Ecosystem History of Central Biscayne Bay Based on Sediment Core Analyses
G.L. Wingard, T.C. Cronin, D.A. Willard, J.B. Murray, R. Stamm
U.S. Geological Survey, Reston, VA
C.W. Holmes
U.S. Geological Survey, St. Petersburg, FL
G.S. Dwyer
Duke University, Durham, NC
S.E. Ishman, C. Williams
Southern Illinois University, Carbondale IL
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.
In order 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 and 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).
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 1600s, 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-20th
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
1996-1999, to determine trends in
salinity, subaquatic vegetation, and
shoreline vegetation for Biscayne Bay.
A comparison to Florida Bay cores will
provide information on ecosystemwide changes. These data will provide
the essential data necessary to establish
performance criteria and restoration
Figure 1. Map showing core locations in
central Biscayne Bay. NN, No Name
Core. FB, Featherbed Core.
targets for CERP (Comprehensive Everglades Restoration Plan) and the agencies
responsible for implementing CERP.
References cited:
Brewster-Wingard, G.L., Stone, J.R., and Holmes, C.W., 2001. Bull. American
Paleo., n. 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., n. 361, p.
159-198.
Dwyer, G.S., and Cronin, T.M., 2001. Bull. American Paleo., n. 361, p. 249-276.
Holmes, C.W., Robbins, J., Halley, R., Bothner, M., Brink, M.T., and Marot, M.,
2001. Bull. American Paleo., n. 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.
Wingard, G. Lynn, U.S. Geological Survey, MS 926A National Center, Reston,
VA 20192. Phone: 703-648-5352, Fax: 703-648-6953, lwingard@usgs.gov,
Coastal Systems.
Figure 2. Ostracode Mg/Ca ratios from No
Name Bank Core, Biscayne Bay.