[Impact of Rainfall-Runoff Events on Methane Emission from Xiangxi Bay of the Three Gorges Reservoir].

Methane is an important greenhouse gas and whether reservoirs act as a source or sink of methane has attracted great attention worldwide. However, unrepresentative sampling periods and a lack of consideration of unfavorable weather conditions have limited the accurate estimation of CH4 emission from reservoirs. This study focused on the middle reach of Xiangxi Bay in the Three Gorges Reservoir to track an entire rainfall-runoff event via on-site measurements in the summer of 2019, and initiatively investigated the impact of rainfall and inflow processes on methane concentration and emission. Results showed that from before to after the rainfall event, methane flux at the air-water interface ranged between 0.011 and 0.326 mg·(m2·h)-1, indicating a net source of methane to the atmosphere. Both wind velocity and rainfall affected methane evasion from the surface by altering the gas transfer velocity, with the effect of wind being more prominent. Methane concentrations at the bottom layer significantly increased when rainfall-induced density flow from the watershed arrived at the sampling section. This was likely due to methane export from upstream and along the flow path. During this event, discharge was too small to destratify the water column, and methane was strongly oxidized as it diffused upwards, having little impact on surface methane concentrations and air-water methane flux.

[Spatio-Temporal Variation of Release Flux of Sediment Nitrogen and Phosphorus in High-Risk Period of Algal Bloom in Lake Erhai].

Spatial and temporal characteristics of release fluxes of sediment nitrogen (N) and phosphorus (P) were investigated in the high-risk period of algal blooms in Lake Erhai. Moreover, the influence factors were examined. Results show that the release flux of N and P increased in recent years, exhibiting a clear increase in the period from 2009 to 2013, and a slight increase in the period since 2013. The release flux of dissolved total nitrogen (DTN) ranged between 11.71-14.15 mg·(m2·d)-1, within which the release flux of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) were 6.39-8.42 mg·(m2·d)-1 and 5.31-5.73 mg·(m2·d)-1, accounting for 58% and 42% of the DTN, respectively. The release flux of dissolved total phosphorus (DTP) ranged between 0.11-0.14 mg·(m2·d)-1, within which the release flux of dissolved organic phosphorus (DOP) and dissolved inorganic phosphorus (DIP) were 0.04-0.05 mg·(m2·d)-1 and 0.07-0.09 mg·(m2·d)-1, accounting for 34% and 66% of the DTP, respectively. The distribution of release flux of N showed a decreasing order:south > north > middle, while P was north > middle > south. The release flux of N increased by 17%, 13% and 23%, and the release flux of P increased by 19%, 28%, and 29% in north, middle, and south part of Lake Erhai from 2009 to 2018. Comparing the years 2009, 2013 and 2018, although the contents of N and P were stable, the release flux of N and P in the sediment was enhanced due to increasing pH and decreasing DO. Therefore, the increasing release of nitrogen and phosphorus from sediments, caused by changes in the water environment factors, should be paid attention to for the protection of Lake Erhai.

[Vertical Distribution Profiles and Release Potential of Mainstream Column Sediments in the Three Gorges Reservoir After Impoundment to 175 m].

