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of sea-level rise (MHWS to MHWS
), water
depths in the estuary increase meaning that all
zones of the marsh are covered more frequently
and for longer periods (Fig. 7.13a). Clearly this
will lead to plants becoming unstable in their
new conditions causing die-back and regrowth
further inland where water depths and periods
of inundation match those of pre-rise conditions.
Hence, the junction between low and mid-
marsh communities moves inland to the depth it
was at prior to the onset of sea-level rise. In the
case of the scenario in Fig. 7.13a, this position
moves from df
Int
to
df
Int
, from df
Low
to
df
Low
and
from df
Mid
to
df
Mid
. With continued sea-level
rise, this relocation is ongoing and gradual, only
ceasing when sea-level reaches a new stable level.
Evidence for such landward migration can be
seen in Maryland, USA, where freshwater forest
communities are being replaced by salt marsh
(Darmody & Foss 1979); and in Chesapeake
Bay, where terrestrial meadows are being invaded
by halophytic vegetation (Bird 1993). In both
of these examples, the process is represented in
Fig. 7.13a by the landward movement of the
high-marsh-terrestrial communities limit.
When an estuary is in its natural, undefended
state, any long- to medium-term rise in sea-level
can be compensated for by these landward
'shifts' in vegetation zonation provided that
suitable back-marsh areas for colonization exist
(Fig. 7.13a). As soon as sea defences are built,
however, any landward shift in these zones is
restricted by a physical barrier (Fig. 7.13b), which
effectively fixes the high water mark. Therefore,
with time, although low and mid-marsh com-
munities can shift landwards, there is no space
for the high marsh, which becomes squeezed
against the sea wall and will eventually disappear
(Fig. 7.13b). As the depth, frequency and period
of inundation increase further, this process will
continue, with the possibility that a marsh will
actually revert to lower marsh species, or even
mudflat. Although such a loss has major implica-
tions for the ecology of an estuarine system, it
can also have implications elsewhere in the sys-
tem. It has already been seen (section 7.1.2) that
vegetation plays an important role in the trap-
ping of sediment and promotes vertical marsh
′
accretion. With the loss of such vegetated sur-
faces, less sediment will be trapped, and so more
will be retained in the water body. This surplus
sediment has, in some situations, caused addi-
tional problems. In Tampa Bay, Florida, for
example, increased turbidity caused by increased
suspended sediment has reduced plant growth
elsewhere in the estuary.
Rising sea-levels are not the only cause of
marsh loss as vegetation may die back for other
reasons. One way, which remains largely un-
explained, is the natural die back in
Spartina
marshes. These are low marshes, in that they
are the first real vegetation to form as the marsh
develops from the mudflat. In the 1930s,
Spartina
marshes along the south coast of England started
to die, leaving large areas of bare mud. In
many areas this has been replaced by other low
marsh vegetation, such as
Zostera
(Haynes &
Coulson 1982; Adam 1990). Although one species
replacing another may not be seen as a prob-
lem, especially when ecologists favour
Zostera
because of its increased biodiversity and better
nutrient dynamics,
Spartina
is by far the more
efficient species in terms of ability to trap and
retain sediment (French 2001).
Although sea-level rise is very much a global
issue, some coastlines are experiencing a rela-
tive sea-level fall. Whereas a sea-level rise will
reduce the space for estuaries and deltas to occupy
unless they can relocate inland, a sea-level fall
will make more space available and, where sedi-
ment input remains high, allow expansion of
the sedimentary and vegetation zones. The delta
front will move further onto the continental
shelf, and intertidal zones in estuaries will start
to encroach on the original intertidal area and
shift seawards.
7.4
PROCESSES AND IMPACTS OF ANTHROPOGENIC
ACTIVITIES IN DELTAS AND ESTUARIES
Although estuaries and deltas are subject to con-
siderable natural sediment dynamics, they also
represent some of the most intensively exploited
environments. The main reason for this is their
proximity to, yet shelter from, the open sea, and
