DESCRIPTION OF THE NATURAL SYSTEM
The South Florida Ecosystem encompasses an area of approximately 28,000 km2 comprising at least 11 major physiographic provinces, including the Everglades, the Big Cypress, Lake Okeechobee, Florida Bay, Biscayne Bay, the Florida Reef Tract, nearshore coastal waters, the Atlantic Coastal Ridge, the Florida Keys, the Immokalee Rise, and the Kissimmee River Valley. The system is dominated by the watersheds of the Kissimmee River, Lake Okeechobee, and the Everglades. The system functions as an interconnected mosaic of wetlands, uplands, coastal areas, and marine areas. Wetlands dominated the predrainage landscape. Prior to drainage, wetlands covered most of central and southern Florida (Fig. 1). The Everglades region was characterized by an extremely low gradient (2.8 cm/km), yet heterogenous, landscape mosaic sculpted by 5000 years' evolution of hydrologic and biologic forces on a Pleistocene limestone platform. Our best template for the predrainage hydrologic structure and elevations is the 1850-era military map (Ives 1856), which defines the "predrainage" system discussed in this report (Fig. 2).
Fundamental Characteristics of the Predrainage System
Predrainage landscapes consisted of swamp forest; sawgrass plains; mosaics of sawgrass, tree islands, and ponds; marl-forming prairies dominated by periphyton; wet prairies dominated by Eleocharis and Nymphaea, cypress strands, pine flatwoods, pine rocklands, tropical hardwood hammocks, and xeric hammocks dominated by oaks. The natural seascapes of South Florida consisted of shallow seagrass beds, riverine and fringe mangrove forests, intertidal flats, coral reefs, hard bottom communities, mud banks, and shallow, open inshore waters. These were all interconnected on a topographic gradient that ranged from about 20 ft at Lake Okeechobee to below sea level at Florida Bay.
The predrainage wetland ecosystems of South Florida had three essential characteristics:
1. a hydrologic regime that featured dynamic storage and sheet flow,
2. large spatial scale, and
3. heterogeneity in habitat.
Dynamic Storage and Sheet Flow
The structure contributing to dynamic storage included the very shallow elevation gradient, the vast expanses of emergent vegetation, the thick peat substrates, sand hills, and highly permeable limestones. The water masses were constantly progressing downslope but so slowly that, in effect, water was banked during one season to use in another. Transport varied between structural elements from on the order of months to years.
Throughout the system, groundwater seepage, driven by hydraulic gradients, provided the base flow of creeks, rivers, and possibly even surface runoff across the mangrove zone. Base flow is the river or stream flow provided entirely by seepage from groundwater sources. For example, in the Kissimmee River in the upper part of the watershed, prior to drainage and channelization, 80% of annual river flow was base flow (Burns 1975, Burns and Taylor 1979).
The all-important extended hydroperiods of the natural system depended more on the large dynamic storage capacity and delayed flow-through that were natural hydrologic features of this system than on the immediate effects of rainfall. Because of the dynamic storage and slow rate of water flow throughout the natural system, wet season rainfall kept the wetlands flooded and maintained freshwater flow to the estuaries well into the dry season. The carry-over effect of the enormous dynamic storage capacity of the natural system was so great that a year of high rainfall maintained surface water in wetlands and freshwater flow to estuaries even into one (Walters et al. 1992), Fennema et al. 1994) or more (Browder 1976) subsequent drought years. The dynamic storage made wetlands and estuaries less vulnerable to South Florida's spatially and temporally variable rainfall.
Large Spatial Scale
The vastness of the predrainage wetland extent made it possible for the natural ecosystem to (1) support genetically viable numbers and subpopulations of species with large feeding ranges or narrow habitat requirements, (2) provide the aquatic production to support large numbers of higher vertebrate animals in a naturally nutrient-poor environment, and (3) sustain habitat diversity through natural disturbance. Population resiliency is undoubtedly proportional to the area of these wetlands because habitat diversity, the amount of seasonal refugia, and the number of dispersal options are proportional to wetland area.
In the predrainage era, the nutrients that were the basis of primary production were derived principally from rainfall. The nutrients in water entering from upstream were scrubbed by the vegetation and soils and not available downstream. Sheet flow enhanced the uptake of nutrients from the water column. The periphyton community, made up of microscopic algae, not only assimilated available nutrients from the water column but also created an environment that precipitated phosphorus, along with calcium carbonate, into the substrate. The system was extremely oligotrophic, given the nutrient loading, spread over the entire areal extent. During seasonal dry-down, topographic depressions (e.g., alligator holes) became areas of concentrated aquatic biomass, producing localized feeding opportunities for large carnivores, including wading birds. The higher vegetation, as well as the periphyton, had adaptations for surviving under low nutrient conditions (Davis 1990).
