The Sediment Record as a Monitor of Natural and Anthropogenic Changes in the Lower Everglades/Florida Bay Ecosystem

Topical Area: Paleoecology

Terri Hood, Carlos Alvarez-Zarikian, Pat Blackwelder and Peter Swart, RSMAS/UM, Miami, FL; Terry A. Nelsen, NOAA/AOML/OCD, Miami, FL 33149; Harold R. Wanless, Department of Geological Science, U. of Miami, Miami, FL; John H. Trefry andWoo-Jun Kang, Division Of Marine & Environmental Systems, Florida Institute of Technology, Melbourne, FL; Lenore Tedesco and Mike O'Neal, Dept of Geology, Indiana/Purdue University @ Indianapolis, Indianapolis, IN

A paleoecological investigation of Florida Bay and adjacent coastal lagoonal areas, such as Whitewater and Coot Bays, was initiated using high-resolution retrospective analysis of regional sediments. Reconnaissance coring, based on evaluation of historical aerial photographs, helped guide core-site selection. Recovered cores, up to 1.5 m long, were quality controlled for stratification/disruption via X-radiography to assure that sediment geochronology would yield interpretable sequences. Once in a temporally constrained context, sediments were subsampled from common 1-cm intervals for chemistry, biology, and palynology. An integrated study of sedimentology, geochronology (Pb²¹⁰, Cs^l37), micropaleontology (foraminifera & ostracods), chemistry (organic carbon, heavy metals, stable isotopes) and palynology allowed a broad scope of characterization. Palynology is being used as an indicator of flushing of freshwater and brackish species, and along with benthic community structure, used to confirm flow out of Shark River Slough. These data were complemented with an analysis of historical rainfall, gauged freshwater flow, and limited near-shore salinity data.

In order to understand the responses of the Florida Bay ecosystem to natural and anthropogenically-induced changes we are addressing several testable hypotheses including the following two that are featured here:

· Anthropogenically-induced changes in freshwater pow across and out of the Everglades, over the last century, have caused changes in the adjacent coastal environment. This has lead to changes in the biogenic community structure which are both recorded in, and interpretable from, the sediment record.

· The biogenic community structure can be used to interpret relative changes in environmental parameters such as paleosalinity.

In overview, results from geochronology supported by other co-sampled parameters, indicated disturbed horizons that temporally correlate with major hurricanes. These recorded disturbances range from essentially undetectable in some sheltered coastal areas to significant in the open waters of Florida Bay. Sediment burdens of heavy metals also showed time-based changes that temporally correlated with documented anthropogenic usage patterns.

Microfaunal analysis of benthic foraminifera and ostracods in the Lower Everglades/Florida Bay has been performed as part of this integrated paleoecological study. Two aspects presented here are: 1) Temporal changes in microfaunal populations in Whitewater Bay and implications regarding water management strategy, and 2) Initial findings from a "modern analog" study of living populations at several Florida Bay sites.

In a high-resolution core in northern Whitewater Bay, a temporal correlation between population characteristics, stable isotopes, and water management policies was observed. Specifically, stable isotope analyses of selected species of foraminifera and ostracods indicate salinity stress events during periods of documented drought and reduced water flow through Shark River Slough. Moreover, foraminifera and ostracod community structures showed changes that temporally correlated with the combination of natural rainfall patterns and anthropogenic effects such as water management practices.

Microfaunal populations currently living in surface sediments throughout Florida Bay are under study. These are being assessed to address certain issues critical in paleoenvironment interpretation using sediment cores. These are: 1) Do post-mortem processes such as transport significantly affect total assemblages?; 2) What relationships exist between the living populations and environmental parameters such as salinity and seagrass presence/absence? Relative to the first question, in surface sediment samples from areas throughout the Bay, total assemblages were determined to be representative of the in-situ living populations for 95% of the benthic foram and ostracod species. Correlation with seasonal salinity data indicate that the remaining 5% are due to seasonal relative abundance changes. Living (vital-stained) microfaunal populations were studied at adjacent grassy/non-grassy sites on a transect from inner to outer Florida Bay. No species in either group was a clear-cut grass indicator bay-wide. Additionally, seagrass-related relative species percentage relationships were found to be site-specific. Data from this transect suggests that only in the inner bay is seagrass an important factor in determining microfaunal diversity. Stable isotopes and relationships between community parameters and salinity are under current investigation.