THE ALTERED SYSTEM
Water quality as well as water quantity problems for the natural systems of the Everglades and South Florida estuaries resulted from the man-made changes in the hydrology of the watershed. These changes began before the turn of the century with dredging by Hamilton Disston that channelized the Caloosahatchee River and connected it to Lake Okeechobee. Changes in the hydrologic structure of South Florida culminated in the creation and implementation of the Central and Southern Florida Project in 1948. The enabling legislation gave the U.S. Army Corps of Engineers the responsibility for construction and oversight of water management structures throughout the Kissimmee-Okeechobee-Everglades basin. The State created the Central and Southern Florida Flood Control District in 1949, which has since become the South Florida Water Management District (SFWMD). Project purposes were (1) flood control, (2) drainage, (3) water supply (municipal, industrial and agricultural), (4) protection against salt water intrusion, (5) preservation of fish and wildlife resources in the Everglades, (6) water supply to Everglades National Park, and (7) recreation and navigation. Initial focus was on flood control, drainage, and water supply. Flood control made possible massive land use changes that decreased the land available for water storage and recharge.
Population Growth
A rapidly expanding population that now exceeds 4 million developed in South Florida, mainly in upland areas such as the southeast coastal ridge but also in the wetlands. This presence, with all its needs and demands, has further changed the South Florida Ecosystem. In addition to hydrologic alterations, the changes include an increasing water demand by agricultural and urban uses, although the water supply has been decreased by the conversion of land to agricultural and urban uses and by the shunting to the coast of freshwater that previously was stored in the wetlands, in the soils, and in the aquifers. Other changes include water treatment and quality problems, and the introduction of non-native plants and animals, some species of which have invasively moved onto remaining open lands, including natural-area parks.
Soil Subsidence
The intensive drainage and associated agriculture south of Lake Okeechobee (the Everglades Agricultural Area, or EAA) caused a tremendous loss of organic soil, which continues today. The compaction and oxidation of organic soils in the agricultural lands south of the Lake was one of the first observed environmentally destructive effects of the large-scale drainage. In most areas, five or more feet of organic soil had been lost by 1984 (Stephens 1984). A recent calculated rate of loss is 3 cm per year. The maximum thickness of this soil was only 12-14 ft initially, and the soil is underlain by limestone. The process of oxidative loss of soil continues today, although the process has been slowed in some locations by reflooding fallow fields and maintaining a high water table.
Soil loss of such magnitude has had many impacts on Everglades hydrology and ecology. The elevation gradient from the upper to the central Everglades has been greatly affected by the soil loss. The loss of elevation has meant a loss of the hydraulic head that once drove water south. The movement of water from north to south now requires pumpage and the pumpage effort necessary to move water continues to increase with time as the soils continue to subside. The soil loss has also meant a loss of water storage capacity, which has meant a reduction in the ability of the area to absorb water and mediate seasonal and longterm variations in rainfall.
The problems caused by soil loss are magnified by the enormous spatial extent over which the loss has occurred. In fact, the loss is not confined to the EAA but actually extends into the northern parts of Water Conservation Areas 1 and 3A, where additional soil loss has occurred due to the diversion of water around these areas and even to the EAA to support agriculture.
Water Quality Implications of Soil Loss
The combination of soil loss in the Everglades Agricultural Area, routing water around the EAA, water demand by the EAA, and materials leaching out of the EAA has caused downstream impacts, many of which have only recently been recognized. The loss of soil may have resulted in the concentration of compounds and minerals such as phosphorus in the remaining soil. It seems likely that soil loss has --or soon will-- proceed to the point at which the binding capacity of remaining soil becomes saturated and, with additional loss, the extremely concentrated compounds and minerals will be released into waters that flow downstream. This problem, in conjunction with the application of pesticides and other chemicals for at least 50 years poses a potential ecological threat. The high concentrations of mercury found in largemouth bass, alligators, panthers, and other top predators demonstrates how contaminants have, in fact, concentrated in aquatic food chains.
The well fields of eastcoast cities are supplied with water from Lake Okeechobee and the Everglades through major canals traversing the Everglades Agricultural Area. Discoloration of the water is evidence of the presence of dissolved organic carbons that are precursors to trihalomethanes formed in the chlorination treatment process for drinking water. In an EPA study, the Miami Preston-Hialeah well field was found to have one of the highest concentrations of trihalomethanes in U.S. drinking water supplies. Trihalomethanes are known cancer causing agents. Dade County water treatment plants have switched to the use of a chloramine-based purifying process, but there may be public health concerns with byproducts of this process also.
