The greater Everglades ecosystem of south Florida exists because of a delicate balance between freshwater influx, low topographic gradient, underlying geology and subtropical climate. This combination has created a unique and diverse ecosystem, supporting many threatened and endangered species of plants and animals that rely on abundant freshwater. Historically, the Shark River Slough served as the primary conduit of freshwater through the central portion of the Everglades southwestward to the coastal region. This natural movement of water, however, has been dramatically reduced over the last century as construction of canals, water conservation areas and the Tamiami Trail either retained or diverted flow (Light and Dineen, 1994). The reduction in flow and changes in water quality through Shark River have had a significant effect on the freshwater marshes and the associated coastal ecosystems. Additionally, the flow reduction may have shifted the balance of fresh to salt-water inflow along coastal zones, resulting in an acceleration of the rate of inland migration of mangroves into the freshwater marshes.

South Florida is currently undergoing a massive restoration effort guided by the Comprehensive Everglades Restoration Plan (CERP, 1999). One of the primary goals of the CERP is to restore the natural flow of water through the terrestrial Everglades and into the coastal zones. Both the CERP and the Department of Interior Science Plan recognize the importance of understanding freshwater flow into south Florida’s estuaries, and the changes incurred in the estuaries due to anthropogenic alterations of freshwater flow. One of the three primary objectives stated in the DOI Science Plan is to “ensure that hydrologic performance targets accurately reflect the natural predrainage hydrology and ecology” (DOI Science Plan, 2004, p. 14). A historical perspective can be obtained by examining the record of the hydrologic and biologic components of the natural system preserved in the sediments of south Florida. USGS researchers have utilized this method to provide restoration planners with information on the ecosystem history of Florida Bay and Biscayne Bay (Brewster-Wingard and others, 2001; Cronin and others, 2001; Ishman and others, 1998; Wingard and others, 2003, 2004).

The importance of Shark River Slough outflow to the health of the coastal ecosystem led the USGS Ecosystem History of South Florida’s Estuaries Project to begin work in the southwest coastal area in 2004. A preliminary series of cores were collected in May 2004 (Wingard, et al., 2005). The cores collected in July 2005 and described in this report form transects with three of the 2004 cores, which allows us to examine changes in flow along a gradient moving towards the coast (Figure 1). The objectives of this research are to document impacts of changes in salinity, water quality, coastal plant and invertebrate communities and other critical ecosystem parameters on a decadal to centennial scale in the southwest coastal region, and to correlate these changes with natural events and resource management practices. Examination of these long-term (centennial scale) data sets will allow us to determine what the natural trends or cycles of change (such as rising sea-level or changes in climatic patterns) within the ecosystem were, and how anthropogenic alteration offset those natural cycles.

Figure 1. Satellite image of the southwest coast of Florida, showing general location of cores. Yellow line marks approximate boundary of Everglades National Park. Yellow dots indicate location of July 2005 core sites; SR1=Shark River, near entrance to Ponce de Leon Bay; SR2=Shark River system, north of main channel; SHR1=South Harney, near mouth of South Creeks; HR2=Harney River, near water station; FBLM1=First Bay of Lostmans River; SBLM2=Second Bay of Lostmans River. Red dots indicate location of May 2004 core sites (reported on in Wingard and others, 2005); AL=Alligator Bay Core; LM=Big Lostmans Bay Core; BR=Broad River Bay Core; TB=Tarpon Bay Core; and RR=Roberts River Core from river mouth. Orange numbered triangles correspond to South Florida Water Management District sites listed in Table 1. Blue lines trace the three river system transects discussed in this report: Lostmans, Harney, and Shark. Image cropped from John W. Jones and others (2001). Table 1 gives geographic coordinates of 2005 core locations and Figure 2 illustrates precise location of cores. [larger image]

