Spatial distribution and influencing mechanism of CO2, N2O and CH4 in the Pearl River Estuary in summer.

Estuaries, considered as the important carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) sources to the atmosphere, are increasingly affected by near-bottom hypoxia. However, the impact of estuarine hypoxic zone development on GHGs production and discharge remains poorly understood due to the seasonal and spatially distributed heterogeneity of estuarine hypoxia occurrence and the lack of simultaneous monitoring of the distribution of bottom hypoxic waters and the vertical distribution of GHGs. Here, we conducted high spatial resolution vertical stratification sampling and analysis of water column GHGs in the Pearl River Estuary (PRE), a large estuary with frequent hypoxia in recent years. Our results showed that Pearl River runoff is the main source of GHGs in the PRE. Strong nitrification is an important N2O production mechanism in the PRE. In situ generation of water and resuspension of surface sediments were the main sources of CH4 in bottom water, while massive organic matter (OM) mineralization is the main driver of CO2 in bottom water. The development of a hypoxic zone in the PRE significantly increased the concentration of N2O and CH4 in the bottom water and thus increased air-water fluxes. The air-water fluxes of N2O, CH4 and CO2 of PRE in summer were 31.9 ± 7.5 µmol m-2 d-1, 192.5 ± 229.4 µmol m-2 d-1 and 51.9 ± 14.1 mmol m-2 d-1, respectively. This study reveals that GHGs fluxes from estuarine waters to the atmosphere will increase significantly with increasing eutrophication caused by human activities and the expansion of hypoxic zones in estuarine waters.

Variation of biogeochemical cycle of riverine dissolved inorganic carbon and silicon with the cascade damming.

To investigate the variation of the biogeochemical cycle of riverine dissolved inorganic carbon (DIC) and silicon (DSi) with the cascade damming, the bicarbonate ([Formula: see text]), dissolved silicon (DSi), and other environmental factors within the cascade reservoirs of the lower reaches of Yalongjiang River, passing through the southeastern Qinghai-Tibet Plateau, were systematically analyzed by collecting water samples during the wet season and dry season from 2018 to 2019, respectively. The results showed that the lower ratio of DSi to[Formula: see text] (0.044 ± 0.001) was mainly controlled by the domination of carbonate mineral in the sedimentary rock of the Yalongjiang River drainage basin. The DSi:[Formula: see text] ratio was positively correlated with discharge (P < 0.05), and negatively correlated with the water retention time (P < 0.01) and chlorophyll a, implying that the variations of DSi:[Formula: see text] ratio were mainly determined by the rock chemical weathering processes and the hydrologic process outside the reservoirs and the biological processes within the cascade reservoirs. The phytoplankton photosynthetic process stoichiometrically assimilated DSi and [Formula: see text], resulted in 3.46 × 104 t·Si a-1 and 1.89 × 104 t·C a-1 sequestering in the cascade reservoirs, respectively. Compared with the situation of dam-free in the lower reaches of Yalongjiang River, the export flux of [Formula: see text] and DSi at the mouth of Yalongjiang River was reduced by 11.87% and 62.50%, respectively; the ratio of DSi:[Formula: see text] decreased by 36.01% for only building the Ertan dam and 53.15% for the cascade damming, respectively. The water renewal time prolonged from 45 to 126.6 days due to the regulation of the cascade reservoirs in the mainstream. Ultimately, a conceptual model on migration-transformation of DIC and DSi within the cascade reservoirs in the lower reaches of Yalongjiang River was established. These findings demonstrated that riverine cascade damming could extend the biogeochemical coupling cycle of DIC and DSi within the inland aquatic ecosystems and ensure the ecological environment security in the hot-dry valley.

Hydro-ecological controls on riverine organic carbon dynamics in the tropical monsoon region.

Transport fluxes and properties of riverine organic carbon in the tropical monsoon region were the vital parameters in the global riverine organic carbon fluxes budget. The study focused on the riverine organic carbon in the Changhuajiang River (CHJR), locating at the mid-west of the Hainan Island, China. Dissolved organic carbon (DOC) concentrations in the CHJR ranged from 0.22 mg/L to 11.75 mg/L with an average of 1.75 mg/L, which was lower than the average of global rivers and had a significantly temporal and spatial variation. Output flux of riverine DOC was calculated as 0.55 t/km2/y, which could be revised up to 1.03 t/km2/y, considering that the riverine discharge before dam construction. A linear model of riverine DOC flux suitable in CHJR basin was established, which involved the factors, such as soil organic carbon, runoff depth and slope, etc. There was a large variation of POC concentrations in the CHJR where the average POC concentration in the dry season was 2.41 times of the wet season. Riverine POC flux in CHJR basin was calculated as 1.78 t/km2/y, higher than the average of global rivers and far lower than those in other domestic larger rivers. About 8.28 × 103 t POC were exported yearly in CHJR, of which, 7.15 × 103 t originated from terrestrial ecosystem and 1.13 × 103 t stemmed from aquatic ecosystem. Meanwhile, about 87.74% of terrestrial source happened in the wet season and 12.26% in the dry season. This research revealed that the riverine organic carbon mainly stemmed from the surface erosion processes in the drainage basin during the wet season.

