More interesting, however, is the potential for mangroves to expand into the area, replacing 

 undeveloped and developed lowland. Although the northern limit of mangrove distribution in 

 eastern Florida is about 80 km south of the Matanzas area, the mangroves here are poorly 

 developed (Odum et al. 1982). The climatic warming that would generate increased sea level rise 

 also would provide favorable conditions for mangrove expansion beyond the center of species 

 distribution in the United States. Because mangrove swamps are modeled as resistant to lateral 

 erosion, a simulation of the area with mangroves is quite different from one with just marsh. 



The potential for mangrove expansion is seen more clearly in the response of the 10,000 

 Islands region of south Florida. Here, under both scenarios through the late twenty-first century, 

 mangroves could become dominant land categories as they moved inland with the advancing tide, 

 replacing marsh and lowlands. 



In the Florida Keys simulations, rapid expansion of mangrove onto areas previously 

 occupied by freshwater marsh in the Everglades is also an artifact of the model. Under the low 

 scenario, the replacement of freshwater marsh by mangrove would not occur until near the end 

 of the simulation period. Before then, limited expansion of mangroves onto undeveloped lowland 

 in the Keys would occur. By 2100, however, the extensive freshwater marsh areas on the 

 mainland adjacent to the Keys are inundated at high tide, assuming no influx of freshwater. 

 Under the high scenario, freshwater marsh areas would be subject to tidal water intrusion and 

 conversion to mangrove swamps by the year 2075 unless significant freshwater discharge would 

 inhibit this trend. However, by the year 2100 mangrove areas would be lost due to complete 

 inundation, and only tidal flats would remain on the higher Keys. 



Along the Florida Gulf Coast north of the mangrove zone, wetlands would expand inland 

 under both the low and high scenarios. Substantial marsh would remain even as late as 2100 in 

 the high scenario by virtue of available adjacent lowland. Expansion of salt marsh, however, 

 would peak in the high scenario by about 2075, to be followed by increased submergence on the 

 seaward side, greatly slowing down the net increase in wetland area. Under the low scenario, salt 

 marsh would still be expanding fairly rapidly in the year 2100. 



Mississippi Delta 



The response pattern for all the Louisiana wetland simulations was remarkably similar for 

 Barataria Bay, Atchafalaya Delta, and the Central Isles Demiere in the Terrebonne Delta. High 

 subsidence rates (11 mm/yr for Barataria Bay; Hatton, DeLaune, and Patrick 1933) are not 

 entirely offset by high accretion rates. Rising seas thus accelerate loss of seaward salt marshes 

 and disequilibrium is introduced into the salt marsh system. By the year 2100, and often even 

 before (e.g., in the Atchafalaya by 2060), the extensive gains in salt marshes are totally flooded by 

 rising seas and converted to sheltered or open water. By that time, the extensive freshwater and 

 brackish marshes would be long gone. 



Although the pattern under the high scenario was similar, trends developed at a faster rate. 

 By 2050 most salt marshes were totally inundated (Figure 4-16). Elsewhere the process was a bit 

 slower but the trends were similar. In those cases complete loss of salt marsh was apparent by the 

 year 2075 in the high scenario (Figure 4-16). These rapid losses occurred despite a simulated 

 accretion rate of 10 mm/yr in marshes. The loss rate of salt marshes in the later decades of each 

 scenario can be attributed partly to a lower accretion rate which can be expected as estuarine 

 conditions prevail. However, even with a constant rate of 10 mm/yr, rapid losses of wetlands in 

 coastal Louisiana would result from accelerated sea level rise, as simulated. 



Chenier Plain-Texas Barrier Islands 



In the sample area considered on the Chenier Plain of Texas, extensive freshwater marshes 

 lie behind lowlands which include small salt marsh areas. Under the low sea level rise scenario, 

 the seafront salt marshes are largely lost to tidal flats by the year 2000, but inland marshes are 

 unaffected. However, in succeeding decades, salt marsh expands onto adjacent freshwater marsh, 



114 



