Response of Coastal Ecosystems to Sea-Level Rise:
Assessing Wetland Elevation Changes, Potential for Submergence, and Management Options

Duration: March 1999 - September 2003

Coastal wetlands, such as tidal marshes and mangrove forests, occur at the boundaries of land and ocean. Because of their unique position in the landscape, they provide many important ecological services that maintain and improve the health of our environment.


Tidal marshes and mangroves store flood waters that run off from upland, thereby improving water quality by filtering out sediments and nutrients. They are critical habitat for juvenile and adult stages of many economically important marine fish and shellfish species that spend part of their life in the oceans. They also provide essential habitat for many species of waterfowl and wildlife. Their overall value to world food production is significant when you combine fisheries with foraging/hunting, agriculture, and aquaculture activities that occur in many wetlands of the world.

Coastal wetlands also provide a buffer to major storm impacts, particularly to damaging storm surges associated with hurricanes. Lastly, coastal wetlands are an important place of human recreation and offer significant aesthetic value to humans. Recent studies estimate the global economic value of all the services coastal wetlands provide to be in excess of $1.6 trillion per year.

How will tidal marshes and mangrove forests along the coasts of the United States be affected by predicted increases in sea level as global warming continues over the next century?

Presently, the surface of the sea is rising about 15 cm every 100 years, or 1.5 mm per year. Most of our wetlands are building up their surfaces at a similar rate through sediment trapping and soil formation, so the potential for wetland submergence is low in most coastal areas (with notable exceptions) so long as the present rate of sea-level rise does not increase. But projections are that sea level will rise from 48 to 66 cm during the next 100 years. We need to know which wetlands will build up soil at a pace equal to this faster rate of sea-level rise, and which wetlands will not.

For those wetlands which cannot keep pace, we need to determine the best management practices for minimizing the effects of sea-level rise. These fundamental issues are the motivation for this project. My research will determine:
How well various coastal wetland types in the United States are keeping pace with current and predicted rates of sea-level rise.
Which critical processes control elevation change in coastal wetlands.
How current wetland management and restoration practices affect wetland elevation and vulnerability to submergence.

This information will aid in predicting the effects of sea-level rise on our coastal wetlands and developing best management practices to minimize sea-level rise effects. This work continues and expands upon global change research initiated in 1992.

Scientific Objectives:

• Establish a comprehensive geographic network, the Advance Warning Network, of elevation monitoring sites on Federal coastal wetlands to give an advance warning of change to land managers.
• Determine the long-term potential for submergence of Federal coastal wetlands through modeling.
• Establish standard measurement protocols among members of the coastal wetland research community to make data from different sites comparable and to allow for evaluation of global trends in the vulnerability of coastal wetlands to submergence.
• Determine the critical subsurface processes (biological, hydrological, and geological) controlling elevation change in coastal wetlands so that appropriate management strategies can be developed.
• Determine the impact of current wetland management and restoration practices on wetland elevation changes and the ability of managed wetlands to build vertically at a pace equal to relative sea-level rise.

Approach:

This research program is based on direct and simultaneous measurements of sediment deposition, also called vertical accretion, and surface elevation change in wetlands. Vertical accretion rates are determined from artificial soil markers laid on the wetland surface. These markers gradually become buried and vertical soil buildup can be determined from soil cores collected through the markers.

Elevation change is determined using a Surface Elevation Table (SET) placed on benchmarks installed in the wetland. These direct measures of elevation change can be compared to the local rate of sea-level rise to determine the potential for submergence of the wetland. In addition, the combination of vertical accretion and elevation change measurements makes it possible to determine the separate influence of surface processes (such as sediment deposition and erosion) and subsurface processes (such as soil compaction, root growth/death, and water storage) on surface elevation. These influences are determined by comparing the sediment deposition and elevation change data within a marsh. Such knowledge is a critical first step in determining which processes are controlling elevation change and developing appropriate management strategies. This approach will be used to address the following questions:

Research Question 1:

What are the rates of vertical accretion and surface elevation change in our Nation’s coastal wetland ecosystems and are they adequate to keep pace with present and predicted future rates of sea-level rise?

