Coastal Wetlands and Global Change: Overview

The potential impacts of climate change are of great practical concern to those interested in coastal wetland resources.

Among the areas of greatest risk in the United States are low-lying coastal habitats with easily eroded substrates which occur

along the northern Gulf of Mexico and southeast Atlantic coasts. The Intergovernmental Panel on Climate Change (IPCC) and

the World Meteorological Organization (WMO) have identified coastal wetlands as ecosystems most vulnerable to direct,

large-scale impacts of climate change, primarily because of their sensitivity to increases in sea-level rise.

Global sea level has risen between

10 and 25 cm over the past 100 years.

The IPCC (1995) expects average sea

level will continue to increase as a

result of thermal expansion and melting

of glaciers and ice sheets. Models

using the IPCCs current best

estimate values of climate sensitivity

and ice melt suggest an increase in sea

level of about 50 cm from the present

to 2100. Continued rise in sea level will

increase the depth of coastal waters

and will cause inland and upstream

salinity intrusion, both of which will

affect fresh and brackish-water

wetlands. While there is uncertainty

regarding the influence of global

warming on the frequency and

intensity of storm events, sea-level rise

alone has the potential for increasing

the severity of storm surge, particularly

in areas where coastal habitats and

barrier shorelines are rapidly deteriorating. These direct consequences of

global change increase the vulnerability of coastal wetlands (including

mangroves and salt marshes) which are

already heavily impacted by human

activities.

Continued loss of coastal wetlands

has tremendous economic and

biological consequences. These areas

provide essential habitat for many

endangered and threatened species as

well as many commercially important

and recreational fisheries. Coastal

areas of Louisiana alone provide the

natural resource base for a $1-billionper-year fish and shellfish industry.

Threatened Habitats

Coastal wetlands will be affected by

large-scale climatic shifts. For example,

rates of subsidence (the sinking of

land below sea level) and sea-level rise

(or eustacy, caused by thermal

expansion of water and melting of polar

ice caps and glaciers) are not being

balanced by accretion (sediment

accumulation) in many gulf coast

wetlands, resulting in increased

flooding, saltwater intrusion into

freshwater wetlands, and erosion of

the coastline (Fig. 1). Although many

coastal wetlands have maintained their

relative elevation in response to

gradual increases of 1-2 mm/yr in sealevel rise over the last several hundred

years, these rates are projected to

increase two- to fourfold within the

next century.

Global change models predict a 1°

to 3.5° C increase in global mean

temperature by 2100 but knowledge is

insufficient to reliably model changes

in regional precipitation patterns

(IPCC, 1995). Variations in patterns of

precipitation and temperature can

affect the growth of individual trees,

tree species population dynamics,

ecosystem structure, and the geographic distribution of low-lying

forests and forest communities.

Underlying the predicted climatic

changes is an overall increase in

carbon dioxide concentrations in the

Fig. 1. Ghost swamps result when saltwater floods baldcypress swamps.
TX

CA

LA

FL

NC

Everglades NP

Ten Thousand 

Islands NP

Rookery Bay NERR

St. Marks NWR

Breton 

NWR

Three Bayous

Bayou

Chitigue Old Oyster 

Bayou

Big Thicket NP

McFaddin NWR

Tijuana

Slough NWR

Cedar Island 

NWR

sea-level rise of 1-2 mm/yr. However,

areas in south Louisiana with high

local rates of soil subsidence are

experiencing relative sea-level rise

rates up to 10 times the global mean

sea-level rise. Localized soil compaction, impacts from human activities,

sea-level rise, and other factors have

contributed to rates of coastal wetland

loss in south Louisiana exceeding 65

km2/yr.

U.S. Geological Survey (USGS)

scientists and their collaborators have

evaluated the potential for the submergence and subsequent loss of a range

of wetlands along the coast of the

United States. Marsh submergence can

only be prevented if sediment accumulation and organic matter production

and accumulation by plants result in a

vertical buildup of the marsh so

surface elevation keeps pace with or

exceeds the rate of relative sea-level

rise.

Surface elevation changes lagged

behind sediment deposition at 7 of the

12 coastal wetlands studied. Four sites

with significant soil subsidence and

lower surface elevations relative to sea

level had high rates of sediment

accumulation. This inverse relationship

suggests that physical and biological

processes occurring in the top few

meters of the soil are as or more

important than sediment accumulation

in determining soil surface elevation

(Fig. 3). It is likely that estimates of

marsh loss based only on tide gauge

atmosphere. Increased atmospheric

carbon dioxide concentrations should

also result in an increase in dissolved

inorganic carbon concentrations in

coastal waters. Such a change may

affect the submerged aquatic plant

communities.

