But a new University of Miami Rosenstiel School of Marine and Atmospheric Science-led study of coral skeletons from Florida Bay suggests that the present poor conditions of bay water were set in motion nearly 100 years ago by the construction of the Florida East Coast Railway, which connected Miami to Key West. By analyzing the oxygen and carbon isotopes in the skeleton of a 160-year-old coral head from the bay, scientists discovered that there has been no clear increase during the last 80 years in the salinity of the southern portion of Florida Bay from which the skeletons were retrieved.

This finding, which is presented in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, runs contrary to a widely-held belief that the decline of Florida Bay is the result of high salinities caused by a man-made diversion of freshwater from the Everglades.

“There is no doubt that the flows of freshwater into Florida Bay have declined, but the geological record from this coral head shows no clear evidence that during the last 80 years this decline has substantially affected the salinity of the portion of the bay where this coral is located,” says principal investigator Dr. Peter Swart, chairman of the Division of Marine Geology and Geophysics at the Rosenstiel School.

“We suggest that the railway construction combined with a reduction in the number of hurricanes over several decades has allowed a buildup of organic material within Florida Bay.”

During construction of the railway, natural gaps between the keys in several locations were filled in to form causeways linking the islands. Swart and his colleagues believe that these unnatural fills reduced the natural exchange of water between Florida Bay and the open sea of the Florida Straits.

“These blockages allowed Florida Bay to retain more of its organic-rich muds,” Swart says. “This organic material decays, reducing levels of free oxygen in the water and causes a build-up of nutrients. The nutrients promote algal blooms, further reducing the oxygen in the bay through the process of eutrophication.”

According to Swart, this build-up of nutrients in the bay has been compounded by the fact that Florida Bay has received relatively few hurricanes during the past 47 years. “Hurricanes have a cleansing effect on the bay,” he says. “They remove organic-rich sediment to the open ocean.”

By analyzing the oxygen and carbon isotopes of cores drilled from the bay corals, Swart and his colleagues determined that there has been no clear increase in the salinity in the lower part of the bay during the past 80 years. The largest change in salinity and nutrients occurred between 1905 and 1910, coincident with the construction of the Florida East Coast Railway. Salinity in that part of the bay has not changed significantly in the years since construction of the South Florida canals that have been diverting water away from the bay.

“A combination of the changes brought about by the railway and the absence of hurricanes has caused this mess,” Swart says. He believes that, in addition to current efforts to redirect freshwater toward the bay, it may also be necessary to restore the natural exchange of water between the bay and the sea by replacing keys causeways with bridges.

The finding does have its detractors, Swart acknowledges. “Some scientists suggest that the salinity changes in the lower bay may not be representative of other portions of the bay, particularly in those areas where the most significant sea grass die-offs are taking place. But other data suggest that salinity in one portion of the bay is strongly correlated to salinity in other parts of the bay.”

Swart and his students Genny Healy and Lisa Greer, together with Richard Dodge of Nova Southeastern University are now undertaking a study for the South Florida Water Management District to correlate salinity records of corals from throughout the bay as a way to assess their current findings as a predictor of the condition of the bay as a whole.

“In many ways corals can be considered to be the trees of the sea,” Swart says. “Many of them form annual bands in their skeletons comprising various densities of calcium carbonate. By counting and measuring these bands, the age and growth rate of the coral can be determined.”

Corals serve as archives of environmental conditions of the past. By analyzing the chemical composition of coral skeletons, scientists can determine water salinity, water temperature and other factors that affected the growth of a coral during its history. Swart points out that coral cores are removed with minimal damage to the coral head. After drilling, cores are plugged with cement, which serves as substrate for the re-growth of corals and prevents damage from organisms that bore into corals.

“There is a saying in geology that the present is the key to the past,” Swart says. “In this case, the past is very much the key to the present.”