Carbon and Nitrogen Stable Isotope Composition of OM From Florida Bay, the Initial Results of a Paleoenvironmental Seagrass Reconstruction

The shallow marine waters of Florida Bay provide an ideal environment for seagrasses, which are the most common benthic community in the region. However, these communities are susceptible to a variety of anthropogenic disturbances, particularly changes in water quality, and environmental conditions in Florida Bay have become a concern due to recent increases in salinity, the frequency of algal blooms, and seagrass die-off. These changes have been attributed to 20th century decreases in freshwater discharge from the Everglades to Florida Bay, deteriorated water quality, and changes in exchange between Florida Bay and the Atlantic Ocean. In order to better understand environmental change over long timescales, sediment cores were collected in the summer, 2002, from four locations in Florida Bay for multiple proxy analyses of seagrass abundance, which is an excellent indicator of water quality. Sediment depths ranged from 96 to 244 cm, potentially representing a 5000-year time series. Cores were sampled in 2-cm increments representing an average of 2-10 years for bulk isotopic analysis of sediment organic content. In 2 cores analyzed, δ¹⁵N values ranged between 3.2 and 7.6‰ , following an oscillating pattern over time. δ13C values ranged between -11.2 and -8.6‰ along a progressive enrichment trend that is inconsistent with the adjacent development of the metro Miami area and agricultural activities. These patterns show evidence of decoupling between carbon and nitrogen isotopic systems, although values throughout suggest that buried organic matter at these 2 sites is seagrass-derived. Further bulk isotopic analyses of remaining cores, together with organic biomarker analyses, diatom and foraminiferal community analyses, and development of an age model for the cores, will allow more definitive interpretation of the isotope patterns with implications to seagrass productivity levels, and thus, water quality, over time in Florida