Figure 1. Percent abundance of key molluscan species from Rankin Core. [larger image]

Preliminary data from analyses of a core (GLBW601-RL1) taken in Rankin Basin in June 2001 indicate significant environmental changes have occurred at the site over the last two centuries. The core was collected at a site of documented seagrass die-off in 1987-1988 (P.R. Carlson, Florida Marine Research Institute; written communication, 2001). The purpose of the core collection was to document the long-term sequences of events leading up to the die-off event. Analyses have been conducted to examine faunal changes in the ostracodes and mollusks, biochemistry of the ostracode shells, geochemical changes in the sediment, and the influx of atmospheric dust. The faunal assemblage analyses provide information on the salinity and benthic habitat at the site. These data, when compiled, will provide a means to test hypotheses of cause and effect in seagrass die-off, and will illustrate decadal-scale patterns of change that may contribute to die-off events.

The molluscan faunal analyses show two important results. First, at some point prior to 1900, a significant influx of freshwater occurred in Rankin Basin. Second, immediately prior to the documented seagrass die-off, the mollusks indicate an increase in the amplitude of salinity fluctuations. Figure 1 shows freshwater mollusks present in the lowest 20 cm of the core, reaching a high of 24 percent of the molluscan fauna at 138 cm. Small percentages of terrestrial gastropods (Polygyridae) and a clam, Polymesoda maritima, also are present in this segment of the core. Polymesoda maritima is found in oligohaline to lower mesohaline waters (5-12 ppt), so it is an indicator of reduced salinities. This assemblage is similar to that seen in the lowest portions of a core from Taylor Creek. At approximately 50 cm in the Rankin core a significant increase occurs in Brachidontes exustus and Anomalocardia auberiana, two species that tolerate fluctuations in salinity; these species are typical of the Northern Transition Zone of Florida Bay. When the geochronology of the Rankin core is complete, these changes can be placed into temporal context and compared to natural and anthropogenic events affecting Florida Bay.

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

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

Back to Project Homepage