To investigate the vertical distribution characteristics of phosphorus in the main stream sediments of the Three Gorges Reservoir (TGR), column sediment samples were collected from 5 sites in October 2010, and sediment particle size, organic matter contents, and mineral compositions were analyzed. Investigation of the contribution rate of phosphorus released from column sediments in the TGR was also conducted. The results show that the sediment pH is between 7.3-7.8. The mainstream column sediment is mainly constituted by silt and clay, which account for 49.4%-78.6% and 20.6%-50.6% of total sediments, while sand represents less than 4.4%. Median grain size of each sampling site presented a phased increase or decrease trend. The organic matter content was between 12.94 g·kg-1 and 53.43 g·kg-1, and it tended to slightly increase from upstream to downstream. The C/N ratio in the sediment was between 4.00 and 11.64, and organic matter content was mainly affected by terrigenous input. Total phosphorus (TP) content was between 861.86 mg·kg-1 and 1024.54 mg·kg-1, and it exhibited negligible change in vertical distribution. There is no obvious enrichment phenomenon of phosphorus for column sediment. The major component of sediment TP is calcium bound phosphorus (Ca-P, 47.83%-73.90%), and there are various trends for phosphorus distribution in different sampling sites. Exchangeable phosphorus (Ex-P), aluminum-bound phosphorus (Al-P), and iron-bound phosphorus (Fe-P) in the 0-4 cm surface sediment of each sampling sites was relatively high. For most sampling sites, no obvious change of phosphorus fractions in 16-20 cm of sediment was detected. Bioavailable phosphorus (the sum of Ex-P, Al-P, and Fe-P) accounted for 2.78%-7.05% of TP, indicating that bioavailability of phosphorus in the column sediments is low. The contents of bioavailable phosphorus and organic matter were significantly and positively correlated (P<0.05, N=50). The distribution and transformation of organic matter will affect the migration and transformation of sediment phosphorus in the TGR.

[Effects and Differences of the Release of Dissolved Organic and Inorganic Phosphorus in Different Sediments Covered by Different Materials of Erhai Lake].

In this work, the effects of four covering materials on the release of total dissolved phosphorus (DTP), dissolved organic phosphorus (DOP), and soluble reactive P (SRP) in different sediments of Erhai Lake were simulated. The results showed that the max release of DTP was reduced in covering material, which attributed to the changes of pH, Eh and characteristics of dissolved organic matter (DOM) by the effect of covering material. The application of iron oxide material significant reduced the release of DTP in the northern and southern part of the lake, with decrease rate of 44.3% and 35.7%, respectively. by contrast, the application of aluminum oxide material significant reduced the release of DTP in the middle part sediment, with decrease rate of 29.6%. Furthermore, the release of SRP and DOP in different sediments has significant difference after added different material. In northern part of sediment, the release of SRP and DOP reduced by 35.6% and 36.2% after added iron oxide material. This is because iron oxide can reduce the pH and Eh but increase the availability of DOM in northern, and then benefits for inhibiting the release of SRP and DOP. In the middle, the release of sediment SRP and DOP reduced by 28.9% and 31.6% after added aluminum oxide material. This is because the aluminum oxide can facilitate the availability of DOM in middle, and then inhibits the release of SRP and DOP. In southern part of the lake, the release of sediment SRP and DOP reduced by 47.4% and 16.5% after added iron oxide material. This is largely attributed to the effect of iron oxide on the pH and Eh. Therefore, to control the release of P in the sediment from Lake Erhai, iron oxide material should be selected in the northern and southern parts, whereas aluminum oxide should be selected in the middle part of the lake.

[Characteristics of Phosphorus Fractions and Phosphate Diffusion Fluxes of Sediments in Cascade Reservoirs of the Huangbai River].

The Huangbai river, with a large phosphorite deposit in the basin upstream, is the drinking-water source of Yichang city, China, and water blooms often break out in it in present times. To investigate the internal contamination load, the distribution and vertical variation of sediment phosphorus and its fractions were analyzed. Phosphorus flux across the sediment-water surface was preliminarily analyzed in three reservoirs in the basin. The results showed that the concentration of total phosphorus (TP) decreased from the upstream to the downstream reservoirs of the basin, and the means of the TP were (8070.0±2251.4), (2681.2±1709.8), and (2656.6±1599.7) mg·kg-1, in the three reservoirs, respectively. This shows that the basin is at a highly polluted level. There were similar tendencies in the concentrations of dissolved oxygen (DO) and chlorophyll a in the surface water. The order of the phosphorus fractions was HCl-P > OP > NaOH-P in most sites. According to Fick's First Law, the flux of PO43--P across the sediment-water surface was 0.0179-0.1825 mg·(m2·d)-1, and it decreased from the upstream to the downstream reservoirs. The flux was positively related to HCl-P, OP, and TP in the high phosphorus concentration basin.

[Characteristics of Water Extractable Organic Nitrogen from Erhai Lake Sediment and Its Differences with Other Sources].

