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

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 mu S 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 (R-2 = 0.66), and salinity vs. DIC (R-2 = 0.94). An independent data set collected in May 2012 also confirmed this finding (DOC: R-2 = 0.79, DIC: R-2 = 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 (SUVA(254)), absorption ratio a(250) : a(365)(E-250 : E-365) and the spectral slope ratio (Sr, S275-295/S350-400)) 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.