Utilization of Stable Isotopes for tracing sources of nutrients in the Caloosahatchee River and Estuary, Florida Samantha Schwippert, James Javaruski, Puspa L. Adhikari The Water School, Department of Marine and Earth Sciences Florida Gulf Coast University, Fort Myers, Florida

The Caloosahatchee watershed is a large area that encompasses the Caloosahatchee River, connecting Lake Okeechobee to the Gulf of Mexico. Much of the Caloosahatchee watershed is occupied by agriculture and rangelands. With precipitation, runoff of anthropogenic nutrients (such as fertilizers and pesticides) can be carried from the surrounding land into the Caloosahatchee River and deposited into the Gulf of Mexico. This runoff contributes to the eutrophication of the watershed, providing an excess of the nutrients necessary for the algae to proliferate. As the amount of precipitation changes seasonally, the anthropogenic inputs of nitrogen, phosphorous and carbon vary. With an aim to determine the extent to which each source (Caloosahatchee River, Gulf of Mexico, Imperial River and groundwater wells) contributes to the nutrient profiles in the Caloosahatchee Estuarine system, water samples were collected from 44 sampling locations in June 2020 (wet season) and February 2021 (dry season). Samples were analyzed for elemental composition (C: N:S), nutrients (dissolved and total N and P) and stable isotope concentrations (13C & 15N). Dry season total nitrogen varied from 0.46-2.85mg/L, with wet season varying from 0.18-2.04mg/L. Dry season total phosphorous varied from 0.02-.688mg/L, with wet season varying from 0.008-0.188mg/L. This is work in progress and the preliminary results show that this region of the Southwest Florida is a very spatially and temporally dynamic system. The isotope tracers showed mixed results by location, indicating that both the groundwater influx and river discharge play important roles in nutrient loading in this region. The results upon completion will provide better understanding of the rates of nutrient enrichment into the watershed, which will lead to improved water quality management and better practices within agriculture and urban land management in Southwest Florida.