Abstract
The Southwest Florida Shelf (SWFS) is a shallow continental margin of great ecological and economic importance yet faces growing challenges from both natural and anthropogenic stressors, such as altered hydrology, eutrophication, urbanization, harmful algal blooms (HABs), and tropical storms. The Caloosahatchee River Estuary (CRE), the region’s primary freshwater source, delivers nutrients and dissolved organic matter (DOM)-rich discharge to the shelf, although, its offshore impact remains unclear. This study investigated spatial and temporal variability in physicochemical parameters (salinity, temperature, dissolved oxygen (DO), pH, turbidity, chlorophyll, fluorescent dissolved organic matter (fDOM)) and nutrients (SiO2, TN, TP, NH3, NOx, and PO43-) along two nearshore-to-offshore transects from August 2022 to August 2023. Stable (δ13C, δ15N) and radioactive (223Ra, 224Ra) isotopes were used to trace nutrient and freshwater sources. Opportunistically, this study also evaluated DOM composition and transformation on the SWFS following Hurricane Ian (September 2022). Radium isotopes indicated submarine groundwater discharge (SGD) as an additional source of nutrients and DOM in nearshore locations. Molecular characterization of DOM revealed a clear compositional shift moving further offshore—from highly aromatic, terrestrially derived material nearshore to more aliphatic, marine-origin DOM offshore. Biogeochemical responses to Hurricane Ian included altered salinity, increased turbidity, nutrient pulses and an initial alleviation of offshore DO with subsequent stratification nearshore. The findings from this study highlight the interactions among riverine input, seasonal dynamics, and tropical disturbances in shaping water quality and carbon cycling on the SWFS. This is especially important as nearshore environments are particularly vulnerable to eutrophication and shifts in microbial and benthic community structure, driven by the combined influence of anthropogenic activity and natural variability.
