Variations in the Oxygen and Carbon Isotope Ratios of Florida Bay Mollusks and Their Environmental Significance

Abstract

The factors of climate and geography which produce salinity gradients and affect the molluscan fauna distribution in Florida Bay also cause variations in the $$\delta O^{18}$$ composition of Florida Bay water. The isotopic composition of bay water varies in response to (1) mixing of Atlantic and Gulf of Mexico waters having a relatively constant isotopic composition with bay water along the western and southeastern margins; (2) evaporation of Florida Bay water which causes isotopie enrichment over the entire bay; (3) the introduction of isotopically enriched fresh water along the northern margin of the bay. The net effect of these processes is to produce a persistent gradient of increasing $$\delta O^{18}$$ from south to north in the bay water while water outside the bay is maintained at a lower and more constant isotopic composition. Isotopie ratios from eight species of Florida Bay mollusk shells reflect clearly the sharp contrast between bay and open water $$\delta O^{18}$$ compositions and, to varying degrees, the gradient of increasing $$\delta O^{18}$$ in the bay itself. Temperature along could not cause the variations found. Measurement of $$\delta C^{13}$$ in Florida Bay mollusk shells reveals a gradient of decreasing $$\delta C^{13}$$ going from open-shelf water into the bay. The gradient is attributed to the equilibration of $$CO_{2}$$ derived locally from the oxidation of organic debris in the sediment with carbonate ions in the water.