The Use of Geographic Information Systems to Assess Change in Salt Marsh Ecosystems Under Rising Sea Level Scenarios Robert Hancock The ecological challenges presented by global climate change are vast, intricate, and difficult to quantify. This is especially true in the coastal zone where influences from both land and sea create a highly dynamic environment. Fortunately, Geographic Information Systems (GIS) technology is increasingly being used to illustrate and model changes in the coastal zone resulting from rising sea level. The technology has proven beneficial in documenting change across large regions of coastline and is increasingly becoming applicable in assessing challenges to localized ecosystems. One habitat that faces particular threats resulting from increased sea levels are tidally influenced coastal salt marshes. The scientific community is beginning to realize the potential for using GIS technology to document and predict changes in salt marsh area and function. GIS, by its very nature as a geospatial technology, does an excellent job documenting landscape change in all ecosystems. This has held true with research focused on the coastal zone and salt marshes. Common practices for assessing ecosystem change are to compare imagery data over time or to compare that data to historical maps, charts, and surveys (Bromberg and Bertness 2005, Hartig et al. 2002, Kastler and Wiberg 1996). Such research has provided quantifiable data for changes in salt marsh extent over the last several decades and approximations over the last few centuries. When this geospatial information is combined with field data the results are even more useful in determining potential for losses in marsh area over time (Kastler and Wiberg, 1996). Another major way in which GIS technology has been applied in the coastal zone is in forecasting and modeling oncoming changes resulting from sea level rise. However, this type of analysis continues to face several hurdles including the uncertainty of many sea level rise prediction models which range from 18 cm to several meters as well as the coarseness of elevation data available for use in illustrating the effects of different sea level rise scenarios. Most regions of the country are reliant on National Elevation Datasets which can offer a vertical accuracy of 1.5 m at best and often rely on a 30 m pixel size. In some regions, 1.5 meters can incorporate the entire tidal range in which a salt marsh would exist. Additionally the 30 m pixel size essentially eliminates the ability to quantify areas of fringing marsh which may only be 3-5 m in diameter but can cover significant lengths of shoreline and thereby contribute considerable acreage to the overall extent of salt marsh habitat in a particular region. Advancements in remote sensing technology, such as Light Detection and Ranging (LIDAR), have led to an increase in the resolution of elevation data where these technologies have been used. In Rhode Island, the availability of high-resolution elevation data is variable but improving with some communities
on the south coast acquiring LIDAR data of their towns (Vinhateiro 2008). This acquisition is particularly beneficial for Rhode Island s South Coast due to the significant acreage of salt marsh within these coastal towns. A second problem faced by researchers when applying GIS technology to model the effects of sea level rise on salt marshes is the dynamic nature of the coastal zone and the many variables that contribute to salt marsh creation and survival. Most salt marshes develop in low energy areas which exist on ocean shorelines only inshore of barrier beaches. Although, these beaches are only a few feet in elevation they provide the protection necessary for salt marsh growth. Accelerated sea level rise may over wash these beaches or increased storminess, another predicted impact of climate change, could accelerate erosion of these protecting beaches. Additionally, the ability of a salt marsh to migrate into upland areas may depend on the current land cover and land use of that upland region. Fortunately research has begun to develop new models built to incorporate these various parameters into the various sea level rise scenarios predicted by climate scientists. One such model is the Sea Level Affecting Marsh Model (SLAMM) originally built upon data from the National Elevation Dataset and National Wetlands Inventory maps (Craft et al 2009). This model considers factors such as predicted sedimentation and erosion, tidal range, ocean depth, and fetch, frequency of large storms, and potential sea level rise scenarios. The development of intricate sea level rise models as well as increasing amounts of high resolution elevation data should lead to a better understanding and forecasting of the threats to salt marsh development and survival. GIS technology will play a critical role in testing these models as well as illustrating the results for the scientific and management community. Additionally, as time passes, GIS technology will help to document changes as they occur in the coastal zone with the onset of accelerated sea level rise.
