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publications > paper > PP 1011 > south florida's hydrologic systems > water quality > water quality of the everglades
South Florida's hydrologic systems
Water quality of the Everglades
Although concentrations of nitrate and phosphorus generally are low in water in the Everglades, they may become excessive during droughts when most of the marshlands dry up and aquatic biota are forced to the deeper water of ponds to survive. Wastes and breakdown products from these organisms accumulate and cause concentrations of oxygen to decrease and concentrations of nitrate and phosphorus to increase in the water. Under anaerobic conditions, nitrogen and phosphorus compounds may be released from the sediment into the water. Evaporation also causes increased concentrations of these and other elements in the water. Nitrate concentrations are also increased in the Everglades by water distributed from the Everglades agricultural area.
Plans to backpump nutrient-rich water from the eastern urban region into the conservation areas (Crowder, 1974h) have raised some concern. The extent to which these nutrients, and possibly other compounds, would alter the water quality and the biota is unknown although aquatic plants and sediments would remove some nutrients. Studies indicate that sawgrass, the dominant emergent marsh plant of the conservation areas, has only a limited capacity for removing nutrients from water (Steward and Ornes, 1973b, c). Controlled tests showed that 1 ha of sawgrass uses 1.8 kg of phosphorus in 1 year's growth. At this utilization rate, 890,000 ha (2.2 million acres) of sawgrass would be required to remove the phosphorus contained in the waste water being discharged into inland canals each year (Steward and Ornes, 1973a). The total area of the three conservation areas, however, is only 348,000 ha (860,000 acres). It is unlikely, therefore, that sawgrass could efficiently renovate backpumped waste water of high nutrient content (Steward and Ornes, 1973a). In addition, water to be pumped into the conservation areas would not be distributed evenly through the areas. In Conservation Areas 1 and 3, the water presently stays chiefly within the peripheral canals, In Conservation Area 2, however, canal water does move into the interior marshes.
The marsh system in general appears to have a limited capacity for assimilating nutrients. Application of phosphorus at a continuous rate of 2.5 kg/ha per week on Everglades test plots caused stress after 3 weeks and extensive biological change and disruption after 8 weeks (Ornes and Steward, 1973). The changes in the composition of the plant community are evidence of the effects that could occur from an increased supply of nutrients into the Everglades ecosystem. Another study, however, has shown that inorganic forms of nitrogen and phosphorus are removed from canal water soon after it moves into the Everglades marshes (Gleason, 1974).
Data collected from many areas in south Florida indicate that pesticide concentrations in water are generally low. This fact is not surprising because most pesticides are insoluble in water and are absorbed readily on sediments. In the sediments of canals or marshes, high concentrations have been found (Klein and others, 1973; Carter and others, 1973). For instance, some soil samples underlying marshes in the Everglades had concentrations of the DDT family (DDT, DDD, and DDE) as much as 1,000 times greater than the concentrations found in water (Klein and others, 1973). Concentrations of the DDT family tend to become higher and accumulate in the higher orders of the Everglades food chain (Klein and others, 1973; Carter and others, 1973; Feltz and Culbertson, 1972). In the Everglades National Park, however, concentrations of the DDT family, dieldrin, and polychlorinated biphenyls (PCB's) in the upper food chain are mostly below the concentrations known to produce either acute or chronic effects (Ogden and others, 1974).
A study of pesticides at 10 sites in the Everglades showed that concentrations of the DDT family in bottom sediments and fish were much more prevalent at sites near agricultural areas than elsewhere. Concentrations of the DDT family in sediment at sites influenced by agriculture averaged 53 µg/kg (micrograms per kilogram) whereas those more remote averaged 31 µg/kg. The difference was most pronounced in fish samples. The average DDT concentration near agricultural areas was 340 µg/kg whereas in more remote areas it was 70 µg/kg (McPherson, 1973b). The data not only show that concentrations of DDT are higher in areas influenced by agricultural activities but also that DDT is more concentrated in organisms in the higher orders of the food chain (Klein and others, 1973).
For comparison, average concentrations of the DDT family in sediment and fish samples collected adjacent to Miami International Airport, an urban-industrial complex, were intermediate between average concentrations in agricultural areas and nonagricultural areas. Average concentrations of the DDT family in sediment and fish near the airport were 34 and 160 µg/kg, respectively (Freiberger and McPherson, 1972).
The toxic effects of pesticides on humans and wildlife are not absolutely clear at this time. It is known that most pesticides are persistent (that is, they are not easily degradable) and that they accumulate in the fatty tissues of both humans and wildlife. A human being may easily accumulate and retain pesticides in his tissues by consuming fish or meat from animals that have accumulated pesticides in their systems.
Much of the information on the effects of pesticides comes from the study of birds. Some species of falcons, hawks, and eagles have laid thin-shelled eggs, probably because of intake of pesticides of the DDT family. Experimental evidence shows that DDE, a metabolite of DDT, reduces the birds' ability to produce calcium for eggshells, thereby causing premature breakage and consequently limited offspring. Concentrations as low as 10,000 µg/kg in bird eggs might result in impaired reproductive success in some species. The highest concentration of DDE in bird eggs found in this study, however, was slightly over 1,000 µg/kg detected in osprey eggs (Ogden and others, 1974).
U.S. Department of the Interior, U.S. Geological Survey
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