Sensitivity of coastal settings to natural and human influences generates interesting problems for scientific study and attracts much public attention. Despite this interest, study of coastal habitats provides many challenges given the dynamic nature of the physical environment and ecosystem. In particular, continental runoff and precipitation create an environment with fluctuating salinity overprinted on seasonal temperature variation. The present study sought to deconvolute some of these complexities and reconstruct temperature, salinity, and landscape evolution in the changing environments of estuaries in southwest Florida.

Faka-Union Canal and its watershed is targeted for restoration. To plan and monitor restoration efforts, pre-canal environmental conditions are required. The lack of historical information recording natural, seasonal conditions necessitates alternative means of acquiring predisturbance environmental information. Environmental archives contained in oyster (Crassostrea virginica) shells potentially contain useful information to reconstruct environmental conditions before channelization of the watershed.

Analysis of oxygen and carbon isotope compositions (delta-18O and delta-13C, respectively) of shell carbonate demonstrate that oysters record modern estuarine conditions, though delta-13C is 1-2‰ more positive than predicted delta-13C of dissolved inorganic carbon of estuarine water during winter months reflecting a vital effect. A radiocarbon-calibrated, amino acid geochronology was established to date pre-canal shells near Naples, Florida. The local reservoir effect (130±30 years) for the Gulf of Mexico near Naples, Florida was determined to correct radiocarbon (AMS) ages of marine samples. Pre-canal shells from Blackwater River are older (ranging 780-1810 years before 1998) than those from Faka-Union Bay (ranging 140-730 years before 1998). Subfossil shells from Blackwater River record winter temperatures that are ~1-5ºC colder than shells recording temperatures in Winter 1999. They also record comparable salinities to modern conditions. However, subfossil shells from Faka-Union Bay suggest a complicated hydrology that cannot be easily deconvoluted based on isotopic compositions alone. Therefore, a Mg/Ca thermometer that is less sensitive to salinity variation was developed to estimate temperature (±2.5ºC). Carbon isotope compositions of subfossil shells suggest change in vegetation from a freshwater prairie-like ecosystem to the saltwater-tolerant mangrove forests of today, possibly due to local influence of relative sea level fluctuation.

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