Global Warming, Sea-level Rise, and Coastal Marsh Survival

Coastal wetlands are among the most productive ecosystems in the world. These wetlands at the land-ocean margin provide many direct benefits to humans, including habitat for commercially important fisheries and wildlife; storm protection;

improved water quality through sediment, nutrient, and pollution removal; recreation; and aesthetic values. These valuable

ecosystems will be highly vulnerable to the effects of the rapid rise in sea level predicted to occur during the next century as

a result of global warming.

A coastal wetland becomes

submerged and disappears if sea level

rises faster than the marsh surface

builds up. Hence, determining the

potential for submergence is a critical

first step for managing these valuable

coastal habitats in the next century.

Research conducted by U.S. Geological Survey scientists has improved

our understanding of the natural

processes controlling wetland elevation and the potential for submergence

of our coastal wetland habitats.

Sea-level Rise

There are two processes by which

sea level can increase relative to the

marsh surface: (1) sea level rises

because of increases in the volume of

the oceans, and (2) the marsh surface

sinks because of soil compaction and

other geologic processes (collectively

called subsidence). The volume of the

oceans increases as global warming

thermally expands seawater and melts

glaciers, resulting in a global change

of sea level. The rate of global sealevel rise is expected to accelerate

during the next century as atmospheric temperatures continue to

increase. Sinking of the soil surface

occurs in nearly every marsh but

varies greatly among locales. River

deltas with thick, unconsolidated

sediment deposits, such as the

Mississippi River delta, have high

rates of sinking, while coastal fringe

marshes with thin sediment deposits

have low rates of sinking. Global sealevel rise and local marsh surface

sinking occur simultaneously, and

their combined effect is referred to as

relative sea-level rise. Vulnerability

to relative sea-level rise and to coastal

submergence varies greatly among

marshes because of local variation in

marsh sinking rate, indicating that sitespecific information is needed to

develop appropriate management

strategies for each wetland.

Coastal Marsh Response

There are two processes by which

marshes build vertically: surface

sediment deposition (sedimentation)

and subsurface accumulation of live

plant roots and decaying plant parts

(Fig. 1). Both processes are controlled

by marsh flooding (hydrology) and

both contribute to soil volume, which

directly influences surface elevation.

Flooding patterns control the delivery

of sediment to the marsh surface and

the oxygen content of the soil, which

influences the rate of plant growth and

decay. Sedimentation enhances plant

growth by delivering plant nutrients to

the soil while vegetation enhances

sedimentation by trapping sediments

suspended in flood waters. Hence, a

rising sea level can influence all of

these processes by changing flooding

patterns. In a healthy marsh under a

moderate rate of sea-level rise, marsh

elevation increases at the same rate as

sea-level rises and the flooding

patterns remain unchanged. In a

deteriorating marsh or under an

accelerated rate of sea-level rise, plant

growth is reduced because of excessive flooding, and soil volume cannot

keep pace with subsidence and sealevel rise. As elevation lags further

behind relative sea-level rise, the plant

stress increases until the plants die,

the soil volume collapses, and the

marsh becomes submerged.

Fig. 1. Relationships influencing marsh elevation.
1992 1993 1994 1995 1996

Year

0

2

4

6

8

10

-2

cm

Accretion

Elevation

Determining Potential for

Marsh Submergence

Traditionally, the potential for

marshes to become submerged has

been determined by comparing the

rate of vertical accretion (surface

sedimentation) to the rate of relative

sea-level rise. If accretion lags behind

sea-level rise, then an accretion

deficit exists and the potential for

submergence is high. The concept of

accretion deficit assumes that vertical

accretion equals marsh elevation

change. Yet we know that marsh

surfaces sink as a result of subsidence

(Fig. 1). Therefore, we tested this

assumption by simultaneously

measuring both vertical accretion and

marsh elevation (Fig. 2). If the

assumption were false (i.e., if elevation gain were less than vertical

accretion), the accretion deficit would

underestimate the potential for

submergence. Therefore, the elevation

data would be used to calculate an

elevation deficit, which would give a

more accurate estimate of the potential for submergence. In addition, we

made the measurements with a level

of accuracy sufficient to distinguish

between the influence of surface

sedimentation and subsurface soil

processes (e.g., root growth and soil

marker horizon and high rate of

accretion indicate that the lack of

elevation gain was not caused by

erosion. Therefore, the lack of

elevation gain must be caused by

sinking of the marsh surface from

shallow subsidence, as indicated by

the distance between the two lines in

Figure 3. The accretion data alone

gave a misleading indication of the

potential for submergence. The

simultaneous accretion and elevation

data indicate that the marsh surface

moved downward 2.28 cm/yr. Estimates of sea-level rise from tide

gauges do not include shallow

subsidence. Therefore, the actual

relative sea-level rise rate for this

marsh is 3.66 cm/yr (1.38 + 2.28),

which explains why the marsh is

rapidly submerging.

The existence of shallow subsidence does not always mean that an

elevation deficit exists. A study in

south Florida found that both fringe

and exposed island mangrove forests

experienced significant shallow

subsidence but soil elevation kept pace

with sea-level rise. The sheltered

island studied, however, appears to be

experiencing an elevation deficit.

Conclusions

The potential for submergence of

some coastal marshes is best determined by calculating elevation deficits

rather than accretion deficits. Moreover, measures of total subsidence

based on tide gauge records are

underestimated by the amount of

shallow subsidence. Trends in elevation of coastal marsh cannot be

extrapolated among sites, and sitespecific data are required to develop

appropriate management practices.

For more information, contact

Donald R. Cahoon

U.S. Geological Survey

National Wetlands Research Center

700 Cajundome Blvd.

Lafayette, LA 70506

318-266-8634

don_cahoon@usgs.gov

http://www.nwrc.gov

Fig. 2. Methods used to measure

marsh elevation change and shallow

subsidence.

U.S. Department of the Interior

U.S. Geological Survey

USGS FS-091-97

June 1997

Fig. 3. Comparison of accretion (soil

buildup) and elevation at Bayou

Chitigue, Louisiana.

compaction) on soil elevation. This

approach allowed us to determine for

each locale whether surface or

subsurface processes were controlling

elevation.

After at least 2 years of measurements, accretion and elevation were

not equal at a majority of the 12

marshes we studied. Shallow subsidence (the difference between vertical

buildup and the effects of surface and

subsurface processes on elevation) at

these marshes typically ranged from 3

to 4 mm/yr. These findings indicate

that soil processes occurring below the

marker horizon had as strong or

stronger an influence on elevation

than surface sedimentation. Therefore,

accretion deficits for these marshes

underestimated the potential for

submergence. This potential was best

estimated by elevation deficits. The

processes which controlled elevation

included sedimentation, plant root

growth, decay of plant matter in the

soil, water storage/drainage, and soil

compaction. Natural forces driving

these processes included seasonal

patterns in hydrology and major

storms.

Findings from a submerging marsh

at Bayou Chitigue, Louisiana, provide

an excellent example of the processes

influencing elevation (Fig. 3). The

relative sea-level rise rate for this

marsh is 1.38 cm/yr, estimated from

local tide gauges. For 4 years, vertical

accretion averaged 2.07 cm/yr,

suggesting that marsh elevation was

keeping pace with sea-level rise.

However, elevation remained virtually

unchanged. The persistence of the zone

Vertical

acretion