Watersheds and Coastal Waters
Coral Reefs and Sea Level
In south Florida, most luxuriant reef growth has long been observed to be located near the shelf edge seaward of the largest Florida Keys. Ginsburg and Shinn (1964, 1994) noted that this distribution, as well as similar relations between reefs and islands in the Bahamas, indicate that reefs grow best where islands protect them from extremely shallow bay waters. Reefs are absent in Florida Bay because the water is too cold in the winter, too hot in the summer, and too variable in salinity to support the growth of reef-building coral (Tabb and others, 1962; Holmquist and others, 1989; Robblee and others, 1989). Similarly, reef growth is limited where bay water passes between the Keys to the shelf (Shinn and others, 1988; Ginsburg and Shinn, 1994).
During a rising sea level, circulation increases between shelves and coastal bays on low-relief carbonate platforms because water depths increase and larger areas of the platforms are flooded, thus increasing the volume of the tidal wedge. On platform tops, water may become increasingly warm, saline, or, in the case of the Bahama Platform, cold in the winter (Roberts and others, 1982). Reefs that become established along the margins of platforms may thrive for thousands of years until the platform is flooded and bays develop water conditions that limit the growth of coral. During thousands of years of sea- level rise, shelf margin reefs may be "shot in the back" by their own bays and lagoons as circulation increases between the platform interior and shelf edge (Neumann and McIntyre, 1985).
Sea level is continuing to rise in south Florida (fig. 22), and the measured rise at Key West has been almost 6 in. since 1913 (Emery and Aubrey, 1991) and about 1-ft since 1850 (Maul and Martin, 1993). Although the result of sea- level rise is not fully understood, a 1-ft increase is thought to have significantly changed the circulation dynamics of Florida Bay. Florida Bay, on average, is about 5 ft deep, so the natural increase represents a 25-percent increase in depth and, presumably, a significant increase in water exchange between the bay and reef tract. Water depth also is determined, in part, by sedimentation rates in the bay that are, to date, undetermined. The relation between current sea-level rise and the health of reefs is a topic that requires more study before specific forecasts of these processes can be applied to the Florida Keys. If sea level continues to rise for geologically significant periods, however, then the long-term fate of the coral reefs of Florida is toward continued decline.