Methane and nitrous oxide annual emissions from an old eutrophic temperate reservoir

Varying water column stability controls the denitrification process in a subtropical reservoir, Southwest China

Reservoirs are regarded as hotspots of nitrogen transformation and potential sources of nitrous oxide (N₂O). However, it remains unclear how the hydrological conditions due to dam construction control the processes of nitrogen transformation in reservoir waters. To address this issue, we examined the spatial-temporal characteristics of nitrate concentrations, δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ¹⁸O-H₂O, relative water column stability (RWCS), and related environmental factors in a subtropical eutrophic reservoir (Hongfeng Reservoir, HFR), Southwest China. We found that denitrification was the most important nitrogen transformation process in the HFR and that higher denitrification intensity was associated with increased RWCS in summer, which suggested hydrological control of the denitrification process. In contrast, low RWCS conditions favored the nitrification process in the HFR in winter. Additionally, dissolved oxygen (DO; p < 0.05) and nitrate concentrations (p < 0.01) had significant impacts on the denitrification rate. We also found that the spatiotemporal RWCS variations were a prerequisite for regulating DO/nitrate stratification and the coupling/decoupling of nitrification-denitrification at the local and global scales. This study would advances our knowledge of the impacts of RWCS and thermal stratification on nitrogen transformation processes in reservoirs.

Coastal reservoirs as a source of nitrous oxide: Spatio-temporal patterns and assessment strategy

Coastal reservoirs are widely regarded as a viable solution to the water scarcity problem faced by coastal cities with growing populations. As a result of the accumulation of anthropogenic wastes and the alteration of hydroecological processes, these reservoirs may also become the emission hotspots of nitrous oxide (N₂O). Hitherto, accurate global assessment of N₂O emission suffers from the scarcity and low spatio-temporal resolution of field data, especially from small coastal reservoirs with high spatial heterogeneity and multiple water sources. In this study, we measured the surface water N₂O concentrations and emissions at a high spatial resolution across three seasons in a subtropical coastal reservoir in southeastern China, which was hydrochemically highly heterogeneous because of the combined influence of river runoff, aquacultural discharge, industrial discharge and municipal sewage. Both N₂O concentration and emission exhibited strong spatio-temporal variations, which were correlated with nitrogen loading from the river and wastewater discharge. The mean N₂O concentration and emission were found to be significantly higher in the summer than in spring and autumn. The results of redundancy analysis showed that NH₄⁺-N explained the greatest variance in N₂O emission, which implied that nitrification was the main microbial pathway for N₂O production in spite of the potentially increasing importance of denitrification of NO₃^--N in the summer. The mean N₂O emission across the whole reservoir was 107 μg m^-2 h^-1, which was more than an order of magnitude higher than that from global lakes and reservoirs. Based on our results of Monte Carlo simulations, a minimum of 15 sampling points per km² would be needed to produce representative and reliable N₂O estimates in such a spatially heterogeneous aquatic system. Overall, coastal reservoirs could play an increasingly important role in future climate change via their N₂O emission to the atmosphere as water demand and anthropogenic pressure continue to rise.

Density currents reduce nitrous oxide emissions in a tributary bay of Three Gorges Reservoir

Reservoirs are a significant source of the potent greenhouse gas nitrous oxide (N₂O), but there are few data on N₂O in the world's largest reservoirs and limited understanding of the factors controlling their emission rates. Here we analyzed high-resolution measurements of dissolved N₂O concentrations and fluxes in a typical tributary bay of Three Gorges Reservoir. The surface water was oversaturated in N₂O during both low and high water level (8.6 -16.4 nmol/L, 107% - 180% saturation) and N₂O fluxes varied nearly tenfold (0.2 and 1.6 μmol/(m² h)). Dissolved N₂O concentrations were characterized by pronounced vertical gradients, which were controlled by bidirectional density currents. The river water with high concentrations entered the bay as an underflow along the riverbed, the upper part of the water column was formed by intrusive backwater of Three Gorges Reservoir having significantly lower N₂O concentrations. In consequence, the N₂O emission potential of the impoundment was reduced compared to pre-impoundment conditions. These results reveal the importance of hydraulic conditions on N₂O emission from large reservoirs and suggest that flow regulation can be a potential tool for mitigating greenhouse gas emissions from manmade impoundments.

