Scott E. Ishman, Thomas M. Cronin, Lynn Brewster-Wingard, and Debra A. Willard
Introduction |
Methods |
Modern |
Downcoare Analysis |
Discussion |
Contact Info
The Biscayne Bay region of South Florida has been affected by natural events such as steadily increasing sea-level, droughts and hurricanes.
Principal components analysis of the foraminiferal data show results similar to the cluster analysis with PC1 and PC2 accounting for the greatest amount of the variability within the data set. Three dominant groups are interpreted from the results, the Open-bay, Restrticted, and Productivity sites. In addition, species specific trends within the modern faunal data set show strong species-environmental relationships in the south Florida region. Above is illustrated the negative correlation between salinity and the relative abundance of Ammonia parkinsoniana within the spatial limits of Florida and Biscayne Bays.
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Significant faunal, floral, and geochemical fluctuations occur throughout
core MB1. The lowermost part of the core (96 -120 cm) contains abundant
limnic to oligohaline (0 -5 ppt salinity) ostracodes and molluscs, as well
as low salinity benthic foraminifera. These taxa are replaced by mesohaline
benthic foraminifera, ostracodes, and molluscs that persist from 106 cm
through 74 cm where a significant decrease in their abundances occurs,
and marine tolerant species first appear in abundance and become prevalent.
Close to euhaline conditions on an annual average are indicated for Manatee
Bay from 74 cm through to recent. This interval also contains abundant
epiphytal species common to sea grasses in the tropics and subtropical
regions, not present in the lower part of the core, indicating a significant
change in substrate conditions. Changes in the faunal compositions throughout
this upper part of core MB1 indicates the possibilities of varying types
and density of sea grasses throughout this interval.
Charcoal analyses of samples from core MB1 indicate significant shifts in the % Charcoal to % TOC ratio (C/T ratio). Increasing ratios indicate an increase in terrestrial fire activity. The lower part of core MB1 (120 to 97 cm) shows C/T ratios fluctuating between approximately 45 and 25. These ratios increase almost two-fold from 87 cm to 77 cm in the core. From 77 cm to the top of the core the values decrease to levels averaging approximately 25. (We thank Dave J. Verardo, University of Virginia, for the Charcoal analyses)
Results from the pollen analyses of core MB1 samples indicate several events. Within the basal peat unit is a high abundance of the fresh water algal cyst Pseudoschizaea. An overall trend observed in the data is the decrease in Pine pollen with a notable shift between 90 and 80 cm. At approximately 80 cm red mangrove pollen first appears, and between 60 and 70 cm is the first appearance of Australian Pine (Casuarina) pollen. This is also coincident with an increase in Aster pollen.
The faunal and floral records from core MB1 show periods of significant
change. In the earliest part of our record (~ 1840) ostracode and molluscan
data indicate nearly fresh-water conditions in the basal peat of core MB1.
The pollen record is consistent with the faunal data, with the basal peat
containing a high abundance of algal cysts thought to be of fresh-water
origin. An overall increase in salinity to average annual mesohaline conditions
with low amplitude annual fluctuations is observed starting in the late
1850's and continuing into the early 1900's. Between 1920 and 1940 a significant
faunal shift, with euhaline taxa becoming more prevalent, indicated further
increase in salinity, and an increase in the amplitude of annual salinity
variability. Euhaline taxa became more prevalent. The overall salinity
increase at this time is also supported by the first occurence, and later
persistence of red mangrove pollen. This salinity increase was accompanied
by an increase in epiphytal taxa, indicating the notable presence of sea
grass. Epiphytal taxa, not present in the early part of the record, persist
to present. The timing of these events coincides with the progression of
flood control efforts and the construction of the Water Conservation and
Everglades Argicultural Areas by the Army Corps of Engineers. These conditions
continued into the 1980's when a slight decrease in average salinity occurs
at approximately 1988, and a slight reduction in the diversity and abundance
of phytal taxa is evident.
The % Charcoal to % TOC record shows a significant increase in the early 1900's. During this same time the pollen record shows a precipitous drop in the relative abundance of pine pollen. The correlation between these two records may reflect development in south Dade County and the removal of the Rockland pines in that region. The pine removal was probably accompanied by burning, that accounts for the spike in the C/T ratio.
Records of spatial and temporal marine and terrestrial changes are possible
by comparing modern and historical data. Additional analyses that include
trace element geochemistry of marine shells and elemental geochemistry
of pore waters and sediments from Biscayne Bay cores will refine and increase
our interpretation of ecosystem change. Data that reflect changes in the
frequency and amplitude of salinity and substrate changes in Biscayne Bay
provides information important for circulation model verification and impact
studies. Determining burn frequency and its effect on the ecosystem through
flow regime alteration is an important consideration for the landscape
modelers and land use managers.
For more information contact:
Scott E. Ishman
U.S. Geological Survey
Reston, VA 20192
(703)648-5316
email: sishman@usgs.gov
Related information:
SOFIA Project: Ecosystem History of Biscayne Bay and the Southeast CoastSOFIA Project: Ecosystem History: Florida Bay and the Southwest Coast
SOFIA Project: Ecoystem History: Terrestrial and Fresh-water Ecosystems of southern Florida
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 03 January, 2005 @ 08:58 AM (KP)