Louisiana Coastal Wetland Loss Continues

July 08, 1997

Several recently released and ongoing studies of coastal wetland loss in Louisiana all reveal that the state is continuing to lose wetlands at a significant rate.

1978-90 Study
New estimates of Louisiana coastal wetland loss for 1978-90 indicate a loss of about 35 square miles a year of freshwater and non-freshwater marshes and forested and scrub-shrub wetlands. That equates to a total 12-year loss of about 420 square miles, an area equal to twice the size of the populated greater New Orleans area.

The estimates are from studies performed by the U.S. Geological Survey's National Wetlands Research Center (NWRC) in Lafayette, La., and the U.S. Fish and Wildlife Service's National Wetlands Inventory (NWI) in St. Petersburg, Fla. Researchers study wetland loss through aerial photography, satellite imagery, and computerized geographic information systems, which can compute losses as small as a third of an acre or less. For Louisiana wetland loss data, they studied areas in the Coastal Zone Management boundary, established by the Louisiana legislature in 1978.

Because studies were limited to these boundaries, however, the statewide coastal loss is probably underestimated and may be closer to 40 square miles a year. According to Dr. James B. Johnston, USGS, the NWRC-NWI study area actually represents only about 80 percent of the state's coast. Missing are 34 percent of Barataria Basin (868 square miles) and 44 percent of Terrebonne Basin (1,694 square miles). These northern portions of the basins were not included in the original Coastal Zone Management boundary. They, however, are experiencing significant losses, according to U.S. Army Corps of Engineers studies and an NWRC analysis of satellite images over the past 15 years.

The 1978-90 losses continue trends reported earlier by an NWI report on the Southeast (North Carolina to Louisiana), which said that from the mid 1970's to mid 1980's, Louisiana experienced nearly all the saltmarsh loss in the Southeast and accounted for 85 percent of the nation's 111-square mile loss of non-freshwater areas (brackish and salt marshes). But Louisiana's coast is unique because it has freshwater marshes associated with the Mississippi River. When the loss of the state's coastal freshwater marshes are added, Louisiana's coastal loss during 1978-90 is four times greater than all of the nation's estuarine coastal land loss from the mid 1970's to mid 1980's.

Based on a report of the National Oceanic and Atmospheric Administration, Louisiana's marshes alone represent more than half of all tidal marshes in the lower 48 states. As of 1990, Louisiana had 5,900 square miles of coastal wetlands left: 3,940 square miles of marsh, about 1,434 square miles of which were freshwater marsh; 1,750 square miles of forested wetlands; and 234 square miles of shrub-scrub wetlands, according to the NWRC data.

In the past, rates of yearly wetland loss in the state have been as high as 50 square miles a year, primarily from the mid 1950's through the 1970's. Overall, scientists estimate that since 1956, Louisiana wetlands have shrunk by more than 1,500 square miles, an area one and a half times the size of Rhode Island.

It should be noted, said Dr. Johnston, that the greater New Orleans area alone lost over 135 square miles of wetlands from 1956 to 1990, with urbanization accounting for more than 50 percent of the loss and hurricane and flood protection being the second greatest contributors.

1990-93 Loss in Selected Areas
Selected areas studied between 1990 and 1993 by the Louisiana Department of Natural Resources (DNR) and Department of Wildlife and Fisheries (DWF), and NWRC indicate that the loss continues into the `90s. Areas of significant loss include Chalmette, Grand Bois (Southeastern Terrebonne), and Western Terrebonne Basin.

Chalmette marshes just below New Orleans have continued ponding, that is, conversion of marshes to water. Between 1990 and 1993, the area loss was four square miles or over nine percent.

The Grand Bois area fresh marshes show continued ponding. During the three-year period, over 1,200 acres or 6 percent of wetlands were lost. The Western Terrebonne Basin's fresh and intermediate (slightly brackish) marshes in the Penchant area have increases in new ponds that may be related to Hurricane Andrew's destruction of flotant (floating) vegetation or continued loss to open water, or a combination of both. Total loss was 41 square miles, or approximately six percent was converted to water.

Nutria Studies
While wetland loss has historically been caused by various natural and human-caused factors such as hurricanes, sea-level rise, subsidence, and leveeing, destruction by nutria is significantly contributing to the loss. Nutria, an exotic species introduced from South America, feed on the roots of marsh vegetation that hold the wetlands together. When the vegetation is destroyed, the wetland loss may be permanent.

A survey in 1996 by DWF with support from NWRC and Barataria-Terrebonne National Estuary Program in Thibodaux, La., detected sites with about 32 square miles of wetlands damaged by nutria in the Barataria-Terrebonne Basin, an increase of eight square miles since 1993. Scientists believe, however, that the destruction is more widespread, perhaps four times as much, or 128 square miles for the basin area. Other areas where DWF has observed large damages but has no data yet are parts of the Chenier Plain and areas near Caernarvon.

Monitoring Gains and Losses
Dr. Robert E. Stewart, NWRC director, said, "The National Wetlands Research Center will continue to work with its partners throughout the state, not only to monitor wetland loss, but more importantly, to monitor wetland gains as a result of the Breaux Act and other restoration efforts in the state."

The Coastal Wetlands Planning, Protection, and Restoration Act of 1990 (Breaux Act) funds coastal restoration projects, primarily in Louisiana. The DNR and NWRC co-chair the monitoring of more than 75 restoration projects that have been funded through the Breaux Act.

US Geological Survey

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