sea ice, land ice, and thermal expansion of the ocean
(Webb III et al., 1993; Wigley and Raper, 1993). There
has been considerable discussion as to how coastal wetlands will develop in the future under climate-enhanced
sea-level rise (Reed, 1990; Simas et al., 2001). Early studies (Titus, 1987; Boorman et al., 1998) predicted the largescale loss of coastal wetlands as a consequence of
sea-level rise exceeding sediment supply. However, there
is some evidence to suggest that, at some locations,
the geomorphic response of salt marshes is not sediment
limited. Many temperate salt marshes built from
allochthonous sediment show a significant excess of vertical sediment accretion relative to sea-level rise (French,
2006; Stupples and Plater, 2007). In the Mississippi delta,
accretion rates greater than 10 mm year
À1 have been measured where there is sufficient sediment input from the
river (Conner and Day Jr., 1991; Cahoon et al., 1995;
Day et al., 2000), and mangroves in many estuaries in
northern Australia tolerated sea-level rise of 8–10 mm
per year in the early Holocene (Woodroffe, 1995). These
accretion rates are higher than most projections and suggest that coastal wetlands can persist at a given location,
in spite of high rates of sea-level rise, if there is sufficient
mineral and organic soil formation.
Nevertheless, human activities alter the ability of wetlands to accrete both at local and regional scales, and
enhanced sea-level rise has led to significant changes on
coastal systems, mainly associated with salinity intrusion
in estuaries and altered sediment transport. There are
numerous examples of detrimental effects of accelerated
sea-level rise on coastal wetlands around the world,
including Chesapeake Bay, the Mississippi Delta, and
other Atlantic estuaries in North America (Stevenson
et al., 1985; Hackney and Cleary, 1987; Day et al., 2003;
Day et al., 2007); Rhone, Ganges, Indus, Nile, and Ebro
deltas ((Snedaker, 1984; Milliman et al., 1989; Stanley
and Warne, 1993; Ibañez et al., 1999; Pont et al., 2002;
Day et al., 2006); and Venice Lagoon (Pirazzoli, 1987;
Day et al., 1999). Deltaic regions are particularly
vulnerable to a relative sea-level rise because of rapid subsidence. Under this scenario, river sediment supply and
human-induced changes in sedimentary fluxes are critical
agents in shaping deltaic evolution. Dam construction and
the increase in water demand for agriculture, industry, and
tourist development have dramatically reduced the sediment load of rivers and are thought to be a major cause
of deltaic degradation when coupled with subsidence
(Stanley and Warne, 1993).
Vertical accretion and progradation resulting from sedimentation are not the only processes supporting wetland
persistence. For coastal areas experiencing a relative rise
in sea level, the different plant associations within the
coastal wetlands continuum are expected to migrate landward, and the future extent of the wetland zone will
depend on the combined effect of seaward vertical accretion, disturbance, and landward transgression (Christian
et al., 2000). There are examples of coastal marshes and
mangroves throughout the world, set against the land as
a fringe parallel to the shore, that seem capable of
responding to sea-level rise by moving inland, but there
are also some exceptions. Wetlands growing on islands
within estuaries have no land to migrate (Kearney and
Stevenson, 1991; Wray et al., 1995). Similarly, the migration of wetlands inland may also be prevented in places
where the landward slope is too steep or where people
have built hard barriers landward of the wetlands. In these
cases where transgression stalls, low sediment supply
results in an eroding seaward margin, and
wetland communities may disappear by erosion over time
(Brinson et al., 1995).
Conclusions
Coastal wetlands have naturally evolved in response to
global changes. Numerous studies show the resilience of
coastal wetlands to natural disturbances. However,
changes in sea level, coupled with anthropogenic changes
on sediment loads, species introduction, nutrient enrichment, and other human alterations are likely to have
a disproportionate impact on these systems.
Bibliography
Boorman, L. A., Garbutt, A., and Barratt, D., 1998. The role of
vegetation in determining patterns of the accretion of salt marsh
sediments. In Black, K. S., Patterson, D. M., and Cramp,
A. (eds.), Sedimentary Processes in the Intertidal Zone. London:
Geological Society, pp. 389–399.