Large Spatial Variations in Diffusive CH4 Fluxes from a Subtropical Coastal Reservoir Affected by Sewage Discharge in Southeast China

Coastal reservoirs are potentially CH₄ emission hotspots owing to their biogeochemical role as the sinks of anthropogenic carbon and nutrients. Yet, the fine-scale spatial variations in CH₄ concentrations and fluxes in coastal reservoirs remain poorly understood, hampering an accurate determination of reservoir CH₄ budgets. In this study, we examined the spatial variability of diffusive CH₄ fluxes and their drivers at a subtropical coastal reservoir in southeast China using high spatial resolution measurements of dissolved CH₄ concentrations and physicochemical properties of the surface water. Overall, this reservoir acted as a consistent source of atmospheric CH₄, with a mean diffusive flux of 16.1 μmol m^-2 h^-1. The diffusive CH₄ flux at the reservoir demonstrated considerable spatial variations, with the coefficients of variation ranging between 199 and 426% over the three seasons. The shallow water zone (comprising 23% of the reservoir area) had a disproportionately high contribution (56%) to the whole-reservoir diffusive CH₄ emissions. Moreover, the mean CH₄ flux in the sewage-affected sectors was significantly higher than that in the nonsewage-affected sectors. The results of bootstrap analysis further showed that increasing the sample size from 10 to 100 significantly reduced the relative standard deviation of mean diffusive CH₄ flux from 73.7 to 3.4%. Our findings highlighted the role of sewage in governing the spatial variations in reservoir CH₄ emissions and the importance of high spatial resolution data to improve the reliability of flux estimates for assessing the contribution of reservoirs to the regional and global CH₄ budgets.

Spatial and seasonal variability of nitrous oxide in a large freshwater lake in the lower reaches of the Yangtze River, China

Aquatic ecosystems are recognized as a source of N₂O in accordance with the flux estimations of rivers and estuaries; however, limited research has been conducted on large lakes. In this study, we report the annual N₂O dynamics of a large eutrophic freshwater lake located in the subtropical zone of East China. The dissolved N₂O concentrations in Lake Chaohu were observed to be between 8.5 and 92.3 nmol L^-1 with emission rates between 0.3 and 53.6 μmol m^-2 d^-1, exhibiting considerable spatiotemporal variability. The average seasonal N₂O concentrations were obtained, with the highest value of 23.4 nmol L^-1 found in winter and the lowest value of 12.7 nmol L^-1 found in summer. In contrast to the N₂O concentrations observed, the highest N₂O emission rates occurred during summer, while the lowest emission rates occurred in autumn. The emissions of N₂O were substantially high in the western part of the lake, which suffers from serious eutrophication. In addition, the hotspots of N₂O emissions have been found around the inflowing mouth of the Nanfei River, which transports large amounts of nutrients into the lake. The results suggest that anthropogenically enhanced nutrient inputs may have a significant role in the production and emission of N₂O. However, the negative relationship between the surface water temperature and the N₂O concentration suggests that, N₂O fluxes might be influenced by other inconspicuous mechanisms. In the future the nitrogen dynamics of water and sediment in the lake should be collated to reveal mechanisms controlling N₂O emissions. In summary, Lake Chaohu acts as a source of N₂O with its most eutrophic part contributing 54.9% of the total N₂O emissions of the whole lake.

Effects of Sulfamethoxazole and 2-Ethylhexyl-4-Methoxycinnamate on the Dissimilatory Nitrate Reduction Processes and N2O Release in Sediments in the Yarlung Zangbo River

The nitrogen pollution of rivers as a global environmental problem has received great attentions in recent years. The occurrence of emerging pollutants in high-altitude rivers will inevitably affect the dissimilatory nitrate reduction processes. In this study, sediment slurry experiments combined with ¹⁵N tracer techniques were conducted to investigate the influence of pharmaceutical and personal care products (alone and in combination) on denitrification and the anaerobic ammonium oxidation (anammox) process and the resulting N₂O release in the sediments of the Yarlung Zangbo River. The results showed that the denitrification rates were inhibited by sulfamethoxazole (SMX) treatments (1-100 μg L^-1) and the anammox rates decreased as the SMX concentrations increased, which may be due to the inhibitory effect of this antibiotic on nitrate reducing microbes. 2-Ethylhexyl-4-methoxycinnamate (EHMC) impacted nitrogen transformation mainly though the inhibition of the anammox processes. SMX and EHMC showed a superposition effect on the denitrification processes. The expression levels of the denitrifying functional genes nirS and nosZ were decreased and N₂O release was stimulated due to the presence of SMX and/or EHMC in the sediments. To the best of our knowledge, this study is the first to report the effects of EHMC and its mixtures on the dissimilatory nitrate reduction processes and N₂O releases in river sediments. Our results indicated that the widespread occurrence of emerging pollutants in high-altitude rivers may disturb the nitrogen transformation processes and increase the pressure of global warming.

