increased, as the model predicted, the upper marsh surface
reached an asymptote lower than that of the highest
predicted tide. Allochthonous sedimentation on vegetated
surfaces occurs in two ways: through capture of sediment
onto leaves and stems and through settling out of sediment
from water stilled by the vegetation (Mudd et al., 2010).
Input of sediment is not the only factor determining marsh
elevation; Cahoon (2006) identified eight processes which
influence changes in elevation, both positively and negatively. Sediment input could be countered by, for example,
compaction or shrinkage (see Salt Marsh Accretion).
The dropping of sediment or accumulation of organic
material creates a topographic gradient. The separation
of species along this gradient to give rise to zonation is
a response to a variety of factors. Conventional wisdom
suggests that the lower (seaward) limit of species is determined by responses to physical factors, while interaction
between species is more important in the upper marsh.
However, even at the lowest limits, marsh biotic interactions can be important. Doubts that the lower limit of the
pioneer Salicornia europaea in southeast England was
set by tidal action were raised by Gerdol and Hughes
(1993), who showed that the lower limit of Salicornia
coincided with the upper limit of the amphipod
Corophium volutator. Removal of Corophium permitted
seedlings transplanted to lower levels to survive,
suggesting that disturbance of the sediment by the amphipod prevented Salicornia from reaching its physically
determined lower limit.
Creeks and pans
Saltmarshes are rarely simple inclined planes; generally,
they have complex internal topographic variation, with
creek and pan systems present. The form of creek systems
varies considerably. On very sandy marshes, the creek systems are simple and less dense than those on muddy substrates (Chapman, 1960). Pye (2000) recognized six
different arrangements of creek forms in southeast
England.
Creeks, as well as providing drainage, represent an
extension of the estuary water body and mudflat habitats
into the marsh. Creeks also generate additional topographic complexity within marshes. As flooding tides
overflow the creek banks, sediment is deposited to create
levees and, consequently, basins between creeks
(Beeftink, 1977; Temmerman et al., 2004). The welldrained soils of the levees provide a contrast to the poorly
drained, frequently anoxic, conditions of the basins. This
environmental difference is reflected either by the occurrence of different species assemblages on the levees or in
the basins (Beeftink, 1977; Adam, 1990) or, in species
poor zones, by taller growth on the levees.
As well as creek systems, the marsh surface may be
interrupted by pans, steep-sided pools. The density of pans
varies considerably between marshes and in some sites is
low. In southeast Australia, where marshes occur as
a zone on the landward side of mangroves, both creeks
and pans are absent from many sites, although shallow
bare areas may occur (Figure 4) (Adam, 1997). Pan development occurs early in succession, with patchy distribution of pioneer plants leading to sediment deposition
around plants, and depressions between them
(Yapp et al., 1917; Goudie, 2013) (Figure 3a). As the vegetated surface continues to rise and consolidate, the
depressions form pans (Figure 3b). In addition, localized
bank slumping produces blockages in creeks resulting in
chains of channel pans (Yapp et al., 1917). If pans were
formed only early in succession, then their density would
be similar regardless of marsh age. Pethick (1974) demonstrated that in eastern England the density of pans
increased in high mature marsh, so the Yapp et al. (1917)
model is not a complete explanation of pan formation.
There are a number of different possible mechanisms of
formation of additional pans. Pethick (1974) suggested
that local deposition of wrack could smother vegetation,
leading to death followed by erosion to create pans. On
some coasts, ice scour might also be a mechanism of pan
formation in high marshes. Importantly, the upper marsh
pans described by Pethick (1974) have the same general
form as those discussed by Yapp et al. (1917) – steep sided
and generally water filled (as illustrated by Figure 2.3 in
Steers, 1977). Also widespread in some high-marsh zones
are extensive shallow depressions, either bare or sparsely
vegetated, which are variously referred to as pans, pannes,
or, as in Clarke and Hannon (1967), “rotten spots.” These
might develop as a consequence of poor drainage
(Ewanchuk and Bertness, 2004) or as a result of the development of a hypersalinity through evapotranspiration in
summer. The extent of these bare areas may vary over
time. Hamilton (1919) described more widespread and
extensive bare areas in the marshes of the Sydney region
(Australia) in the early twentieth century than are present
in the early twenty-first century. Loss of vegetation may
also be caused by the use of recreational vehicles
(Kelleway, 2005), and naturally bare areas may be
extended, or have recolonization prevented, by vehicle
use (Figure 4).
The environment
Environmental conditions within saltmarshes are very
much determined by the tide. The tidal regime varies
between sites, and estuarine saltmarshes experience the
full range of tidal regimes. Tidal ranges vary from macro,
including locations with the largest ranges in the world
(Bay of Fundy, the Bristol Channel), to negligible. In
some regions, there is a seasonal variation in water level,
for example, in the Baltic (Gillner, 1965), or the estuaries
of southwest Western Australia (Brearley, 2005). ICOLLs
may, when closed, be nontidal for extended periods, but,
nevertheless, the conditions established during opening
periods determine the structure and function of any fringing saltmarshes.
