modify flows. The navigation and oil field access canals 

 together account for over one-half of the total. These 

 were built to provide the most direct and/or cheapest 

 route from one point to another and were constructed 

 without regard to hydrologic effects. There is httle 

 documentation on the hydrologic effects of these 

 canals the relationship between the hydrologic altera- 

 tions and wetland loss and salinity changes in the 

 Chenier Plain are not clear. The complexity of the 

 ecosystem has made it difficult to draw a causal con- 

 nection. A study is needed similar to the modeling ef- 

 fort in the upper reaches of Barataria Bay (Light 

 1976). Light's model suggested that dredged canals in 

 the basin have increased peak discharge rates by nearly 

 100%; consequently, runoff occurs more rapidly than 

 it would normally, and low-water stages have been 

 lowered about 15 cm (6 in). These changes result in a 

 higher suspended load capacity and more wetland in- 

 undations of shorter duration. 



At the basin level correlations between habitat 

 modifications (such as erosion) and human activities 

 (such as canal density) have been made. Although the 

 approach does not concern itself with the mechanisms 

 of the response-that is, the way the hydrologic regime 

 is modified and in turn modifies habitats— it has pro- 

 duced useful insights that are discussed in part 3.4.6. 



A correlation has recently been drawn between 

 canal density and eutrophication. Bedient and Gate- 

 wood (1976) showed that in Florida, as agricultural 

 drainage canal density increased, phosphorus concen- 

 trations in receiving waters also increased. Gael and 

 Hopkinson (1978) reported a similar phenomenon in 

 the Barataria Basin, southeastern Louisiana, for oil 

 field access canals (fig. 3-23). They found that the 

 eutrophic state indices (high index infers a dangerous 

 eutrophic state) for various areas in the basin (as mea- 

 sured by an index of four water quality parameters— 



Cumulaliv* Orkinsg* Ocntity 



m/fc 



Figure 3-23. The relationship of the trophic state 

 index of water to drainage density in 

 tJie upper part of Barataria Basin, Loui- 

 siana. The regression Hne accounts for 

 59% of the variation among points and 

 is highly significant statistically (Gael 

 and Hopkinson 1978). 



chlorophyll a, total nitrogen, turbidity, and total 

 phosphorus) were directly proportional to canal den- 

 sity. Canals speed the runoff of sediment- and nutrient- 

 rich agricultural water, and water from cleared forests 

 and urban lands. Instead of fiowing slowly over wet- 

 lands, where much of the sediment and nutrient load 

 is captured, this water flows directly through the canals 

 into downstream lakes and bays where the nutrients 

 stimulate the plankton growth that results from in- 

 creased eutrophication. 



3.4 HABITATS AND LAND-MODIFYING 

 PROCESSES 



3.4.1 INTRODUCTION 



Except for the major commercial and sports spe- 

 cies, relatively little is known about the standing stock, 

 life history, and ecological importance of the many 

 species inhabiting the Chenier Plain. Normal year to 

 year population fluctuations are wide, and a basin- 

 level inventory at any one time (if it were possible) 

 would yield litfle infomiation about the factors that 

 control population size. 



Populations of individual species result from the 

 interaction of many factors. A broad evaluation of 

 living resources requires the use of describable units; 

 the habitat is used for that purpose in this report. Re- 

 gardless of what is not known about living organisms, 

 it is known that they require a place to live-a habitat. 

 (Part 4.1 considers further development of the con- 

 cept.) 



Certain attributes of habitats make them useful 

 indices of living resources. First, they are objectively 

 defined landscape units whose areal extent can be de- 

 temiined. Second, if the ecology of individual species 

 is determined for small, representative habitat areas, 

 the results can be extrapolated to similar habitats else- 

 where. Third, tlie habitat as a unit contains an entire 

 spectrum of species, many virtually unknown yet all 

 functional parts of the habitat. Since our understand- 

 ing of these non-resource species (Ehrenfeld 1976) 

 and of their importance to ecosystems is fragmented, 

 emphasis should be on interactions between species 

 and their habitats. Finally, since a habitat is an irre- 

 ducible requirement of a species, changes in extent of 

 habitats can be expected to reflect long-term popula- 

 tion shifts for species of interest. The habitat acts as 

 an integrator of information about individual species. 



The habitats and their location in a basin result 

 from the interaction of geomorphic, cUmatic, hydro- 

 logic, and biotic processes on the geologic template 

 of the Chenier Plain. Habitats are dynamic, and the 

 interactions among habitats change constantly under 

 the influence of these forces. The physical processes 

 have been described in parts 2.0 and 3.3; biotic effects 

 are dealt with in part 4.0. Documentation of the rates 

 of change and processes responsible for major changes 

 are provided in tlie following section. This section 

 does not address the internal dynamics of these habi- 

 tats (for instance, changes in productivity of existing 

 habitats; part 4.0 deals with that topic). 



74 



