nutrients may be taken up during plant growth or 

 by bacteria during detritus formation. Some of 

 these nutrients may be exported later as organic 

 detritus, a form more compatible with natural 

 populations. Phosphorus may physically bind with 

 sediments, and nitrogen may be denitrified. 



The natural resource productivity of a basin is 

 thus a function of the particular mix of habitat 

 types, especially the relative proportions of natural 

 wetlands and water bodies, and the degree of 

 human perturbation. 



LAND MODIFYING PROCESSES (C) 



Submodel C (fig. 10) represents the dynamic 

 habitat area changes that occur within a basin of 

 constant area. Over the past several thousand years, 

 the dominant trend has been the growth of the wet- 

 land habitat concurrent with the formation of new 

 chenier ridges. The aerial gain of these habitats was 

 at the expense of aquatic habitats (nearshore Gulf 

 and inland water bodies). During the past 50 years, 

 however, the major change has been loss of natural 

 wetland (C^), either to open water (C2), or by im- 

 poundment for waterfowl and/or agriculture (C3). 

 Basically two processes cause loss of natural wet- 

 land: hydrologic changes resulting from canalling, 

 marsh burning, or impounding; and natural sub- 

 sidence and erosion. Hydrologic changes are not 

 always local phenomena. For example, artificial 

 maintenance of the present Mississippi River course 

 on the eastern side of the delta means that very 

 little new sediment is reaching the area. 



SOCIOECONOMIC FACTORS (D) 



Submodel D represents human effects at the 

 basin level (fig. 10). Socioeconomic factors have 

 been lumped into five main components: 



1. The tolid human population in a basin (D^), 

 its energy and material requirements and its 

 waste production; 



2. Commerce and industry (D2) such as manu- 

 facturing, refining, retail sales, etc., that 

 occur in a basin, along with the concomi- 

 tant waste release; 



3. Mineral resources in a basin (D3), primarily 

 petroleum and natural gas (port and naviga- 

 tion facilities are included here); the extrac- 

 tion of minerals and maintenance of naviga- 

 tion channels entails release of waste, as 

 well as extensive disruption of natural habi- 

 tats (dredging, etc.); 



4. Fishery' and wildUfe resources harvested by 

 man (D4) both commercially and for sports 

 purposes; and 



5. All agricultural activity (D5), especially rice 

 and cattle. This activity also entails signifi- 

 cant waste release, especially nutrients and 

 pesticides. 



D^, D9, and D5 all require large quantities of fresh 

 water. Some species in D4, especially waterfowl, 

 are limited by freshwater bodies, and D3 requires 

 fresh water for some processes. 



BASIN SYNTHESIS 



The water requirement of the socioeconomic 

 submodel (fig. 10) is a convenient place to begin 

 a discussion of the connections among the four 

 basin submodels. The basin natural resource fresh 

 water (B) is required by all five components of sub- 

 model D, as indicated by the broad-branched arrow. 

 Many of these water needs are met by groundwater 

 pumping, but surface fresh water is also used, 

 especially for rice irrigation and waterfowl habitat. 

 The other input to submodel D from submodel B 

 represents the harvest of commercial and sports 

 fisheries and wildlife, which is a function of basin 

 quality or natural resource productivity. 



Effects of the socioeconomic sector on other 

 submodels ai'e broken down into waste effects, 

 effects on hydrology, and developmental decisions 

 based on market conditions (economics) that lead 

 to habitat changes. 



Wastes, which include nutrients, toxins, and 

 dredged spoil, affect the natural resource pro- 

 ductivity of a basin. Nutrient wastes, such as 

 sewage or fertilizer, can decrease NRP by causing 

 eutrophication, or if applied judiciously to wet- 

 lands, can actually increase NRP. Toxins such as 

 pesticides and heavy metals generally lower NRP, 

 and may selectively reduce higher consumers with- 

 out affecting lower trophic levels. Another form of 

 waste is dredged material which can create silting 

 problems, e.g., destruction of oyster beds by silta- 

 tion. 



The socioeconomic sector affects basin hy- 

 drology via activities that disturb natural circula- 

 tion patterns, especially by dredging canals or navi- 

 gation channels (Stone and McHugh 1977). Fresh- 

 water pumping can also affect hydrologic change 

 by lowering the water head relative to sea level and 

 causing salt water intrusion. Freshwater availability 

 is so critical to all socioeconomic sectors that it can 

 set ultimate limits to economic growth and develop- 

 ment in a given basin. 



42 



