The Use of Proxy Chemical Records in Coral Skeletons to Ascertain Past Environmental Conditions in Florida Bay

This paper will discuss the use of chemical proxies in coral skeletons to reconstruct the history of salinity (from the δ ¹⁸ O of the skeleton) and nutrients in the water (from the δ ¹³ C) in Florida Bay between 1824 and 1994. Monthly salinity and water temperature data collected since 1989 were used to establish a correlation between salinity, temperature, and the δ ¹⁸ O of the skeleton of the coral Solenastrea bournoni from Lignumvitae Basin in Florida Bay. This relationship explains over 50% of the variance in the δ ¹⁸ O of the skeleton. Assuming that interannual variations in the temperature of the water are small, we have applied this relationship to the δ ¹⁸ O measured in the coral skeleton collected from Lignumvitae Basin which has a record between 1824 and 1993. These data provide a revised estimate of salinity variation in Lignumvitae Basin for the period when historical records for salinity were not available, and show that the highest salinity events occurred in the past 30 yr. Using the relationships between the salinity in Lignumvitae Basin and other basins, obtained using a modern dataset, we are able to estimate ranges in salinity for other portions of Florida Bay. Skeletons of specimens of the coral species Siderastrea radians collected from other areas of Florida Bay show similar patterns in the δ ¹⁸ O over the past 10 yr, indicating that corals in most portions of Florida Bay are recording salinity variations in their skeletons and therefore support the idea that salinity variations in different portions of Florida Bay can be related. Fluorescence analysis of the coral from Lignumvitae Basin shows a large change in the magnitude of the 10-yr signal coincident with the construction of the railway, confirming that this event had a significant impact upon Florida Bay. The δ ¹³ C of the coral skeletons reveals a long-term history of the oxidation of organic material, fixation of carbon by photosynthesis (algal blooms), and the intrusion of marine water into the bay. Since the construction of the railway from Miami to Key West there has been a long-term decrease in the δ ¹³ C of the coral skeleton from Lignumvitae Basin, suggesting the increased oxidation of organic material in this area. This decrease in δ ¹³ C appears to have reached a minimum value around 1984 and has increased since this time in the western portions of Florida Bay. The increase may be related to the algal blooms prevalent in the area or alternatively could result from intrusion of more marine water. In the eastern areas, a small increase in the δ ¹³ C between 1984 and 1988 was followed by further decline suggesting more oxidation of organic material. We have also attempted to use the concentration of barium in the coral skeleton as a proxy indicator of the nutrient status in Florida Bay.