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a paleoenvironmental record from manatee bay, barnes sound, florida
A Paleoenvironmental Record from Manatee Bay, Barnes Sound, Florida
Scott E. Ishman, Thomas M. Cronin, Lynn Brewster-Wingard, and Debra A. Willard
Introduction
The Biscayne Bay region of South Florida has
been affected by natural events such as steadily increasing sea-level,
droughts and hurricanes.
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In addition, human alterations of the South Florida region: urbanization,
increased agricultural activity, dredging of natural channels and water
management of the Everglades have changed the setting of South Florida.
These events and modification of the South Florida geography have driven
the evolution of the Biscayne Bay ecosystem. However, the magnitude at
which each of these factors affect the ecosystem remains unknown.
The objective of the "Ecosystem History of Biscayne
Bay and the Southeast Coast" project of the U.S. Geological Survey is to
determine historical changes in the ecosystem of Biscayne Bay and adjacent
regions, and correlate the timing of the changes with natural events and
human alterations to the South Florida region. This will be accomplished
through the analysis of modern distributions of marine invertebrates and
plants throughout the South Florida region (Everglades, Florida and Biscayne
Bays) to determine their environmental preferences. Results of these analyses
will be used to interpret historical changes in the ecosystem by analyzing
animal and plant remains collected from sediment cores that date back 150
to 200 years before present.
Methods
A wide diversity of organisms make up the living
ecosystem of Biscayne Bay. These include the mangroves of the coastal wetlands,
molluscs and other invertebrates found on the hard and barren bottom of
central and northern Biscayne Bay, and sea-grass beds rich in invertebrates
in southern Biscayne Bay. A variety of conditions in the Bay (salinity,
nutrients, bottom type, oxygen availability, and pollution)are responsible
for the presence and health of these organisms, and thus the ecosystem
itself. By studying where the modern day organisms are living in the bay,
and under what conditions, scientists can use this information to determine
what conditions existed in the past.
Samples of sediments (mud, sand, vegetation) are
being collected from the bottom of Biscayne Bay twice a year. Where each
sample is collected a variety of environmental information is collected
that includes water salinity, temperature, clarity, oxygen content, and
nutrient levels. Each sediment sample is washed through a sieve to remove
the very fine mud. The coarse material is saved, dried and observed for
a variety of marine invertebrates (molluscs, foraminifera, and ostra-codes).
A portion of the original sample is processed and observed for various
plants and plant remains (pollen, diatoms, seeds, and dinocysts). The results
of these observations are recorded and analyzed with the environmental
information to determine what factors control where different organisms
are found within Biscayne Bay.
Short sediment cores (up to 2 meters) collected
from Biscayne Bay contain a historical record of ecosystem change. Sediment
samples collected from cores are processed for marine invertebrate (foraminifera,
ostracode, and mollusc) shells, plant fragments, pollen, diatoms, and dinocysts.
The occurrences of the marine invertebrates and plant materials in the
core samples are compared to their distributions in the modern samples
to determine the most similar modern conditions represented in the core
sample. In addition, charcoal abundance in marine sediment cores samples
indicate fire history in the south Florida region. Measurement of trace
element ratios (Mg/Ca) in the ostracode shell material collected in the
core samples will be used to determine salinity estimates for the past.
Measurement of the radioisotope Lead-210 in the core samples will be used
to provide an age for each sample. Using these methods, USGS scientists
are able to reconstruct the evolution of the Biscayne Bay ecosystem through
time, and compare the timing of the ecosystem changes with various natural
and human induced events to determine causal factors.
Modern Biscayne
Bay
Benthic foraminiferal analyses of 24 surficial sediment
samples from Biscayne Bay
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resulted in the identification of modern sample groups (biotopes) and faunal
groups (biofacies). R-mode cluster analysis of the foraminiferal data are
used to group the samples into biotopes. Two biotopes are clearly defined
by the clustering. Biotope 1 includes samples from the open-bay where there
is unencumbered exchange between Biscayne Bay and the Atlantic Ocean. Biotope
2 includes samples from coastal localities and sites north of the southern
end of Key Biscayne. Two outliers are present, BB02 that is the northernmost
site located in Dumbfoundling Bay, and BB48 that is located in Card Sound.
Q-mode cluster analysis of the foraminiferal data results in four biofacies.
The Productivity biofacies is distiguished by its high relative abundance
of Bolivinids. The Open-bay coarse sediment biofacies contains abundant
miliolid taxa, as well as Archaias angulatus. The Open-bay grass biofacies
is represented by several miliolids and Articulina mucronata. The Restricted
biofacies is represented by the abundance of Ammonia parkinsoniana, Elphidium
galvestonense, and species associated with high salinity tolerances.
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.
Downcore Analysis
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A large diameter (4 inches) piston core, MB1, was collected from Manatee
Bay, Barnes Sound (25 15.69" N and 80 24.06" W) and sampled at 2 cm intervals
for faunal, floral, sedimentological, and geochemical analyses. Five distinct
sedimentologic units are recognized in core MB1, a basal peat unit, a shelly-marl,
a shelly-mud, mud, and a ashelly-mud rich in vegetation debris.
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.
Discussion
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.
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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
Coast
SOFIA Project: Ecosystem History: Florida Bay and the Southwest
Coast
SOFIA Project: Ecoystem History: Terrestrial and Fresh-water
Ecosystems of southern Florida
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