Table 3.63. Annual water balance for Chenier Plain basins. 



Rainfall 30 year average from U.S. Weather Bureau records; Riverine input long term average USGS 1977; evapotranspira- 

 tion calculated from Borengasser 1977; agriculture and industry use from table 3.41 and figure 3-7 assuming all industrial 

 water and 2/3 agricultural water from surface. It is assumed all water was evenly layered over each basin. 



broad evaluation. Therefore, eutrophication is evalu- 

 ated by using phosphorus as a general index of water 

 quality. "EutropWcation" refers to the natural or 

 artificial addition of nutrients to bodies of water, and 

 the effects of these additional nutrients on the biota 

 (National Academy of Sciences 1969). The ecological 

 consequences of nutrient additions, beneficial and/or 

 deleterious to the aquatic habitat, are discussed in 

 part 4.8. Generally, excess nutrient addition results in 

 aquatic community changes that may be detrimental 

 to sport and commercial fisheries. 



Phosphorus (P) is the critical limiting nutrient in 

 freshwater ecosystems (Likens 1972). It is a conve- 

 nient indicator of eutrophication because it is not lost 

 as a gas through biological decomposition or chemical 

 change. Furthermore, it is a common constituent in 

 most of the common sources of materials that cause 

 eutrophication: municipal sewage, urban runoff, 

 drainage from agricultural land, and natural sources 

 (detritus, waterfowl wastes, eroded minerals). It is 

 a poor index of industrial wastes which can contain a 

 wide variety of toxins. Because P is a normal compo- 

 nent ofmost pollutants, its analysis within the Chenier 

 Plain waters allows a useful evaluation of water qual- 

 ity. This general analysis will be supplemented by in- 

 formation about other pollutants that appear to be 

 important in individual basins. 



Because of the rapid transformations of P (and 

 other nutrient elements) in the water column, the 

 concentration of inorganic P alone is a poor index of 

 eutrophication (Hutchinson 1969). A more significant 

 indicator is the total amount of P contained in inor- 

 ganic and organic dissolved forms. Phosphorus drains 

 into the major water bodies of each basin. This input 

 load, expressed in grams per cubic meter (g/m^), pro- 

 vides anindexof theeutropliic state of different water 

 bodies. The "input load" retained in the water, the 

 biota, and the underlying sediments of a basin, con- 

 stitute the "storage" compartment of an aquatic eco- 

 system. 



In this analysis the nutrients are assumed to be 

 homogeneously distributed throughout the water 

 bodies. Point sources of discharges are identified and 

 their magnitudes are listed when available (plates 5A 

 and 5B). Concentrations generaUy decrease as a func- 

 tion of distance from the source and of the volume of 

 the receiving waters because of mixing and dilution. 

 Circulation patterns determine how well these nu- 

 trients become mixed throughout the system. As a re- 

 sult, localized areas of high biotic production (ad- 

 vanced stages of eutrophication) can occur even 

 though the entire water body may not exhibit over- 

 enrichment. If nutrient inputs continue to exceed the 

 capacity of the system to assimilate them over time, 

 advanced stages of eutrophication proceed from point 

 sources and affect more extensive areas. Thus, the 

 average input load provides an indication of nutrient 

 enrichment of a water body even though the eutrophic 

 state may vary from place to place (Craig et al. 1979). 



It is assumed in this report that processes occurring 

 in saline waters are similar to those in freshwater sys- 

 tems and that it is valid to average quantities over an 

 entire basin, regardless of the fact that differences oc- 

 cur in salinity. For instance, salt water flocculates col- 

 loidally suspended nutrients so that they sink. The re- 

 lationship between the loading rate and P retention in 

 the sediments was worked out for freshwater lakes 

 and may not apply equally in brackish areas. Finally, 

 nitrogen (N) is more often limiting as a nutrient than 

 P in coastal marine systems (Ryther and Dunstan 

 1971). This, however, does not invalidate the use of P 

 as a tracer of eutrophication, since N and P appear to- 

 gether in equal amounts in most pollutants. 



Input loading rate. Critical P levels were taken as 

 indicators of eutrophication from previous studies 

 (table 3.64). The values of Shannon and Brezonik 

 (1971) stated on a volumetric basis are used in this re- 

 port. They considered P loads less than 0.12 g/m^ 

 (1.2 X 10''' oz/ ft"' )/yr as permissible, and loads greater 

 than 0.22 g/m' (2.2 x lO"'* oz/ft^)/yr as dangerous. 



91 



