Phosphorus availability as a primary control of dissolved organic carbon biodegradation in the tributaries of the Yangtze River in the Three Gorges Reservoir Region

Control mechanisms of water chemistry based on long-term analyses of the Yangtze River

Long-term series data can provide a glimpse of the influence of natural and anthropogenic factors on water chemistry. However, few studies have been conducted to analyze the driving forces of the chemistry of large rivers based on long-term data. This study aimed to analyze the variations and driving mechanisms of riverine chemistry from 1999 to 2019. We compiled published data on major ions in the Yangtze River, one of the three largest rivers in the world. The results showed that Na+ and Cl- concentrations decreased with increasing discharge. Significant differences in riverine chemistry were found between the upper and middle-lower reaches. Major ion concentrations in the upper reaches were mainly controlled by evaporites, especially Na+ and Cl- ions. In contrast, major ion concentrations in the middle-lower reaches were mainly affected by silicate and carbonate weathering. Furthermore, human activities were the drivers of some major ions, notably SO42- ions associated with coal emissions. The increased major ions and total dissolved solids in the Yangtze River in the last 20 years were ascribed to the continuous acidification of the river and the construction of the Three Gorges Dam. Attention should be given to the impact of anthropogenic activities on the water quality of the Yangtze River.

Effects of dissolved organic matter from sediment and soil samples on the growth and physiology of four bloom-forming algal species

Algal blooms negatively impact the water quality of reservoirs; however, the role of dissolved organic matter (DOM) in bloom formation in reservoirs has not been investigated. Therefore, we assessed the compositions of sediment- and soil-derived DOM and their effects on the growth, physiology, and photosynthetic activity of Microcystis aeruginosa, Anabaena sp., Chlamydomonas sp., and Peridiniopsis sp. (bloom-forming species). Sediment DOM promoted the growth of all algal species, whereas soil DOM significantly promoted the growth of Chlamydomonas sp. and Peridiniopsis sp.; this effect was due to enhanced stress tolerance and photosynthetic efficiency exhibited by these algae under DOM treatment. However, soil DOM slightly inhibited the growth of Anabaena sp. by increasing reactive oxygen species levels and inactivating some photosystem II reaction centers. The tyrosine-like substance, humic acid-like substances, and unsaturated aliphatic compounds were the main DOM components that affected algal growth. The findings of this study will provide a theoretical foundation for the development of bloom-prevention strategies for river-type reservoirs.

Effects of dissolved organic matter derived from two herbs on the growth, physiology, and physico-chemical characteristics of four bloom-forming algae species

While algal blooms occur frequently in lakes and reservoirs worldwide, the effects of dissolved organic matter (DOM) from lakeside and riparian zones on bloom formation are not well understood. In this study, we characterized the molecular composition of DOM from Cynodon dactylon (L.) Pers. (CD-DOM) and Xanthium sibiricum Patrin ex Widder (XS-DOM) and assessed their effects on the growth, physiology, volatile organic compounds (VOCs), and stable carbon isotope in four bloom-forming algae species (Microcystis aeruginosa, Anabaena sp., Chlamydomonas sp., and Peridiniopsis sp.). Stable carbon isotope analysis showed that the four species were affected by DOM. Both DOM types increased the cell biomass, polysaccharide and protein contents, chlorophyll fluorescence parameter values, and VOCs release of Anabaena sp., Chlamydomonas sp. and Microcystis aeruginosa, suggesting that DOM stimulated algal growth by increasing nutrient sources, photosynthetic efficiency, and stress tolerance. And in general, these three strains exhibited better growth at higher DOM concentrations. However, DOM treatment inhibited the growth of Peridiniopsis sp., as indicated by the increases in reactive oxygen species, damage in photosystem II reaction centers, and blockage in electron transport. Fluorescence analysis showed that tryptophan-like compounds were the main DOM components that affected algal growth. Molecular-level analysis suggested that unsaturated aliphatic compounds may be the most important DOM components. The findings indicate that CD-DOM and XS-DOM promote the blue-green algal blooms formation and thus should be considered in the management of natural water quality.

