SOFIA - Poster - 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

The Biscayne Bay region of South Florida has been affected by natural events such as steadily increasing sea-level, droughts and hurricanes.

Project area

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

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.

diagram showing Q-mode cluster analysis foraminiferal data

Downcore Analysis

photo of scientists with coring equipment
diagram showing core composition
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.

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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Last updated: 03 January, 2005 @ 08:58 AM (KP)