Introduction
Scientists over the past few decades have noticed that the South Florida ecosystem has become increasingly stressed. Declining fisheries, changes in natural vegetation patterns, and apparent industrial and agricultural pollution have plagued the Everglades, Florida Bay and Biscayne Bay. In 1994, the Everglades Forever Act was passed in order to address these concerns and develop a restoration plan for the South Florida ecosystem. Restoration efforts include changing agricultural and land development practices in the Everglades, where drainage is controlled by a series of levees and canals. The regulated, channelized flow controlled by the canal system is the main source of freshwater input to Florida and Biscayne Bays. Before restoration can take place, scientists must determine the natural versus human-induced variability of the South Florida ecosystem. Results from this research will help managers with restoration of the region to a more pristine, natural state.
For more information, contact:
Tom Cronin
MS 926A
USGS National Center
Reston, Virginia 20192
tcronin@usgs.gov
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Biscayne Bay is of interest to scientists because of the rapid urbanization that has occurred in the Miami area and includes Biscayne Bay National Park. Dredging, propeller scars, and changes in freshwater input have altered parts of Biscayne Bay. The main freshwater input to Biscayne Bay is through the canal system, but many scientists believe subsurface springs in the Bay bring in groundwater from the mainland. Study of modern sediments and core sediments from Biscayne Bay will provide important information on past salinity and seagrass coverage, which will be useful for predicting future change within the Bay.
Methods
Modern grab samples and water quality data (temperature, salinity, water clarity, etc.) are collected from 23 sites within Biscayne Bay twice a year. Seven piston cores have been collected within the past 2 years. The cores are split in half and sampled every 2 centimeters. Both the modern samples and one half of each core sample are washed through a sieve stack and examined for mollusks, forams, and ostracodes. Mollusks are picked out of the >850 micrometer size fraction and approximately 300 forams and ostracodes are picked from the >63 micrometer size fraction. The remaining half of the core is sampled for pollen, dinoflagellates, and diatoms.
Ages for the cores are established using isotopic dating methods, primarily the analysis of the radioisotope lead-210 (210PB). Radiocarbon (14C) dating is used on deeper parts of the core because the sediment is too old to be dated with lead-210.
Modern Distribution of Benthic Foraminifera
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Photo of seagrass in Biscayne Bay.
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Presently, benthic foraminifers have been studied in detail throughout Biscayne Bay. An in-depth report describing the benthic foraminifers of Biscayne Bay can be found in the USGS Open-File Report 97-34 titled "Modern benthic foraminifer distributions in Biscayne Bay: analog for historical reconstructions". There are three main foraminifer assemblages ound in Biscayne Bay; the
Archaias-miliolid (A-M) assemblage, the
Ammonia-Elphidium (A-E) assemblage and the
Bolivinid assemblage. The A-M assemblage dominates the unrestricted and open marine waters of Central and South Biscayne Bay, where there is an abundance of seagrass. Salinities in this assemblage are greater than 37 ppt. The A-E assemblage is mainly found in Northern Biscayne Bay, Barnes Sound, and at freshwater discharge points such as canals. Salinites are typically less than 34 ppt. The Bolivinid assemblage is located in the northernmost section of Biscayne Bay in organic-rich sediments and thrives in high productivity waters.
Evidence of Past Changes
To date, one piston core from Manatee Bay (SEI196-MB-1) has been analyzed for benthic foraminifers, ostracodes, molluscs, and pollen. The results appear in "A record of ecosystem change, Manatee Bay, Barnes Sound, Florida," Journal of Coastal Research, special issue no. 26. Ages for this core were determined using the first appearance of pollen from the exotic taxa Casuarina that was recorded in the southern Florida region in the early 1900's. The age of the sediment at the bottom of the core was 148 years, assuming a constant sedimentation rate of 0.80 cm/yr.
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The Everglades, seen from the air.
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There are 3 main shifts of the fauna and flora in the MB-1 core. Near the turn of the century, Manatee Bay was nearly a freshwater environment, dominated by the
Ammonia-Elphidium (A-E) foraminifer assemblage. Around 1910 there was an increase in salinity and a shift in substrate conditions to predominantly seagrass. Hence, the foraminifer assemblage shifted from the A-E assemblage to the higher salinity,
Quinqueloculina-Miliolinella (Q-M) assemblage. This shift corresponds with the construction of the Flagler Railway and the drainage of the Everglades/Lake Okechobee area by the canal system. The second shift, observed in the core, was a change from relatively stable annual salinity variability to a system that has large fluctuations in salinity throughout the year. The foraminifer assemblage shifted from the Q-M assemblage to the more resistant
Quinqueloculina-Articulina (Q-A) assemblage. This change represents an increase in water management practices. In the late 1980's, there was a dramatic seagrass and invertebrate dieoff in Manatee Bay, due to a decrease in salinity. In response, the A-E assemblage dominated the region again. The dieoff is coincident with a short freshwater influx to Barnes Sound, due to the opening of a canal plug. Overall, in Biscayne Bay, there has been an increase in salinity through the past 150 years due to the lack of freshwater inputs coming from the Everglades. The salinity and faunal shifts identified above also correspond to changes found throughout Florida Bay, and the Everglades.
Publications
Some of the following files are available as PDFs. You will need the free Aobe Acrobat Reader in order to view these files.
Ishman, S.E.., Cronin, T.M., Brewster-Wingard, G.L., Willard, D.A., and Verardo, D.J., 1998. A record of ecosystem change, Manatee Bay, Barnes Sound, Florida. Journal of Coastal Research, 26, 125-138.
Ishman, S.E., 1997. Ecosystem history of South Florida: Biscayne Bay sediment core descriptions. US Geological Survey Open-file Report, 97-437, 13. (525 K download)
Ishman, S.E., Graham, I., D'Ambrosio, J., 1997. Modern benthic foraminfer distributions in Biscayne Bay: Analogs for historical reconstruction. US Geological Survey Open-file Report, 97-34, 23. (242 K download)
Stone, J. R. Cronin, T.M., Brewster-Wingard, G.L., Ishman, S.E., Wardlaw, B.R., and Holmes, C.W., 2000. A Paleoecologic Reconstruction of the History of Featherbed Bank, Biscayne National Park, Biscayne Bay, Florida, Open File Report, 00-191 (463 K download)
