Development of a two-dimensional model to assess carbon dynamics and anthropogenic effects on CO <subscript>2</subscript> emissions in the Tan river, southern China.

Tidal rivers are key biochemical reaction channels along the land-ocean aquatic continuum, receiving carbon from wastewater and agricultural drains, which can considerably affect CO 2 emissions. We developed a two-dimensional hydrodynamic and ecological model coupled with an inorganic carbon module along the Tan River in southern China. The simulations of and observations regarding discharge, temperature, total organic carbon (TOC), total inorganic carbon (TIC), and other common water quality variables were generally in good agreement. Based on the validated model, we employed statistical and scenario analyses to evaluate the carbon distribution, TOC and TIC budgets, and the imbalances induced by climatic and anthropogenic changes, providing insights into their potential greenhouse effect. The Tan River was consistently supersaturated with CO 2 with an annual mean air-water CO 2 emission flux (FCO 2 ) of 226.1 ± 84.9 mmol m -2 d -1 , and significant temporal and spatial variations of FCO 2 , TOC, and TIC were observed. Urban small streams tended to emit additional CO 2 during wet seasons, and rural tributaries usually had an increase in TOC concentrations during the dry season. FCO 2 was significantly positively correlated with air temperature and negatively correlated with total nitrogen, total phosphorus, and TOC. The annual riverine input of carbon to the urban river network was 17.37 Gg C yr -1 , with 59.82% of TOC, and carbon output was 15.31 Gg C yr -1 , with 66.87% of TOC. The retention rates for TOC and total carbon were 50.7% and 11.8% in the urban branch, respectively. Furthermore, warming and wastewater treatment could prevent urban river networks and downstream rivers from becoming carbon sources. Therefore, our findings suggest that riverine management strategies change the global CO 2 release from tidal rivers and estuarine systems under climate change.

Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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