RESPONSE OF LOUISIANA GULF COAST MARSHES 

 TO SALTWATER INTRUSION 



S.R. Pezeshki, R.D. DeLaune, W.H. Patrick, Jr., and B.J. Good 



Laboratory for Wetland Soils and Sediments 



Center for Wetland Resources 



Louisiana State University 

 Baton Rouge, LA 70803-7511 



ABSTRACT 



A review is presented of carbon assimilation response of wetland vegetation from brackish marsh, 

 freshwater marsh, and bottomland hardwood to flooding and salinity for several species 

 representative of wetland habitats from the Louisiana coastal area. The combination of salinity and 

 soil anaerobiosis adversely affected photosynthetic rates for many wetland plant species. Results 

 suggest that saltwater intrusion and brine discharge, common problems along the Louisiana coast, 

 will adversely affect normal plant physiological functions. Reduction in carbon assimilation, leaf 

 tissue death, and other responses will lead to stressed plants with decreased productivity followed 

 by habitat changes. Plant successional patterns and marsh management strategies in relation to 

 changes in salinity and marsh soil physicochemical properties are also discussed. 



INTRODUCTION 



Louisiana contains vast (3.2 million ha) coastal marshes representing 41% of those in the 

 continental United States (Turner and Gosselink 1975). The marshes extend inland from the 

 Gulf of Mexico for distances ranging from 24 to 80 km (Figure 1) and reach their greatest width 

 in southeastern Louisiana (Chabreck 1972). Features of the Louisiana coast are closely related to 

 the geological history of the Mississippi River. 



Deterioration of the coastal wetlands began in the early 20th century when the Mississippi River 

 was leveed. In this century, decreased sedimentation has resulted partly from efforts to maintain 

 the Mississippi River in its present channel. This loss of sediment and compaction of existing 

 sediment is a primary factor in the loss of Louisiana wetlands. Currently marshes are deteriorating 

 at the rate of over 130 km 2 /yr (Gagliano 1981). 



The marshes of the coastal wetlands, both deltaic and chenier plains, exhibit a striking zonation 

 of emergent plants (Figure 2). Vegetation types range from saline near the Gulf of Mexico to 

 brackish, then freshwater, then bottomland hardwood with increasing distance from the gulf. Water 

 levels in the marshes are affected by rainfall, tides, and local drainage patterns. Vegetation types 

 are influenced by hydrology, salinity, and the type of sediment involved. The mixing of saltwater 

 from the gulf and freshwater from inland sources provides a spatial zonation in water salinities. 

 Water salinities are high (20 to 25 ppt) near the coastline and gradually decline inland until a zone 

 of freshwater is reached along the northern perimeter of the marsh region (Chabreck 1981). 



The vegetation is confronted with progressively rising water levels (submergence) in many 

 wetland habitats of the Louisiana coast primarily because of the rapid subsidence. Water-level 



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