Coastal ecosystems are more easily envisioned 

 than described. The rocky coast of Maine, the 

 extensive low-lying wetlands and bayous of Louisi- 

 ana, the mangrove-coral systems of tropical Florida, 

 and the barrier island coast of Texas can each be 

 conceived as unique coastal ecosystems and adja- 

 cent to other coastal systems. 



Forces molding the structure of the system 

 include weather, wave energy', sediment transport, 

 and the long-term processes of subsidence as well 

 as climatic and geologic change. These physical 

 processes result in the establishment of the envi- 

 ronment and substratum upon which biological 

 communities develop. In turn, the communities 

 influence the continued evolution of the system. 



The Chenier Plain ecosystem, for example, is 

 considered a transition zone between the active 

 Mississippi River delta to the east and the relatively 

 stable barrier island system to the west. Fluctua- 

 tions of sediment availability from the Mississippi 

 River over the past 5,000 years have resulted in the 

 accretion of a vast coastal system composed of 

 emergent wetlands, lakes, ponds, estuaries, tidal 

 channels, and slightly elevated stranded beach 

 ridges. Like similar extensive estuaries, the Chenier 

 Plain owes its existence to the relative stability of 

 sea level over the past several millennia and to the 

 abundant sediment supply of a major river. Eco- 

 system boundaries, although defendable, have been 

 somewhat arbitrarily estabUshed and reflect the 

 functional differences between adjacent systems. 

 With this natural background plus a 30-year history 

 of onshore and offshore oil and gas and other 

 development activities, the Chenier Plain provides 

 an ideal setting for piloting the implementation of 

 the ecosystem characterization concept. 



CONCEPTUAL ECOSYSTEM MODEL 



After the boundaries of the system have been 

 established, the next step is the development of a 

 conceptual ecosystem model. The model guides the 

 entire characterization effort by providing the 

 framework for identifying important natural 

 resource components of the system and the 

 functional processes which affect their survival and 

 productivity. 



The modeling approach for the Chenier Plain 

 involves a four-level analysis of the system. At the 

 first level, a broad regional model considers the 

 entire ecosystem, emphasizing geomorphology and 

 the geologic processes responsible for the origin of 

 the system, and the long-term system changes. 



Most natural changes at the ecosystem level occur 

 on the order of thousands of years and it is diffi- 

 cult to incorporate this information into planning 

 and iinpact analysis procedures. The framework is 

 useful, however, for providing a proper perspective 

 to the other components of the system (fig. 3). 



Time Scale of 

 Natural Change 



1,000 + Years 



1-100 + Years 



0.01-10 Years 



Open Water 



Salt Marsh 



Figure 3. Stratified organization of conceptual 

 model of Chenier Plain ecosystem. 



At the second level, the Chenier Plain ecosys- 

 tem is subdivided and modeled as six subsystems 

 generally representing different drainage basins or 

 hydrologic units. Hydrologic processes dominate 

 basin function and provide a mechanism for inte- 

 grating basin components. Natural change occurs 

 on the order of one to several hundred years, a use- 

 ful scale for planning and impact analysis. 



The relatively homogenous units which vari- 

 ously might be termed communities, associations, 

 or habitats are the third level of resolution. Basins, 

 therefore, emerge as spatially heterogeneous areas 

 composed of a number of interacting habitats. At 

 the community level, change is constant, seasons 

 come and go, plants and animals live and die and 

 man's impact on the environment is most apparent. 

 It is the habitat which is altered by dredging, pol- 

 luted by oil spills, or drained for agricultural, urban, 

 or industrial development. Most environmental 

 changes are viewed in respect to these habitats. The 

 conceptual model identifies functional relation- 

 ships between habitats, which would then permit 

 planning and cumulative impact analysis, at the 

 basin level, for the Chenier Plain. 



At the fourth level of the hierarchy, the natural 

 history, growth dynamics, and environmental 

 limits arc considered for species of commercial, 

 recreational, or functional importance in the 

 Chenier Plain region. 



