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Case Study: Interplay of Carbonate Islands, Coral Reefs, and Sea Level

Introduction
Setting
Pleistocene Geology
Holocene Geology
Hydrogeology
Water Resources
Case Study
Concluding Remarks
References

The most significant reef growth along the Florida reef tract is seaward of the largest Keys. Reefs are generally absent opposite channels between islands. In an 1851 report to the Superintendent of the Coast Survey, Louis Agassiz described the relation between reefs and Keys as follows (Agassiz, 1880, p. 5):

"Though continuous, the outer reef is, however, not so uniform as not to present many broad passages over its crest, dividing it, as it were, into many submarine elongated hillocks, similar in form to the main keys, but not rising above the water, and in which the depressions alluded to correspond to the channels intersecting the keys."

More than a century later, Newell and Rigby (1957) observed a similar relation between channels in Andros Island, Bahamas, and the barrier reef along that island's east side. Newell and Rigby (1957) surmised that the distribution of reefs east of Andros might be due, in part, to the eastward flow of bank water unfavorable to coral growth.

Ginsburg and Shinn (1964, 1994) noted that the relation between islands, channels and reefs in the Florida Keys and Bahamas suggests that reefs grow best where they are protected 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 reef-building coral growth (Holmquist et al., 1989; Robblee et al., 1989). Similarly, reef growth is limited where bay water passes between the Keys to the shelf (Shinn et al., 1989; Ginsburg and Shinn, 1994).

During a rising sea level, circulation between shelves and coastal bays increases on low-relief carbonate platforms. As water depths increase and larger areas of the platforms are flooded, the volume of the tidal wedge increases. On platform tops, waters may become increasingly warm, saline, or, in the case of the Bahamian platforms, cold in the winter (Roberts et al., 1982). Reefs that become established along the margins of platforms may thrive for thousands of years until the platform is flooded. Healthy reefs may grow rapidly enough to keep up with all but the largest rates of sea-level rise (Schlager, 1981). Eventually, flooded lagoons or platforms develop water conditions that limit coral growth. After thousands of years of sea-level rise, shelf-margin reefs may be "shot in the back" (Neumann and McIntyre, 1985, p. 107) by their own bays and lagoons as circulation increases between the platform interior and shelf edge. Thus harmful platform and lagoon waters may be an important factor in terminating reef growth on time scales of thousands of years.

Today, sea level continues to rise in the Florida Keys. The measured rise at Key West has been almost 30 cm since 1850 (Maul and Martin, 1993). Although the impact of this sea-level rise is not fully understood, it is thought that an increase of 30 cm would have been sufficient to change the circulation dynamics of Florida Bay significantly. Florida Bay, on average, is about 1.5 m deep now, so the historical rise represents a 25% increase in bay depth and, presumably, would have resulted in a significant increase in tidal exchange between the bay and reef tract. However, water depth is also affected by sedimentation rates in the bay. For much of the bay, sedimentation rates are undetermined. The detailed relation between the current sea-level rise and reef health will require more study before the concept that reefs are "shot in the back" by their own lagoons can be applied predictively to the Florida reef tract and Florida Bay.

Continued sea-level rise will diminish the size of the Keys and their freshwater to brackish lenses, eventually eliminating both. Lidz and Shinn (1991), projecting the rise of the recent geologic past, have forecast how rising sea level might flood the islands in the future. A rise of a little more than 5 m would completely drown the Keys. Other factors such as global temperature change and disease might affect reef health, but the long-term fate of Florida's Acropora palmata coral reefs is toward continued decline if sea level continues to rise and the protective islands are diminished.

The Florida Keys provide evidence that the long-term relation between sea-level rise and declining reef health is not continuous. When sea level is 5-6 m above its present position, reef growth will probably start anew on the hard surfaces of the drowned Keys, initiating the next episode of reef growth, such as that observed in the late Pleistocene sequence. At that time, as in the past, the Florida shoreline will have transgressed 200 km to the north, the Florida Current may meander much farther onto the shelf, and the passage of severe winter cold fronts may be less frequent. At this future time, reef growth may change style, but a healthy, if faunally distinct, reef may flourish again on the Florida Keys.




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