Abstract Introduction Acknowledgments Methods Results Discussion Conclusions References

CONCLUSIONS

Changes in ecosystems take place on many scales, from daily to decadal to millennial. Molluscs generally are tolerant of short-term perturbations in their environment, but they will respond to seasonal and annual changes in salinity. Thus, changes recorded in molluscan populations are significant. A comparison of molluscan faunal distributions in modern Florida Bay to down-core faunal distributions has revealed important details about the last 100 years of Florida Bay history.

All four cores examined show changes during the period of time represented by deposition at the individual sites, yet only the Bob Allen site recorded a complete change in molluscan assemblages. The molluscan fauna that occur throughout the cores are the same fauna that are present at those sites today. Changes that have occurred within the cores are fluctuations in patterns of dominance and diversity within a single assemblage, not complete replacement of the assemblage. For example, the Turbo/Tegula/Columbella assemblage indicative of more marine conditions has not existed at Bob Allen mudbank or at Russell Bank during the last 100-200 years. Nor has the northern transitional assemblage, typical of the Little Madeira Bay sites, dominated at Bob Allen or Russell Bank. Even the change that occurs around 1900 at Bob Allen mudbank does not represent complete replacement of a fauna. We have not identified a modern analogue for the Transennella assemblage that existed at Bob Allen prior to the turn of the century, but we do know that Transennella sp. occur at Bob Allen today.

The changes indicated by shifts in dominance patterns, however, do suggest the influence of some environmental factors on molluscan fauna in northern, eastern and central Florida Bay. The patterns in core T24 clearly demonstrate increasing salinity at the mouth of Taylor Creek. Freshwater outflow through Taylor Creek has been controlled by water management during the later half of this century. The change in salinity at this site may be due to water management practices, fluctuations in average annual precipitation, changes in sea level, or a combination of all of these. The record from Pass Key (core PK37) is not as clear, due in part to the rapid sedimentation rates at that site, but fluctuations in the abundance of Anomalocardia auberiana and other typical northern transition species at Pass Key probably reflect fluctuations in salinity. This conclusion is consistent with Lyons' findings (1996, 1998).

The age models for Russell Bank (core RB19B) and Bob Allen mudbank (core BA6A) allow us to compare changes in the molluscan fauna to historical data on human activities and rainfall (Text-figs. 9 and 10). The changes that occur between 1900 and 1910 at Bob Allen and between 1913 and 1933 at Russell indicate a shift toward less stable conditions. The disappearance of Anomalocardia auberiana from Russell Bank after 1922 may indicate more saline conditions. It is possible, given the increasing error in the age model with depth, or the possibility of varying sedimentation rates with depth, that these events occurred simultaneously. During this time period, average rainfall did not fluctuate significantly in southern Florida, but substantial human alteration of the environment occurred with the construction of the Flagler Railroad in the Keys. Given the current data, it is impossible to determine if the construction contributed to changes in the molluscan fauna, but evidence indicates that the railroad restricted the natural exchange of water between the bay and the Atlantic (Swart et al., 1996).

Between approximately 1930 and 1980, the molluscan fauna at Bob Allen and Russell both went through a period of fairly rapid and dramatic fluctuations. These fluctuations correspond to a period of relatively dramatic shifts in regional rainfall and to a period of significant alteration of the terrestrial environment with the construction of canals, the implementation of water management practices, and a rapidly growing population. Lithopoma americanum, a typically western Florida Bay euhaline species, is occasionally present at Bob Allen from approximately 1930 to 1980. The presence of Lithopoma may be indicative of periods of increased salinity or of fluctuating marine influence caused by the environmental perturbations.

Beginning around 1980, Brachidontes exustus becomes the most dominant species at Russell and Bob Allen, and it increases in dominance in the upper portion of the cores from Pass Key and Taylor. Brachidontes exustus is a euryhaline species that can tolerate diminished water quality, and it is nearly ubiquitous at sites in central and eastern Florida Bay today. The concentration of this species in the upper portion of all the cores implies that salinity fluctuations have increased in the last 20 years, and/or water quality has diminished.

Additional studies of the autoecology of molluscan species will refine our knowledge of the environmental parameters that control their distribution and will increase their utility as bioindicators. We have established that the following molluscan species can serve as important biological indicators of conditions in Florida Bay during restoration: Anomalocardia auberiana, Brachidontes exustus, Cerithidea spp., Pteria longisquamosa, Polymesoda maritima, Turbo castanea, Tegula fasciata, and Hydrobiidae. Additional cores from eastern and central Florida Bay are being examined to geographically extend our coverage, and determine if the patterns seen here persist in other areas as well.

It is critical for effective restoration of the Florida Bay ecosystem that we understand the dynamics of the system and the degree to which physical, chemical and biological components of the system have varied in the past. Studies of modern fauna and flora provide a means to interpret biological data preserved in cores, and to determine the physical and chemical variations in the environment indicated by the biota. Once the patterns of the past are understood, we can better predict the impact of future change on the environment.