publications > circular > Circular 1134 > water and environmental stress > degradation of water quality > agricultural lands and everglades region U.S. Department of the Interior U.S. Geological Survey Circular 1134 Water and Environmental Stress Degradation of Water Quality Agricultural Lands and Everglades Region The marshes in the northern Everglades have been replaced, for the most part, by the EAA which includes about 700,000 acres of rich organic soils, most of which has been converted to intensively managed agriculture; the crops are primarily sugar cane and vegetables. Nutrients from the EAA soils are released as the result of periodic drying and oxidation of these organic soils by aerobic bacteria (soil subsidence). Once the soil is oxidized, large quantities of nitrogen and phosphorus are released and transported from these fields during subsequent rains. Nutrients are carried from the field through drainage ditches, water-control structures, and flood-control pumps into EAA canals. Water that drains the EAA farmlands contains low dissolved-oxygen concentrations; high concentrations of nitrogen, phosphorus, chloride, sodium and trace metals; high color; high specific conductivity; and occasional pesticides. Water quality generally is worse during periods of pumping than during those of no discharge. Present water-quality conditions in some northern parts of the Everglades are dramatically different than conditions that existed at the turn of the century. These changes are a result of the disruption of the natural flow patterns by drainage canals and the development of agriculture south of Lake Okeechobee. Nutrient concentrations in water discharged from canals that drain the EAA are significantly higher than those in the marsh at locations remote from the canals. Average flow-weighted phosphorus concentrations at canal- control structures that discharge to the northern Everglades from the EAA range from 0.10 to 0.25 mg/L and represent a tenfold increase in nutrient levels compared with water at the marsh sites remote from the canals (South Florida Water Management District, 1992). A number of studies have identified the Everglades marshes of the WCA's as a natural filtration system or nutrient sink that has a purifying, or "kidney effect," by reducing inorganic forms of nitrogen and phosphorus to background levels as the waters flow through the marsh (Jones and Amador, 1992). Much of the introduced nutrients are assimilated or incorporated into sediment and marsh vegetation. The marsh vegetation of the Everglades, however, has a limited capacity for nutrient uptake and is sensitive to small increases in nutrients which are measured in the parts per billion range. Historically, the Everglades was a low-nutrient system, limited primarily by phosphorus. The algae and vascular plants that comprise the marsh system have developed under conditions of low nutrient inputs that are characteristic of pristine rainfall. Sawgrass, a major plant component of the Everglades system, has a low nutrient requirement and is competitive with other vascular plants in a low nutrient environment (Steward and Ornes, 1973 a,b). Sawgrass has been replaced by cattail in parts of the northern Everglades, particularly in WCA-2 where nutrient concentrations are high. Cattails thrive under high nutrient conditions and have an advantage over sawgrass under nutrient-enriched conditions (Davis, 1991). Algal species and bacterial populations also have changed at nutrient enriched locations in the northern Everglades. These changes include loss of the native periphyton mat, changes in algal species present, phytoplankton blooms, and changes in microbial populations that result in prolonged low dissolved-oxygen concentrations or anoxic conditions (Steward and Ornes 1973 a,b; Swift and Nicholas, 1987). These changes in Everglades vegetation appear to be due to combined effects of nutrient enrichment, hydroperiod change, and fire (South Florida Water Management District, 1992). There is concern that the nutrient enrichment and ecological disruption in the northern Everglades will spread south and adversely affect the southern Everglades, which includes Everglades National Park. Long-term water-quality data have shown an increase in specific conductance and major ion concentrations within the Shark River Slough in the southern Everglades. These increases are due to changes from the natural sheetflow regime to one dominated by canal delivery (McPherson and others, 1976; Flora and Rosendahl, 1982). No changes in metal concentrations have been observed. Trace levels of pesticides occasionally have been detected in waters that enter the park. However, trace metals and pesticides are only slightly soluble in water and their presence in the park may be mainly in sediments and biota (Ogden and others, 1974). Average flow-weighted phosphorus concentrations discharged into the park from 1979 through 1988 was 0.011 mg/L for the four control structures (S-12's). Although these concentrations are much less than those in the northern Everglades, there is concern that nutrient concentrations at the S-12's may increase unless delivery of water by canals, such as C-67A, is replaced or supplemented by natural sheetflow (South Florida Water Management District, 1992). Water quality in the southeastern Everglades (the northeastern part of the Shark River Slough, C-111 basin, and the Taylor Slough) is typically good. Flora and Rosendahl (1982) and Waller (1982) reported low nutrient concentrations within the northeastern part of the Shark River Slough and the Taylor Slough. Current data collected by the SFWMD from 1979 through 1988 indicate that flow-weighted total phosphorus concentrations at S-333 in the northeastern part of the Shark River Slough (fig. 23) averaged 0.026 mg/L over the 10-year period of record. Concentrations were highest (0.043 mg/L) during the drought of 1985 as a result of water being routed from Lake Okeechobee to western Dade County well fields through L-29 for water- supply purposes. Total phosphorus values within C-111 at S-18C averaged 0.007 mg/L from 1985 through 1987 and were comparable to values recorded at interior marsh sites. C-111 water, however, contained moderate concentrations of dissolved minerals compared with the Miami and Hillsborough Canals. Canals that drain the urban and agricultural lands that lead to C-111 had detectable levels of such pesticides as atrazine and chlordane (South Florida Water Management District, 1992). The most significant water-quality effects observed within the C-111 basin has been the periodic removal of the S-197 structure from the mouth of the canal to alleviate upstream flooding. This removal released large amounts of freshwater into Manatee Bay, Barnes Sound, and other surrounding estuarine areas and caused severe impacts to marine biota (South Florida Water Management District, 1992). Next: Degradation of water quality in Lake Okeechobee Go to top Return to Publications Page

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