Coastal Wetland Loss

The relative rates of vertical marsh accretion and submergence determine the long-term stability of Louisiana coastal marshes. Coastal marshes are highly susceptible to submergence associated with a rise in relative sea level (Penland and Ramsey, 1989). Louisiana coastal marshes are undergoing rapid subsidence and currently experiencing rapid increases in the water level. Research conducted over the past quarter century in coastal Louisiana has shown that marsh accretion at many 

FIGURE 18.2 Wetland loss rates over the entire Lonisiana coastal area and in the Mississippi River deltaic plain. 

FIGURE 18.3 Impact of increase in submergence on wetland ecosystem process along the Louisiana Gulf Coast. 

FIGURE 18.4 Factors influencing carbon cycling in the rapidly subsiding Mississippi River deltaic plain. 

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Fig. 2. Rates of coastal wetland loss in the United States. From Gosselink and Baumann (1980).

Tornqvist, T.E., and Gonzalez, J.L. (2002) Reconstructing background rates of sea-level rise as a tool for forecasting coastal wetland loss, Mississippi Delta. EOS 83, 530-531. 

Turner, R.E. (1990) Landscape development and coastal wetland loss in the northern central Gulf of Mexico. Am. Zool. 30, 89-105. 

Wells, J.T. (1996) Subsidence, sea-level, and wetland loss in the lower Mississippi River Delta. In Sea-Level Rise and Coastal Subsidence (Milliman, J.D., and Elaq, B.U., eds.), pp. 281-311, Kluwer Academic Dordrecht, The Netherlands. 

Much of the coastal sedimentation accretion work has been conducted in the Mississippi River deltaic plain, where vertical accretion rates are large, and where there are also very high rates of subsidence and coastal land loss. In other coastal areas of the United States and Europe, a wide range of results have been found, with sedimentation rates varying from 0 to 1.5 cm year". Many coastal marshes are not accreting at a rate sufficient to compensate for the present rates of sea level rise. Many factors affect accretion rates in wetlands. These include plant community, density of vegetation, tidal elevation, sediment input from riverine, estuarine and marine sources, proximity to sediment sources, total organic matter input from primary productivity of wetland, and relative sea level rise. 

A series of diversion projects have been implemented to reintroduce freshwater and sediment from the Mississippi River into Louisiana coastal wetlands (Figure 18.9). A recent study examined the impact of Mississippi River freshwater diversion on enhancing vertical marsh accretion (mineral and organic matter accumulation) in Breton Sound estuary, a coastal wetland experiencing marsh deterioration as a result of subsidence and saltwater intrusion (DeLaune et al., 2003). The Caernarvon diversion has positively impacted marsh accretion in Brenton Sound estuary helping to slow or reverse wetland loss. Several hundred hectares of new marsh have been created by the introduction of Mississippi River water into the system (Villarrubia, 1998). 

Much of the wetland loss occurring in coastal Louisiana is due to the deterioration of highly organic marsh soil. As discussed earlier, conversion of coastal marshes to inland open water is associated with plant stresses such as saltwater intrusion into nonsaline marshes and increased soil waterlogging as a result of subsidence. Marsh elevation decreases rapidly following plant mortality because of the structural collapse of the living root networks (DeLaune et al., 1994). The peat collapse and the associated erosion result in the conversion of marsh into open water. Conversions to open water system releases a considerable amount of carbon into the estuary where it is either decomposed or... 

Leibowitz, S. G. and J. M. Hill. 1987. Spatial analysis of Louisiana coastal land loss. In R. E. Turner and D. R. Gaboon (eds.) Causes of Wetland Loss in the Coastal Central Gulf of Mexico. Volume II Technical Narrative. Final report submitted to minerals Management Srvice, New Orleans, LA. Contract No. 14-12-001-30252. OCS Study/MMS 87-0120. 400 pp. 

Land-use change in the coastal zone has accelerated the rate of pollutant loading for three reasons. First, removal of vegetation mobilizes materials, such as sediment. Second, the emplacement of impervious surfeces, such as roads and roofs, enhances pollutant transport as part of stormwater runoff Third, loss of natural habitats, such as wetlands, eliminates important ecosystem services, such as pollutant uptake and degradation. 

Lower terrestrial and coastal emission estimates combined with the increasing loss in wetlands 165-681. although it may not significantly impact the global sulfur cycle, may be an important consideration in local contributions of natural emissions to acid precipitation. 

Barras j. a., Bourgeois P. E. and Handley L. R. (1994) Land loss in coastal Louisiana 1956-1990. National Biological Service, National Wetlands Research Center, Open File Report 94-01, 4 p, 10 color plates. 

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