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open file report > OFR 94-276 > introduction
During the 1980s, scientists and Florida Keys coral reef user groups became alarmed by increasing coral mortality and explosive growths of algae. One well-documented cause of increased algal growth was the disappearance of the herbivorous sea urchin Diadema. Although Diadema suffered near extinction throughout the Caribbean in 1983 (Lessios, 1984), coral mortality and algal proliferation appeared most pronounced on Florida's reefs (Dustan, 1985). While Diadema were dying in unprecedented numbers, reefs, especially in the Florida Keys, were also experiencing accelerating human exploitation. Along with tourism, the resident human population increased dramatically. Unrelated to urban stresses, corals throughout the Caribbean and Florida suddenly expelled the symbiotic algae necessary for their growth and color. This, the first major "bleaching event," as it became known, began during the unusually warm and calm summer of 1987 (Causey, 1988; Ogden and Wicklund, 1988; Porter et al., 1989). Bleaching was especially pronounced on Florida's reefs. Though bleaching caused reduced growth rates, mortality was not significant. For the most part, affected corals recovered and regained symbiotic algae (zooxanthellae) and normal color with the return of cooler water temperatures. Bleaching had occurred in Florida before 1987 (Jaap, 1979, 1985; Glynn 1984), but such a severe case had never been reported and it did not end in 1987. Bleaching reoccurred in the summer of 1990. The hydrocoral, Millepora sp., suffered severe mortality, especially on the tops of reefs (B. Causey, pers. commun., 1994). At the same time, massive corals, namely Montastrea annularis, experienced severe mortality caused by black-band disease (Rutzler and Santavy, 1983; Rutzler et al., 1983, Richardson and Carlton, in press). Dead corals were quickly colonized by turf algae, which flourishes in the continuing absence of Diadema herbivory. As algae became more prominent, many respected coral specialists suggested growth was stimulated by excessive nutrification. The consensus of many reef scientists was that increased nutrification is linked to accelerating urbanization in south Florida and in the Keys specifically (EPA, 1992).
NOAA, EPA, the Audobon Society, the Nature Conservancy, and the University of Miami each conducted coral reef workshops to assess the problem. All workshops have concluded that a change in water quality is the most likely cause of reef mortality and algal proliferation.
The newly created Florida Keys National Marine Sanctuary (FKNMS), as directed by enabling legislation, established an advisory council composed of lay citizens and scientists. This council, like previous workshops, recognized water quality to be of major concern. Thus, concurrent with the FKNMS advisory council, a water-quality steering committee was created with EPA and the State of Florida Department of Environmental Regulation (now Department of Environmental Protection, DEP) taking the lead. The task of the steering committee was, through scientific consensus, to devise a water-quality protection program for the FKNMS. The resulting plan (EPA, 1991, 1992) again concluded that water quality was the target of concern.
During early preparation stages of the water-quality protection program document, EPA/DEP and many others became concerned not only with the more obvious nutrient sources, such as surface runoff, outfalls, and live-aboard boats, but also with the unknown fate of treated sewage effluent entering the porous limestone beneath the Florida Keys. These effluents enter the ground water from septic tanks and through shallow injection wells in what are termed Class V disposal wells. In 1991 DEP records show there were 619 permitted Class V wells in the Florida Keys. DEP data for well and casing depth have been tabulated and displayed as frequency plots in Appendix A. An unknown number of disposal wells, permitted by HRS (Housing and Rehabilitative Services) for family-owned restaurants and private residences, also exist in the Florida Keys. In addition, there were an estimated 24,000 septic tanks and 5,000 cesspools in the Florida Keys as of 1990 (EPA, 1992).
The possibility of nutrients reaching the reefs through groundwater movement and seepage was stimulated by a discovery of near-fresh water seeping from bottom sediment in 130 ft (40 m) of water off Key Largo (Simmons, 1986). More recent work (Simmons and Netherton, 1987) suggested that seepage of ground water off Key Largo is "evidence of a new biogeochemical cycle." Simmons (1992) emphasized that "the movement of water across sediment/water interfaces is very important to the ecology of aquatic habitats." All of the above work was biologically oriented and focused on submarine groundwater discharge from Holocene sediments. Sediments off the Florida Keys are relatively impermeable, especially where they are fine grained. However, the underlying limestone, which would be the primary pathway for submarine fluid movement offshore, had never been investigated. To do so requires equipment and techniques not previously used in reef areas of Florida.
This report, the result of a one-year investigation, addresses the fate of sewage nutrients injected into the porous limestone beneath the Florida Keys. The specific questions addressed include: 1) are nutrient levels elevated in the ground water beneath the Florida Keys? 2) are nutrient levels elevated in ground water beneath offshore reef areas? 3) are ground waters migrating laterally and diffusing upward into areas of coral growth? and 4) if the answers to the above are yes, what controls lateral and upward movement of ground waters?
In an attempt to answer these and other questions, 24 water-quality monitoring wells (average depth 35 ft, 10.7 m) comprising three major transects, Lower Keys, middle Key Largo, and northern Key Largo, were installed and sampled quarterly for one year. The study was a collaborative effort between the Geologic and Water Resources Divisions of the U.S. Geological Survey aided by the NOAA Undersea Research Center (NURC), NOAA's Sanctuaries Reserves Division (including Key Largo National Marine Sanctuary, Florida Keys National Marine Sanctuary and the FKNMS Advisory Council), State of Florida Department of Environmental Protection (DEP), and the Federal Region IV Environmental Protection Agency (EPA).
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 10 September, 2014 @ 12:20 PM (KP)
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