Greenhouse gases concentrations and emissions from a small subtropical cascaded river-reservoir system

The damming of rivers can significantly intercept carbon transportation and regulate dynamic biogeochemistry, which then greatly influence carbon gases (CO2 and CH4) metabolism and emissions. However, little is known about the spatial–temporal pattern and controls of carbon emissions from a cascaded river-reservoir system. In this study, we investigated the spatial and seasonal partial pressure of CO2 and CH4 (i.e. pCO2 and pCH4) dynamics along a subtropical cascaded dammed river (Wubu River) located in Southwest China. The pCO2 and pCH4 in the Wubu River basin showed significantly spatial–temporal and were generally supersaturated with gas fluxes of 89.5 ± 89.7 mmol CO2 m−2 d-1 and 1.19 ± 1.18 mmol CH4 m−2 d-1. The pCO2 and pCH4 were all increasing from upstream to downstream. And, most of river sections had significantly higher pCO2 and CO2 than their downstream reservoir sections, while the pCH4 and CH4 flux were the opposite. Meanwhile, released waters had invariably high pCO2 and pCH4 and were carbon gas emission hotspots in the river-reservoir system. The intercepts of carbon and nutrients by cascade damming, created a series of eutrophic hotspots and enhanced primary productivity, could inhibit riverine CO2 emissions but promote CH4 production. Thus, the spatial patterns of riverine pCO2, pCH4 and the “pCO2/pCH4” in cascade hydroelectric river were importantly altered. It was evaluated that cascade damming would reduce about 17.9 % of the total CO2 evasion amount but increase approximately 16.3 % of CH4. The global warming potential of rivers would be enhanced when organic carbon was captured and mineralized to CH4 rather than to CO2 in cascade damming river system. pH, dissolved oxygen and chlorophyll a (chl-a) were indicated as natural controls of pCO2 in such a cascade river-reservoir system, while dissolved organic carbon, dissolved organic carbon and chl-a were significantly correlated with pCH4 and acted as good predictors of CH4 emissions. The temperature and autotrophic production were responsible for the different seasonal patterns of carbon gas concentrations observed between river sections and reservoir sections. Our study highlighted that cascaded damming decreased CO2 emissions and created CH4 hotspots in river systems and may have significantly increased the complexity of the spatial–temporal pattern and construction of riverine carbon gases emissions. Given the increasing damming of rivers worldwide for energy demand, researches are needed to quantify the role of cascaded damming in the riverine C budget.