Short-Lived Radium Isotopes as Tracers of Estuary-Shelf Transport of Dissolved Inorganic Carbon and Nutrients

Dissolved Inorganic Carbon (DIC) is an important part of marine carbon cycle with implications of climate change. The four major species of DIC in the marine carbonate system (CO2, H2CO3, HCO3- and CO32-) act as major buffers in the ocean, making them integral to the study of ocean acidification. The short-lived naturally occurring radium isotopes,223Ra (t1/2 = 11.4 d) and 224Ra (t1/2 = 3.6 d) can be used to estimate the residence times, mixing rates, and transport fluxes of water and associated DIC as well as nutrients in the coastal marine environments. In this study, we used radium isotopes to track the transport of DIC and nutrients (nitrate, nitrite, ammonia, and orthophosphate) from the Caloosahatchee River to the Gulf of Mexico (GOM). Due to the paucity of regional data in Southwest Florida, it is important to identify whether the influx of nutrients and DIC from the river plume affects the Gulf. In both wet and dry seasons, we collected water samples for DIC, alkalinity, and nutrients from 20 sites along two transects throughout the Caloosahatchee and the GOM. For radium isotope analysis at each site, 40 liters of water samples were collected and then passed through manganese coated fibers. 223Ra and 224Ra isotopes on the Mn fibers were measured using Radium Delayed Coincidence Counter (RaDeCC). The spatial and temporal variability in preliminary DIC data shows that this region is a dynamic system. The activities of 223Ra varied from 3.2 to 40.7 dpm/100L and 2.1 to 29.3 dpm/100L in the wet and dry season respectively. Similarly, the activities of 224Ra varied from 8.4 to 126.3 dpm/100L and 5.9 to 180.7 dpm/100L in wet and dry season respectively. The concentrations of total nitrogen (TN), orthoPhosphate (SRP) and DIC varied from 1.6 to 0.25 µg/L, 63.0 to 0.378 µg/L, and 23.3 to 26.7 mg/L. Similarly, TN, SRP and DIC in the wet season from 160.2 to 2000 µg/L, 0.47 to 1.2µg/L, and 23.4 to 50.0 mg/L. The radioisotopes, 223Ra and 224Ra, showed mixed results by location, indicating that both groundwater influx and river discharge play an important role in DIC and nutrient loading in this region. The results from this study will help determine the importance of freshwater plume discharge in transporting DIC.