The number of tides reaching particular positions on the
marsh surface varies with elevation. The lower marsh may
SALTMARSHES
519
reached an asymptote lower than that of the highest
predicted tide. Allochthonous sedimentation on vegetated
surfaces occurs in two ways: through capture of sediment
onto leaves and stems and through settling out of sediment
from water stilled by the vegetation (Mudd et al., 2010).
Input of sediment is not the only factor determining marsh
elevation; Cahoon (2006) identified eight processes which
influence changes in elevation, both positively and negatively. Sediment input could be countered by, for example,
compaction or shrinkage (see Salt Marsh Accretion).
The dropping of sediment or accumulation of organic
material creates a topographic gradient. The separation
of species along this gradient to give rise to zonation is
a response to a variety of factors. Conventional wisdom
suggests that the lower (seaward) limit of species is determined by responses to physical factors, while interaction
between species is more important in the upper marsh.
However, even at the lowest limits, marsh biotic interactions can be important. Doubts that the lower limit of the
pioneer Salicornia europaea in southeast England was
set by tidal action were raised by Gerdol and Hughes
(1993), who showed that the lower limit of Salicornia
coincided with the upper limit of the amphipod
Corophium volutator. Removal of Corophium permitted
seedlings transplanted to lower levels to survive,
suggesting that disturbance of the sediment by the amphipod prevented Salicornia from reaching its physically
determined lower limit.
Creeks and pans
Saltmarshes are rarely simple inclined planes; generally,
they have complex internal topographic variation, with
creek and pan systems present. The form of creek systems
varies considerably. On very sandy marshes, the creek systems are simple and less dense than those on muddy substrates (Chapman, 1960). Pye (2000) recognized six
different arrangements of creek forms in southeast
England.
Creeks, as well as providing drainage, represent an
extension of the estuary water body and mudflat habitats
into the marsh. Creeks also generate additional topographic complexity within marshes. As flooding tides
overflow the creek banks, sediment is deposited to create
levees and, consequently, basins between creeks
(Beeftink, 1977; Temmerman et al., 2004). The welldrained soils of the levees provide a contrast to the poorly
drained, frequently anoxic, conditions of the basins. This
environmental difference is reflected either by the occurrence of different species assemblages on the levees or in
the basins (Beeftink, 1977; Adam, 1990) or, in species
poor zones, by taller growth on the levees.
As well as creek systems, the marsh surface may be
interrupted by pans, steep-sided pools. The density of pans
varies considerably between marshes and in some sites is
low. In southeast Australia, where marshes occur as
a zone on the landward side of mangroves, both creeks
and pans are absent from many sites, although shallow
bare areas may occur (Figure 4) (Adam, 1997). Pan development occurs early in succession, with patchy distribution of pioneer plants leading to sediment deposition
around plants, and depressions between them
(Yapp et al., 1917; Goudie, 2013) (Figure 3a). As the vegetated surface continues to rise and consolidate, the
depressions form pans (Figure 3b). In addition, localized
bank slumping produces blockages in creeks resulting in
chains of channel pans (Yapp et al., 1917). If pans were
formed only early in succession, then their density would
be similar regardless of marsh age. Pethick (1974) demonstrated that in eastern England the density of pans
increased in high mature marsh, so the Yapp et al. (1917)
model is not a complete explanation of pan formation.
There are a number of different possible mechanisms of
formation of additional pans. Pethick (1974) suggested
that local deposition of wrack could smother vegetation,
leading to death followed by erosion to create pans. On
some coasts, ice scour might also be a mechanism of pan
formation in high marshes. Importantly, the upper marsh
pans described by Pethick (1974) have the same general
form as those discussed by Yapp et al. (1917) – steep sided
and generally water filled (as illustrated by Figure 2.3 in
Steers, 1977). Also widespread in some high-marsh zones
are extensive shallow depressions, either bare or sparsely
vegetated, which are variously referred to as pans, pannes,
or, as in Clarke and Hannon (1967), “rotten spots.” These
might develop as a consequence of poor drainage
(Ewanchuk and Bertness, 2004) or as a result of the development of a hypersalinity through evapotranspiration in
summer. The extent of these bare areas may vary over
time. Hamilton (1919) described more widespread and
extensive bare areas in the marshes of the Sydney region
(Australia) in the early twentieth century than are present
in the early twenty-first century. Loss of vegetation may
also be caused by the use of recreational vehicles
(Kelleway, 2005), and naturally bare areas may be
extended, or have recolonization prevented, by vehicle
use (Figure 4).
The environment
Environmental conditions within saltmarshes are very
much determined by the tide. The tidal regime varies
between sites, and estuarine saltmarshes experience the
full range of tidal regimes. Tidal ranges vary from macro,
including locations with the largest ranges in the world
(Bay of Fundy, the Bristol Channel), to negligible. In
some regions, there is a seasonal variation in water level,
for example, in the Baltic (Gillner, 1965), or the estuaries
of southwest Western Australia (Brearley, 2005). ICOLLs
may, when closed, be nontidal for extended periods, but,
nevertheless, the conditions established during opening
periods determine the structure and function of any fringing saltmarshes.
The number of tides reaching particular positions on the
marsh surface varies with elevation. The lower marsh may
SALTMARSHES
519