Landscapes
Habitat displacement and sea-level change -- the Blackwater model
More than 130 U.S. National Wildlife Refuges and 50 National Parks are found in the coastal zone of the United States. National Wildlife Refuges encompass 3 million acres of estuarine and 1.5 million acres of marine wetland habitat. Extensive wetland habitat also exists within National Parks and National Seashores. Along the Atlantic and Gulf coasts, these low-lying lands are being threatened by rising sea level. The projected rates of inundation due to sea level rise are thought to exceed the capability of the ecosystems to respond to increasing water depth and salinity, thus limiting the availability of critical habitat and triggering a need for expensive engineered schemes for restoration.
Rates of sea-level rise along the Atlantic and Gulf coasts of the United States may exceed the ability of the ecosystems to adjust to increasing water depth and salinity. One example is Blackwater National Wildlife Refuge, Maryland, where the USGS has assessed the impact of sea level rise on the coastal wetlands. The model developed for this assessment forecasts that most of the refuge will be in open water in approximately 50 years.
Tidal zonation of salt-marsh vegetation. As sea level rises, the plant communities migrate inland to higher elevations. MHW=mean high water and MLW=mean low water.
Given suitable land surface and response time, marsh habitat will respond to rising sea level by migrating inland and reestablishing in new areas. Although this is the geological norm through time, we do not know how current marsh surfaces are responding on a decadal time scale to current rates of sea-level rise and specific environmental processes. A clearer scientific understanding of ecosystem response is needed by Federal, State, and local land managers so that management strategies can be put into place that ensure long-term sustainability.
The Blackwater National Wildlife Refuge (NWR) is an example of "wetland loss" resulting from rising sea level. In 1877, the area was mapped as a forest; today much of the land is less than one meter above sea level.
Confluence of Blackwater and Little Blackwater Rivers, Blackwater National Wildlife Refuge, shows steady inundation of the marsh over time. Photo 1, taken in 1938, shows discreet ponds and channels within the marsh system.
Photo 2, taken in 1974, shows evidence of significant open water expansion; ponds are breached and integrity of channels is compromised.
Photo 3, taken in 1989, shows the expansion of open water; discrete features have disappeared. Approximately 130 acres of marsh are inundated each year at Blackwater National Wildlife Refuge.
In 2002, the USGS acquired light detection and ranging (lidar) data for the Blackwater NWR in order to assess the long-term impact of rising sea level on the refuge. Lidar data can be used to see through the vegetation cover of an area and map the topography to within approximately 15 cm accuracy.
Lidar image, Blackwater National Wildlife Refuge, 2002.
Blue areas are open water, light blue are intertidal marsh,
and light yellow are high marsh.
Our model assumed that the long-term rate of sea-level rise would continue to be about 3 mm/year. Increments this small can have profound impacts on low altitude wetland surfaces.
Projected changes in land cover at Blackwater National Wildlife Refuge, 2050 (left) and 2100 (right). Blue areas are open water, light blue are intertidal marsh, and light yellow are high marsh. The increase in area of open water is relatively constant until about 2050 and then shows a marked increase owing to topography.
The model shows that the area of habitat managed for migratory waterfowl (the high marsh) stays relatively constant until 2050, when it almost disappears and is converted to intertidal marsh. Models similar to this one, but on a broader scale, will show coastal land managers where critical wetland habitats will be destroyed and new habitats developed in time and space as sea level rises. This modeling effort is a prototype for other coastal wetlands and part of a broader integrated study that the USGS has been conducting at the refuge.
Future applications
USGS recently entered into a cooperative agreement with the Maryland Department of Natural Resources to develop a lidar-inundation model for the Maryland coastal areas, including the Chesapeake Bay. This effort should also contribute directly to USFWS planning in the Chesapeake Bay region. The new work will enable us to place the Blackwater National Wildlife Refuge in a broader geospatial context of wetlands in the area. Land managers will be able to identify which low-lying lands will be converted to critical waterfowl habitat over the next century. Wildlife managers will be able to view a total region on a decade-by-decade basis and make decisions from a database that will calculate areas of high marsh, intertidal marsh, and shallow water habitats. Such a planning tool will provide flexibility in decision making not currently available.