Control of the Hydraulic Load on Nitrous Oxide Emissions from Cascade Reservoirs

Nitrous oxide (N₂O) emissions show large variability among dam reservoirs, which makes it difficult to estimate N₂O contributions to global greenhouse gases (GHGs). Because river damming alters hydraulic residence time and water depth, the hydraulic load (i.e., the ratio of the mean water depth to the residence time) was hypothesized to control N₂O emissions from dam reservoirs. To test this hypothesis, we investigated N₂O fluxes and related parameters in the cascade reservoirs along the Wujiang River in Southwest China. The N₂O fluxes showed obvious temporal and spatial variations, ranging from -7.86 to 337.22 μmol m^-2 d^-1, with an average of 12.76 μmol m^-2 d^-1. Nitrification was the main pathway of N₂O production in these reservoirs, and seasonal dissolved oxygen (DO) stratification played an important role in regulating the N₂O production. The reservoir N₂O flux had a significant negative logarithmic relationship with the hydraulic load, suggesting its control of the N₂O emission. This was because the hydraulic load was a prerequisite for regulating the nitrification-denitrification and the DO stratification in the dam reservoirs. This empirical relationship will help to estimate the contribution of reservoir N₂O emissions to global GHGs.

Contrasting methane emissions from upstream and downstream rivers and their associated subtropical reservoir in eastern China

Subtropical reservoirs are an important source of atmospheric methane (CH4). This study investigated the spatiotemporal variability of bubble and diffusive CH4 emissions from a subtropical reservoir, including its upstream and downstream rivers, in eastern China. There was no obvious seasonal variation in CH4 emissions from the main reservoir, which increased slightly from the first half year to the next half year. In the upstream river, CH4 emissions were low from February to June and fluctuated widely from July to January due to bubble activity. In the downstream river, CH4 emissions were lowest in February, which was possibly influenced by the low streamflow rate from the reservoir (275 m3 s-1) and a short period of mixing. There was spatial variability in CH4 emissions, where fluxes were highest from the upstream river (3.65 ± 3.24 mg CH4 m-2 h-1) and lowest from the main reservoir (0.082 ± 0.061 mg CH4 m-2 h-1), and emissions from the downstream river were 0.49 ± 0.20 mg CH4 m-2 h-1. Inflow rivers are hot spots in bubble CH4 emissions that should be examined using field-sampling strategies. This study will improve the accuracy of current and future estimations of CH4 emissions from hydroelectric systems and will help guide mitigation strategies for greenhouse gas emissions.

Distribution and emission of N2O in the largest river-reservoir system along the Yellow River

Rivers and reservoirs are affected by human activities and are sources of the greenhouse gas nitrous oxide (N₂O). Concentrations of N₂O in the middle and lower reaches of the Yellow River and Xiaolangdi Reservoir, China, were measured in June and December 2017. Fluxes were estimated by boundary layer method to explore their controlling factors, especially the impact of damming and reservoir operation. In the middle and lower reaches of the Yellow River, N₂O concentrations in surface waters were 26.65 ± 14.67 nmol L^-1 in summer and 21.16 ± 5.35 nmol L^-1 in winter. In comparison, the concentrations of N₂O in the reservoir were 32.94 ± 17.32 nmol L^-1 in summer and 23.73 ± 5.60 nmol L^-1 in winter. The longitudinal distribution of N₂O along the river exhibited different patterns with surface N₂O decreasing downstream towards the dam in summer but increasing in winter. Vertical profiles of N₂O concentrations in the reservoir showed an increase with depth in summer but were almost vertically uniform in winter. In winter, N₂O that had accumulated in the bottom water in summer was transported to the surface by vertical mixing and released into the atmosphere. Dissolved oxygen (DO), water temperature, and in situ biological production were the main factors affecting the distribution of N₂O. The mean emissions rates of N₂O from the surface waters were 13.7 ± 8.8 μmol m^-2 d^-1 in summer and 13.2 ± 7.6 μmol m^-2 d^-1 in winter. Approximately 1.31 × 10⁶ mol N₂O was released from the reservoir surface in 2017, which represents 0.12% of the annual N₂O emissions from global reservoirs. The construction of dams increased N₂O emission from the lower reaches of the river by 4.53 × 10⁵ mol and 1.22 × 10⁵ mol due to the discharge of the bottom water and the water and sediment regulation, respectively. This study demonstrates that the construction of dams and reservoir operation practices have made the Xiaolangdi Reservoir a key area for N₂O emissions.

Greenhouse Gas Emissions from Freshwater Reservoirs: What Does the Atmosphere See?

Freshwater reservoirs are a known source of greenhouse gas (GHG) to the atmosphere, but their quantitative significance is still only loosely con- strained. Although part of this uncertainty can be attributed to the difficulties in measuring highly variable fluxes, it is also the result of a lack of a clear accounting methodology, particularly about what constitutes new emissions and potential new sinks. In this paper, we review the main processes involved in the generation of GHG in reservoir systems and propose a simple approach to quantify the reservoir GHG footprint in terms of the net changes in GHG fluxes to the atmosphere induced by damming, that is, 'what the atmosphere sees.' The approach takes into account the pre-impoundment GHG balance of the landscape, the temporal evolution of reservoir GHG emission profile as well as the natural emissions that are displaced to or away from the reservoir site resulting from hydrological and other changes. It also clarifies the portion of the reservoir carbon burial that can potentially be considered an offset to GHG emissions.