Brinson, M. M., 1993. Changes in the functioning of wetlands along
environmental gradients. Wetlands, 13, 65–75.
Brinson, M. M., Christian, R. R., and Blum, L. K., 1995. Multiple
states in the sea-level induced transition from terrestrial forest
to estuary. Estuaries, 18, 648–659.
Cahoon, D., Reed, D., and Day, J., 1995. Estimating shallow subsidence in microtidal salt marshes of the southeastern United
States: Kaye and Barghoorn revisited. Marine Geology, 128,
1–9.
Cavallotto, J. L., Violante, R. A., and Parker, G., 2004. Sea-level
fluctuations during the last 8600 years in the de la Plata river
(Argentina). Quaternary International, 114, 155–165,
doi:10.1016/s1040-6182(03)00050-8.
Christian, R. R., Stasavich, L., Thomas, C. R., and Brinson, M. M.,
2000. Reference is a moving target in sea-level controlled wetlands. In Weinstein, M. P., and Kreeger, D. A. (eds.), Concepts
and Controversies in Tidal Marsh Ecology. The Netherlands:
Kluwer Press, pp. 805–825.
Conner, W. H., and Day, J. W., Jr., 1991. Variations in vertical accretion in a Louisiana swamp. Journal of Coastal Research, 7,
617–622.
Day, J., Rybczyk, J., Scarton, F., Rismondo, A., Are, D., and
Cecconi, G., 1999. Soil accretionary dynamics, sea-level rise
and the survival of wetlands in Venice Lagoon: a field and
modeling approach. Estuarine, Coastal and Shelf Science, 49,
607–628.
Day, J., Shaffer, G., Britsch, L., Reed, D., Hawes, S., and Cahoon,
D., 2000. Pattern and process of land loss in the Mississippi
delta: a spatial and temporal analysis of wetland habitat change.
Estuaries, 23, 425–438.
Day, J. W., Yáñez-Arancibia, A., Mitsch, W. J., Lara-Dominguez,
A. L., Day, J. N., Ko, J. Y., lane, R., Lindsey, J., and
SEA-LEVEL CHANGE AND COASTAL WETLANDS
547
(Webb III et al., 1993; Wigley and Raper, 1993). There
has been considerable discussion as to how coastal wetlands will develop in the future under climate-enhanced
sea-level rise (Reed, 1990; Simas et al., 2001). Early studies (Titus, 1987; Boorman et al., 1998) predicted the largescale loss of coastal wetlands as a consequence of
sea-level rise exceeding sediment supply. However, there
is some evidence to suggest that, at some locations,
the geomorphic response of salt marshes is not sediment
limited. Many temperate salt marshes built from
allochthonous sediment show a significant excess of vertical sediment accretion relative to sea-level rise (French,
2006; Stupples and Plater, 2007). In the Mississippi delta,
accretion rates greater than 10 mm year
À1 have been measured where there is sufficient sediment input from the
river (Conner and Day Jr., 1991; Cahoon et al., 1995;
Day et al., 2000), and mangroves in many estuaries in
northern Australia tolerated sea-level rise of 8–10 mm
per year in the early Holocene (Woodroffe, 1995). These
accretion rates are higher than most projections and suggest that coastal wetlands can persist at a given location,
in spite of high rates of sea-level rise, if there is sufficient
mineral and organic soil formation.
Nevertheless, human activities alter the ability of wetlands to accrete both at local and regional scales, and
enhanced sea-level rise has led to significant changes on
coastal systems, mainly associated with salinity intrusion
in estuaries and altered sediment transport. There are
numerous examples of detrimental effects of accelerated
sea-level rise on coastal wetlands around the world,
including Chesapeake Bay, the Mississippi Delta, and
other Atlantic estuaries in North America (Stevenson
et al., 1985; Hackney and Cleary, 1987; Day et al., 2003;
Day et al., 2007); Rhone, Ganges, Indus, Nile, and Ebro
deltas ((Snedaker, 1984; Milliman et al., 1989; Stanley
and Warne, 1993; Ibañez et al., 1999; Pont et al., 2002;
Day et al., 2006); and Venice Lagoon (Pirazzoli, 1987;
Day et al., 1999). Deltaic regions are particularly
vulnerable to a relative sea-level rise because of rapid subsidence. Under this scenario, river sediment supply and
human-induced changes in sedimentary fluxes are critical
agents in shaping deltaic evolution. Dam construction and
the increase in water demand for agriculture, industry, and
tourist development have dramatically reduced the sediment load of rivers and are thought to be a major cause
of deltaic degradation when coupled with subsidence
(Stanley and Warne, 1993).
