Coastal Services Center

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Keeping Wetlands Healthy

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The Goal:

Prevent the decline of coastal marshes.

The Issue:

How do you identify and track deteriorating wetlands?

The Answer:

A change analysis of the area created by comparing classified Landsat Thematic Mapper (TM) imagery and marsh baseline maps derived from remote sensing data for an inventory of existing wetlands.

The Example:
Louisiana's Mermentau River Basin

Louisiana's lower Mermentau River Basin fronts the Gulf of Mexico and is vital to many sport and commercial fisheries. In fact, as much as 16 percent of the nation's fisheries harvest, including shrimp, crabs, crayfish, oysters and many fin fish, comes from Louisiana's coast (U.S. Department of Commerce 1996). In addition, migrating birds, especially ducks and geese, winter in the basin. The river basin is marked by numerous national and state refuges and protected areas that serve as critical habitat for hundreds of thousands of waterfowl and other wildlife.

Across Louisiana, critical wetlands, such as those in the Mermentau River Basin, are at risk. Louisiana has lost up to 40 square miles of marsh a year for several decades. If the current rate of loss is not slowed, by the year 2040 an additional 800,000 acres of wetlands will disappear, and the Louisiana shoreline could advance inland as much as 33 miles in some places. Changes like these could seriously impact the health of Louisiana's fisheries and wildlife, while increasing the risks of coastal communities to erosion and other hazards.

The state's wetlands are being damaged by human activities such as intensive development, nonpoint source pollution, agricultural runoff, and oil exploration. Storms, sea-level rise, subsidence or the gradual deterioration of soil, and other natural causes also threaten marshes. Recently, a new problem has appeared.

1996 land cover data for Louisiana with the location of the Mermentau River Basin outlined in yellow.

Coastal marsh dieback, also known as "brown marsh," is the rapid and unusual browning of smooth cordgrass (Spartina alterniflora). This browning first began during the spring of 2000, long before marshes usually turn brown in the fall. The phenomenon has been observed in parts of Texas and Florida, but is mostly found in coastal Louisiana. In some cases, dense vegetation has been converted to open mud flats with only a few plants. If coastal marsh dieback continues or intensifies, the loss of vegetation could result in rapid subsidence and erosion of the unstable marsh soils. This, in turn, would leave coastal communities vulnerable to direct exposure from hurricanes and tidal surges.

Applying Land Cover Data:

For southwestern Louisiana's Mermentau River Basin, the 1990 to 1996 land cover classifications were derived from Landsat Thematic Mapper (TM) imagery from three different years: 1990, 1993, and 1996. Coastal Change Analysis Program (C-CAP) change analysis methodology was used to characterize changes to the landscape over these years by comparing the landscape in one year to that of the next year. The resulting change data were analyzed to identify changes to the wetlands.

By conducting a change analysis for the affected areas and comparing that to a baseline map of Louisiana's coast, managers can determine what is happening to the wetlands on a large scale, what the wetlands were converted to, and how fast and in what areas the changes have occurred.

Unlike most of Louisiana, estuarine (saltwater) marshes in the Mermentau River Basin actually increased during the last decade. Palustrine (freshwater) marshes and mature forested wetlands remained relatively stable. This is good news for coastal managers in the Mermentau River Basin, at least for the moment. For now the basin is free of brown marsh and wetland loss, and the managers can use the satellite-derived land cover data as a baseline data set for comparison if the disease should ever impact the area. Land cover data helped the managers sketch the big picture of the health of the marsh. Managers in Louisiana have already started (or plan to continue) using Landsat imagery to look at the larger picture of the health of Louisiana's coast.

Using the big picture as a guide, managers can identify wetlands that need a closer look and supplement the satellite-derived data with other remote sensing technologies. Some of these methods include fixed-wing transects, helicopter transects, and aerial photography. Fixed-wing and helicopter surveys are key in locating and determining the extent of marsh dieback. Aerial photography is important in providing high-resolution imagery for studying marsh dieback in relation to adjacent wetland areas.

The Result:

At the moment, "brown marsh" has not spread to the Mermentau River Basin. By using satellite-derived land cover data, managers can determine the presence and extent of the wetlands, establishing a baseline that will be very helpful if coastal marsh decline or some other problem threatens the region in the future.

Works Cited:

U.S. Department of Commerce. 1996. Current Fishery Statistics. National Oceanic and Atmospheric Administration, National Marine Fisheries Service.

To learn more about Louisiana's coasts and brown marsh, visit www.LAcoast.gov or www.brownmarsh.net.

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