Optical properties of dissolved organic matter in a monsoonal headwater stream, China: Insights for structure, source and riverine pCO2

Dissolved organic matter plays a fundamental geochemical role in aquatic systems. However, not much is known about interactions of dissolved organic matter characteristics, source, and partial pressure of carbon dioxide in the monsoonal streams of Asia. The ultraviolet visible and fluorescence spectroscopic indices were used as proxies for dissolved organic matter quantity and quality, and consequently associations and controls of dissolved organic matter quality, nutrients and partial pressure of carbon dioxide were examined in a monsoonal headwater River Jinshui. The source, composition, and molecular weight of dissolved organic matter showed temporal variations, mainly contributable to hydrological seasonality and potentially biological productivity. The allochthonous (land-derived) high-molecular weight components (aromatic, humification and lignin substances) were the dominant source of dissolved organic matter in the River with more terrestrial contributions in the wet season (much higher absorption coefficient at a wavelength of 350 nm and specific ultraviolet-visible absorption at 254 nm, and lower biological index and freshness index). The autochthonous low-molecular weight fractions contributed more in the dry season (significantly higher biological index and freshness index). Dissolved organic matter quality indices rather than dissolved organic carbon concentration showed significant relationships with partial pressure of carbon dioxide, this observation coupled with correlation seasonality could demonstrate the origins and controls of partial pressure of carbon dioxide. The new findings advanced the interactions of dissolved organic matter geochemistry, nutrients and partial pressure of carbon dioxide in rivers. This is critical for water environmental assessment and regional sustainable development. Further studies on discrimination and controls of dissolved organic matter quality, dissolved carbon dioxide and nutrients remain to be conducted. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Seasonality has a significant impact on the characteristics of dissolved organic matter, with different sources in different seasons. The quality indices of dissolved organic matter show significant relationships with the partial pressure of carbon dioxide, helping to understand the origins and control mechanisms of carbon dioxide.