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Plant and animal communities in the South Florida ecosystem have undergone striking changes over the past few decades. In particular, Florida Bay has been plagued by seagrass die-offs, algal blooms, and declining shellfish and sponge populations. These alterations in the ecosystem have traditionally been attributed to human activities and development in the region. Currently, under the South Florida Initiative of the Ecosystem Program, scientists at the U.S. Geological Survey are studying the paleoecologic changes taking place in Florida Bay in hopes of understanding the physical environment and restoring the region to a a more pristine, natural state.
Cores collected from Florida Bay are first sampled for pollen, dinoflagellates and diatoms, washed through a seive stack to remove fine-grained mud, and analyzed for bottom-dwelling, or benthic, organisms, such as mollusks, foraminifera, and ostracodes. These organisms indicate the salinity and substrate types present when the animal was alive. Dinoflagellate cyst analysis indicates the nutrient supply and current while pollen grains reveal the terrestrial vegetation present. By correlating the marine habitat with the terrestrial realm, scientists can learn about regional changes that may have affected the Everglades ecosystem, such as climate and nutrient supply.
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 samples in the parts of the cores where the sediments are too old to be dated with lead-210.
The cores studied included Bob Allen 6A (158cm), Russell Bank 19B (142cm), Pass Key 37, (74cm) and Taylor Creek T24 (86cm). Each core was sampled at 2cm intervals for benthic foraminifers, mollusks, and lead-210 data to provide an age model.
A comparison of the pre-1900 record to the post-1900 record, however, indicates that human development in South Florida has influenced the natural pattern of salinity fluctuations and benthic biota. Prior to 1900, variations in salinity were part of the natural hydrlogic system, with faunal indicators oscillating around 15-20% of the mean. Faunal distributions show subtle shifts around 1910, which may be linked to the construction of the Flagler Railroad. The railroad, built between 1905 and 1912, has significantly impacted Florida Bay by cutting off the flow of water between the Atlantic Ocean and the Gulf of Mexico. Between 1910 and 1940, benthic foraminiferal and molluscan data indicate an increase in salinity. The shifts in salinity become even more drastic after 1940, when the amplitude of the shifts increased from 15-20% around the mean pre-1900 to 40-60% post-1940. After 1940, faunal indicators show a peak in euhaline conditions around 1958, followed by a decline throughout the 1970s and a subsequent rise in the 1980s.
In particular, Bob Allen 6A exhibits this increase in salinity. At present, polyhaline to euhaline conditions exist at these sites.Vegetation patterns are also an integral component to understanding the ecosystem of Florida Bay. The distribution of epiphytal species reveals that changes in seagrass occur regularly and that, in the past, natural causes have profoundly affected seagrass development. All four cores show a general increase in seagrass population during the twentieth century. These data imply that seagrass is more abundant in Florida Bay after 1900 than in the past, although additional cores dating back to at least the 1800s would be necessary to substantiate these conclusions. The cores also reveal that macro-algal mats have become more abundant during this century as well. The patterns in salinity and biota identified in these for cores correspond to changes found throughout South Florida, Manatee Bay, and the terrestrial Everglades.
Like the deeper piston cores, the modern cores are sampled for pollen, dinoflagellates, and diatoms, washed through a seive stack, and examined for mollusk, foraminifera, and ostracode content. Mollusks, including any recognizable fragments, are picked out of the >850 micrometer size fraction while at least 300 foraminifera and ostracodes are picked from the >63 micrometer size fraction. Species abundances are determined by calculating relative percent abundance.
The data obtained from these cores indicate changes in substrate and salinity conditions and the associated fauna and flora. Indicator species for low salinity conditions, seagrass beds, and other parameters of the environment have been identified for use in down-core analysis.
Eleven samples taken in 1995 were dominated by four ubiquitous taxa of mollusks, including Transenella spp.,Cerithium spp., Brachiodontes sp., and Bitium varium. Anomalocardia sp., Parastarte triquetra and a group of terrestrial and freshwater gastropods were also present in significant numbers. Combined, these groups make up 72% of all the specimens examined. Among the pelecypods, Anomalocardia sp., Chione cancellata, and Lima sp. show seasonal variation at five sites, although these patterns may be related to seasonal spawning for the individual species. Otherwise, no seasonal pattern of increasing or decreasing overall abundance could be determined through analysis of the total number of species at each site both in February and July.
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U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology
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Last updated: January 15, 2013 @ 12:42 PM (HSH)
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