[Biogeochemical processes of the major ions and dissolved inorganic carbon in the Guijiang River].

Within the drainage basin, information about natural processes and human activities can be recorded in the chemical composition of riverine water. The analysis of the Guijiang River, the first level tributary of the Xijiang River, demonstrated that the chemical composition of water in the Guijiang River was mainly influenced by the chemical weathering of carbonate rocks within the drainage basin, in which CO2 was the main erosion medium, and that the weathering of carbonate rock by H2SO4 had a remarkable impact on the water chemical composition in the Guijiang River. Precipitation, human activities, the weathering of carbonate rocks and silicate rocks accounted for 2.7%, 6.3%, 72.8% and 18.2% of the total dissolved load, respectively. The stable isotopic compositions of dissolved inorganic carbon (delta13C(DIC)) indicated that DIC in the Guijiang River had been assimilated by the phytoplankton in photosynthesis. The primary production of phytoplankton contributed to 22.3%-30.9% of particulate organic carbon (POC) in the Guijiang River, which implies that phytoplankton can transform DIC into POC by photosynthesis, and parts of POC will sink into the bottom of the river in transit, which leads into the formation of burial organic carbon.

[Study on heavy metals in soils contaminated by acid mine drainage from Dabaoshan mine, Guangdong].

Mining activities in the Dabaoshan area in the upper reach of the Hengshihe River have caused severe environmental changes, the waste water of milling and refining drained directly into the Hengshihe River, which contaminated the soils along the river severely. It is shown that Pb, Zn, Cd and Cu have contaminated the soil, the Cd contamination was more severe, and the contaminated level of Pb, Zn reached moderately to strongly polluted. The pH value of river and soil affected directly the heavy metals concentration of total and exchangeable ions, and presented negative pertinences. The levels of Pb, Zn, Cu and Cd in the surface soil of Shangbacun village in the lower reach of the river were found as high as 257.762, 350.235, 5.083 and 186.901 mg x kg(-1) respectively, which were relatively higher than those of the background values of soil 1.03, 1.75, 16.9 and 3.7 times respectively, and the result on the soil profiles showed that the contaminations have infiltrated into lower layer soil, ecological environment was harmed severely.

Dynamics of CO2 partial pressure and CO2 outgassing in the lower reaches of the Xijiang River, a subtropical monsoon river in China.

The partial pressure of carbon dioxide (pCO(2)) in surface water was surveyed monthly at 6 sampling sites along the entire length of the lower reaches of the Xijiang River, a subtropical monsoon river in China, and at the mouths of its major tributaries, over a whole hydrological year from April 2005 to March 2006, to reveal the seasonal and spatial dynamics of pCO(2). Intensive sampling and measurements were also conducted at Wuzhou gauge station in June and July to investigate the impact of floodwater on pCO(2) and to further explore the relationship between river discharge and pCO(2). The pCO(2) levels were well above atmospheric equilibrium (380 microatm) during the entire survey period with obvious seasonal and spatial variations, ranging from 600 microatm to 7200 microatm for the mainstream and from 700 to 11000 microatm for tributaries, respectively. The pattern of pCO(2) seasonal variation across 6 sites was almost consistent with each other with little difference. The pCO(2) levels in the dry season were relatively low, with relatively slight temporal and spatial fluctuations that were predominantly controlled by in situ biogenic activities. While the pCO(2) in the wet season greatly varied with river discharge, both annual maximum and minimum pCO(2) levels occurring in this period. The much higher pCO(2) in the early wet season were mainly induced by increasing baseflow and interflow that flushed significant soil CO(2) into the streams, whereas the lower pCO(2) observed after floods from July to September, some even lower than pCO(2) levels in the dry season, potentially resulted from in situ plankton blooms. The annual minima pCO(2) levels occurring in this period were caused by the dilution effect of floodwater. There was no obvious downstream trend in pCO(2) variation during the whole survey period, probably a consequence of disturbance from tributaries or spatially distinct channel characteristics and water environments. Based on measurements, we estimate that the water-to-air CO(2) flux in the lower reaches of the Xijiang River is about 8.3-15.6 Mg C ha(-1)y(-1). The role of the Xijiang River as a net source of atmospheric CO(2) is undoubted.

[Advances in studies on transported flux and properties of riverine organic carbon].

Some lately advances in the study of riverine organic carbon were summarized in this paper. The transported flux of organic carbon from terrestrial ecosystems to the oceans via rivers, which is one of the most sensitive land surface processes in global climate change, has been changed in quantity due to the anthropogenic disturbance to it. The properties of riverine organic carbon, even in the same drainage, changed notably with the changes of hydrological processes in the drainage. Riverine organic carbon may become aged since they have been metabolized by the riverine microbes in the processes of being transported to the seas from the land, i.e., the radiocarbon in the riverine organic carbon was partially degraded by the microbes.