Table 1: Summary of salinity data from river systems where cores were located. Data from South Florida Water Management District's Water Quality Monitoring Project (data from http://www.sfwmd.gov/org/ema/envmon/wqm/wqprojects.html, accessed August 2, 2006). Position of stations are shown on Figure 1 (orange triangles) Note: data were collected approximately once/month and generally two measurements were taken for each collection. [Click here to see entire table]

The Shark River Slough, flowing southward through Everglades National Park, delivers freshwater from Lake Okeechobee and the terrestrial ecosystem to the southwest coastal area of Florida. In addition, the Fakahatchee Strand and the Okaloacooochee Slough provide freshwater to the Ten Thousand Islands area north of Lostmans River. The southwest coastal region is a complex network of channels, bays, coastal prairies, and mangroves (Figure 1). In 2004, a series of cores were collected from the inner bays that form the transition between the coastal and terrestrial ecosystems (cores described in Wingard, et al., 2005). The six sites cored in July 2005 and reported on here were collected downstream from 2004 cores, forming three transects from the inner bays out to the coast along Lostmans River, Harney River, and Shark River (Figure 1). This area receives the most direct influence of the overland flow from Shark River Slough.

Characterized as a mangrove estuary (or mangrove and coastal glades (McPherson and Halley, 1996)), salinity increases downstream in the channels towards the coast, but can vary tremendously on a seasonal basis, and inter-annually with changes in rainfall, tide, and freshwater flow from the terrestrial Everglades (Davis and others, 2005). Measured salinity along these transects over the last decade shows a high range of variability from <5 ppt to >30 ppt at most sites (Table 1; data from South Florida Water Management District). Ponce de Leon Bay experiences slightly higher salinities; the minimum measured salinity there is 9.8 ppt over the last decade.

Topographic highs throughout the southwest mangrove estuarine zone are less than one meter in elevation (Desmond, 2005). Due to the low relief and the location of the region at the terminus of the freshwater drainage, the mangrove estuaries of the southwest coast are “particularly vulnerable to changes in sea level and freshwater flow” (Davis and others, 2005, p. 833). These two factors – sea level and freshwater flow – are the primary drivers controlling “all ecological processes and attributes in the mangrove coastline of the southern Everglades” (Davis and others, 2005, p. 833). A general comparison of 1940 aerial photos and digital orthophoto quadrangles (DOQs) from the 1990s (Figures 2 and 3) indicates that the area has not undergone large scale geomorphic changes in the latter half of the 20^th century.

Figure 2. Comparsion of 1940 and 1995 landscape for region near Tarpon Bay and Harney River. Yellow X on map shows location of Harney River core (GLW705-HR2) from near water monitoring station on both images. A. 1940 aerial photo obtained from Smith and others (2002; section 34, line 6, image 3; available at http://sofia.usgs.gov/exchange/aerial-photos/). B. Composite of four 1995 Digital Orthophoto Quadrangles (DOQs) from Harney River area. DOQ images obtained from http://edcsns17.cr.usgs.gov/EarthExplorer/ [click on the images above to view larger versions]

Figure 3. Comparsion of 1940 and 1994 landscape for region near Lostmans River. Yellow X on map shows location of Lostmans River, Second Bay core (GLW705-SBLM2) on both images. A. 1940 aerial photo obtained from Smith and others (2002; section 27, line 14, image 182; available at http://sofia.usgs.gov/exchange/aerial-photos/). B. Composite of two 1994 Digital Orthophoto Quadrangles (DOQs) from Lostmans River area. DOQ images obtained from http://edcsns17.cr.usgs.gov/EarthExplorer/ [click on the images above to view larger versions]

We would like to thank Everglades National Park for their cooperation in this study; the work described here was conducted under Study #EVER-00141 and permit #EVER-2004-SCI-0099. This work was funded by the U.S. Geological Survey (USGS) Greater Everglades Priority Ecosystems Studies. Marci Marot, USGS, assisted in the core collection; and supervised the core cutting process. We would like to thank our reviewers Debra Willard and Bruce Wardlaw, U.S. Geological Survey, for their thoughtful and thorough reviews of the manuscript.