Melissa W. Southwell

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.

Oysters provide a suite of important ecosystem services, and recent research shows that oyster restoration rapidly enhances sedimentary organic carbon deposition. In 2012, an oyster reef enhancement project began in the GTM National Estuarine Research Reserve in Northeast FL, USA. We analyzed the spatial and downcore variability in sedimentary organic matter (OM) and particle sizes in the intertidal zone between the reefs and the marsh, along with oyster reef characteristics, to better understand physical and/or biological influences on sediment. Our data indicate that OM in the top 20 cm of sediment cores was negatively correlated with reef age. Similar decreases in particles <63 μm suggest remobilization of sediment, likely driven by the degradation of the reef structure over its approximately 9-year lifetime. Likewise, a survey of surface sediments showed that adjacent reef structural metrics were the best predictor of sediment OM and particle size. These results highlight the importance of reef structure as a control on sedimentary organic carbon deposition and stability in areas where physical energy is relatively high. This result is discussed in the context of implications for carbon budgets and biogeochemical ecosystem services of oysters as a part of living shorelines.