Reservoir NO3- pollution and chemical weathering: by dual isotopes of δ15N-NO3-, δ18O-NO3- and geochemical constraints

Reservoir dams alter the nutrient composition and biogeochemical cycle. Thus, dual isotopes of δ¹⁸O-NO₃^- and δ¹⁵N-NO^-₃ and geochemical signatures were employed to study the NO₃^- pollution and chemical weathering in the Three Gorges Reservoir (TGR), China. This study found that the TGR dam alters the δ¹⁵N-NO₃^- composition and is enriched in the recharge period. Values of δ¹⁵N-NO₃^- varied from 4.5 to 12.9‰ with an average of 9.8‰ in the recharge period, while discharge period δ¹⁵N-NO₃^- ranged from 3.2 to 12.5‰, with an average of 9.3‰. δ¹⁸O-NO₃^- varies (1.2-11.3‰) with an average of 6.5‰ and (2.4-12.4‰) with an average of 7.5‰, in the recharge and discharge periods, respectively. Stable isotopic values sharply decreased from upstream to downstream, indicating the damming effects. δ¹⁸O-NO₃^- and δ¹⁵N NO₃^- confirm that sewage effluents, nitrification of soil organic material, and NH₄⁺ fertilizers were the primary sources of NO₃^- in the reservoir. Carbonate weathering mainly provides ions to the reservoir. HCO₃^- + SO₄^2- and Ca²⁺ + Mg²⁺ represent 90% of major ions in the TGR. Downstream sampling sites showed low solute concentration during the recharge period, indicating the dam effect on solute concentration. Ca-Mg-Cl^-, Ca-HCO₃^- and Ca-Cl^- were the main water types in the TGR. The average percentage of solutes contribution revealed the carbonate weathering, evaporites dissolution, silicate weathering, and atmospheric input were 51.9%, 41%, 7.8%, and 1.7% for the recharge period. In contrast, the discharge period contributed 66.4%, 29.2%, 10%, and 4.3%, respectively. TGR water is moderately suitable for irrigation, and hardness is high in drinking water. This study provides new insight into the dual isotopic approach and geochemical signatures to interpret the NO₃^- cycle and chemical weathering process under dam effects in the TGR. However, this isotopic application has some limitations in source identification, isotope fractionation, and transformation mechanisms of nitrate. Thus, further studies need to be done on these topics for a better undestanding.

The synergy of environmental and microbial variations caused by hydrologic management affects the carbon emission in the Three Gorges Reservoir

The synergy of environmental and microbiological changes caused by hydrologic management on carbon emissions of river reservoirs remains unknown. Here, we investigated physiochemistry parameters, compositions of dissolved organic matter (DOM), carbon fluxes (CH₄ and CO₂), and microbial communities in the surface waters of the Three Gorges Reservoir (TGR) within one whole hydrological year. The results showed that hydrologic management significantly changed physiochemistry and DOM composition of the TGR water, and further influenced microbial community composition and functions. DOM content during the drainage period was much lower than during the impoundment period. During the impoundment period, humification extent of DOM became decreasing, while biotransformation extent became increasing compared with the drainage period. DOM composition and water pH exhibited significant correlation with the fluxes of CH₄ and CO₂, respectively. Microbial community composition and function significantly differed between the drainage and impoundment periods. Most of the differential microbial taxa were affiliated with functional groups involved in carbon cycle such as methanotrophy and phototrophy, which showed significant correlation with carbon fluxes. CH₄ and CO₂ fluxes can be mostly explained by synergy of microbial function with DOM composition and water pH, respectively. Such synergistic effect may account for the observed temporal variations of CH₄ fluxes and spatial variations of CO₂, and for the relatively low annual carbon emissions in the TGR. In summary, the synergy of environmental and microbial variations caused by hydrologic management affects carbon emissions from river reservoirs.

Water quality and the biodegradability of dissolved organic carbon in drained boreal peatland under different forest harvesting intensities

