Dissolved Organic Carbon Dynamics in a Subtropical Estuarine System: A Study of Ogun Estuary

Dissolved organic carbon plays a central role in estuarine biogeochemistry as a substrate for microbial production, a vector for pollutant transport, and a significant contributor to coastal ocean carbon budgets. This study investigates the sources, composition, and transformation dynamics of dissolved organic carbon in the Ogun River Estuary, Ogun State, South West Nigeria, a subtropical estuary receiving substantial anthropogenic dissolved organic carbon inputs from the Ogun Dam reservoir release and from urban wastewater in the Abeokuta-Lagos metropolitan corridor. Bi-monthly sampling at 14 stations spanning a salinity gradient from zero to 35 practical salinity units is conducted during 2023 to 2024. Dissolved organic carbon concentrations are measured by high-temperature catalytic oxidation, and optical characterisation is achieved using ultraviolet-visible spectrophotometry and excitation-emission matrix fluorescence coupled with parallel factor analysis. Fluorescence indices and specific ultraviolet absorbance values distinguish between terrestrially derived humic-like dissolved organic carbon and autochthonous protein-like and microbial-derived components. Isotopic signatures (stable carbon-13 and radiocarbon) of bulk dissolved organic carbon are used to constrain source apportionment. Mixing diagrams at each sampling period identify conservative versus non-conservative dissolved organic carbon behaviour across the salinity gradient. Bacterial production rates determined from leucine incorporation are related to dissolved organic carbon lability indices. Results are expected to reveal non-conservative dissolved organic carbon removal at salinities below five, attributable to flocculation and photodegradation of terrestrially derived humic material. Keywords: dissolved organic carbon, estuarine biogeochemistry, Ogun Estuary, fluorescence spectroscopy, carbon cycling