Literature Review Coastal salt marshes are an important ecological ...

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22 févr. 2014 (il y a 3 années et 8 mois)

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Alistair Lockett

Impact of grazing on two Merseyside Salt marshes

263145


Literature Review


Coastal salt marshes are
an important
ecological h
abitat

for all kinds of flora and
fauna
.

Salt

marshes are defined as areas with either sediment or peat deposits that are
colonized by herbaceous and/or small shrubby plants and are frequently washed over
by saline waters (Long, Mason, 1983).

They form in mainly sheltered bays or
estuaries due to th
e dispersi
on and lowered wave energy
. This leads to s
edimentation

which

is the most important factor of a salt marsh as
without
it
colonizat
ion cannot
occur
. Sedimentation occurs when water movement is slowed and the suspended
particles can settl
e
. This

can take from minutes to as long as a couple of days
depending on the size of

the particle (stokes law)(
Garzanti
et al.
2008)
. Due to the
constant water movement in an estuarine salt marsh, only the larger particl
es such as
sand can settle
. Once sedimen
t has accumulated above the mean high water at neap
tide (MHWN)
halophytic vegetation
, such as
Spatina

species

can

start to co
lonize the
newly laid land
. Once
vegetation

begin
s

to colonize, they increase the rate of
sedimentation in a number of ways. Roo
t/rhizome networks trap sediment and
encourage particles to settle, above ground stalks and leaves both lower

wave energy
and trap particles and as a secondary outcome the halophytic plants add organic
material which appears to deter filter feeders that ma
y rework t and de
-
stabilize the
sediment,
all of these processes increase sedimentation further

(Shen
et al.
2008)
.

Holland and Coles (1972, 1979 cited in Long, Mason, 1983) show that a
nother factor
that increases the sedimentation is the mucus from certa
in
micro floral

species such as
Diatoms. This binds particles together increasing their mass and so accelerating
sedimentation


D
ue to the increase in sedimentation the land rises and so is subject to
a
decrease
in
saline inundation, this allows
more
halophytic plants

to colonize
increasing biodiversity of the marsh. Eventually the high zone become
s

colonized by

glycophytes
, such as sedges and rushes.

A
t that stage only freak weather could de
-
stabilise the marsh.
Salt marshes are most on very flat g
round, ranging 1
-
2m at most,
this adds to the sedimentation as wave action is reduced due to length of travel over
such a flat area. Due to the nature of salt marshes most are found around the coasts of
Western

Europe including the United Kingdom, Sweden,

Norway and France. Other
salt marsh sites include both East and West coast of North America, South America
around Chile and
Argentina
, New Zealand and a few around the Russian coast line.
The total distribution is unknown but an estimation has been take
n from Long and
Mason (fig.1).



Figure 1


Estimated worldwide distribution of salt marshes.




Alistair Lockett

Impact of grazing on two Merseyside Salt marshes

263145


Salt marshes can usually be split into three zones. The first is the pioneer or low zone,
this is typically between the mean high water at neap tides (MHWN)

and the mean
high water at normal tide (MHW)
. The pioneer zone is mainly occupied by
Spartina
anglica
, with a few others including
Aster tripolium, Salicornia europaea

and
Puccinellia maritim
a
. All of these species are halophytic meaning there can toler
ate
complete submergence in saline water. This is vital and only these types of plants can
colonize the pioneer zone. This does mean the diversity of flora in the pioneer zone is
usually very low compared to the mid and high zones.
Common
floras

that oc
cupy
the mid marsh are

Triglochin martima
,
Halimione portulacoides
and also some
species from the pioneer zone such as
Spartina anglica
and
Puccinellia maritime
.
This is not surprising that vegetation does overlap from zone to zone and there is no
cl
ear c
ut line between the zones. Some species such as
Salicornia europaea

can
actually inhabit all three zones on the same marsh.

The high marsh is rarely
submerged with saline water and so the floras do not have to be halophytes. High
marsh is mainly occupie
d by shrubby plants such as
Suaeda fruticosa

and
Agropyron
pungens
. The plants, if allowed, can grow higher and faster due to the fact they are
not covered with water
or sediment
twice a day.


Salt marshes are home to many different animals,
from invertebrates such as
nematodes, periwinkles
littorina littorea
, many different types of worms and also
clam
s

Macoma balthica

to vertebrates such as lapwing

species
Vanellinae
, avocet
species
Recurvirostra
, pink
-
footed geese

Anser brachyrhynchus
, whoo
per swans

Cygnus cygnus

and many more.

Salt marshes provide different micro
-
habitats for
these different species

and also different resources. The mud flats provide
invertebrate filter feeders
a stable environment in which they can dig themselves
down in
to and due to the almost constant cover by water food is abundant. Further up
the salt marsh the edges of the creeks also provide a suitable habitat for filter feeders,
again for the soft, muddy substrate and a high amount of water flowing through the
cre
eks.