The 47 samples from Erhai Lake surface sediments were collected in different seasons. The distribution and structure characteristics of sediment water extractable organic nitrogen(WEON) were investigated by using the combined techniques of UV-Vis absorption and three-dimensional excitation-emission matrix spectra(3DEEMs). The differences in DON of various sources(overlying water, pore water, inflow Rivers and wet deposition) were explored to analyze its effects on sediment. The results showed that:1the temporal distribution followed the pattern of summer > spring > autumn > winter, with the spatial WEON distribution of northern > southern > central. 2 The humic degree of Erhai sediment WEON was relatively high and mostly composed of fulvic acid, which mainly contained UV-like humic-like fluorescence peak A and high-excited tryptophan fluorescence peak B. This indicated that it was mainly affected by terrestrial input and microbial activity. 3 There were two fluorescent components(C1, C2) in the sediments and other sources of Erhai Lake. The component C1 was the endogenous visible ultraviolet peak formed by biodegradation, while the component C2 was the tryptophan peak. The bioavailability of wet deposition samples was comparatively high, greatly impacting Erhai Lake in heavy rainfall. The DON bioavailability in the inflow river was the lowest, which was prone to be accumulated in sediments. 4The DON component C1 and C2 in overlying water had significant negative correlation with Erhai sediment WEON contents(r=-0.79, P<0.01;r=-0.944, P<0.01). This suggested that the overlying water DON components could indirectly reflect the sediment WEON content of Erhai Lake, namely the higher the fluorescence components C1 and C2 in overlying water DON were, the lower the sediment WEON content was.

[Gas Transfer Velocity of CH4 at Extremely Low Wind Speeds].

Thin boundary theory equation (TBL) is widely used to determine gas fluxes across water-air interfaces, and the gas transfer velocity (k600) is the key environmental factor in the equation. A monthly field campaign was carried out during one year to measure CH4 flux and to probe its exchange rate across the air-water interface in a drinking reservoir and 5 adjacent ponds. The ranges of wind speed and surface water temperature were 0-0.75 m·s-1 and 6.3-30.9℃respectively, and their average values were 0.19 m·s-1 and 19.3℃ respectively. The gas transfer velocity of CH4 varied from 0.20 to 1.99 cm·h-1 with an average of 0.50 cm·h-1. Correlation functions between the gas transfer velocity and the wind speed at 10 m height (U10) and surface water temperature (Tw) were given here to quantify k600. There were significant correlations between the fitted values and actual values both for original and bin-averaged data.

[Partial Pressure of Carbon Dioxide and Methane from Autumn to Winter in Xiangxi Bay of the Three Gorges Reservoir].

The Concentrations of dissolved CH4 and CO2 in Xiangxi Bay of the Three Gorges Reservoir from autumn to winter in 2014 were determined with headspace gas chromatography technology. Then their partial pressures of CH4 and CO2 were calculated according to the Henry's law. Their temporal variation and the effects of environmental parameters were also discussed. The results indicated that the CH4 partial pressure in the surface water ranged 0.64-4.43Pa, with an average of (1.69±0.94)Pa. The CO2 partial pressure varied from 49.90 to 868.91Pa, with the average of (328.48±251.63)Pa. The pCO2 and pCH4 had a strong negative correlation (r=-0.618,P<0.01). During the period of monitoring, the pCO2 and pCH4 in surface water were significantly correlated with pH, DO, chlorophyll a, TP, surface water temperature and water level. Compared with pCH4, pCO2 was more closely correlated with various environmental factors.

[Characteristics of Methane Flux Across the Water-air Interface in Subtropical Shallow Ponds].

Five shallow ponds of Yichang were selected to illustrate the characteristics of methane(CH4) in subtropical eutrophic shallow ponds. CH4 flux across the water-air interface was quantified with static floating chamber method for one year. Annual CH4 fluxes of the five ponds were 4.495, 12.702, 6.827, 8.920, 17.560 mg·(m2·h)-1 respectively. Diffusive CH4 fluxes were 0.075, 0.087, 0.118, 0.086, 0.151 mg·(m2·h)-1 respectively and bubble emissions were 4.420, 12.616, 6.709, 8.834, 17.409 mg·(m2·h)-1 respectively. Over 98% of total CH4 flux was bubble emission and CH4 flux was apparently higher than other aquatic ecosystems. So the CH4 flux of shallow waters was high and bubble emission was the dominant way. CH4 emission would be largely underestimated if the research only focus on the diffusion discharge and ignore the bubble emission.

