Spatiotemporal characterization of dissolved carbon for inland waters in semi-humid/semi-arid region, China

Abstract. Spatiotemporal variations of dissolved organic carbon (DOC) and inorganic carbon (DIC) in 26 waters across the semi-humid/semi-arid Songnen Plain, China, were examined with data collected during 2008–2011. Fresh (n = 14) and brackish (n = 12) waters were grouped according to electrical conductivity (threshold = 1000 μS cm⁻¹) Significant differences in the average DOC and DIC concentrations were observed between the fresh (5.63 mg L⁻¹, 37.39 mg L⁻¹) and the brackish waters (15.33 mg L⁻¹, 142.93 mg L⁻¹). Colored dissolved organic matter (CDOM) and DOC concentrations were mainly controlled by climatic–hydrologic conditions. The investigation indicated that the outflow conditions in the semi-arid region had condensed effects on the dissolved carbon, resulting in close relationships between salinity vs. DOC (R² = 0.66), and salinity vs. DIC (R² = 0.94). An independent data set collected in May 2012 also confirmed this finding (DOC: R² = 0.79, DIC: R² = 0.91), highlighting the potential of quantifying DOC and DIC via salinity measurements for waters dispersed in the plain. Indices based on the CDOM absorption spectra (e.g., the DOC-specific CDOM absorption (SUVA₂₅₄), absorption ratio a₂₅₀ : a₃₆₅ (E₂₅₀ : E₃₆₅) and the spectral slope ratio (Sr, S_275−295/S_350−400) were applied to characterize CDOM composition and quality. Our results indicate that high molecular weight CDOM fractions are more abundant in the fresh waters than the brackish waters.