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Recent studies suggest that hypolimnetic respiration may be responsible for greenhouse gas (GHG) emissions from deep reservoirs. Currently, quantitative evaluation of aerobic vs. anaerobic processes and priming (enhanced processing of organic matter due to the addition of labile carbon) in regulating GHG production and emissions across the reservoir-downstream continuum remains largely unknown. High-resolution, annual time-series observations in a large, subtropical reservoir (Shuikou) experiencing seasonal hypoxia in southeast China indicate that aerobic hypolimnetic CO2 production dominated in most periods of the stratified spring/summer with higher rates at higher temperatures. In addition, anaerobic production of hypolimnetic CO2 occurred in the late stratified spring/summer period, which stimulated hypolimnetic production of CH4 and N2O. Incubation experiments showed that priming in spring enhanced both aerobic and anaerobic production of excess GHGs. A late spring flood event generated the highest daily efflux of CO2 through the flushing of GHG-enriched hypolimnion waters. Turbine degassing contributed 59%, 93%, and 63% of annual CO2, CH4, and N2O effluxes, respectively. Moreover, annual downstream GHG emissions were similar to those in the transition/lacustrine zone of the Shuikou reservoir. Diurnal variation observations revealed net CO2 emissions even during algal bloom seasons. The reservoir-downstream river continuum was a year-round source of GHGs (218.5 ± 18.9 Gg CO2-equivalent yr−1; CO2 contributed 91%). However, the loss of oxygen also leads to increased production and storage of recalcitrant dissolved organic carbon (RDOC). Thus, identifying mechanisms controlling both GHG emissions and RDOC production is crucial to constrain the carbon neutrality issue of hydroelectric reservoirs in the context of climate change mitigation strategies.

Recent studies suggest that hypolimnetic respiration may be responsible for greenhouse gas (GHG) emissions from deep reservoirs. Currently, quantitative evaluation of aerobic vs. anaerobic processes and priming (enhanced processing of organic matter due to the addition of labile carbon) in regulating GHG production and emissions across the reservoir‐downstream continuum remains largely unknown. High‐resolution, annual time‐series observations in a large, subtropical reservoir (Shuikou) experiencing seasonal hypoxia in southeast China indicate that aerobic hypolimnetic CO2 production dominated in most periods of the stratified spring/summer with higher rates at higher temperatures. In addition, anaerobic production of hypolimnetic CO2 occurred in the late stratified spring/summer period, which stimulated hypolimnetic production of CH4 and N2O. Incubation experiments showed that priming in spring enhanced both aerobic and anaerobic production of excess GHGs. A late spring flood event generated the highest daily efflux of CO2 through the flushing of GHG‐enriched hypolimnion waters. Turbine degassing contributed 59%, 93%, and 63% of annual CO2, CH4, and N2O effluxes, respectively. Moreover, annual downstream GHG emissions were similar to those in the transition/lacustrine zone of the Shuikou reservoir. Diurnal variation observations revealed net CO2 emissions even during algal bloom seasons. The reservoir‐downstream river continuum was a year‐round source of GHGs (218.5 ± 18.9 Gg CO2‐equivalent yr−1; CO2 contributed 91%). However, the loss of oxygen also leads to increased production and storage of recalcitrant dissolved organic carbon (RDOC). Thus, identifying mechanisms controlling both GHG emissions and RDOC production is crucial to constrain the carbon neutrality issue of hydroelectric reservoirs in the context of climate change mitigation strategies. [ABSTRACT FROM AUTHOR]

• The N 2 emission rate was 1.23 ± 0.83 mg N m−2 h−1 in Chaohu Lake. • The upper and lower limits of uptake velocity were 3.33 m yr−1 and 2.67 m yr−1. • The N removal efficiency via denitrogen was around 26.14% in Chaohu Lake. • The total N removal efficiency of the Chaohu lake was 31.45%. • N removal efficiency of reservoirs has a power relationship with water discharge. Lakes and reservoirs are important inland waterscapes in linking nitrogen (N) transport and removal from the terrestrial to marine ecosystems. N removal processes eliminate N permanently from lakes and reservoirs, including denitogen, permanent burial and biomass harvest. The N removal efficiency (R _removal , i.e. the fraction of N removal amount against total N loads to the lake) is an important parameter for the quantification of N removal from lakes and reservoirs. Intrinsically, R _removal depends on hydrologic load and total N uptake velocity (V f_removal). However, it is a challenge in determining V f_removal which can be used to model R _removal of lakes and reservoirs. Furthermore, there is less studies in distinguishing the N uptake velocity of individual processes from each other. Here, we estimated the magnitude of each uptake velocity of N removal processes with field observed data, and further presented an integrated V f_removal to estimate R _removal of a typical eutrophic lake (the Chaohu Lake) in Eastern China. The mean dinitrogen emission rate was 1.23 ± 0.83 mg N m−2 h−1 in Chaohu Lake in 2016. The uptake velocity of denitrogen, permanent N burial and biomass harvest was 2.67 m yr−1, 0.25 m yr−1 and 0.23 m yr−1, respectively. The removal efficiency of denitrogen, permanent N burial and biomass harvest was 26.14%, 2.76% and 2.56%, respectively. The integrated value of V f_removal was 3.33 m yr−1. We finally compiled hydrological data of 566 large reservoirs in Changjiang River network, and modelled the N removal by combining our estimated value of V f_removal with hydrological data. We found a significant power relationship between N removal and the reservoir discharge, which can predict the variation of N removal of each reservoir in Changjiang River network. Our research helps to understand N removal in lakes and reservoirs at an entire river network scale. [ABSTRACT FROM AUTHOR]