Autochthonous organic matter input in reservoirs: Limited methane oxidation in sediments fails to suppress methane emission.

The interception of rivers leads to the accumulation of substantial organic matter in reservoirs, exerting a significant influence on greenhouse gas emissions. The diverse imported organic matter, coupled with sedimentary heterogeneity and intricate microbial processes, gives rise to seasonal variations in methane emissions from reservoirs. In this study, sediment cores were supplemented with terrestrial or autochthonous carbon to emulate reservoir carbon input across different seasons, thereby investigating methane emission potential and associated microbial mechanisms within the sediment cores. Results demonstrated that autochthonous organic matter enhanced sediment organic content, thereby providing more substrates for the methanogenic process and fostering the proliferation of methanogens (with a relative abundance of 47.17 % to 60.66 %). Notably, the dominant genera of Methanosaeta, Methanosarcina, and Candidatus Methanomethylicus were boost on the surface layer of sediment. Concurrently, the introduction of autochthonous organic carbon spurred an increase in methane-oxidizing microbe, reaching up to 5.59 %, with Methylobacter and Candidatus Methanoperedens as the predominant species, which led to a downward migration of the functional microbial group in the sediment. Under the priming impact of autochthonous carbon, however, the methane oxidation probably doesn't consume the substantial methane produced in sediment. Consequently, the sediment functions as a hotspot for methane release into the overlying water, highlighting the necessity to include summer as critical periods for integrated assessments, particularly during algae bloom.

Microbial methanogenesis in aerobic water: A key driver of surface methane enrichment in a deep reservoir.

Significant amounts of the greenhouse gas methane (CH4) are released into the atmosphere worldwide via freshwater sources. The surface methane maximum (SMM), where methane is supersaturated in surface water, has been observed in aquatic systems and contributes significantly to emissions. However, little is known about the temporal and spatial variability of SMM or the mechanisms underlying its development in artificial reservoirs. Here, the community composition of methanogens as major methane producers in the water column and the mcrA gene was investigated, and the cause of surface methane supersaturation was analyzed. In accordance with the findings, elevated methane concentration of SMM in the transition zone, with an annually methane emission flux 2.47 times higher than the reservoir average on a large and deep reservoir. In the transition zone, methanogens with mcrA gene abundances ranging from 0.5 × 103-1.45 × 104 copies/L were found. Methanobacterium, Methanoseata and Methanosarcina were the three dominate methanogens, using both acetic acid and H2/CO2 pathways. In summary, this study contributes to our comprehension of CH4 fluxes and their role in the atmospheric methane budget. Moreover, it offers biological proof of methane generation, which could aid in understanding the role of microbial methanogenesis in aerobic water.

Development of subsurface chlorophyll maximum layer and its contribution to the primary productivity of water column in a large subtropical reservoir.

The phenomenon of subsurface chlorophyll maximum (SCM) layer emerging at a certain water depth is commonly found in stratified water bodies. Also, it is a crucial contributing region to the primary productivity of the water column. Currently, there is a lack of concern about the occurrence of SCM phenomena in studies targeting inland water bodies such as natural lakes and artificial reservoirs. This led to a significant underestimation of the level of primary productivity in these water bodies and their trophic state. In this study, a subtropical reservoir (the Xinanjiang Reservoir, XAJR) was investigated, to understand the characteristics of SCM layer in deep-large reservoir and its contribution to the primary productivity of the water column. Water sampling were conducted from September 2020 to August 2021, and in September 2022. Buoy station data for this reservoir between 2019 and 2021 were also collected. Based on the detailed observations of the water column profile in riverine area (X1), transitional area (X2), and central area (X3 and X4) of this reservoir, it was found that there was an obvious SCM phenomenon, which was closely related to the characteristics of seasonal thermal stratification. The SCM layer of XAJR appeared at depth around 3-5 m underwater from May to August, and as the thermal stratification strength increased, so did the depth and thickness of the SCM layer. It was estimated that gross primary productivity of euphotic layer of XAJR ranged from 347.9 to 4508.6 mgC·m-2·d-1. The average primary productivity level of the SCM layer reached 1411.7mgC·m-2·d-1, accounting for about 40-90% of the gross primary productivity of euphotic layer. This study contributes to a better understanding of the factors influencing changes in the development of the SCM layer in large reservoirs, as well as its critical role in the inland water carbon cycle.

Shifts of soil archaeal nitrification and methanogenesis with elevation in water level fluctuation zone of the three Gorges Reservoir, China.

