Biotic and chemical proxies for salinity in Florida Bay provide a wealth of information for understanding the historic interplay of rainfall and runoff in influencing the salinity of the Bay.

Biotic proxies, the distribution and abundance of ostracodes, mollusks, foraminifers, and diatoms, provide reasonable trends in salinity variation on decadal and slightly finer scales. The trends show agreement with a strong negative correlation with rainfall amount and strong positive correlation with frequency. An overall trend for a slight increase in the salinity of the Bay is indicated as well as a subtle change in the response to rainfall frequency patterns over the last 30 years.

Chemical proxies derived from the analysis of the tests of the biotic proxies appear very promising in providing finer and finer resolution of salinity variability in the Bay. An understanding of the life cycle, specifically, the growth history of some biotic proxies allows for very accurate salinity determinations.

The ostracode Loxoconcha matagordensis grows its adult shell in the late Spring-early Summer (May through July). The Ca/Mg ratio of the test reflects the salinity and temperature of the day that test was secreted during this time interval. Analysis of many tests from a single horizon, then, gives an insight into the salinity and temperature through the growth season. Detailed analysis of the Loxoconcha matagordensis from two cores compared to the average January through May rainfall from NOAA data compiled over the last 100 years yields predictive, site specific, linear equations for salinity values.

The clam, Chione cancellata, has a relatively robust shell in comparison to other Florida Bay inhabitants. Initial growth studies indicate that this species lives for about 1.5 years and secretes calcite to its prismatic shell layer on a daily basis. We are actively conducting growth experiments to quantify the growth rate of this species and to use as standards for chemical analysis including elemental mapping and carbon and oxygen isotope analysis on a SHRIMP mass spectrometer, where resolution should be down to 2-3 prisms or 2-3 days. Specimens of Chione cancellata have been collected live from or near monitoring stations from the Bay, so that we can calibrate the carbon and oxygen isotopes from those shells with the known salinity and temperature record. Oxygen isotopic ratios are sensitive to the salinity and temperature of the water and carbon isotopic ratios are sensitive the to amount of rainfall and runoff that contributed to the particular water mass at time of secretion. We plan to due our first analyses in December and January and hope to present the initial results at the conference.

(This abstract was taken from "Programs and Abstracts - 2001 Florida Bay Science Conference". (PDF, 6.8 MB))

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