Historical Salinity and Seagrass Trends in Florida Bay Derived From Benthic Faunal Data

Topical Area: Paleoecology

G. Lynn Brewster-Wingard, Scott E. Ishman and Thomas M. Cronin, U.S. Geological Survey, Reston, Virginia

Benthic fauna preserved in shallow cores from Florida Bay record the history of changes in salinity and seagrass over the last 150 years. These data allow us to recognize long-term trends, to determine the natural range of variability, and to determine if changes in the environment correlate to natural events, to human activities, or to a combination of factors. Seven cores from four sites in eastern and central Florida Bay have been examined: Bob Allen mudbank, Russell Bank, Pass Key, and the mouth of Taylor Creek in Little Madeira Bay.

The benthic faunas present in the cores indicate that fluctuations in salinity and seagrass are part of the natural system. These natural changes appear to be driven by climatic effects. Beginning around 1900, however, the natural patterns of salinity fluctuation began to change slightly. Initially, the faunas indicate an increase in salinity, followed by a period of decreased fluctuations in the low salinity range. After 1940, the changes represent a substantial departure from the pre-1900 pattern. The percent abundance of key faunal indicators shifts from 15-20% about the mean pre-1900, to 40-60% about the mean post-1900. These changes may be a response to the construction of canals that altered the delivery of freshwater to Florida Bay. These data are consistent with the findings of Smith et al. (1989). In a study of fluorescence patterns in coral they found that a significant and persistent decrease in freshwater flow occurred around 1932, and that the natural periodicity seen prior to 1932 disappeared.

Our representations of salinity change in Florida Bay are derived from the analysis of the distributions of modern benthic fauna at 26 monitoring sites in Florida Bay. A strong association between the relative abundances of the benthic foraminifers Ammonia parkinsoniana and Elphidium galvestonense and salinity has been established. Typically A. parkinsoniana is most abundant (>20%) where average salinities are less than 18ppt. Above 18 ppt the miliolid group of benthic foraminifers becomes dominant. Among the molluscs, a typical northern transitional zone mesohaline assemblage has been identified; it is composed predominantly of hydrobiids, Physa sp., Cyrenoida floridana and Polymesoda sp. Anomalocardia sp. and Bittium varium are relatively abundant molluscs in mesohaline-polyhaline salinities. A diverse group of molluscs including Modulus modulus, Pinctada radiata, Transennella spp., Laevicardium sp., Chione cancellata, Rissoina sp., and Acteocina canaliculata, represents polyhaline to euhaline salinities. The ostracode Malzella floridana and others are indicative of polyhaline to euhaline salinities.

Modern distributions of ephiphytes have provided the analogue data for interpreting the down-core changes in substrate. The molluscs Cerithium spp., Bittium varium, Modulus modulus, and Crepidula spp., and the ostracode Loxoconcha matagordensis, are found primarily on Thalassia at our monitoring sites. The mollusc Brachiodontes sp. and the ostracode Xestoleberis sp. are predominantly associated with macro-algal mats. The mollusc Pinctada radiata is found on both Thalassia and macro-algal mats.

The distribution patterns of epiphytal species in the cores indicate that fluctuations in seagrass occur regularly; however, all four cores show a general increase in the percent abundance of epiphytal species during this century. This implies that the seagrasses themselves have undergone an increase post-1900. The data also indicate an increase in the presence of macro-algal mats (Laurencia, Polysiphonia, Chondria, etc.) during this century. Natural causes appear to have profound affects on seagrass distribution at specific sites. Epiphytal species are absent for approximately 50 years at the Bob Allen site. Following periods of decreased abundance, the epiphytal species rebound rapidly, suggesting that the seagrasses themselves have the ability to recover rapidly.

We believe that analysis of historical trends of salinity and substrate change provides the data necessary to analyze the effects of human-induced changes on the south Florida ecosystem. By determining the natural range of variability within the system, we can examine the overprint of human-induced change. Knowledge of the natural patterns will allow land-managers to make sustainable, cost-effective decisions about restoration goals. Additional work this year will include completion of analysis of a core from Whipray Basin, analysis of a core from Park Key, and a more detailed examination of the trends in the upper portion of the cores examined to date. We will continue to monitor our 26 sites within Florida Bay to refine our analogue data, and to provide a database that can be used by restoration managers to determine the effects of changes upstream.

Smith, T.J., III, Hudson, J.H., Robblee, M.B., Powell, G.V.N., and Isdale, P.J., 1989, Freshwater flow from the Everglades to Florida Bay: A historical reconstruction based on fluorescent banding in the coral Solenastrea bournoni: Bulletin of Marine Science, v. 44, pp. 274-282.