Salt pans provide a habitat for both vertebrates and invertebrates depending
on location. A mid marsh salt pan can be used as a nursery for both invertebrates,
such as beetles, and vertebrates, such as fish. It is vital in this instance that the sa
lt
pan is in the mid marsh zone as to avoid it completely drying out. High salt pans are
used by ground nesting birds such as redshank
Tringa tetanus
, Tringa erythropus

and
oystercatcher species
Haematopus.

Salt marshes act as a perfect environment for
t
hese types of birds because as well as providing breeding habitats, salt marshes also
provide these birds with a good feeding habitat in the form of the mud flats. Salt
marshes also provide a safe haven for many birds from the genus
Anas

during the
winter

months. Again it provides a good feeding habitat and also due to the flatness
of the marsh predators are easily seen and can be avoided.


The main technique used to management a salt marsh is to moderately graze it. This
keeps important habitats availab
le, such as around the edges salt pans for the breeding
birds.
However grazing does have impacts on the other aspects of the salt marsh
ecosystem. For instance the primary production of the salt marsh is reduced due to
the loss in amount of flora, detris

production is reduced and also invertebrate
populations (crabs) are reduced (Reimold
et al.

1975). A study carried out in
Germany by Kiehl
et al,
in 1996 also states that when grazing was stopped, the salt
marsh developed a higher biodiversity over a two

year period. Contradicting these
studies are studies carried out by Esselink
et al.

who found that moderate grazing of a
salt marsh (40
-
80 individua
ls per hˉ¹) increased the floral diversity. Also grazing is a
Alistair Lockett

Impact of grazing on two Merseyside Salt marshes

263145


natural management scheme (Dijkema, 1990) a
nd so may seem the ideal way to
management a salt marsh. Other techniques include spraying and replanting.
Spraying is mainly used when a salt marsh is overrun with invasive species such as
Spatina
species, by killing off the invasive species there is a
chance for native species
to reclaim the salt marsh. On the opposite side replanting of invasive species have
been used as a form of management to speed up the marsh formation however once a
marsh has been developed invasive species do become a problem.


As salt marshes are an important habitat for many different plants and animals it is
important to know how the salt marsh ecosystem interacts and develops. Also more
information is needed on how grazing affects the secondary consumers such as the
soil inv
ertebrates. As every single salt marsh differs slightly in composition or
formation or both it is important to look at each individual site and record data that
may be used with other data to create the best possible management plan.


Reference list




Coles, S.M. (1979) “Benthic microalgal populations on intertidal sediments and their
role as precursors to salt marsh development.”
Estuarine Interactions

(ed. M. L. Wiley),
Academic press, New York, pg 93
-
113

Cited in: Long, S.P. and Mason C.F. (1983)
Sa
ltmarsh Ecology
. Blackie


USA:
Chapman and Hall, New York



Dijema, K.S. (1990) “Salt and brackish marshes around the Baltic Sea and adjacent parts
of the north sea: Their vegetation and management.”
Biological Conservation,
Vol.51,
pg 191
-
209



Esselin
k, P., Fresco, L.F.M. and Dijkema, K.S. (2002) “Vegetation change in a man
-
made salt marsh affected by a reduction in both grazing and drainage.”
Applied
Vegetation Science
, Vol. 5, pg 17
-
32



Frid
, C and James, R. (1989) “The marine invertebrate fauna of a British coastal salt
marsh.”
Holarctic Ecology
, Vol. 12 (1) pg 9
-
15.



Garzanti, E. Andò, S. And Vezzoli, G. (2008) “Settling equivalence of detrital
minerals and grain
-
size dep
endence of sediment composition.”
Earth and Planetary
Science letters,

Vol. 273 (1
-
2)

pg138
-
151
.



Holland, A.F., Zingmark, R. G. And Dean J.M. (1974) Quantitative evidence concerning
the stabilization of sediments by marine benthic diatoms.
Marine Biolog
y.

Vol. 27 (3), pg
191
-
196

Cited in: Long, S.P. and Mason C.F. (1983)
Saltmarsh Ecology
. Blackie


USA:
Chapman and Hall, New York



Kiehl, K., Eischeid, I., Gettner, S. And Walter, J. (1996) “Impact of different sheep
grazing intensities on salt mars
h vegetation in northern Germany.”
Journal of Vegetation
Science
, Vol. 7, pg 99
-
106.



Long S.P. and Mason C.F. (1983)
Saltmarsh Ecology
. Blackie


USA: Chapman and

Hall, New York



Reimold, R.J., Linthurst
, R.A. and Wolf, P.L. (1975) “Effects of grazing on a salt marsh.”
Univeristy of Georigia



SHEN, Yong
-
Ming. YANG, Jing
-
Song. WANG, Yan
-
Hong. FENG, Nian
-
Hua. ZHOU,
Qin and ZENG, Hua, (2008) “Impact of sediment supply on
Spatina

Salt Marshes.”
Soil
Science so
ciety of China,

Vol. 18 (5) pg 593
-
598.