STABLE ISOTOPES OF MODERN PLANTS AND ALGAE AS PROXIES FOR PAST ENVIRONMENTAL CHANGE IN FLORIDA BAY, EVERGLADES NATIONAL PARK

Stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) and their elemental ratios (C/N) can be used to assess the origins of organic matter in sediments of coastal environments, such as tracking long-term sea-level changes, due to the influx of estuarine-derived sediment and vegetation into a terrestrial environment. To evaluate their utility as a proxy for past storm events, we measure δ¹³C, δ¹⁵N, and C/N from a representative assemblage of modern plants and algae from in and around Florida Bay, Everglades National Park. We divided samples into five functional groups according to their physiology and representative environment: estuarine plant (e.g. Thalassia), estuarine algae (e.g. Halimeda), terrestrial C₃-photosynthetic plant (e.g. Avicennia, Cladium, Batis), terrestrial C₄-photosynthetic plant (e.g. Sporobolus), and terrestrial mat algae. These groups can be distinguished based on their isotopic and elemental composition and ultimately comprise the organic component of sediments, thereby providing information on the origin of those sediments (i.e., estuary, island). Following the passage of Hurricane Irma in September 2017 (Category 4 storm), we sampled overwash sediments and collected shallow (50 cm) cores in January 2018 from four carbonate mud islands in Florida Bay to allow for characterization of storm deposits and to assess whether our reference stable isotopes and C/N data can be used to identify storm signatures in Holocene sediments. Preliminary results suggest that terrestrial C₃-photosynthetic, terrestrial C₄-photosynthetic, and estuarine plant groups have unique δ¹³C values and C/N ratios. Estuarine and terrestrial algae overlap in their δ¹³C, δ¹⁵N, C/N values. Hurricane Irma and pre-hurricane overwash sediments are distinct (δ¹³C, C/N) from the three plant functional groups, but indistinct (δ¹³C, δ¹⁵N, C/N) from the estuarine and terrestrial algae groups and from one another. Finally, we will use stable isotopes and C/N ratios of late Holocene (up to ca. 750 years before present) sediments from Florida Bay islands to identify potential signals of past storms. Paired with data on the effects of storms, past storm history provides land management agencies with key information to develop plans for future climate scenarios.