Everglades waters were naturally clear. The organic soils in the EAA and elsewhere that are oxidating due to drainage are the obvious source of the dissolved organic compounds that are causing problems in water treatment. In fact, concentrations of low molecular weight dissolved organic compounds decrease dramatically with distance south from the EAA (R. Jones, Florida International University, pers. comm.).
Nutrient Enrichment and Contamination
Accompanying the drainage and development of much of the South Florida wetland system, the increased rates of runoff into and through the system, and the diminished hydroperiods in remnant wetlands has been the introduction or transport of water-borne eutrophication-causing nutrients, contaminants such as mercury and other toxins, and introduced, invasive, non-native species, into remnant wetlands. Nutrient-laden agricultural runoff water has resulted in altered macrophyte and algal communities with diminished support capacities as food chain bases and habitats.
Elevated concentrations of chlorinated hydrocarbon pesticides or their derivatives (e.g., DDE) have been found in the tissues of Great Egrets and other wading birds from Water Conservation Area 1 (M. Mafei, unpublished data).
Mercury Contamination
High levels of mercury in fish and wildlife throughout much of the South Florida wetlands represents a particularly alarming ecological threat. Roughly one million acres of the Everglades are under health advisories recommending that anglers completely avoid consumption of largemouth bass and several other species of fish (Lambou et al. 1991). Mercury body burdens in largemouth bass collected in the Everglades were higher than in those taken at SuperFund sites noted for mercury contamination (D. Scheidt, EPA, Athens, Georgia, pers. comm.). Alligators harvested from the Water Conservation Areas cannot be sold for human consumption because of elevated mercury levels in their tissue. In 1989 a Florida panther died from mercury toxicosis (Lambou et al.). Mercury is suspected as the causative agent in the deaths of two other panthers. Analyses of racoons, a major prey organism of panthers, from certain areas of South Florida revealed very high concentrations of mercury in liver and muscle tissue (Lambou et al).
Fragmentation of Landscapes and Habitat
The South Florida Ecosystem is now highly fragmented with diminished habitat diversity. Four major wetland landscapes are reduced to remnants: the cypress strands fringing the western side of Atlantic coastal ridge, the pondapple forest/swamp on the southern shore of Lake Okeechobee, the tall sawgrass plain of the now Everglades Agricultural Area, and the biologically important peripheral wet prairies in southeastern Dade County (Davis et al. 1994).
On the eastcoast ridge, most of natural areas have been replaced by urban development. Only 10% of the former rockland pinelands and 10% of the tropical hardwood hammocks persist. Both are seriously stressed by a combination of lowering of the water table and invasion by introduced species, which makes them much more vulnerable to natural disasters such as hurricanes.
Compartmentalization of much of the remaining Everglades fragmented the system by creating a series of poorly connected wetlands. Similarly, urbanization fragmented the upland systems. The former role of these now diminished systems in regulating both the hydrology and ecology of the South Florida ecosystem no doubt was enormous; yet now the urban area exerts a stress on both the water supplies and water storage capacity of the ecosystem.
Loss of Wetland Area
Roughly 50% of the predrainage wetland area has been lost to agricultural, industrial, and residential development (Fig. 3). In particular, the critical peripheral, or short hydroperiod, wetlands on the eastern side of the Everglades have been diminished. Loss of wetland area has significantly reduced the landscape heterogeneity, habitat options, and long-term population survival for vertebrate species with large spatial requirements. Wading birds, snail kites, and panthers, for instance, have become increasingly stressed by the fragmentation and loss of habitat. Decreasing the spatial extent of South Florida wetlands has reduced the solar collector area that becomes transformed into aquatic productivity. Reducing topographic heterogeneity at regional and local scales has narrowed survival options. By any measure of species richness, there has been a drastic erosion of the biodiversity of the South Florida ecosystem.
Accompanying the decrease in wetland area has been a loss of wetland function, sheetflow, and base flow through water management, which produced a significant change in volume and timing of water flow and overland flow patterns across wetlands and into the estuaries of South Florida. The Everglades and other wetland systems of South Florida were naturally flowing, or lotic, systems that not only covered greater area but also had longer periods of inundation and more sustained outflows to estuaries than exist today under managed conditions.
