eutrophication results in undesirable 

 changes in water quality, destabiliza- 

 tion of natural communities, algal 

 blooms, a takeover by pollution-tolerant 

 species, and frequent anoxic conditions. 



Studies have shown that many of the 

 water bodies of the MDPR are eutrophic 

 (Craig and Day 1977; Day et al. 1977; 

 Gael and Hopkinson 1979; Hopkinson and 

 Day 1979, 1980a, 1980b; Seaton and Day 

 1979; Craig and Day 1981; Witzig and Day 

 1982). This is the cumulative result of 

 natural and cultural factors. Domestic 

 wastes are an increasing problem because 

 of inadequate sewage treatment and 

 population growth. Urban runoff is also 

 a significant pollution source from 

 cities. Asphalt and concrete increase 

 surface runoff into storm sewers that 

 pour nutrients (as well as toxins) into 

 water bodies. Large quantities of 

 nutrients from eroded sediments, ferti- 

 lizers, and animal manure enter lakes 

 via drainage canals. Wastes from in- 

 dustrial sites are also often sources of 

 nutrients. Natural sources of nutrients 

 are precipitation, excrement from 

 waterfowl and wading birds, and organic 

 mineralization. 



The most comprehensive studies of 

 eutrophication in the MDPR have been 

 made in the Barataria and Pontchartrain 

 basins. Witzig and Day (1982) developed 

 a trophic state index (TSI) and classi- 

 fied the water bodies in the two basins 

 based on existing water quality data. 

 Their TSI uses several water quality 

 parameters (total organic nitrogen, 

 total phosphorus, Secchi depth, and 

 chlorophyll a) in a multivariate sta- 

 tistical approach (Brezonik and Shannon 

 1971). Results of the TSI analyses are 

 presented in Table 24. Negative TSI 

 scores indicate low nutrient enrichment 

 and positive scores high enrichment. 

 The various stations are also assigned 

 to one of the classical trophic cate- 

 gories (eutrophic, mesotrophic, oligo- 

 trophic) after the approach of Brezonik 

 and Shannon (1971). 



The waters of the two basins range 

 from hypereutrophic to meso-oligotrophic . 

 Generally, the most eutrophic stations 

 are those most affected by upland run- 

 off. It has been shown that upland 



runoff that flows via canals to water 

 bodies is a cause of eutrophication (Day 

 et al. 1977, Hopkinson and Day 1979, 

 Kemp and Day 1981). This channelized 

 flow contrasts with natural hydrology, 

 where upland runoff flows slowly through 

 wetlands before entering water bodies. 

 The relationship of canals to water 

 quality was demonstrated by Gael and 

 Hopkinson (1979) who showed a signifi- 

 cant correlation between the density of 

 canals and the TSI. Witzig and Day 

 (1982) correlated algal productivity and 

 TSI. Craig and Day (1977) showed that 

 large areas in the Terrebonne basin have 

 been closed to oyster fishing because of 

 domestic wastes from the city of Houma . 

 These wastes reach the affected areas 

 via the Houma Navigation Canal. 



Eutrophication can occur in all 

 salinity regimes, so all aquatic habi- 

 tats are susceptible. The effects of 

 eutrophication are similar in all areas, 

 although the characteristic species 

 recognized as biological indicators may 

 be different. Microcystis , Anabaena , 

 Anabaenopsis , Anacystis , and Spirulina 

 are common freshwater bluegreen algae 

 that are associated with eutrophic con- 

 ditions. Brackish water and marine algae 

 that indicate eutrophication tend to be 

 small, like Monodus , Nanochloris , and 

 Stichoco.ccus . 



In general, freshwater areas tend 

 to be more susceptible to eutrophica- 

 tion, because flushing rates are lower 

 in these areas than they are near the 

 coast where there is tidal action. 

 Also, the bulk of cultural eutrophica- 

 tion occurs in fresh or slightly 

 brackish areas. Finally, a nutrient 

 trapping process characteristic of 

 brackish waters tends to reduce nutrient 

 levels in the lower basin. This trap 

 mechanism involves the flocculation and 

 sedimentation of clays with adsorbed 

 nutrients as fresh water first encoun- 

 ters measurable salinity. The sediments 

 in brackish water lakes thus become 

 repositories for excess nutrients, up to 

 a saturation point at least. Lake 

 Salvador may presently be acting as such 

 a nutrient trap. 



Nutrient sources to water bodies 

 fall into point and diffuse (nonpoint) 



155 



