Definitions
Coastal wetlands are wetlands within the zone of hydrologic influence of sea level.
The perimarine zone is an area where freshwater
nontidal wetlands persist under the control of sea level.
Introduction
Sea-level-controlled wetlands comprise a wide variety of
environments from salt marshes to freshwater marshes
and swamps, fens, or barren salt flats, in a continuum of
increasing elevation from a shoreline to the upland. In
the case of the intertidal zone, the primary abiotic control
on wetland structure and function is a combination of
tidal inundation frequency, depth, and duration, known
as hydroperiod (Brinson, 1993). Wetland environments
below the highest astronomical tide experience direct tidal
inundation, with decreasing frequency and duration as
a function of increasing elevation within the tidal frame.
For coastal wetlands landward, the water table is linked
to the sea-level influence, which is an important control
on the groundwater position that provides the waterlogged
conditions necessary for their development (Hageman,
1969). The result of the interaction between hydrodynamics and elevation is a shore-parallel zonation of plants,
where each zone tends to move both vertically and horizontally in response to changing sea level and associated
stressors (Hayden et al., 1995).
Changes in relative sea level
While the globally averaged sea level has been rising from
the last glacial maximum to the present, the relative height
of the sea with respect to land (relative sea level) can vary
from place to place due to local tectonic and hydrographic
effects . As the mass of the continental ice melted, a huge
weight was released from continental shelves, which rose
by isostatic rebound of the land. In those areas where the
ice load was the greatest and the largest rebound occurred,
the land rose faster than the sea, the relative sea level
decreased, the coast prograded, and new land emerged
over the last 10,000 years (e.g., Fennoscandia, Finland,
Labrador). In other areas, the coast initially receded from
a rising sea until the relative sea level reached a transgressive maximum, after which the coast prograded, as the relative sea level decreased to its present elevation (e.g., the
east coast of South America, Western Australia, and East
China). Where the relative sea level fell rapidly, new land
constantly emerged, the coastal wetlands continuum
migrated seaward, and the Holocene estuarine environments became part of the terrestrial landscape. Where
sea level rose, the Holocene estuaries were drowned and
new wetlands formed landward.
A change in relative sea level produces an alteration in
the ecological state of wetlands, and the different plant associations within the coastal wetlands continuum are expected
to migrate in response to different hydrologic conditions.
Coastal wetlands developing under a rising relative sea
level during the Holocene have been largely studied along
the eastern coast of North America, as well as marshes in
northern Europe. The process of wetlands landward migration under a rising sea level was early described by Dutch
geologists. Hageman (1969) termed the area where freshwater wetlands persist under the control of relative sea level
as the perimarine zone, and he studied the evolution of
freshwater swamps in the western Rhine/Meuse delta, in
response to the rise in sea level during the Holocene. There
are examples of sedimentary records (Waller, 1994; Kirby,
2001) showing that peat-forming perimarine wetlands
accumulated deep layers of organic matter between about
6,000 and 2,000 years BP, and palynological analysis of
these peat deposits showed sequences of salt marshes, reed
swamps, fens, and woodland carr communities developing
under a rising sea level, which maintained a near-surface
water table (Waller et al., 1999). A similar model of transgression was described for the Virginia coast (Virginia,
USA), a typical coastal barrier ecosystems extending along
the seaward margin of the Delmarva Peninsula (Oertel
et al., 1989). In this system, a sustained sea-level rise during
the Holocene set up a similar sequence of state changes
in wetlands along the mainland edge (Brinson et al.,
1995). As transgression occurs, upland forest is replaced
with high marsh, high marsh with low marsh, low marsh
with mudflats, and mudflats with open water (Christian
et al., 2000).
In contrast to these well-studied examples of continuous
rising in relative sea level, little is known about wetland
response in coastal environments that developed under different conditions after the last glacial age. Some examples
from the Gulf of Bothnia describe a downward migration
of plant zones in response to the continuous land uplift
(Vartiainen, 1988; Ecke and Rydin, 2000) and the seaward
expansion of pioneer plant communities (Zobel and Kont,
1992). A more complex dynamics characterizes wetland
environments in the Atlantic coasts of southern South
America, where the relative sea level reached
a transgressive maximum during the Holocene (Cavallotto
et al., 2004; Violante and Parker, 2004). In these systems,
the late Holocene marine regression resulted in wide
low-lying coastal landforms inherited from the former estuarine dynamics. These coastal environments are commonly
occupied by perimarine wetlands, which undergo increasing inundation under the current rising trends in relative
sea level. In the Bahía Blanca Estuary, Argentina, a rising
sea level is a major cause of wetlands loss in elevated Holocene surfaces (Pratolongo et al., 2013), but the accelerated
erosion of soft sediments is also the main source of
suspended solids to the tidal sediment budget, allowing
deposition and seaward expansion of low salt marshes
(Pratolongo et al., 2010).
Accelerated sea-level rise
A major concern related to climate change is the recently
accelerated sea-level rise associated with the melting of
546
SEA-LEVEL CHANGE AND COASTAL WETLANDS
Coastal wetlands are wetlands within the zone of hydrologic influence of sea level.
