Figure 1. Location of transects across ridges and sloughs, Water Conservation Area 3A, Everglades, Florida. [larger image]

The Ridge and Slough landscape is a widespread habitat type in the Everglades, consisting of a system of dense sawgrass ridges separated by relatively open waterlily sloughs. The ridges run approximately parallel to each other, oriented in the direction of flow. Using pollen and chronological analyses of sediment cores, we tested the hypotheses that 20^th century compartmentalization and water-management practices have altered the ridge and slough landscape and that ridges are expanding at the expense of sloughs.

A suite of surface samples collected in different subenvironments of the landscape were analyzed to determine which vegetation types were distinguishable in the pollen record. These include: dense Cladium (sawgrass) with scattered Cephalanthus (buttonbush) in the central ridge; less dense, shorter Cladium, with abundant Cephalanthus and occasional Crinum and Sagittaria in the ridge-slough transition zone; and Nymphaea, Utricularia, Panicum, Pontederia, and Eleocharis in the slough. Using Mann-Whitney tests and cluster analyses, it was shown that ridge and slough assemblages differ significantly, primarily in abundance of Cladium pollen. Incorporation of these data into the existing database of 170 sites and eight vegetation types provides a tool reconstruct past vegetation and its response to hydrologic changes based on pollen analysis of sediment cores.

Transects of sediment cores were collected across ridges and sloughs in Water Conservation Area (WCA) 3A and 3B (fig. 1); these transects include cores collected in the central part of the dense sawgrass ridge, the ridge-slough transition zone, and central slough. Analysis of the slough core indicates that slough vegetation occupied the site for >2,000 years; however, increased abundance of sawgrass pollen during the 20th century indicates the onset of drier conditions. Likewise, the central ridge site was occupied by sawgrass for approximately 2,000 years. The central ridges and sloughs appear to have been stable over long time scales. At the ridge-slough transition site, however, slough vegetation was present for >2,000 years before expansion of sawgrass ridge sometime in the last 200 years. Completion of geochronological analyses will document the timing of the expansion more precisely.

These preliminary data provide evidence that sawgrass ridges have expanded laterally into sloughs. Future coring strategies will address the issue of north-south stability of ridges and sloughs and will expand into WCA 2A to determine whether the ridge and slough landscape previously occupied more northerly sites and, if so, when it shifted to predominantly sawgrass vegetation.

Contact: Debra A. Willard, U.S. Geological Survey, 926A National Center, Reston, VA 20192, Phone: 703-648-5320, Fax: 703-648-6953, dwillard@usgs.gov, Oral, Hydrology and Hydrological Modeling

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

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