Core collection methods

Using a 10 cm diameter piston-coring device, we collected 22 cores in 2002 and two cores in 2000 (Appendix A) within a series of transects in the historic ridge and slough landscape (Figure 2). Within each transect, cores were collected based on the distribution of current vegetation types. Typically, we collected one core from the central sawgrass ridge, one in the ridge approximately 10 meters from the ridge-slough transition, one in the transition, and one in the central slough. We sampled the cores in 1 cm increments in the upper 20 cm and in 2 cm increments for the remainder of the core.

Palynological Processing and Analysis

We isolated pollen from both sediment cores and surface samples using standard palynological preparation techniques (Traverse, 1988; Willard and others, 2001a). For each sample, one tablet of Lycopodium spores was added to between 0.5 grams to 1.5 grams of sediment. Samples were processed with HCl and HF to remove carbonates and silicates respectively, acetolyzed (1 part sulfuric acid: 9 parts acetic anhydride) in a boiling water bath for 10 minutes, neutralized, and treated with 10% KOH for 10 minutes in a water bath at 70° C. After neutralization, residues were sieved with 149 µm and 10 µm nylon mesh to remove the coarse and clay fractions, respectively. When necessary, samples were swirled in a watch glass to remove mineral matter. After staining with Bismarck Brown, palynomorph residues were mounted on microscope slides in glycerin jelly. At least 300 pollen grains were counted from each sample to determine percent abundance and concentration of palynomorphs.

Chronology

Figure 3. Age Models for Selected Cores. [click on images above for larger versions]

Chronology of these cores is based on radiocarbon dating, lead-210, and the biostratigraphic indicator Casuarina equisetifolia (Australian pine). Radiocarbon dates were obtained on bulk sediments by Beta Analytic (Appendix B) and lead-210 analyses were conducted in the USGS geochronology lab, St. Petersburg, FL (Appendix C, Appendix D, Appendix E, Appendix F, Appendix G). The pollen of C. equisetifolia, an exotic species introduced to South Florida about 1900 AD, first occurs in sediments in 1910 +/- 15 years (Langeland, 1990; Wingard and others, 2003) after 1940, C. equisetifolia is common. Age models are based off of calibrated years before present (calyr BP) radiocarbon dates and lead-210 (Figure 3).

Statistical Analysis

To determine whether the pollen abundance of dominant vegetation (Cladium in ridges and Nymphaea in sloughs) significantly differed in sediment from each vegetation type, we used the Mann-Whitney test. The null hypothesis is: regardless of vegetation type the percentage of species specific pollen does not vary in spite of changes in site-specific vegetation (i.e. the amount of Cladium pollen is the same in both ridge and slough surface samples).

Reconstruction of past plant communities is based on statistical comparison of fossil and modern assemblages from different wetland communities throughout the Everglades (Willard and others, 2001a). Using the modern analog technique (Overpeck and others, 1985), we calculated dissimilarity coefficients (d_ij) using the squared chord distance formula (SCD):

d_ij= _k (p^1/2_ik-p^1/2_jk)²

Down-core samples with dissimilarity coefficients equal or less than the critical value 0.15 are considered to share similar vegetation and environmental parameters with their modern analog (Willard and others, 2001a).