Boreal peatlands are major sources of nitrogen (N), phosphorus (P) and dissolved organic carbon (DOC) to downstream aquatic ecosystems, and forest harvesting generally further increases the loading of DOC and nutrients. Continuous cover forestry (CCF) is proposed to be an environmentally more sustainable management option for peatland forests than conventional even-aged clear-cutting. However, the impacts of CCF on water quality, the biodegradability of DOC and consequent CO₂ emissions from inland waters are poorly known. We studied the concentrations of N, P and DOC, the quality of DOC, and the mineralization of DOC to CO₂ in ground water and ditch water in clear-cut, partially harvested, i.e. CCF, and uncut drained forests in Finland. Groundwater total N, NH₄-N and PO₄-P concentrations were significantly lower in CCF and uncut forest than in the clear-cut forest. Groundwater DOC concentrations were often highest in the clear-cut forest, where the water table was closer to the soil surface. Ditch water DOC and N concentrations were lowest next to the clear-cut area. DOC aromaticity in ground water was higher in the uncut forest than in the clear-cut and CCF, whereas ditch water aromaticity did not differ between the treatments. The biodegradation of DOC was studied by incubating water (at 15 °C for 24 h) 1, 3, 7 and 21 days after sampling. The results indicated that the majority of the CO₂ production took place during the first three days, and CO₂ fluxes were considerably higher from the ditch water than from the groundwater. The CO₂ emissions were lower in summer than in the other seasons. Ditch water and groundwater CO₂ production were generally significantly higher in the clear-cut than in the uncut forest. The results suggest that CCF can decrease the nutrient concentrations as well as CO₂ emissions from inland waters compared to conventional clear-cutting.

Anaerobic and aerobic biodegradation of soil-extracted dissolved organic matter from the water-level-fluctuation zone of the Three Gorges Reservoir region, China

The biodegradation of dissolved organic matter (DOM) in natural environments is determined by its molecular composition and reactivity. Redox oscillations are common in the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR). As a consequence, the soil DOM released is degraded under both anaerobic and aerobic conditions. The DOM compounds available for degradation under contrasting redox conditions and the resulting DOM composition still need to be elucidated. By combining laboratory experiments with an in-depth characterization of DOM optical properties, we show that different pathways controlled the depletion and enrichment of the DOM optical components under different oxygen regimes. In particular, 28-day dark biodegradation assays showed that up to 39.5 ± 4% DOM was degraded under anaerobic conditions, while 55.5 ± 6% DOM was biodegraded under aerobic conditions. Aerobic biodegradation resulted in a higher aromaticity and degree of humification of the DOM compared to anaerobic degradation. The specific UV absorbance at a wavelength of 254 (SUVA₂₅₄) and biological index (BIX) could be used to track DOM biodegradation under anaerobic conditions. Under aerobic conditions, the SUVA₂₅₄, BIX and concentration of coloured DOM (CDOM, reflected by a (355)) could track DOM biodegradation, and significant amounts of CDOM could be aerobically biodegraded.

A novel strategy to alleviate ultrafiltration membrane fouling by rotating membrane module

Integrated ultrafiltration (UF) membrane technology has attracted extensive attention in drinking water treatment due to its excellent performance and small footprint. However, membrane modules normally are static in membrane tanks, which cause a gradual increase in the cake layer thickness over time, thus resulting in severe membrane fouling. To overcome this shortcoming, we report an effective strategy to regulate cake layer thickness by rotating the membrane module in the presence of flocs. The results showed that the cake layer thickness can be effectively reduced because of the floc looseness, resulting in the alleviation of membrane fouling. The higher the module rotation speed, the higher the flow velocity in the membrane tank and the larger the shearing force on the cake layer surface. As a result, the membrane fouling was considerably mitigated, and it was interesting that the pollutant removal efficiency was hardly influenced. With module rotation, we found that acid solutions displayed a better performance in removing pollutants (even low molecular weight pollutants) and alleviating membrane fouling compared to the alkaline conditions because of the smaller floc size, larger floc specific surface area, and higher floc positive charge. Additionally, an excellent UF membrane performance was also observed with the raw water taken from the South-North water in China. Collectively, this study demonstrated that floc-based cake layers can be effectively regulated with module rotation, which has a great potential in drinking water treatment application, particularly in small water plants.

Spatial and temporal comparisons of dissolved organic matter in river systems of the Three Gorges Reservoir region using fluorescence and UV-Visible spectroscopy

Understanding optical characteristics, composition and source of dissolved organic matter (DOM) in rivers of the Three Gorges Reservoir (TGR) region is important for region and global carbon cycle. However, chemical compositions and source of DOM from the tributary to mainstream in the TGR region are not well studied. Consequently, 126 water samples were collected from rivers in different land use region, and these rivers covered the main tributaries of the Yangtze River in the TGR region. The temporal and spatial variations of DOM structure and source identification in different land use region were investigated using UV-visible absorbance and fluorescence spectroscopy. Overall, there were higher ratio of humic acid to fulvic, aromaticity, molecular weight and proportion of colored humic substances in DOM in the wet season than in the dry season. The weaker biologic/microbial and stronger terrestrial sources in DOM were observed in the wet season than in the dry season. DOM comparison in variable land use demonstrated the higher terrestrial sources and weaker biologic/microbial sources in DOM in the forest-affected rivers irrespective of hydrological seasonality, as well as in the wet season irrespective of land use types. DOM in the farmland-affected rivers showed more protein-like signal. We found that monsoonal precipitation, anthropogenic activities and land use were important drivers for the DOM quality variations. These findings will be beneficial to unravel riverine DOM structure and sources in relation to anthropogenic activities and also improve our understanding of DOM biogeochemical cycle in the rivers.