The water level fluctuation zone is a unique ecological zone exposed to long-term drying and flooding and plays a critical role in the transport and transformation of carbon and nitrogen materials in reservoir-river systems. Archaea are a vital component of soil ecosystems in the water level fluctuation zones, however, the distribution and function of archaeal communities in responde to long-term wet and dry alternations are still unclear. The community structure of archaea in the drawdown areas at various elevations of the Three Gorges Reservoir was investigated by selecting surface soils (0-5 cm) of different inundation durations at three sites from upstream to downstream according to the flooding pattern. The results revealed that prolonged flooding and drying increased the community diversity of soil archaea, with ammonia-oxidizing archaea being the dominant species in non-flooded regions, while methanogenic archaea were abundant in soils that had been flooded for an extended period of time. Long-term alternation of wetting and drying increases methanogenesis but decreases nitrification. It was determined that soil pH, NO3--N, TOC and TN are significant environmental factors affecting the composition of soil archaeal communities (P = 0.02). Long-term flooding and drying changed the community composition of soil archaea by altering environmental factors, which in turn influenced nitrification and methanogenesis in soils at different elevations. These findings contribute to our understanding of soil carbon and nitrogen transport transformation processes in the water level fluctuation zone as well as the effects of long-term wet and dry alternation on soil carbon and nitrogen cycles. The results of this study can provide a basis for ecological management, environmental management, and long-term operation of reservoirs in water level fluctuation zones.

Chlorophyll maxima layer in a large subtropical reservoir (Xinanjiang Reservoir): Spatial development process and limitation by CO2 and phosphorus.

In marine investigations, the maximum chlorophyll-a (Chla) concentration is often reported to occur at a specific depth below the ocean surface, a phenomenon known as subsurface Chla maxima (SCM). However, SCM has long been overlooked in artificial reservoirs, which may lead to a serious underestimation of the primary productivity level and trophic status of reservoirs. To better understand the temporal and spatial variability of SCM and the mechanisms leading to SCM development, this study conducted a detailed survey in a large subtropical reservoir (Xinanjiang Reservoir, XAJR) from September 2020 to August 2021. The seasonal thermal stratification, in situ variables (WT, pH, DO and Chla), nutrient concentrations (DSi, NO3-, DIP and DCO2), Chla maxima depth and magnitude of the riverine region (S1), transition region (S2) and the central part of the XAJR (S3 and S4) were all thoroughly investigated. Thermal stratification and SCM in XAJR exhibited significant seasonal and spatial heterogeneity. Phytoplankton biomass in the epilimnion was limited by dissolved CO2 from June to October in the warm seasons, while it was primarily limited by phosphorus in the other seasons, according to the nutrient limitation analysis. Along the water column, dissolved CO2 limitation occurred mainly above the SCM layer, and the water column below the SCM layer gradually transitioned from dissolved CO2 limitation to phosphorus limitation. Furthermore, as the thermal stratification developed, the upstream water mass moves along the middle of the water column as density flow toward the reservoir, providing nutrients for the development of the SCM. This research contributes to a better understanding of the temporal and spatial variation of SCM and nutrient supply in deep and large stratified reservoirs.

Study on carbon dioxide emission from reservoirs with different regulation types and its empirical prediction model.

The construction of artificial reservoirs with various regulation types on river is currently an important form of comprehensive utilization of water energy and water resources in river basins. The type of regulation is important in controlling the residence time, which in turn affects the photosynthesis-respiration balance in the water. This process has a significant impact on carbon dioxide (CO2) emissions from reservoirs. In this study, seasonal observations were carried out from September 2020 to July 2021 at five artificial reservoirs in the Qiantang River Basin, eastern China, to reveal the characteristics of CO2 emission from the water-air interface of reservoirs with different regulating types. The results showed that the annual average CO2 emission flux of the studied reservoirs varied significantly, ranging from 4.2 to 155.3 mmol m-2 day-1 with an average of 48.4 mmol m-2 day-1, which also had a significant negative correlation with the hydraulic retention time. While downstream of the dam, the annual average CO2 emission flux was quite high with a range of 105.8 to 543.0 mmol m-2 day-1, averaging 381.6 mmol m-2 day-1. This is mainly due to the release of water with high-concentration CO2 from the bottom of the reservoir. Additionally, using related data of reservoirs around the world, a CO2 emission model with hydraulic retention time, air temperature, and reservoir age as the primary parameters was developed, which was conducive to evaluate reservoir CO2 emissions on a larger scale and provided theoretical support for effective reservoir management.