The Kissimmee-Lake Okeechobee-Everglades basin has been compartmentalized by impoundment of Lake Okeechobee, drainage of the Everglades Agricultural Area, and construction of levees around both the EAA and the Water Conservation Areas. This compartmentalization, plus the channelization of the Kissimmee River and construction of a network of major canals from Lake Okeechobee to the coast, has almost eliminated the wetland- and estuarine-sustaining sheet flow characteristic of the natural system. Now water conveyance networks capture much of the rainfall that the system receives during the wet season and deliver freshwater to the estuaries in huge pulses that drastically lower salinities, stressing estuarine life and lowering estuarine productivity. Wetland productivity is lost as well, as the elimination of large volumes of fresh water from the system results in shorter hydroperiods and lower wetland carrying capacity. Lost, during the dry season, are the important delayed flows originating mainly from wet season rainfall throughout the system, including the dense sawgrass plain that formerly covered the present Everglades Agricultural Area. Lost also are the base flows that, in the natural system, sustained the estuaries during the dry season, extending the period of salinities ranging from 18-24 ppt.
Uncoupling of Wetlands and Estuaries from Rainfall
Water supply releases and regulatory releases involved in the management of stage levels in Lake Okeechobee, the Water Conservation Areas, and the East Everglades, have decoupled water flows from rainfall. Flows to the Everglades from Lake Okeechobee have shifted from primarily wet season flows in response to rainfall to dry season flows in response to urban and agricultural water demands. Impoundment of water in the Water Conservation Areas and diversion of surface water flows to the Atlantic coast, combined with groundwater and levee seepage losses eastward in the modified system, have significantly reduced flows to the southern Everglades, shortening hydroperiods in this area. These changes have resulted in larger intraannual flow variations.
Large volumes of rainwater are drained to sea annually that did not occur historically because of the loss of wetland area and reduction in dynamic storage capacity in remaining wetlands. This diversion of water eastward results in a loss of several hundred thousand acre feet per year to sea.
The reduction in flow from upstream has reduced the maximum area covered by water each year and the duration of flooding. Peak flows are higher following major rain events and flow rates decline more abruptly following the end of the wet season than in the natural system. Channelization and impoundment have disrupted the annual pattern of rising and falling water depths in the remaining wetlands of South Florida. In particular, the effects of dry season rainfall have been aggravated by increases in the depth and duration of reversals in natural drydown process--causing a rainfall event to have a greater disruptive effect on the concentration of secondary production upon which the whole system depends.
Altered Hydroperiods
One result of the changes is that hydroperiods are reduced in most South Florida wetlands compared to pre-drainage conditions. In most South Florida landscapes, development has accelerated the rate of runoff, resulting in sporadic water flow and increased frequency and spatial extent of wetland drying.
Reduced hydroperiods in wetlands appear to adversely affect aquatic production at all levels of the food chain. Surface water refugia to support populations of aquatic fauna and their predators during drought are smaller and fewer and are relocated and subdivided in the currently-managed system compared with the predrainage system.
In a few areas, such as the southern parts of the Water Conservation Areas, channelization, coupled with impoundment, has increased depth and hydroperiod. Resulting regulation water releases from the Water Conservation Areas have caused unseasonable flooding of alligator nesting sites in Everglades National Park, causing nest failure. In addition, these releases have disrupted wading bird nesting, which depends upon concentrated food supplies.
Encouragement of Invasive Introduced Species
Invasive, non-native plant species introduced by man are changing the South Florida landscape and affecting hydrologic conditions and ecosystem function. Prolific non-native animal species are changing animal community structure. Water management has encouraged the spread of these invasive species. The canal networks of the managed system provide a type of deep-water refugia that may cause a completely different community composition--particularly in the predatory fish community--than in the natural system. Furthermore, the water conveyance system may be a conduit for the dispersal of invasive introduced species. Canals also serve as artificial conduits for the transport of waterborne substances such as nutrients. Probably the most important way that water control structures encourage invasive introduced species is by creating spaces where conditions are more favorable to various introduced species than to natives. For instance, altered hydrologic regimes within remnant wetlands have increased their vulnerability to invasion by Melaleuca.
Loss of Hydraulic Head and Recharge Value
One effect of drainage was to drastically lower the water table and increase water table recession rates on the eastcoast ridge. This had a major effect on water flow to both the interior wetlands and the estuaries. It also affected the plant communities of the ridge, the salt/fresh interface, and water supply.
