Temporal-spatial variation of DOC concentration, UV absorbance and the flux estimation in the Lower Dagu River, China

Abstract

Dissolved organic carbon (DOC) is an important component for both carbon cycle and energy balance. The concentration, UV absorbance, and export flux of DOC in the natural environment dominate many important transport processes. To better understand the temporal and spatial variation of DOC, 7 sites along the Lower Dagu River were chosen to conduct a comprehensive measurement from March 2013 to February 2014. Specifically, water samples were collected from the Lower Dagu River between the 26th and 29th of every month during the experimental period. The DOC concentration (C_DOC) and UV absorbance were analyzed using a total organic carbon analyzer and the ultraviolet-visible absorption spectrum, and the DOC export flux was estimated with a simple empirical model. The results showed that the C_DOC of the Lower Dagu River varied from 1.32 to 12.56 mg/L, consistent with global rivers. The C_DOC and UV absorbance showed significant spatial variation in the Dagu River during the experiential period because of the upstream natural processes and human activities in the watershed. The spatial variation is mainly due to dam or reservoir constructions, riverside ecological environment changes, and non-point source or wastewater discharge. The seasonal variation of C_DOC was mainly related to the source of water DOC, river runoff, and temperature, and the UV absorbance and humification degree of DOC had no obvious differences among months (P<0.05). UV absorbance was applied to test the C_DOC in Lower Dagu River using wave lengths of 254 and 280 nm. The results revealed that the annual DOC export flux varied from 1.6 to 3.76×10⁵ g C/km²/yr in a complete hydrological year, significantly lower than the global average. It is worth mentioning that the DOC export flux was mainly concentrated in summer (~90% of all-year flux in July and August), since the runoff in the Dagu River took place frequently in summer. These observations implied environment change could bring the temporal-spatial variation of DOC and the exports, which would further affect the land-ocean interactions in the Lower Dagu River and the global carbon cycle.

Keywords

DOC / temporal-spatial variation / UV absorbance / export flux / Dagu River

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Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 41101080) and Shandong Natural Science Foundation (No. ZR2014DQ028 and No. ZR2015DM004). We thank Prof. D. Yang and Dr. S. Sun for advice on the language and conclusions.

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