Planning—The Fragile Fringe

Summary: Coastal salt marshes may be at risk when sea level changes at a rate that is more rapid than normal. While coastal wetlands usually build up sediments and vegetation at rates similar to the rates that they subside (sink) or erode, the expected rate of sea level rise over the next few decades may flood or erode some wetlands before they can refill. Today, researchers are studying how salt marshes grow so that they can help land managers predict the wetlands’ response to elevated sea level. In this topic guide, students use a model to demonstrate wetland subsidence, and learn about the importance of sediment deposition and vegetation growth to marsh survival.

Concepts to teach:


  1. Wetland subsidence is a gradual sinking of land with respect to its previous level.
  2. Wetland accretion is the deposition of organic material that leads to a vertical buildup of wetland area.
  3. Wetlands need to accumulate new sediments and vegetation to remain elevated and healthy.

Standards: NGSS Performance Expectations

  • MS-LS2-4. Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.

Specific Objectives:
Students will be able to:

  1. Define subsidence and demonstrate the resulting effects on wetlands.
  2. Understand how sea level rise can result in wetland loss.
  3. Identify factors that can help keep wetlands vertically elevated.

Activity Links and Resources:

  • Activity: Loss of Wetlands – Subsidence in The Fragile Fringe from the National Wetlands Research Center—In this classroom experiment, students use a plastic box, soil and water to simulate a wetland and the effects water on the “elevation” of the soil.
  • Presentation: Sea Level Rise—from Maryland Chesapeake and Coastal Program. This model of salt marsh migration shows both how salt marshes stabilize shorelines and how tidal communities may respond to sea level rise.
  • Field experience: Visit a wetland area and identify high marsh and low marsh areas.
    • Using observations of plant composition and other cues, identify the mean low water (MLW) and mean high water (MHW) lines.
    • Compare your observations of the field site with the models discussed in class. What local factors could cause the marsh area at your field site to change (sink, rise, migrate)?
  • The Coastal Ecosystem Response to Climate Change (CERCC) webpage from USGS Western Ecological Research Center is a resource that describes how scientists monitor tidal marsh processes and responses to environmental changes such as sea level rise.


  • In the subsidence model, what happened to the level of the soil when water was added? What could be done to keep the soil elevation in the model constant?
  • How do shoreline structures help or hinder the amount of plant material and sediment that can build up on a marsh?
    • What types of shoreline structures allow for sediment to build up on a marsh?
    • What types of shoreline structures inhibit sediment build up on a marsh?