Changes in Fire Regimes
The role of fire may have changed from one of increasing habitat diversity in the natural system to reducing diversity in the current managed system because of altered seasonal burning patterns accompanied by overdrying of wetlands. Fragmentation has interfered with the ability of fire to maintain natural mosaics. Fire patterns have also changed because of prescribed burning, a practice that dampens the annual and interannual variability in the number and severity of fires. Man's tendency to replace natural variations and extremes in disturbances like fire with a regular schedule of variation can lead to loss of biological diversity because species tend to be adapted to natural variations in environmental conditions. Any regularization of physical driving forces may favor some species over others and affect species composition.
Lost Wetland Function Greater than Lost Wetland Area
While South Florida wetlands have been reduced by one half, wading bird populations have been reduced to less than 10% of their former size. This suggests either (1) that the particular wetlands that were lost were especially critical to wading bird feeding and nesting success and/or (2) that the remaining wetlands are so degraded that their carrying capacity for wading birds is only 20% of what it was formerly. The estuarine system serves as a foraging ground for many of the wading birds and loss of estuarine feeding opportunities may also have decreased the carrying capacity of the South Florida Ecosystem for wading birds.
Estuarine Impacts
Water management has resulted in more short duration, high volume water flow to estuaries and less life-sustaining base flows to estuaries. Regulatory releases to control lake and groundwater levels according to prescribed flood-preventive formulae result in pulses of fresh water entering estuaries, causing rapid, drastic decreases in salinity that stress estuarine organisms. In addition, water flows have been diverted from one receiving basin to another, changing the longterm salinity regimes in both systems. As a result of both diversion and the increased runoff rate, Florida Bay receives less water flow than it did historically and salinities greatly exceeding oceanic concentrations are widespread and chronic. Hypersaline conditions in Florida Bay are extreme, frequently reaching 50 ppt over large areas, with known maxima of 70 ppt during severe drought. Biscayne Bay sometimes exhibits abnormal negative, or reverse, salinity gradients, with hypersaline conditions inshore. On the other hand, salinities in Manatee Bay have dropped from 36 ppt to 0 ppt in a matter of hours due to abrupt regulatory releases from the South Dade Conveyance System. This occurrence is particularly disruptive to Manatee Bay because the bay ordinarily experiences extremely high salinities because of loss of natural freshwater inflow. The same is true in northeastern Florida Bay.
Long-term changes in freshwater inflow rates to many South Florida estuaries have shifted salinity zones upstream or downstream from where they originally prevailed each season. As a result, areas within the optimum salinity ranges for various species may no longer coincide with structural features of the estuary that favor the growth and survival of the species. In addition, salinity gradients may have become more spatially compressed (steeper), providing less overall area within some salinity zones and less opportunity to overlap with favorable structural habitat. The shifts in salinity zones and changes in area within various salinity ranges may have reduced the optimum habitat available to each species and even have altogether eliminated the habitat of some species in an estuary. Therefore, changes in freshwater flow have had spatially related consequences for estuarine, as well as wetland, habitat.
Declines in Estuarine and Reef Resources
The fisheries productivity of South Florida marine waters and estuaries is dependent upon habitat quality and quantity. One measure of habitat carrying capacity is recruitment, which is reported as the abundance of age 0 or age 1 fish. Given this, fisheries productivity is directly related to habitat productivity. Decreased fisheries productivity may be reflected by declines in catches and catch rates, although these declines are complicated by fisheries regulations, which reduce total catches (e.g., spotted seatrout, gray snapper, snook). Productivity of spiny lobster, as indicated from recruitment, is habitat-limited to the extent that sponge density is adequate. The impact of the recently documented and continuing sponge die-off on lobster productivity has not yet been observed but is likely to be observed via declines in catches and catch rates in the near future. Landings in the valuable Tortugas pink shrimp fishery, dependent upon Florida Bay nursery grounds, have declined sharply since the mid 1980s. Long-term catch rates, standardized for vessel power increases, declined from the 1960s through the 1970s. Catch rates (unstandardized) declined precipitously beginning in the mid 1980s (Browder 1985).
Fish displaying abnormal dorsal fins and misaligned scales are common in North Biscayne Bay (Browder et al. 1993) and also are present in the St. Lucie Inlet and the lower Indian River (Kandrashoff pers. comm.). The same abnormalities have been seen in at least 10 species. The multiple species occurrence of the abnormalities suggests that something common to the environment of all of the species is causing the problem.
On the reef tract, coral bleaching, coral diseases, including black band disease, and a decline in coral cover and recruitment are some indications of a declining reef community. DDE and other chlorinated hydrocarbons have recently been found in coral reef tissue (Skinner and Japp 1986).