Long-term accretion and elevation monitoring stations have been established in coastal wetlands on five National Wildlife Refuges and National Estuarine Research Reserves. This set of monitoring sites will be expanded into a more comprehensive network with standard sampling protocols in order to provide an advance warning of change to the Nation’s coastal wetland ecosystems during predicted changes in sea level. Up to ten additional sites will be located on national refuges, parks, seashores, research reserves, and NSF-funded Long-Term Ecological Research projects along the U.S. coasts (Figure 1, below). The network will focus primarily on salt marshes and mangroves since these wetland systems are most immediately vulnerable to sea-level rise. The network will be expanded into lower salinity and fresh wetland settings in future funding cycles.

The ability of our Nation’s wetlands to keep pace with predicted future increases in sea-level will be determined by the Relative Elevation Model (REM). Relative Elevation Model is a powerful tool for examining the response of wetlands to increasing rates of sea-level rise. The model simulates sediment processes over decades, incorporates feedback mechanisms, and is calibrated with site-specific field measurements of accretion, elevation change, and soil profile analyses. Those wetland sites which will not be able to keep pace with predicted increases in sea level can be identified now, several decades prior to their deterioration, and prioritized for management. Relative Elevation Model was developed by our collaborators, Drs. John Day and John Rybczyk. All wetland sites in the Advance Warning Network will be modeled by Day and Rybczyk during this funding cycle.

Research Question 2:

What are the critical processes controlling wetland elevation?

If the comparison of vertical accretion and elevation change measurements suggests that surface processes are controlling wetland elevation, then the interpretation is straight forward. Elevation is being controlled by either sediment deposition or erosion. If the comparison suggests, however, that subsurface processes are controlling elevation, then the interpretation is not easily determined. Separating the influences of biological processes (root growth), geological processes (soil compaction) and hydrological processes (groundwater storage) is difficult at best. As a first step toward separating them, we will design and install new versions of the surface elevation table (SET) that attach to deeper and shallower benchmarks than the current design. In this way, we can determine where in the soil profile most of the subsurface influence is occurring (e.g., in the root zone versus deeper down) and make a first approximation of which process is dominant.

Research Question 3:

How do current wetland management practices affect wetland elevation and the potential for wetland submergence?

Common wetland management and restoration practices include burning, hydrologic impoundments, and restoration of tidal circulation to diked marshes. The influence of these practices on soil elevation and subsurface soil processes has rarely been studied and is poorly understood. In 1999 and 2000, we will establish long-term accretion and elevation monitoring stations in burned and impounded marsh at McFaddin NWR in Texas and in restored impounded marshes in Merritt Island NWR in Florida.

Application of Results:

This project will provide:

• Information on the sensitivity of coastal wetland ecosystems to sea-level rise and identify those wetland systems which are most vulnerable. These findings can be used to develop a strategic response that will span several decades and preserve our most vulnerable wetland systems.
• An evaluation of the underlying causal mechanisms controlling elevation change in coastal wetlands. This information will be vital in the formulation of best management practices for our most vulnerable wetland systems.
• An evaluation of the effectiveness of various management practices at minimizing the undesired effects of rising sea level on coastal ecosystems. This information will be useful at improving current and developing new management practices.

Products:

Products of our research will include written reports, scientific publications, models and model outputs, and popular articles which will provide information on the stability and viability of these wetland communities to sea-level rise and management practices.

Co-Investigators:

Dr. John Day, Louisiana State University
Dr. John Rybczyk, California University of Pennsylvania
Dr. James Morris, University of South Carolina

	Primary Contact: Donald R. Cahoon, USGS Patuxent Wildlife Research Center