Reservoir CO2 evasion flux and controlling factors of carbon species traced by δ13CDIC at different regulating phases of a hydro-power dam

As the world largest hydropower reservoir, the Three Gorges Reservoir (TGR) significantly impacted on the carbon cycle since reservoirs are sources of carbon sink. This study was carried out to investigate the effects of damming on the carbon cycle. δ¹³C_DIC and δ¹³C_DOC were used to trace the origin of dissolved organic (DOC) and inorganic carbon (DIC). The estimated CO₂ evasion flux in two regulating phases (discharge and recharge) with averages of 111 mg/m² h and 264 mg/m² h, respectively. At the basin scale, average CO₂ flux was about 188 mg/m² h and varies from -158 mg/m² h to 1092 mg/m² h. The highest average pCO₂ (1294 ppmv) was observed during the discharge period, which was oversaturated than atmospheric equilibrium value; hence, the TGR act as a considerable sink of atmospheric carbon. The δ¹³C_DIC varies from -8.95‰ to 0.00‰ with mean -1.87‰; these enrich isotope values indicated that metabolic process (photosynthesis and respiration) and the rapid kinetics of carbonate weathering by soil CO₂ control the pCO_2. The low pCO₂ of reservoir water caused the rapid dissolution of CO₂ from the atmosphere during the recharge period. The δ¹³C_DOC varies between -30.64‰ to -23.05‰, which is similar to the values of C₃ vegetation; thus, the source of DOC would be the degradation of soil organic matter. Overall, this study revealed the δ¹³C_DIC signature coupled with soil CO₂ dissolution and admixture of atmospherically equilibrated waters resulting in the sink of atmospheric CO₂ of the reservoir and impoundment of the dam alters the carbon cycle and aquatic carbon budget in TGR. The findings of this study provide a global image on the contribution of reservoirs to the carbon cycle and aquatic carbon budget. Coupling with isotope signatures and elemental concentrations, investigation of the biogeochemical cycle of the carbon can be effectively traced.

Temporal controls on dissolved organic carbon biodegradation in subtropical rivers: Initial chemical composition versus stoichiometry

Dissolved organic carbon (DOC) plays an indispensable role in biogeochemical cycles and ecosystem services in rivers. However, little is known about the seasonal variations of DOC biodegradation in subtropical rivers. Here, we investigated the concentrations of DOC, dissolved total nitrogen (DTN), and dissolved total phosphorus (DTP), humification index (HIX), fluorescence index (FI), and DOC biodegradation in 57 rivers in the dry and wet seasons in the Three Gorges Reservoir area, China, and the aims were to clarify the temporal changes in DOC biodegradation and its driving factors in these subtropical rivers. Compared with dry season, DTN and DTP concentrations, and HIX value were greater, and FI value was lower in the wet season. However, DOC biodegradation remained unchanged across the two sampling seasons. Further, DOC biodegradation negatively correlated with DOC:DTP ratio, DTN:DTP ratio, and FI in the dry season, but only with HIX in the wet season. These findings emphasis that, despite unchanged DOC biodegradation, the key factors driving DOC biodegradation shift from C:N:P stoichiometry in the dry season to initial chemical composition in the wet season in subtropical rivers. Our results regarding the temporal patterns of DOC biodegradation and the underlying mechanisms bear important implications for a better understanding of C dynamics in subtropical river ecosystems.

