We are investigating the response of the Everglades/Florida Bay ecosystem to hydrologic changes over the last century using floral and faunal assemblages as proxies for vegetation and environmental parameters such as hydroperiod, salinity, and substrate. Data from more than twenty cores in the Everglades and seven cores in Florida Bay provide biotic records covering the last 2,000 years. The long-term record of the Everglades is provided by peat cores, which provide century-scale resolution over the last 2,000 years. Cores from Florida Bay and Biscayne Bay have much higher sedimentation rates and provide decadal-scale resolution for the last century. Here, we present data on the natural variability of the system over the last few millennia as well as the impact of hydrologic changes on the system as a whole.

Analyses of pollen assemblages from Everglades cores from Loxahatchee NWR, Water Conservation Area (WCA) 2A, 3A, and 3B, and Everglades National Park indicate that water depths and hydroperiods have been greater than today from at least 2,000 yr. BP until about 1500 AD, with an interval of drier conditions from about 1200-900 AD. Sites in the northern Everglades were characterized by slough vegetation for most of this period, and southern Everglades sites typically consisted of sawgrass marshes and wet prairies. At the southernmost Everglades site near Florida Bay, invertebrate faunas indicate a change from fresh-water to brackish conditions at about 1200 AD. Concomitantly, vegetation shifted from fresh-water to brackish marshes. This salinity shift in the Bay and vegetational shift from sloughs to sawgrass marshes in the Everglades indicates a system-wide response to climatic changes during the Medieval Warm Period (900-1300 AD), when droughts and increased sea-surface temperatures have been recorded elsewhere in the region. Although terrestrial environments became wetter again after about 1200 AD, the site near Florida Bay never returned to fresh water and appears to have become more saline through time. Following an increase in tree-pollen abundance in the 16th century, vegetation remained stable until the mid-19th century, indicating stable, somewhat drier conditions during this time.

Biotic and environmental changes of the last century are recorded in cores from the Everglades, Florida Bay, and Biscayne Bay. In the Everglades, major changes in the vegetation occurred between the 1920s and 1940s, when water depths and hydroperiods were altered throughout the region. At most sites, these changes resulted in drier conditions and a shift from slough to sawgrass marsh vegetation. Additional, more localized changes occurred after 1950, including the increased abundance of cattails at a nutrient-enriched site in WCA 2A, shallower water depths and greater abundance of weedy annual species at sites in WCA 3A, and a shift from fresh-water and brackish marsh taxa to mangroves near Florida Bay. Correlative changes are recorded in Florida Bay salinity and seagrass abundance based on evidence from ostracodes, molluscs, benthic foraminifers, and diatoms.

The combined terrestrial and marine records indicate that the major, system-wide biotic changes occurred by 1940, which coincides with the construction of primary canals and the Tamiami Trail. The resulting disruption of sheet flow apparently had a broad regional impact, particularly when compared to the more localized impacts of changes associated with the C&SF project. These paleoecological reconstructions indicate the mid-19th century provides a reasonable approximation of the "natural" state of the Everglades ecosystem over the last few centuries and may be a realistic goal for restoration planning.

(This abstract was taken from "Programs and Abstracts - 1999 Florida Bay and Adjacent Marine Systems Science Conference". (PDF, 1 MB))

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