Goals

The coastal wetland global change

research program seeks to 1) identify

sensitive ecosystems and critical

processes, 2) understand the current

condition of these ecosystems, 3)

develop predictive tools which model

the vulnerability of habitats to environmental changes, and 4) develop

management recommendations to

ameliorate the potential impacts. This

work has concentrated on land

managed by the Department of the

Interior (Fig. 2). Climate change is

projected to have significant regional

impacts on resource management and

use of these public lands. Southeastern coastal wetlands are also high

priority areas for state fish and wildlife

agencies because of the renewable

resource base associated with these

habitats.

Coastal Marshes

Over the past few decades, the rate

at which coastal wetlands have been

lost world-wide has averaged 0.5-1.5%

per year. In many coastal wetlands, the

level of the marsh surface does keep

pace with the current global average

records and rates of sediment accumulation underestimate the actual rates of

coastal wetland submergence.

As sea level rises, coastal marshes

can also persist by extending inland

and occupying formerly upland sites.

As lower elevation sites become

submerged, marsh build-up and

expansion may occur up the slope of

the landward marsh boundary. This

horizontal migration can be impeded if

the slope of the surface is too steep or

if barriers to new marsh formation exist,

such as sea walls, roads, or buildings.

Duke University and USGS scientists recently modeled the movement of

the marsh edge in North Carolinas

Pamlico Sound. Field data indicate that

movement of the marsh edge occurs in

a series of discrete events. These

events are separated by longer periods

of relatively little marsh movement.

Inland movement of the marsh is

associated with disturbance of the

upland vegetation from major storms or

fire. The general model implies that

upslope marsh movement is controlled

not only by sea-level rise but also by

Fig. 2. Study sites for the coastal marsh research. local conditions.

2

Fig. 3. Methods used to measure marsh

elevation change and shallow

subsidence.
Submerged Aquatic Plants

Communities of submerged aquatic

vegetation found in marine, estuarine,

and coastal freshwater environments

provide critical habitat for fish, shrimp,

wintering waterfowl, and endangered

species such as sea turtles and

manatees, as well as improve water

quality and provide erosion protection.

Field and laboratory experiments

addressed the response of submerged

plant species to elevated dissolved

inorganic carbon concentrations in the

water column, sea-level rise/salinity

intrusion, and physical disturbance by

storms.

The photosynthetic activity of three

freshwater submerged aquatic plant

specieswild-celery, coontail, and

hydrillaas well as a seagrass species,

shoalgrass , exposed to higher

concentrations of dissolved carbon

dioxide were measured in the laboratory. All four species showed an

increase in photosynthetic activity in

response to higher carbon dioxide

concentrations. The results of experiments with shoalgrass are particularly

significant because seagrass photosynthesis was not thought to be

limited by dissolved inorganic carbon

concentrations.

Longer term growth experiments

were conducted with wild-celery and

shoalgrass grown in laboratory

mesocosms aerated with carbon

dioxide-enriched air. Both species

exhibited changes in biomass allocation and an increased ratio of carbon to

nitrogen in certain plant tissues but did

not respond with increased growth.

Higher ratios of carbon to nitrogen in

plant tissue tend to provide poorer

quality forage for wintering waterfowl

that rely on aquatic plant species for

their food supply. Increased epiphytic

growth occurred on shoalgrass and

resulted in a decline in its dominance

because of shading by the epiphytes

(plants which depend upon other

plants for structural support).

We can associate general groups of

submerged aquatic vegetation species

with broad salinity zones. But as sea

level continues to rise, salt water will

move farther inland, subjecting fresh

and brackish marsh communities to

salinity stress. Experiments in a

greenhouse showed that species

known to be strong competitors for

light and nutrients dominated at low

salinities but did not grow well at

higher salinities because of a physiological intolerance to high salinity.

Species tolerant of high salinities

proved to be weak competitors at low

salinities. At moderate to high salinities

community structure was determined

by the salinity tolerance of the species.

Results indicate that salt tolerant

species including wild-celery, widgeon

grass, and sago pondweed will

dominate those areas where saltwater

intrusion results in habitat with higher

salinities. An increase in the severity of

tropical storms associated with climate

change can also have acute impacts on

seagrass communities.

Turtlegrass was found in protected

areas of Chandeleur Sound located off

coastal Louisiana which are characterized by nutrient rich sediments of high

organic content and low sand content.

Manatee grass occurred in patches

characterized by sandy sediments. An

increase in the deposition of sandy

beach and offshore sediments in

seagrass beds is expected to cause

shifts in community structure by

promoting an increase of manatee

grass.

Coastal Forested Wetlands

Forested wetlands at low elevations

are already threatened by logging and

alterations in hydrology. USGS

scientists and their collaborators

examined the effects of potential

increases in flood duration and salinity

level through a series of growth and

physiological experiments on 10 major

wetland tree and shrub species.

Physiological measures can be

good predictors of whole-plant growth

responses that may not be evident for

years or decades. In greenhouse

experiments, baldcypress was

relatively tolerant to permanent

flooding but relatively intolerant to

exposure to saline waters. Oaks, which

usually occur on ridges no more than

30-60 cm above the surrounding

swamps, were vulnerable to a combination of flooding and salinity stress.

