Carbon dioxide and methane dynamics in a human-dominated lowland coastal river network (Shanghai, China)

Evasion of carbon dioxide (CO₂) and methane (CH₄) in streams and rivers play a critical role in global carbon (C) cycle, offsetting the C uptake by terrestrial ecosystems. However, little is known about CO₂ and CH₄ dynamics in lowland coastal rivers profoundly modified by anthropogenic perturbations. Here we report results from a long-term, large-scale study of CO₂ and CH₄ partial pressures (pCO₂ and pCH₄) and evasion rates in the Shanghai river network. The spatiotemporal variabilities of pCO₂ and pCH₄ were examined along a land use gradient, and the annual CO₂ and CH₄ evasion were estimated to assess its role in regional C budget. During the study period (August 2009 to October 2011), the overall mean pCO₂ and median pCH₄ from 87 surveyed rivers were 5846 ± 2773 μatm and 241 μatm, respectively. Internal metabolic CO₂ production and dissolved inorganic carbon input via upstream runoff were the major sources sustaining the widespread CO₂ supersaturation, coupling pCO₂ to biogeochemical and hydrological controls, respectively. While CH₄ was oversaturated throughout the river network, CH₄ hot spots were concentrated in the small urban rivers and highly discharge-dependent. The Shanghai river network played a disproportionately important role in regional C budget, offsetting up to 40% of the regional terrestrial net ecosystem production and 10% of net C uptake in the river-dominated East China Sea fueled by anthropogenic nutrient input. Given the rapid urbanization in global coastal areas, more research is needed to quantify the role of lowland coastal rivers as a major landscape C source in global C budget.