Freshwater Marsh Extent
This layer is one of the South Atlantic LCC indicators in the tidal and nontidal freshwater marsh ecosystem. It captures the overall acres of freshwater marsh in the South Atlantic geography.
Reason for Selection
Overall acreage of existing freshwater marsh provides an indicator of whether tidal freshwater marsh being inundated by sea level rise is being replaced or restored somewhere else. It is also well monitored and resonates with a diversity of audiences. Since the area occupied by freshwater marsh shifts over time due to land use change, this extent indicator complements the freshwater marsh ecosystem map. The ecosystem map attempts to include other potential sites for freshwater marsh, including areas altered by people (for example, for agriculture or urban development), while the extent indicator is intended to depict only the area currently occupied by intact freshwater marsh.
Input Data
-- 2011 National Land Cover Database (NLCD)
-- Blueprint 2.1 estuarine marsh and estuarine open water ecosystem maps (based on NWI data)
Mapping Steps
-- We resampled the 2011 NLCD from 30 m pixels to 200 m pixels using a majority resample in ArcGIS.
-- Pixels identified as "emergent herbaceous wetlands" were considered existing freshwater marsh.
-- Pixels identified as estuarine marsh or estuarine open water in the Blueprint 2.1 ecosystem map were removed.
Indicator values were assigned as follows:
1 = Tidal or nontidal freshwater marsh
Defining the Spatial Extent of Ecosystems
This indicator has been clipped to the freshwater marsh ecosystem. Visit the Blueprint 2.1 ecosystem maps page for an explanation of how each ecosystem’s spatial extent is defined.
Known Issues
-- The extent indicators are intended to capture the current area of an ecosystem, while the ecosystem maps are intended to include the potential area of an ecosystem as well. For example, a drained forested wetland currently used for agriculture should not show up in the ecosystem extent indicator, but should be included in the ecosystem map since restoration could return it to a forested wetland state. However, due to differences in how this freshwater marsh extent layer and the Blueprint 2.1 freshwater marsh ecosystem map were created, the freshwater marsh ecosystem map does not include all the areas of current freshwater marsh depicted by this indicator. When calculating ecosystem integrity scores for the freshwater marsh ecosystem, we use the freshwater marsh ecosystem map as a mask. As a result, some pixels identified as freshwater marsh by this ecosystem extent layer are are classified as other ecosystems as shown in the Blueprint 2.1 ecosystem maps and scored according to the corresponding indicators. These freshwater marsh extent pixels that occur outside the ecosystem map do not influence the Blueprint. We hope that future improvements to our ecosystem maps will help correct this issue.
-- The ability of National Wetland Inventory (NWI) data to distinguish estuarine marsh from freshwater marsh is relatively unknown.
Disclaimer: Comparing with Older Indicator Versions
While this indicator has changed since the version of freshwater marsh extent used in Blueprint 2.0 and the version of freshwater marsh extent used in Blueprint 2.1, this only reflects differences in the way it was calculated and should not be compared to measure change over time.
Indicator Overview
The South Atlantic ecosystem indicators serve as the South Atlantic LCC's metrics of success and drive the identification of priority areas for shared action in the Conservation Blueprint. To learn more about the indicators and how they are being used, please visit the indicator page. Check out the Blueprint page for more information on the development of the Blueprint, a living spatial plan to conserve our natural and cultural resources.
Literature Cited
Homer, C.G., Dewitz, J.A., Yang, L., Jin, S., Danielson, P., Xian, G., Coulston, J., Herold, N.D., Wickham, J.D., and Megown, K., 2015, Completion of the 2011 National Land Cover Database for the conterminous United States-Representing a decade of land cover change information. Photogrammetric Engineering and Remote Sensing, v. 81, no. 5, p. 345-354.
