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

Abstract

• Partial pressure and fluxes of CO 2 and CH 4 in a subtropical cascaded damming river were measured. • Cascade damming decreased riverine CO 2 emissions, while promoted CH 4 fluxes. • Cascade damming altered the spatial patterns of riverine p CO 2 , p CH 4 and “ p CO 2 / p CH 4 ”. • Released waters from reservoirs were C gas emission hotspots in the cascade damming river. • p CO 2 and p CH 4 and fluxes were affected by autotrophic production and carbon trapping. The damming of rivers can significantly intercept carbon transportation and regulate dynamic biogeochemistry, which then greatly influence carbon gases (CO 2 and CH 4 ) 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 CO 2 and CH 4 (i.e. p CO 2 and p CH 4 ) dynamics along a subtropical cascaded dammed river (Wubu River) located in Southwest China. The p CO 2 and p CH 4 in the Wubu River basin showed significantly spatial–temporal and were generally supersaturated with gas fluxes of 89.5 ± 89.7 mmol CO 2 m −2 d -1 and 1.19 ± 1.18 mmol CH 4 m −2 d -1 . The p CO 2 and p CH 4 were all increasing from upstream to downstream. And, most of river sections had significantly higher p CO 2 and CO 2 than their downstream reservoir sections, while the p CH 4 and CH 4 flux were the opposite. Meanwhile, released waters had invariably high p CO 2 and p CH 4 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 CO 2 emissions but promote CH 4 production. Thus, the spatial patterns of riverine p CO 2 , p CH 4 and the “ p CO 2 / p CH 4 ” in cascade hydroelectric river were importantly altered. It was evaluated that cascade damming would reduce about 17.9 % of the total CO 2 evasion amount but increase approximately 16.3 % of CH 4 . The global warming potential of rivers would be enhanced when organic carbon was captured and mineralized to CH 4 rather than to CO 2 in cascade damming river system. pH, dissolved oxygen and chlorophyll a ( chl-a ) were indicated as natural controls of p CO 2 in such a cascade river-reservoir system, while dissolved organic carbon, dissolved organic carbon and chl-a were significantly correlated with p CH 4 and acted as good predictors of CH 4 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 CO 2 emissions and created CH 4 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.