CO2 oversaturation and degassing using chambers and a new gas transfer velocity model from the Three Gorges Reservoir surface

Reservoirs are considered as important carbon source of the atmosphere, whilst, regional and global reservoir CO₂ quantification is hampered by data limitation and bias in spatial and temporal sampling. By deploying chamber measurements and employing the newly developed model of gas transfer velocity, CO₂ partial pressure (pCO₂) and evasion in the main stem of the Three Gorges Reservoir (TGR) were investigated. The pCO₂ ranged from 429 to 8668 μatm with an average of 2511.6 ± 1721.3 μatm, 6.1-fold higher than the ambient air pCO₂ (mean: 410 μatm). All the samples were net CO₂ sources via water-air interface, displaying pronounced spatial and monthly variability. The CO₂ areal flux averaged 212.5 ± 120.1 mmol/m²/d in June, 123.3 ± 78.5 mmol/m²/d in July in the lotic TGR main stream, much higher than its lentic system, i.e., 79.6 ± 41.3 mmol/m²/d in November, and 76.3 ± 88.1 mmol/m²/d in March. Much lower k levels in the lentic reservoir surface resulted in lower CO₂ evasion rates. Furthermore, dam impoundment considerably altered the riverine carbon cycle, as reflected by the changing magnitude of CO₂ efflux and environmental controls of dissolved CO₂. Precipitation and concurrent soil CO₂ influx exhibited a central role in controlling riverine pCO₂, and respiration of allochthonous organic carbon was a secondary factor in the TGR lotic system, whilst, both in-stream metabolism and terrestrial inputs played crucial roles in controlling aqueous CO₂ in the TGR lentic system. In comparison, we provided key findings of k model and more reliable CO₂ quantification with a consideration of water level shifts and a complete coverage of spatial sampling. Our higher CO₂ emission (1.47 (1.16-2.13) Tg CO₂/y) than previous studies called more field measurements to assess the resulting changes in CO₂ flux owing to dam operation and changing environment, and their implications for regional carbon budgets should be warranted.

Temperature sensitivity of biodegradable dissolved organic carbon increases with elevating humification degree in subtropical rivers

Biodegradable dissolved organic carbon (BDOC) plays a key role in C cycle in inland waters. However, the magnitude of temperature sensitivity (Q₁₀ value) of BDOC is still unclear, and the effect of DOC quality on Q₁₀ value of BDOC is not well verified in these aquatic systems. Here, we used a laboratory incubation experiment to determine the Q₁₀ value of BDOC in 57 rivers in the Three Gorges Reservoir area, China, and then tested whether C quality-temperature hypothesis could be applied to BDOC in inland waters. We observed approximately twofold variations in Q₁₀ values of BDOC (1.42-2.67) in these rivers. Moreover, the tight positive relationship between the Q₁₀ values of BDOC and DOC humification index indicated the applicability of C quality-temperature hypothesis in subtropical rivers. In addition, the Q₁₀ values of BDOC exhibited a negative relationship with pH. These findings suggest that DOC quality and pH are powerful predictors of temperature sensitivity of BDOC in subtropical rivers. In conclusion, our results would help to improve the C models and predict the feedback between climate warming and C dynamics in inland waters.

Long-term phosphorus addition enhances the biodegradability of dissolved organic carbon in a nitrogen-limited temperate freshwater wetland

Phosphorus (P) enrichment is expected to strongly influence dissolved organic carbon (DOC) biodegradation. However, the relationship between P availability and DOC biodegradation is largely unknown in nitrogen (N)-limited ecosystems. Here, we investigated the changes in the ratio of DOC to dissolved total nitrogen (DTN), specific UV absorbance at 254nm (SUVA₂₅₄), and DOC biodegradation in surface water and soil pore water (0-15cm depth) following eight years of multi-level P addition (0, 1.2, 4.8, and 9.6gPm^-2year^-1) in an N-limited freshwater marsh in Northeast China. We found that P addition caused an increase in DOC biodegradation in surface water and soil pore water, irrespective of the P addition levels. Compared with the control treatment, the P addition rates of 1.2, 4.8, and 9.6gPm^-2year^-1 increased DOC biodegradation by 20.7%, 15.2%, and 14.5% in surface waters, and 11.3%, 9.4%, and 12.0% in soil pore waters, respectively. The DOC biodegradation was separately negatively correlated with the DOC:DTN ratio and SUVA₂₅₄, indicating that the positive effect of P addition on DOC biodegradation was caused by the elevated N concentration and the reduced DOC aromaticity. Our findings suggest that P enrichment enhances the biodegradability of DOC through increased N availability and altered DOC chemical composition, which would accelerate DOC loss from the waters and alter ecosystem C balance in N-limited temperate wetlands.