Map of south Florida showing distribution of federal lands, Water Conservation Areas, and Everglades Agricultural Area.

Ecosystems are communities of organisms, often including humans, and the associated physical and chemical environments in which they live. Eco-systems are a complex natural resource that need to be understood, carefully managed, and prudently conserved. Human modification of the environment, such as changing water drainage patterns and introducing pollutants (such as mercury) and nutrients (such as nitrates and phosphates), has altered critical ecosystems around the globe, and the south Florida region is now considered to be one of the most threatened ecosystems in the Nation. The south Florida ecosys-tem has both a land component, the Everglades (including all fresh-water wetlands south of Lake Okeechobee), and an estuarine component, Florida Bay. The two components are closely linked by hydrologic cycles and the plants and animals that live within the ecosystem.

The broad expanse of wetlands that composes the Everglades formed about 5,000 years ago as a vast, flat region that tilts slightly to the south and drains into the shallow Florida Bay and Gulf of Mexico. Early Native Americans settled south Florida between 10,000 and 12,000 years ago and began cultivating maize (corn) as early as 2,400 years ago. Strik-ing changes in this wetland-estuarine ecosystem have been attributed largely to increasing urban and agricultural activity during the past century. One set of environmental changes affecting the ecosystem may have resulted from con-struction of a complex canal and levee system to control flooding and provide a steady supply of fresh water to a growing population and to agriculture. This system has drained over half of the Ever-glades wetland and has altered the flow of fresh water into Florida Bay to an unknown extent.

Water plays a key role in regulating vegetation and animal life throughout south Florida. Top- Wading birds in a sawgrass marsh, Big Cypress National Preserve. Bottom- Mangroves bordering Florida Bay

Many people believe that these alterations of the system are responsible for a decline in populations of wading birds and a collapse of nesting activities, decrease in biodiversity, and major changes in plant communities as introduced taxa and "weedy" species invade the wetlands. In Florida Bay, die-off of seagrass populations, declining numbers of shellfish, and frequent algal blooms may be related to onshore drain-age changes. However, until the range of natural variability in the ecosystem is understood, it is premature to attribute cause-and-effect relationships to these changes. Plans are being developed to restore the hydrologic regime of the south Florida ecosystem to a more natural state, such that water patterns in parts of the historic Everglades more closely resemble those that existed about 150 years ago, prior to significant human development. In order to develop realistic goals for the restoration, however, it is ecessary to determine what the natural ecosystem was like historically through analyses of changes in plant and animal distribution over time. It also is necessary to determine when major changes occurred in the communities because the timing of these changes then can be compared to records of human activities, allowing managers to evaluate and, ultimately, predict the response of the natural system to human intervention.

Geologists collect a peat core in a cypress strand

Anecdotal evidence indicates that changes in water levels and water chemistry (salinity and nutrients) may have occurred over the last century. U.S. Geological Survey (USGS) geologists and geochemists are conducting studies to quantify these changes by examining shallow cores (less than 2m) from Florida Bay, Everglades National Park, Big Cypress Preserve, and the Water Conservation Areas of the historic Everglades wetland. These cores provide a record of sediments deposited over at least the last 150 years and include the remains of plants and animals (bioindicators). By comparing changes in abundance of different bioindicators and evidence of chemical and sedimentological changes in the cores, scientists can interpret the roles of changing rates of fresh-water flow, nutrient levels, sedimentation patterns, and fire frequency in controlling plant and animal composition over the last 150 years.

Environmental Indicators

Changes in the biota, nutrient supply, and fire frequency are interpreted by study of different components in the cores. For example, the past distribution of plants in the region can be determined through analysis of pollen and larger plant remains in the cores. Analysis of small invertebrates, such as mollusks, ostracodes, and foraminifers, and algae, such as diatoms and dinoflagellate cysts, provides details on changes in salinity, nutrient levels, and substrate conditions. Studies of the sedimentology of the cores provide information on broad-scale changes of substrate in the region. Biogeochemical analyses provide information on changes in chemical and nutrient input through time, as well as insights into broad-scale vegetation changes. Analysis of charcoal content of cores tracks changes in fire frequency and associated vegetation and nutrient patterns. Integration of all these types of data enables scientists to understand the processes that formed the ecosystems and to interpret cause-and-effect relationships between biotic changes and alteration of physical and chemical characteristics of the environment.

Age Indicators

This research requires careful dating of samples by several techniques. Dating using radioactive isotopes, such as cesium-137 (137Cs) and lead-210 (210Pb), provides good chronological control, particularly for the last 150 years. 137Cs, produced by atmospheric testing of nuclear weapons, is useful in determining whether the material was deposited since the late 1950's. A longer time scale, for the last 150 years, is provided by 210Pb, which is constantly supplied to the atmosphere through the decay of uranium in naturally occurring materials. A cross-check on radiometric dates is provided by the record of the pollen in the sediments, because several exotic (non-native) plants have been introduced in south Florida in the last 200 years. Because the timing of the introduction of these species is known through historical records, the first appearance of their pollen in the sediments provides a time marker.

USGS scientists and collaborators are quantifying both biotic and chemical changes that occurred during the last 150 years in the south Florida ecosystem. These data are being used to verify the validity of the study methods for reconstruction of changes documented by the historical record. Additionally, ongoing work on deeper parts of cores is yielding new information on earlier events that have not been well-documented. Invertebrates preserved in a core from one of the Florida Keys indicate an increase in salinity around 1930 as well as a change from a sandy substrate to one populated by higher abundances of seagrasses. The pollen record also indicates minor changes in the Everglades vegetation at about the same time. Ongoing analyses of cores from elsewhere in the bay will help to determine whether these patterns are consistent and whether these changes are correlated with known environmental changes in the region. In parts of the Water Conservation Areas, water is being impounded, and cattail vegetation gradually is increasing in abundance and displacing the original sawgrass community. Analysis of pollen and geochemistry from a core from this area indicates that major changes occurred shortly after 1960. The pollen record reflects increased abundance of pollen of cattails and the saltwort family; both groups tend to become more abundant with increased disturbance. Peat accumulation rates also have increased, along with the concentrations of various nutrients and metals. Increases in phosphorus, sulfur, copper, and zinc probably result from increased agricultural activities and application of fertilizer. Increases in mercury and other metals also are noteworthy, and possible causes for the increases currently are under investigation.

Integration of these data types enables USGS scientists to reconstruct the historic south Florida ecosystem and to under-stand past changes in the system. Such information is particularly useful for modeling the ecosystem in efforts to determine the "best" restoration plan. It also provides a foundation for develop-ment of indicators for measuring the success of the restoration efforts. By providing impartial data on sites throughout the Everglades and Florida Bay ecosystem to modelers, resource managers, and other interested parties in south Florida, the U.S. Geological Survey is playing a key role in restoring the natural ecosystem functions of the region.