Annotated Bibliography Bromberg, K. and M. D. Bertness. 2005. Calculating the Loss of New England Salt Marshes from Historical Maps. Estuaries 28:823-832. While not specifically dealing with changes in salt marsh areas as a result of sea level rise, this article creatively uses GIS software and aerial photography in conjunction with historical maps and nautical charts to asses historical losses in salt marsh area throughout New England. Due to the inability to verify the spatial accuracy of historical maps and charts, this comparison is at most an approximation of the loss of salt marshes but it provides an important, if not exact, extension of the record of change in marshes by adding another 200-300 years to the current record based on aerial photography. Hartig, E. K., V. Gornitz, A. Kolker, F. Mushacke, and D. Fallon, 2002. Anthropogenic and Climate-Change Impacts on Salt Marhes of Jamaica Bay, New York City. Wetlands, 22:71-89. This research by Hartig et al. uses a standard format of salt marsh assessment in which a GIS analysis of aerial photography is combined with field work to build a comprehensive understanding of a changing salt marsh system. The GIS analysis used in the article provides the ability to quantify the changes in salt marsh area in Jamaica Bay while the field data offers a better understanding of the causes of the change including relative sea level rise. The authors also used GIS technology to create a rough projection of the future of the marsh under a variety of sea level rise and marsh accretion scenarios. Simas, T., Nunes, J.P., Ferreira, J.G., 2001. Effects of Global Climate Change on Coastal Salt Marshes. Ecological Modelling, 139:1-15. This project, based on salt marsh changes in Portugal, applied small scale biogeochemical and demographic modeling to the whole marsh area using GIS and bathymetric data. The modeling component focused on two pathways for carbon assimilation, C3 and C4 in order to determine the main ecological processes of salt marsh vegetation per unit area. GIS was used to determine salt marsh area and to develop sea-level rise scenarios. The model was then upscaled using GIS to determine results of changes to C3 and C4 vegetation across the full salt marsh area. Austin, G.E. and Rehfisch, M.M., 2003. The Likely Impact of Sea Level Rise on Waders (Charadrii) Wintering on Estuaries. Journal for Nature Conservation, 11:43-58. One significant concern when considering potential losses of salt marsh area is the effect on waders that use these estuaries as wintering grounds. In this survey of two British estuaries, the investigators used models to determine sea level rise scenarios which were then applied to LIDAR elevation data using ArcView GIS software. This project also took into consideration potential sea level rise management strategies and determined that while there will be significant changes to the morphology of the estuaries, waders may end up
gaining as much new marsh area as they lose. This article shows the importance of illustrating the detailed effects of sea level rise on particular ecosystems functions within a salt marsh using GIS. Kastler, J.A. and Wiberg, P.L., 1996. Sedimentation and Boundary Changes of Virginia Salt Marshes. Estuarine, Coastal, and Shelf Science, 42:683-700. The survival of salt marshes over time is a balance between the rate of sediment accumulation on the surface of the marsh and the erosional properties of the coastal zone as well as submergence by a rising sea or subsiding land. In this article, the investigators use color infra-red and black and white aerial photography to map the changes in marsh area dating back almost 70 years. This data was reviewed in conjunction with sedimentation data collected in the various marshes of concern. This combination led to the determination of changes in marsh area as well as the likely cause of that change. Titus, J.G., Richman, C., 2001. Maps of Lands Vulnerable to Sea Level Rise: Modeled Elevations Along The U.S. Atlantic and Gulf Coasts. Climate Research, 18:205-228. The changes to individual salt marsh ecosystems can be significant to that region or estuary but large scale changes to coastal habitats are also critical to the nation as a whole. In this project, the investigators used GIS technology and data from USGS and NOAA to create maps of large regions of the coastline. These small scale maps are helpful in projecting the significant changes that may result from sea level rise in the coastal zone. However, the coarseness of the data does not allow for effective analysis of individual marsh ecosystems. Vinhateiro, N. 2008. Sea Level Rise and the Current Status of Digital Terrain Data for the South Shore of Rhode Island: A White Paper in Integrated Coastal Science for the Rhode Island Coastal Resources Management Council. Available online at www.crmc.state.ri.us/. This white paper provides an excellent catalog of the available digital terrain data for the communities along the south shore of Rhode Island. Included in this review is the vertical and spatial resolution of various datasets as well as the map accuracy standards used for each dataset. The author also illustrates the usefulness of the various datasets with regard to their extent and availability. A graphic representation of the difference in modeling sea level rise with National Elevation Dataset 1/3 arc second elevation and LIDAR data demonstrates the critical need for elevation data the is accurate within the projected ranges of sea level rise. Craft et al., 2009. Forecasting the Effects of Accelerated Sea-Level Rise on Tidal Marsh Ecosystem Services. Frontiers in Ecology and the Environment. Available online at www.fronteirsinecology.org. Available in print in early 2009. This article makes use of the Sea Level Affecting Marsh Model (SLAMM) version 5 to forecast changes in salt marsh extent along the Southern Atlantic Coast.
This run of the model utilized USGS National Elevation Data, NOAA Tidal Data, and FWS National Wetlands Inventory Maps and output the results to GIS software to forecast changes in salt marsh extent across sea level rise scenarios from the IPCC. In addition the researchers collected field data to quantify the predicted changes in the delivery of ecosystems services from salt marshes. This is an excellent application of this promising model to use geospatial technology to illustrate oncoming changes from sea level rise.