Vertical accretion and progradation resulting from sedimentation are not the only processes supporting wetland
persistence. For coastal areas experiencing a relative rise
in sea level, the different plant associations within the
coastal wetlands continuum are expected to migrate landward, and the future extent of the wetland zone will
depend on the combined effect of seaward vertical accretion, disturbance, and landward transgression (Christian
et al., 2000). There are examples of coastal marshes and
mangroves throughout the world, set against the land as
a fringe parallel to the shore, that seem capable of
responding to sea-level rise by moving inland, but there
are also some exceptions. Wetlands growing on islands
within estuaries have no land to migrate (Kearney and
Stevenson, 1991; Wray et al., 1995). Similarly, the migration of wetlands inland may also be prevented in places
where the landward slope is too steep or where people
have built hard barriers landward of the wetlands. In these
cases where transgression stalls, low sediment supply
results in an eroding seaward margin, and
wetland communities may disappear by erosion over time
(Brinson et al., 1995).
Conclusions
Coastal wetlands have naturally evolved in response to
global changes. Numerous studies show the resilience of
coastal wetlands to natural disturbances. However,
changes in sea level, coupled with anthropogenic changes
on sediment loads, species introduction, nutrient enrichment, and other human alterations are likely to have
a disproportionate impact on these systems.
Bibliography
Boorman, L. A., Garbutt, A., and Barratt, D., 1998. The role of
vegetation in determining patterns of the accretion of salt marsh
sediments. In Black, K. S., Patterson, D. M., and Cramp,
A. (eds.), Sedimentary Processes in the Intertidal Zone. London:
Geological Society, pp. 389–399.
Brinson, M. M., 1993. Changes in the functioning of wetlands along
environmental gradients. Wetlands, 13, 65–75.
Brinson, M. M., Christian, R. R., and Blum, L. K., 1995. Multiple
states in the sea-level induced transition from terrestrial forest
to estuary. Estuaries, 18, 648–659.
Cahoon, D., Reed, D., and Day, J., 1995. Estimating shallow subsidence in microtidal salt marshes of the southeastern United
States: Kaye and Barghoorn revisited. Marine Geology, 128,
1–9.
Cavallotto, J. L., Violante, R. A., and Parker, G., 2004. Sea-level
fluctuations during the last 8600 years in the de la Plata river
(Argentina). Quaternary International, 114, 155–165,
doi:10.1016/s1040-6182(03)00050-8.
Christian, R. R., Stasavich, L., Thomas, C. R., and Brinson, M. M.,
2000. Reference is a moving target in sea-level controlled wetlands. In Weinstein, M. P., and Kreeger, D. A. (eds.), Concepts
and Controversies in Tidal Marsh Ecology. The Netherlands:
Kluwer Press, pp. 805–825.
Conner, W. H., and Day, J. W., Jr., 1991. Variations in vertical accretion in a Louisiana swamp. Journal of Coastal Research, 7,
617–622.
Day, J., Rybczyk, J., Scarton, F., Rismondo, A., Are, D., and
Cecconi, G., 1999. Soil accretionary dynamics, sea-level rise
and the survival of wetlands in Venice Lagoon: a field and
modeling approach. Estuarine, Coastal and Shelf Science, 49,
607–628.
Day, J., Shaffer, G., Britsch, L., Reed, D., Hawes, S., and Cahoon,
D., 2000. Pattern and process of land loss in the Mississippi
delta: a spatial and temporal analysis of wetland habitat change.
Estuaries, 23, 425–438.
Day, J. W., Yáñez-Arancibia, A., Mitsch, W. J., Lara-Dominguez,
A. L., Day, J. N., Ko, J. Y., lane, R., Lindsey, J., and
SEA-LEVEL CHANGE AND COASTAL WETLANDS
547