, USA, and
found that the velocity in the salt marsh area even at
spring tide never exceeded 0.01 m s
−1
. Shi et al. (
2000
)
found by means of electromagnetic current meter
measurements on a mudfl at/salt marsh transition in
the Changjiang Estuary, China, an average reduction
in current velocity between the bare fl at and the salt
marsh canopy by 16%. Likewise by means of electro-
magnetic current meters, van Proosdij et al. (
2000,
2006a
) and Davidson-Arnott et al. (
2002
) studied the
near-bed velocity conditions in a macrotidal and wave-
exposed salt marsh in the Bay of Fundy. Their results
Fig. 8.10
Aerial photo of the central part of the Skallingen
backbarrier. There is a clear difference in the vegetation cover
between grazed (
lower left side
) and ungrazed (
upper right side
)
parts. The coloured bullets correspond to analysed plant associa-
tions (see the text) (Modifi ed from Kim et al.
2010
)
photo (Fig.
8.10
). In the low marsh zone (light blue
bullets), this species is followed by a number of other
species of which
Puccinellia maritima
,
Limonium vul-
gare
,
Plantago maritima
,
Triglochin maritima
and
Aster tripolium
are most abundant. This is in contrast
to the middle marsh (dark blue bullets) whereby the
same standard
Halimione
is followed by
Artemisia
maritima
,
Aster tripolium
and
Limonium vulgare
.
Other plants are present but these are the most abun-
dant with an overwhelming abundance of
Halimione
.
In the lowest part of the high marsh (black bullets), the
dominating plant is
Artemisia maritima
followed by
Juncus gerardii
,
Halimione portulacoides
and
Festuca
rubra
. Further up (landward) in this zone, the plant
community gradually changes into less salt-tolerant
species. The fi rst of the four mentioned salt-tolerant
plants to disappear during this transformation is
Halimione
. Notice that with some scatter, the vegeta-
tion zones follow the concave topographic profi le from
a relative high outer part close to the sediment source
(the salt marsh rim) through the lowest central part in
some distance from this that further inland merges
with the higher central parts of the peninsular. In this
case, this part is only reached in the northern line in the
form of an isolated ridge.
It is not the purpose of this chapter to go into further
details about plant community variations in various types