Figure 1. Map showing core locations in central Biscayne Bay. NN, No Name Core. FB, Featherbed Cove. [larger image]

During the last century, Biscayne Bay has been greatly affected by anthropogenic alteration of the environment through urbanization of the Miami/Dade County area and alteration of natural freshwater flow. Changes in the sources, timing, delivery, and quality of freshwater flow into the Bay have altered shoreline and sub-aquatic vegetation to unknown degrees. Land management agencies that focus on the restoration of natural flow of freshwater into Biscayne and Florida Bays can benefit from paleoecological and geochemical analyses documenting the impacts of past changes in freshwater input.

To establish targets and performance measures for restoration, research is being undertaken to determine pre-alteration baseline conditions and natural variation within the system. The USGS, in conjunction with South Florida Water Management District, Biscayne National Park, Duke University, Southern Illinois University, and University of Miami, is examining the natural patterns and causes of temporal change in salinity, water quality, vegetation, and benthic fauna in Biscayne Bay over the last 100-300 years.

The current studies extend prior knowledge of Biscayne Bay environmental history by obtaining cores from new sites, obtaining series of radiocarbon dates, evaluating lead-210 dating of Biscayne Bay sediments, utilizing multiple paleoenvironmental proxies, and applying quantitative methods of faunal and geochemical analyses. These analyses build on an expanding methodology utilized in research conducted in Florida Bay (1995 - ongoing) and Biscayne Bay (1996-2000) (Brewster-Wingard, et al, 2001; Cronin, et al., 2001; Dwyer and Cronin, 2001; Holmes, et al, 2001; Ishman, 1998).

Figure 2. Ostracode Mg/Ca ratios from No Name Bank Core, Biscayne Bay. [larger image]

Three sets of replicate cores were collected in March 2002 from sites in central Biscayne Bay, Featherbed Bank, and an unnamed bank [herein referred to as No Name Bank, equivalent to Black Shoal of Wanless (1969)] (fig. 1), and southern Biscayne Bay at Card Bank. Analyses of the two cores from central Biscayne are nearly complete. A preliminary age model using lead-210 geochronology places the base of the 2002 Featherbed Bank Core at 1741 and the base of No Name Core at 1874. Age models developed from radiocarbon and lead-210 dating of the 1997 and 2002 cores indicate mean sedimentation rates for Card, Featherbed, and No Name Banks were 0.25 cm yr^-1, 0.85 cm yr^-1 and 1.22 cm yr^-1, respectively. The Card, Featherbed, and No Name cores yield paleoecological and geochemical records spanning the past 900, 500, and 150 years, respectively.

Analyses of fossil ostracode and molluscan faunal assemblages indicate a major shift in benthic communities in both central and southern Biscayne Bay during the late 1500's through 1600's, when epiphytal species indicative of seagrass (Thalassia) habitats became abundant. Moreover, there is preliminary evidence that a decline in seagrass-dwelling epiphytal species, unprecedented in the past 400 years, occurred at both No Name and Featherbed Banks during the mid-20^th century. It is unclear what caused this decline; however, it may be due to changes in mean salinity, increased turbidity, nutrient influx, or other factors.

Ostracode shell magnesium to calcium (Mg/Ca) ratios, a proxy for salinity, indicate considerable short-term variability in Mg/Ca at No Name Bank during the past 150 years ( fig. 2). The paleosalinity patterns are generally similar to those observed at Featherbed Bank and from Russell Bank in Florida Bay.

Compilation of the data from the 2002 cores will provide additional data on the timing and causes of salinity variability and its relation to benthic faunal variability and benthic habitats. Comparisons will be made to cores collected in Biscayne Bay between 1996-99, to determine trends in salinity, subaquatic vegetation, and shoreline vegetation for Biscayne Bay. A comparison to Florida Bay cores will provide information on ecosystem-wide changes. These data will provide the essential data necessary to establish performance criteria and restoration targets for CERP (Comprehensive Everglades Restoration Plan) and the agencies responsible for implementing CERP.

REFERENCES

Brewster-Wingard, G.L., Stone, J.R., and Holmes, C.W., 2001, Bull. American Paleo., no. 361, p. 199-232.

Cronin, T.M., Holmes, C.W., Brewster-Wingard, G.L., Ishman, S.E., Dowsett, H.J., Keyser, D., and Waibel, N., 2001. Bull. American Paleo., no. 361, p. 159-198.

Dwyer, G.S., and Cronin, T.M., 2001, Bull. American Paleo., no. 361, p. 249-276.

Holmes, C.W., Robbins, J., Halley, R., Bothner, M., Brink, M.T., and Marot, M., 2001, Bull. American Paleo., no. 361, p. 31-40.

Ishman, S.E., 1998, Jour. Coastal Research, sp. issue 26, p. 125-138.

Wanless, H.R., 1969, Sediments of Biscayne Bay - Distribution and depositional history: University of Miami Institute of Marine and Atmospheric Sciences, Technical Report.

Contact: Wingard, G. Lynn, U.S. Geological Survey, MS 926A National Center, Reston, VA 20192. Phone: 703-648-5352, Fax: 703-648-6953, email: lwingard@usgs.gov, Coastal Systems

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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