found in all seven MDPR hydrologic 

 units, over 70% of this habitat exists 

 within the Barataria, Terrebonne, and 

 Vermilion units. All of the 4,000 ha 

 (10,000 acres) of scrub-shrub in the 

 Barataria and Vermilion units have de- 

 veloped since the 1950' s as a result of 

 human modifications to basin hydrology. 



MANGROVES (11) 



The black mangrove ( Avicennia 

 germinans ) occupies a small area in the 

 MDPR; only 2,955 ha (7,299 acres) in 

 1978 (Table 5). It occupies a fringe at 

 the southernmost end of the region on 

 the inside edge of the barrier islands 

 (Figure 28). There are also stands of 

 black mangroves on the Isles Dernieres, 

 Timbaliers, Breton Islands, and the 

 Chandeleurs. 



Functionally, the black mangrove is 

 an ecological analog of salt marsh cord 

 grass that grows in similar high salini- 

 ty intertidal areas, produces organic 

 matter (most of which decomposes to 

 detritus), and has been linked to the 

 support of fishery species. In addi- 

 tion, mangrove roots and stems stabilize 

 sediments and absorb storm wave energy, 

 as do the stems of Spartina . 



Mangroves are dominant in coastal 

 zones at lower latitudes (e.g., southern 

 Florida) but their range is limited to 

 about latitude 30 deg. N because they 

 cannot tolerate hard freezes. In the 

 MDPR, irregular freezes every 7 to 10 

 years result in the dieback of the man- 

 grove fringe. Thus, in the MDPR, this 

 plant is at the northern edge of its 

 range and particularly vulnerable to 

 cultural stress. Because of its intol- 

 erance to cold, the only occurrence of 

 the black mangrove in the north central 

 Gulf coast is in the MDPR. One has to 

 travel east near Cedar Key on the 

 Florida gulf coast, or to the Texas 

 coast near Galveston (Sherrod and 

 McMillan 1981) before encountering man- 

 groves again. On the Atlantic coast of 

 Florida black mangroves occur as far 

 north as Daytona Beach, about latitude 

 29 deg. N. 



The distribution of mangroves with- 

 in the MDPR reflects the salinity regime 

 as well as climate; mangroves are scarce 

 in the Mississippi Delta and the Atcha- 

 falaya hydrologic units because of high 

 freshwater inputs to these areas, which 

 favors other plants. Most of the man- 

 grove acreage in the MDPR occurs in the 

 Pontchartrain, Barataria, and Terrebonne 

 basins. 



Other than mapping, there has been 

 little research on mangroves in the 

 MDPR. However, there is a large body of 

 information on mangroves in the tropics 

 which lends insight into the mangroves 

 of this region. The black mangrove that 

 occurs in the MDPR, Avicennia germinans , 

 is one of three major species that are 

 found in the new world tropics. The 

 other two species are red mangroves 

 ( Rhizophora mangle ) and white mangroves 

 ( Laguncalaria racemosa ) . 



Mangrove production rates as high 

 as 3,000-4,000 g dry wt/m 2 /yr have been 

 reported. As with salt marshes, the 

 main factors that limit productivity are 

 nutrients, salinity, and drainage char- 

 acteristics of the soil. Most of the 

 fauna of mangrove swamps are detriti- 

 vores, and much of the production is 

 exported. Review papers on mangrove 

 swamps have been written by Walsh 

 (1974), Lugo and Snedaker (1974), and 

 Kuenzler (1974). 



The mangrove zone serves an addi- 

 tional functional role over salt 

 marshes. The mangrove is a woody shrub 

 that grows in the MDPR to a height of 

 about 3 m (10 ft). It provides ex- 

 tremely valuable elevated nesting sites 

 over water for a number of species of 

 wading birds, including egrets, herons, 

 and ibises. Pelicans formerly nested 

 (until 1961) on North Island (Chandeleur 

 Sound) . A fairly large colony of wading 

 birds nests in mangroves on the Isles 

 Dernieres (Raccoon Pt. Island). Queen 

 Bess Island behind Grand Terre Island is 

 a tiny mangrove cay that is densely 

 populated by pairs of nesting wading 

 birds each spring. Unfortunately this 

 island and many of its neighbors are 

 diminishing in size each year because of 



58 



