High productivity on the Florida Platform during the Pliocene has been ascribed to upwelling and to freshwater input of nutrients. To test these hypotheses, high-resolution stable isotopic and Sr/Ca analyses have been performed on 14 Conus and Turritella gastropod shells collected from the middle Pliocene Pinecrest Beds (Units 7 and 4) and the Plio-Pleistocene Caloosahatchee Formation. Assuming a published Pliocene seawater δ18O of 1.02‰ derived from a coupled ocean-atmosphere general circulation model (OAGCM), reconstructed paleotemperatures of Units 7 and 4, and the Caloosahatchee are respectively 25.1 ± 1.4 °C, 16.1 ± 0.6 °C, and 22.4 ± 0.5 °C. Unit 7 paleotemperatures are similar to, and Caloosahatchee paleotemperatures slightly lower than, modern sea surface temperatures (SSTs) in the Sarasota Bay (24.5 ± 0.4 °C). In contrast, Unit 4 paleotemperatures are unrealistically low. Sr/Ca ratios, however, suggest no significant paleotemperature difference between Pinecrest Units 7 and 4 and the Caloosahatchee Formation, indicating seawater δ18O variations, rather than temperature differences, are responsible for δ18O differences. High δ18O and low δ13C values of these samples likely reflect highly evaporated freshwater input combined with oxidation of terrestrial debris, as a brackish environment is indicated by marine and freshwater fauna in Unit 4. This isotopic pattern is similar to that for modern Florida Bay mollusks, which are influenced by discharge of Everglades waters. Furthermore, episodic enrichments in Fe, U, and P in some shells suggest nutrient input from submarine groundwater discharge. The data, therefore, support the contention that the dominant cause of high productivity in this region was enhanced nutrient input from freshwater influx.