Salt Marsh Die-Back in Coastal Louisiana

This past summer scientists and natural resource managers from several governmental agencies and universities participated in tracking the extent and progression of a massive die-back of salt marsh, mainly Spartina alterniflora, in coastal Louisiana. The greatest extent of die-back has occurred in the Barataria and Terrebonne basins, where out of 389,160 total acres of salt marsh, 105,570 acres are estimated to be dead or severely stressed, and an additional 145,935 acres have been categorized as having a moderate impact. (For more information, see www.Lacoast.gov). Thus, in Barataria and Terrebonne basins, only about 35% of salt marshes are unaffected. Some marshes in other portions of Louisiana and in Texas have also experienced marsh die-back. In many of the worst die-back sites, other drought-and-salt tolerant species like black mangrove (Avicennia germinans) seedlings and saltwort (Batis maritima) have survived and show no signs of stress. In some locations, patches of salt marsh hay (Spartina patens) and saltgrass (Distichlis spicata) also survive in the midst of dead or dying S. alterniflora.

Coastal marshes of Louisiana are experiencing extremely high rates of loss due to a variety of factors, and this loss could be accelerated if a hurricane were to occure while the marches were affected by die-back (peak hurricane season is September-November). This marsh die-back is therefore being viewed as a serious environmental emergency. Louisiana governor Mike Foster formally requested federal financial aid to help determine the cause(s) of the die-back and to develop a plan of action.

Even before the emergency declaration, a number of scientists from several agencies and universities had diverted some of their time from their ongoing studies in order to help address this critical problem as it was occurring. Several groups are tracking the die-back in field studies in various locations. In addition, other university and agency scientists are re-analyzing existing water quality and hydrological data, examining flow records of the Mississippi River, analyzing weather and climatic patterns of the region, and developing conceptual models relating various environmental factors (e.g., drought, pore water salinity, soil hydrogen sulfide, etc.) to the spatial and temporal patterns of die-back.

Salt Marshes Die; Murder Weapon Still Missing

No "smoking gun" has yet been revealed in the limited studies to date, although several hypotheses have sifted to the top as likely explanations for the die-back. Most center around the extreme drought conditions that have existed in southern Louisiana for several years. High temperatures and low water levels might combine, for example, to lead to elevated pore water salinities. Although this combination seems like a fairly intuitive answer to the problem, few if any of the many sites sampled have yielded salinities high enough to be lethal to S. alterniflora. Some afflicted sites, especially locations where only short stubble or bare ground remain, appear to be low areas; this has lead the hypotheses that sediment saturation and hydrogen sulfide accumulation are involved as causative agents in the die-back. However, it is not clear if the areas were lower in elevation than the surrounding marsh before the vegetation died. If so, the higher levels of hydrogen sulfide found in some samples could be a result of root decomposition rather than a cause of the die-back.

Plant pathologists working with S. alterniflora samples from the Louisiana die-back as well as smaller similar die-backs in earlier years from Florida and Texas, have isolated several different taxa of pathogenic fungi. Some of the taxa involved have been implicated in disease in other plants, but have not yet been studied in S. alterniflora. Although the fungi have not been formally charged, they remain suspects, pending further investigation.

Other possible causes suggested for study include the effects of a low flow year in the Mississippi River, geologic faults causing increased loss of elevation, and changes in soil metal toxicity because of biogeochemical changes wrought by the drought conditions.

Although concerns for accelerated marsh loss are of paramount importance, other potential problems that could result from the die-back include severe reductions in annual primary productivity in the Louisiana salt marshes and loss of habitat for various birds, juvenile fishes, and invertebrates.

Spring into Action

No one knows what next spring will bring. If indeed the primary culprit is some environmental factor(s) associated with the drought, and if spring rains come, then recovery should occur in some of the stressed marshes. In other sites, though, samples of root and rhizome tissue showed that all or most of the below ground material was dead and rotting. These sites would obviously (will probably) not re-sprout from belowground tissue and would require colonization from other marshes before any natural recovery would occur. If the drought does not break as hoped, or if the cause is a pathogen, then next spring and summer may lead to increased marsh loss.

To avoid the worst-case scenario, various groups such as the Coastal Wetlands Planning, Protection, and Restoration Act (or Breaux Act) Task Force (a multi-agency entity that oversees coastal restoration), the Louisiana Dept. of Natural Resources, the U.S. Dept. of Agriculture's Natural Resources Conservation Service (which has extensive experience in developing plants for restoration), L.S.U. School of Agriculture's Rice Research Institute (which besides the name, also works on S. alterniflora culture and seed production and storage), the Governor's Office, and scientists from our lab (the USGS, National Wetlands Research Center) are coming together to further discuss the need for restoration and to develop innovative techniques for restoring massive areas in very remote locations.

Before spring, we hope to have plans for several different programs of restoration to implement in a rapid fashion if warranted. Projects might include such activities as advancing the implementation schedule of the Davis Pond diversion of water from the Mississippi River to feed more fresh water into parts of the Barataria-Terrebonne area; modifying agricultural techniques and practices to allow planting or seeding of the massive areas involved; identifying "hot spots" of land loss in marsh die-back areas for targeted planting either into or along water's edge; identifying and propagating any "resistant" genotypes for demonstration plantings.

All in all, the response from the scientific and resource management communities has been very positive and, given the magnitude of the problem and the speed at which it has developed, reasonably focused and coordinated. If you are interested in more information, go to the LACoast web site (www.LACoast.gov) or to the NWRC web site www.nwrc.gov. Also, you can email me (Edward_Proffitt@usgs.gov) with various questions or suggestions.