[Estimation of releasing fluxes of sediment phosphorous in the Three Gorges Reservoir during late autumn and early winter].

In order to investigate the internal contamination load sources in the Three Gorges Reservoir (TGR), a field sampling campaign was carried out in November and December 2010. Phosphate contents (PO4(3-) -P) were determined in 6 mainstream and 9 estuary sediment cores of sediment-water interface in the TGR. The results showed that the PO4(3-) -P concentrations of pore water in sediments from tributaries and the corresponding overlying water were higher than those from the mainstream. The PO4(3-) -P contents in tributaries and mainstream ranged from 9.59-29.79 microg x L(-1) and 9.01-25.36 microg x L(-1), respectively. Based on calculations using the Fick's First Law, sediments located at GuoJiaba and Xiaojiang estuaries were the 'sink' of P, and the fluxes of sediment PO4(3-) -P were estimated to be -0.63 mg x (m2 x a)(-1) and -0.60 mg x (m2 x a)(-1), respectively. In other areas, PO4(3-) -P diffused from the pore water to the overlying water, and the diffusive fluxes were in the range of 0.15-2.47 mg x (m2 x a)(-1). With the assumption that molecular diffusion was the main process by which nutrients were transported from the pore water to the overlying water and the water body in the TGR was evenly mixed, the contribution of sediment phosphorus to the water body was only -0.011-0.098%. So far, with the Three Gorges Reservoir sediments as the internal contamination load sources, the phosphorous release didn't significantly influence the water quality. The sediments in the TGR may be a large P source in a period of future time after the external P source is controlled efficiently.

[Phosphorus output characteristics under different rainfall-runoffs in Gaolan River].

Through capturing the dynamics of water quantity and quality during varied intensities of rainfall, the variation characteristics of different forms of phosphorus and flux during the producing of runoff were studied systematically in Gaolan River watershed (a secondary tributary of Three Gorges Reservoir). Meanwhile, the phosphoric loadings from point and non-point sour,-were identified, respectively. The results show that: the variation of rainfall-runoff under " squat" rain was relatively slow, while un,c "pointed thin" rain it presented quickly both during rising and recession. Total phosphorus concentration increased quickly because soil carried particulate phosphorus into water under the heavy rainfall, while the relative variations of dissolved total phosphorus and orthophosphate concentration were slightly elevated. Compared with "squat" rain, the maximum value of total phosphorus concentration and flux were both higher in "pointed thin" rain, which led to the more heavily soil erosion. The dynamics of flux and concentratiorn of phosphorus were generally consistent with that of the flow rate between the two different types of rain. Phosphorus from non-point source accounts for 90% during the whole heavy rain. Because of the long lasted time during " squat" rain, the contribution of the "squat" rain outweighs the "pointed thin" rain.

[Effects of thermal stratification on spring blooms in Xiangxi Bay of the Three Gorges Reservoir].

Frequent spring blooms have been observed in tributary bays of the Three Gorges Reservoir (TGR) since its initial filling. In order to identify the primary factor controlling spring blooms, a field monitoring plan was carried out in Xiangxi Bay, the largest tributary of TGR head area in spring 2010. The environmental factors such as water temperature, Secchi depth, underwater illumination and nutrients were monitored. The results showed that the sudden decrease of mixing depth resulting from the development of thermal stratification was the direct and primary contributor to the onset of spring blooms (Pearson correlation coefficient of the ratio of euphotic depth to mixing depth (Z(eu)/Z(mix)) and chlorophyll-a (Chl-a) is 0.934, P < 0.01). The significant response of spring blooms to thermal stratification indicates that Critical Depth model can be applied to tributary bays of TGR, which will provide important references for the investigators and managers who are working on the prediction and control of algal blooms in the bays.