Modeling Dissolved Organic Carbon in an Estuary Using Optical Properties and Salinity

UV-Visible spectroscopy provides qualitative and quantitative information on colored dissolved organic matter (CDOM) that can be used as a proxy for dissolved organic carbon (DOC). We developed an absorbance-based linear model of DOC for the San Sebastian River estuary in NE Florida. We compared linear and mixed models, with and without salinity as an additional fixed effect. All models exhibited strong correlations (R2 = 0.88–0.97) with measured DOC values for the training dataset. The model with the strongest performance on the testing dataset was a linear model containing the absorption coefficient at 254 nm, the spectral slope at 275–295 nm, and salinity. The range of measured DOC was 0.5 to 52.3 mg/L, and the model was able to predict DOC concentrations for an independent testing dataset with a relative mean absolute error of 17%. Incubation experiments indicated that aging and photolysis altered absorption coefficients and spectral slopes, which negatively affected the model performance, particularly for photolysis. However, predicted DOC was still well-correlated (R2 > 0.9) with measured DOC, even for photolyzed samples. Spectral slope ratios indicate that DOM in the San Sebastian River is mainly terrigenous, and that hydrologic variability, possibly associated with freshwater inflow from rainfall, influences DOC/CDOM concentration and composition.