Heterogeneity in Habitat
Habitat heterogeneity maintained by micro-topographic features, small-scale climatic variation, and natural disturbances such as freezes, fire, and storms, acting on the large spatial scale of the wetlands, was a major contributor to biotic diversity and the persistence of populations. The mosaic of habitat types and water depths provided the spatial framework for the production and survival of animals under a wide seasonal and annual range of hydrologic conditions.
The vegetative landscape resulting from this vast, low relief, low gradient landform was a diverse mosaic of plant communities. These communities varied in extent from patches on the order of tens of meters to areas approaching physiographic provinces. The larger expanses had more long term resiliency than the patches. Large spaces were necessary to maintain resiliency under conditions that changed on scales from seasons to decades. To some extent, maps from the 1800s, when compared with maps of the 1980s, reveal large scale persistence of landscape patterns, even in the face of major anthropogenic disturbance.
Relationships with Spatial and Temporal Variation
The diverse and large number of aquatic biota that these systems once supported are maintained by the complex annual and long-term hydrologic patterns of the natural system, as expressed in wet-dry cycles, drying and flooding rates, surface water and water depth patterns, annual hydroperiods, flow volumes, and, at the coast, salinity and mixing patterns. For most animals, annual patterns of production, dispersal, and survival were seasonally regulated by the annual periodicity of wet-dry cycles and by the rates of drying and flooding. Primary and secondary production, including that in the key periphyton communities, depended on depth and duration of surface water.
The production of food for consumption by larger predators was largely a function of surface water area and flooding duration during the annual wet period. Food availability was then determined by the amount of forage produced and the rate and degree that it became concentrated into a smaller space during the annual dry season. The distribution and persistence of large animal populations was further influenced by the seasonal patterns of surface water distributions and, in the coastal wetlands and estuaries, salinity patterns, superimposed over major habitat, or plant community, patterns. For example, large, historic wading bird nesting colonies were once clustered in wetlands adjacent to estuaries, presumably because the prey base for these birds was greatest and most reliable at the estuarine/freshwater interface.
Colonial wading birds were extremely abundant in predrainage South Florida and were conspicuously present even into the 1960s and, to a lesser extent, the 1970s. Wood Storks, White Ibis, Great and Snowy Egrets, and other species nested in vast numbers in mangrove swamps along the southern rim of the Everglades and at various interior locations, particularly Corkscrew Swamp in southwest Florida and along the Kissimmee. Other large predators such as alligator, panther, and bear were common. Fish such as snook, tarpon, sea trout, and red drum were abundant in the estuaries. The Florida Reef Tract supported a healthy living coral reef and a rich diversity of associated fish and other organisms, including snappers, groupers, and spiny lobster.
Relationships with Spatial and Temporal Variation in the Estuaries
The estuaries of South Florida had salinities naturally ranging from about 18 ppt to 36 ppt or slightly greater and lower salinities (0-18 ppt) in the mangrove zone. These estuaries were naturally well mixed, rather than stratified. They had horizontal salinity gradients, with salinities increasing in an offshore direction and a lower salinity range throughout the estuary during the wet season. Parts of the more enclosed estuaries may have infrequently experienced salinities of between 36 and 40 ppt during the dry season. The estuaries received freshwater across a broad front, flowing across the mangrove zone, as well as from creeks and rivers, which provided some freshwater inflow throughout the year.
Salinities shifted gradually from high flow to low flow conditions because of the enormous dynamic storage capacity of the upstream system. Shallow, oligotrophic waters promoted the growth of seagrass beds, which supported a resident fauna and the juveniles of many species that spawn offshore but depend upon estuarine nursery grounds. The proproots of mangroves lining the estuaries and tidal creeks also provided habitat for estuarine life. Schooling coastal migratory species such as Spanish mackerel, bluefish, and pompano entered the estuaries during higher salinity times of the year.
Natural resources in the estuaries and on the reef tract supported recreational and commercial fisheries. A major fishery for pink shrimp, based on nursery grounds in or near Florida Bay, became established in the Dry Tortugas. A lucrative charter and guide-boat trade and other tourist-related water-dependent industries developed, particularly in the Florida Keys.