Chinese tallowtree, a highly invasive

exotic species, was tolerant to the

combined salinity and flooding

stresses associated with simulated

storm surges. Flooding was more

important than small increases in

salinity in the growth and survival of

most tree species tested, whereas

chronic or large increases in salinity

were very harmful to all of the species

tested regardless of flooding regime.

Large-scale shifts and/or losses in

wetland forest communities are likely

to occur over the next 50 years due to

salinity increases if current trends in

sea-level rise continue. However,

considerable variation in salt tolerance

existed among natural populations of

baldcypress. Greenhouse and field

experiments demonstrated that this

tolerance has a genetic component and

is inheritable. These findings imply

that new varieties of baldcypress can

be developed and used in reforestation

efforts where existing populations

have been killed by saltwater intrusion.

Long-term monitoring at Big Thicket

National Preserve in southeast Texas is

helping us understand coastal forest

response to climate change. Permanent

study plots help quantify the dynamics of forest structure and response to

changes in climate. Resulting data

suggest that increases in drought

associated with changing climate may

significantly alter understory seedling

populations in bottomland forests and

recruitment into the sapling layers, and

ultimately influence overstory canopy

structure.

Increased disturbance associated

with flooding and storms may favor

early successional, shade-intolerant

species at the expense of shadetolerant species. Perhaps more

importantly, increased disruptions to

the forest canopy will provide recruitment opportunities for exotic woody

species, enhancing their rate of

invasion into natural stands. The

costs to control exotic species will

continue to rise in national parks and

other managed natural areas.

Landscape simulation models

confirmed that hurricane disturbance

also plays a major role in the structural

composition of mangrove forests

across south Florida and Everglades

National Park. Future climate change

scenarios using these models suggest

that damage associated with strong

storms will likely result in future

mangrove forests of smaller stature

3
4

Fig. 4. Predicted shoreline changes by 2100 showing coastal inundation of the St.

Marks National Wildlife Refuge area, Florida, for low, mid (best), and high

projections of sea-level rise.

and that shifts in species composition

to stands dominated by red mangrove

are likely.

Net Loss of Coastal Habitat

The ability to predict landward

movement of coastal marsh caused by

sea-level rise depends on knowledge

of the current vegetation distribution

and the relationship to topography.

A model based on elevation

measurements recorded from different

locations in St. Marks NWR was used

to analyze the topography and track

the process and pattern of coastal

inundation for given sea-level rise

projections (Fig. 4). All three sea-level

rise projections adopted by the

Intergovernmental Panel on Climate

Change indicate that major portions of

the coastal zone would be permanently

flooded, bringing about a movement of

vegetation community types and loss

in total area and proportion of some

habitats. The model suggests that a

large area of land will be quickly

converted from coastal salt- and

freshwater marsh to open water on the

scale of marsh loss experienced in

south Louisiana. There will be some

shift of emergent marsh into forested

zones as wetlands extend inland, but a

net loss of marsh habitat will occur.

Site geography and the slope of the

land in St. Marks refuge will present an

effective barrier to the establishment

and growth of some plant communities

because of lack of suitable habitat

conditions. The landform in this area

increases sharply in slope from the sea

inland, effectively squeezing area

available for forest and marsh.

Conclusions

The research of USGS scientists

and their collaborators reveals both the

scale and scope of potential global

change impacts on plant communities

of low-lying coastal areas. While there

remain many uncertainties regarding

regional and local responses to

changes in climate, current studies of

plants and landforms of the U.S.

southeastern coastal zone provide

strong evidence of the effects of sealevel rise and tropical storms. In the

southeastern coastal plain, USGS

scientists have documented trends in

U.S. Department of the Interior

U.S. Geological Survey

USGS FS-089-97

June 1997

95 cm - high estimate

50 cm - mid (best estimate)

15 cm - low estimate

Based on IPCC sea-level rise model (1995)

Red line approximates present shoreline

the changing dynamics of plant

communities, loss of coastal wetlands,

and erosion of coastal habitats. Future

research should provide a more

thorough understanding of wetland

response to sea-level rise and other

consequences of long-term environmental change.

For more information, contact

Glenn Guntenspergen

Beth Vairin

or

Virginia R. Burkett

U.S. Geological Survey

National Wetlands Research Center

700 Cajundome Blvd.

Lafayette, LA 70506

318-266-8500

beth_vairin@usgs.gov

http://www.nwrc.gov

Cooperators:

Rice University

Louisiana State University

Duke University

Clemson University

University of Southwestern

Louisiana

University of Georgia

The College of William and Mary

Louisiana Universities Marine

Consortium

East Carolina University

Lamont - Doherty Geological

 Observatory

Tall Timbers Research Station