The objectives of the Ecosystem History of Biscayne Bay and Southeast Coast project are to: (1) assess the spatial variability in the distribution of modern biota (foraminifera, ostracodes, molluscs, diatoms, and palynomorphs) in Biscayne Bay; (2) determine how these distributions relate to the present environmental conditions (fresh water input, salinity, nutrient supply, seagrass, and contaminants) in the Bay; (3) interpret the temporal variability in the measured environmental conditions by applying modern biotic and environmental associations and geochemical data to biotic and geochemical data from downcore sediments; and (4) analyze the timing of environmental changes as determined by ²¹⁰Pb dated samples and relate the changes to historical events in South Florida.

This project is using faunal (foraminifera, ostracodes, and mollusks), floral (palynology and diatoms), and geochemical (stable isotopes and trace element geochemistry) data to examine the ecosystem history of Biscayne Bay and adjacent regions. Surface sediments collected from sites within Biscayne Bay and adjacent regions are being analyzed for their faunal and floral compositions. These data are correlated to water column data (temperature, salinity, pH, dissolved oxygen, nutrients, and redox potential) from each collection site and ground-water data (salinity, nutrient, stable isotope and trace element) collected at specific sites. Calcareous shells of living organisms are analyzed at Duke University for their trace element composition. In addition, the National Oceanic and Atmospheric Administration is currently gathering surficial sediment geochemistry data on samples collected near or at many of the ecosystem sites. Quantitative analyses of these data are used to determine controls on the modern faunal and floral distributions in the Bay. The data are also used to identify organisms that are useful environmental indicators, derive trace element (Mg/Ca ratio) calibration equations for modern bay salinities and temperatures, and determine the source areas for specific submarine ground-water flows into the Bay.

Distinct faunal distributions are identified within Biscayne Bay that are strongly associated with salinity, nutrient, and seagrass distributions. Trace element geochemical analyses of water and shells indicate a strong correlation between salinity and shell trace element geochemistry. Faunal distributions and shell geochemistry from sediment core samples, dated using ²¹⁰Pb, indicate significant environmental changes in Biscayne Bay over the past 150 years. The mid-1800's were characterized by low salinities and upper estuarine conditions. At the turn of the century, a significant faunal and floral shift occurred, indicative of higher salinity and lower estuarine to near marine conditions. This was accompanied by an increase in seagrass abundance. Bay waters became more saline (lower estuarine conditions) in the early 1900's. At about 1940, another faunal shift is evident suggesting a change to highly fluctuating annual salinity conditions with episodic periods of hypersaline conditions. Seagrass abundance remained persistent throughout this period. The most recent period (late 1980's to present) shows a slight decrease in relative annual salinity and reduced seagrass density. The carbon geochemical record from lower Biscayne Bay shows distinct periods of increased terrestrial charcoal sedimentation in the mid-1800's, at the turn of the century, and at about 1940. These events indicate increased charcoal production from fire events, giving us a proxy for burn frequency in southern Florida.

An understanding of pre-existing ecosystem conditions in Biscayne Bay and the evolution of the Bay is necessary to establish the range of natural variability and the impact of human influences in the region. The faunal and floral events detailed in Biscayne Bay show natural variability in salinity with a progressive increase in salinity throughout the last 150 years. This trend is punctuated by discrete events attributed to construction of the Flagler Railway, implementation of water management practices, and channelization of freshwater flow in South Florida. These results suggest that conditions within Biscayne Bay and adjacent regions during the past several decades may not represent the norm, and that significantly different salinity and seagrass conditions existed before intense water management practices and land development in southern Florida began. Restoration efforts need to reflect this possibility and incorporate this information into the present hydrologic and circulation models for Biscayne Bay and adjacent regions. This record reflects the importance of volume and rate of freshwater flow into Biscayne Bay and adjacent regions for the stability of the ecosystem and suggests an increase in freshwater inflow to maintain and improve the natural health of the Bay.

Results to date for this project have been provided to the Central and South Florida Restudy Project describing the impact of reduced freshwater flow to Biscayne Bay and surrounding regions. This project will establish methodologies for application to restoration of other Fragile Environments within our Nation. Spatial reconstructions of environmental conditions for the last 150 to 200 years will be used to produce synoptic maps of salinity, nutrient, and substrate conditions for Biscayne Bay. The data are compiled and can be accessed at http://geology.er.usgs.gov/gmapeast/fla/home.html (note: this site has moved to http://sofia.usgs.gov/flaecohist/), and a summary publication is being prepared on the ecosystem history of Biscayne Bay. These data are necessary for setting restoration objectives, supplying circulation modelers with control points to test their circulation models and modeling future outcomes of land and water management decisions, and locating sites at which restoration objectives can be monitored.

This project has benefited from collaboration with Biscayne National Park, Metro-Dade Department of Environmental Resources Management (Metro-Dade DERM), South Florida Water Management District (SFWMD), National Oceanic and Atmospheric Administration (NOAA), and Duke University.

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