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

Topical Area: Paleoecology

Terry A. Nelsen, NOAA/AOML/OCD, Miami, FL; Harold R. Wanless, University of Miami, Miami, FL; Pat Blackwelder, Terri Hood, Carlos Alvarez-Zarikian, and Peter Swart, RSMAS/University of Miami, Miami, FL; John H. Trefry and Woo-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, IN

Florida Bay is a shallow subtropical estuary (~ 2200 km2, mean depth ~ 2 m) that lies at the southern end of peninsular Florida and is subject to both natural and anthropogenically-induced changes. Nearshore areas receive not only atmospheric input but also freshwater input from natural sheetflow, major and minor creeks (Shark River and Taylor Sloughs respectively), an unquantified input from groundwater and direct precipitation as well as anthropogenic sources (C-111 canal).

A paleoecological investigation of this region 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, Cs137), micropaleontological (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 complimented 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 focusing our work on the following testable hypotheses:

Sedimentary sequences from Whitewater Bay, Coot Bay and Florida Bay have been collected and analyzed. The former was collected based on its position directly adjacent to the outflow of the Shark River Slough. This area is not only the major source of freshwater to the south Florida coastal environment and subsequently into Florida Bay, but one directly controlled by water management policies that are of prime concern to studies of coastal environmental changes. Coot Bay provided an intermediate site and one potentially influenced by canal construction while sediments from Florida Bay were central to paleoenvironmental reconstruction therein. Placed within a temporal context by geochronology, sediment accumulation rates of ~ 1 cm per year in recovered sequences permitted evaluation back to about the turn of the century. This allowed high-resolution evaluation of other co-sampled parameters over a time period that encompasses both natural and anthropogenic changes.

Results from geochronology, that were 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. Moreover, foraminifera and ostracod community structures showed changes that temporally correlated with both natural rainfall patterns and anthropogenic effects such as water management practices. Sediment burdens of heavy metals also showed time-based changes that temporally correlated with documented anthropogenic usage patterns. Stable isotope analyses, of selected species of foraminifera and ostracods from the Whitewater Bay sediments, indicate salinity stress events during periods of documented drought and reduced water flow through Shark River Slough.

During the past phase of research we have developed and refined valuable techniques for analyzing layered sediment sequences to document the regional natural and anthropogenic influences on the chemistry and benthic communities of the coastal lagoons of south Florida. For the next two-year cycle we will continue this highly successful approach and continue to document the influences of water-flow/salinity changes and events (hurricanes) on the regional environments of Florida Bay.