The perimarine zone is an area where freshwater
nontidal wetlands persist under the control of sea level.
Introduction
Sea-level-controlled wetlands comprise a wide variety of
environments from salt marshes to freshwater marshes
and swamps, fens, or barren salt flats, in a continuum of
increasing elevation from a shoreline to the upland. In
the case of the intertidal zone, the primary abiotic control
on wetland structure and function is a combination of
tidal inundation frequency, depth, and duration, known
as hydroperiod (Brinson, 1993). Wetland environments
below the highest astronomical tide experience direct tidal
inundation, with decreasing frequency and duration as
a function of increasing elevation within the tidal frame.
For coastal wetlands landward, the water table is linked
to the sea-level influence, which is an important control
on the groundwater position that provides the waterlogged
conditions necessary for their development (Hageman,
1969). The result of the interaction between hydrodynamics and elevation is a shore-parallel zonation of plants,
where each zone tends to move both vertically and horizontally in response to changing sea level and associated
stressors (Hayden et al., 1995).
Changes in relative sea level
While the globally averaged sea level has been rising from
the last glacial maximum to the present, the relative height
of the sea with respect to land (relative sea level) can vary
from place to place due to local tectonic and hydrographic
effects . As the mass of the continental ice melted, a huge
weight was released from continental shelves, which rose
by isostatic rebound of the land. In those areas where the
ice load was the greatest and the largest rebound occurred,
the land rose faster than the sea, the relative sea level
decreased, the coast prograded, and new land emerged
over the last 10,000 years (e.g., Fennoscandia, Finland,
Labrador). In other areas, the coast initially receded from
a rising sea until the relative sea level reached a transgressive maximum, after which the coast prograded, as the relative sea level decreased to its present elevation (e.g., the
east coast of South America, Western Australia, and East
China). Where the relative sea level fell rapidly, new land
constantly emerged, the coastal wetlands continuum
migrated seaward, and the Holocene estuarine environments became part of the terrestrial landscape. Where
sea level rose, the Holocene estuaries were drowned and
new wetlands formed landward.
A change in relative sea level produces an alteration in
the ecological state of wetlands, and the different plant associations within the coastal wetlands continuum are expected
to migrate in response to different hydrologic conditions.
Coastal wetlands developing under a rising relative sea
level during the Holocene have been largely studied along
the eastern coast of North America, as well as marshes in
northern Europe. The process of wetlands landward migration under a rising sea level was early described by Dutch
geologists. Hageman (1969) termed the area where freshwater wetlands persist under the control of relative sea level
as the perimarine zone, and he studied the evolution of
freshwater swamps in the western Rhine/Meuse delta, in
response to the rise in sea level during the Holocene. There
are examples of sedimentary records (Waller, 1994; Kirby,
2001) showing that peat-forming perimarine wetlands
accumulated deep layers of organic matter between about
6,000 and 2,000 years BP, and palynological analysis of
these peat deposits showed sequences of salt marshes, reed
swamps, fens, and woodland carr communities developing
under a rising sea level, which maintained a near-surface
water table (Waller et al., 1999). A similar model of transgression was described for the Virginia coast (Virginia,
USA), a typical coastal barrier ecosystems extending along
the seaward margin of the Delmarva Peninsula (Oertel
et al., 1989). In this system, a sustained sea-level rise during
the Holocene set up a similar sequence of state changes
in wetlands along the mainland edge (Brinson et al.,
1995). As transgression occurs, upland forest is replaced
with high marsh, high marsh with low marsh, low marsh
with mudflats, and mudflats with open water (Christian
et al., 2000).
In contrast to these well-studied examples of continuous
rising in relative sea level, little is known about wetland
response in coastal environments that developed under different conditions after the last glacial age. Some examples
from the Gulf of Bothnia describe a downward migration
of plant zones in response to the continuous land uplift
(Vartiainen, 1988; Ecke and Rydin, 2000) and the seaward
expansion of pioneer plant communities (Zobel and Kont,
1992). A more complex dynamics characterizes wetland
environments in the Atlantic coasts of southern South
America, where the relative sea level reached
a transgressive maximum during the Holocene (Cavallotto
et al., 2004; Violante and Parker, 2004). In these systems,
the late Holocene marine regression resulted in wide
low-lying coastal landforms inherited from the former estuarine dynamics. These coastal environments are commonly
occupied by perimarine wetlands, which undergo increasing inundation under the current rising trends in relative
sea level. In the Bahía Blanca Estuary, Argentina, a rising
sea level is a major cause of wetlands loss in elevated Holocene surfaces (Pratolongo et al., 2013), but the accelerated
erosion of soft sediments is also the main source of
suspended solids to the tidal sediment budget, allowing
deposition and seaward expansion of low salt marshes
(Pratolongo et al., 2010).
Accelerated sea-level rise
A major concern related to climate change is the recently
accelerated sea-level rise associated with the melting of
546
SEA-LEVEL CHANGE AND COASTAL WETLANDS