preparation) . Hurricane stress aids in 

 the maintenance of wetland plant commu- 

 nities at a successional stage of low 

 maturity and high net production. In 

 addition, the threat of hurricane damage 

 may discourage coastal development and 

 indirectly prevent habitat destruction. 



The pathways of hurricanes that 

 have impinged on the gulf coast in the 

 past are illustrated in Figure 12. Be- 

 cause a storm may be as wide as 200 km 

 (124 mi) , any location along the coast 

 of the MDPR has about a 14% chance of 

 sustaining hurricane effects during a 

 given year (Baumann et al. in prepara- 

 tion) . 



Hurricanes and less severe tropical 

 storms are characterized by low baro- 

 metric pressure, which results in a 

 significant rise in sea level. At the 

 same time, onshore winds of up to 200 or 

 more km/hr (124 mi/hr) can produce a 

 storm surge in the MDPR as great as 7 m 

 (23 ft) in elevation, as did Hurricane 

 Camille, driving ocean water far inland 

 (USACE 1970). Torrential rains that 

 accompany hurricanes compound the flood- 

 ing problem. These effects can drasti- 

 cally change the shape of the coastline; 

 dunes may be washed away and new inlets 

 may be created across barrier islands. 



Major storms supply significant 

 amounts of the total average rainfall in 

 some areas. About 15% of the precipi- 

 tation in the MDPR is related to trop- 

 ical disturbances and 70% to frontal 

 passages (Muller and Wax 1978). 



The mean distribution of wind speed 

 and direction in the MDPR during four 

 months of a typical year is shown as 

 four wind roses in Figure 13. 



BIOLOGICAL SETTING 



setting of the MDPR is affected and 

 constrained by both the physical and 

 climatic settings. At the same time, 

 the physical and climatic settings are 

 affected by the biological setting. 



The regional biological setting is 

 most appropriately visualized as it ap- 

 pears to an observer flying over the 

 coastal zone at an altitude at which 

 major plant communities can be per- 

 ceived. This setting is a broad flat 

 zone of lush emergent wetland vegeta- 

 tion, intersected by wooded natural 

 levee ridges along river courses and 

 bayous. Numerous open water bodies are 

 turbid, algal rich, and the more inland 

 lakes may be covered with floating 

 plants. Flocks of wading birds and 

 waterfowl will be seen feeding in the 

 wetlands. Large numbers of commercial 

 and private fishing boats will be seen 

 plying the waters. The overall impres- 

 sion is one of a biologically rich and 

 fertile region. O'Neil (1949) mapped 

 the dominant coastal marsh communities 

 across the Louisiana portion of the 

 MDPR, including saline, brackish, inter- 

 mediate, and fresh marshes, but exclud- 

 ing forested swamps. The distribution 

 of these communities was again mapped by 

 Chabreck (1972) and later updated by 

 Chabreck and Linscombe (1978) and by 

 Wicker (1980), who included Mississippi 

 wetlands as well. 



At the regional level, the biolog- 

 ical components in the MDPR can be 

 visualized the way they appear on high 

 level imagery: bands of vegetation that 

 completely cover almost all subaerial 

 and intertidal surfaces. Such photo- 

 graphs exhibit roughness and color dif- 

 ferences that indicate differences in 

 biomass and/or productivity. This per- 

 spective indicates the relative role of 

 biological processes in determining the 

 topography of the regional landscape. 



The biological setting of the MDPR 

 is defined here as the broad pattern of 

 distribution of the organismal communi- 

 ties that occupy the region, the pro- 

 cesses that affect and limit their 

 distribution, and the regional effects 

 of these communities. The biological 



Organisms and topography are inter- 

 related in several ways. The production 

 of organic carbon by wetland vegetation 

 results in the net deposition of peat 

 during and after the long growing sea- 

 son. The carbon that remains after 

 losses from leaching, grazing, and 



23 



