Temporal–Spatial Variability of Dissolved Carbon in the Tributary Streams of the Lower Yangtze River Basin

Dissolved carbon has been widely investigated in natural rivers worldwide. However, it has been rarely studied in riverine system of farming regions, where small streams have been usually modified by a water gate and flood levee. This study was conducted to investigate dissolved organic and inorganic carbon (DOC, DIC) in artificially modified tributary streams, namely the Desheng and Shuangqiao-Shiba streams, in a farming region of the lower Yangtze River basin. The results showed that the DOC and DIC concentrations had remarkable temporal–spatial variability in the Desheng and Shuangqiao-Shiba streams. The mean DOC concentrations were 5.4 and 6.7 mg L−1 in the Desheng and Shuangqiao-Shiba streams during the period of about 1 year, while the DIC concentrations were 14.0 and 9.6 mg L−1 in both streams, respectively. The DOC and DIC concentrations mainly showed the linear decreasing trends from source through to export in the spring, summer, autumn, and winter. The DOC concentration was almost always lower than the DIC concentration. The DOC concentration was positively correlated with the DIC concentration in the Desheng stream over the four seasons, indicating that the DOC and DIC potentially had similar sources. The DOC and DIC concentrations increased with sewage discharge, which introduced a potential hazard to human health. The water gate and flood levee in the Desheng and Shuangqiao-Shiba streams were constructed for discharging floods and recharging irrigation in the farming region, and their effects on dissolved carbon require further research.