Microplastics contamination in the surface water of the Yangtze River from upstream to estuary based on different sampling methods.

In this study, we investigated microplastic contamination of the Yangtze River from the upper reaches to the estuary using different sampling methods to understand extensive information on microplastic pollution. The microplastic samples were collected at 10 sites using two conventional methods: trawling and filtering water. The results showed that the average abundance of microplastics ranged from 1.62±0.61 × 105 to 4.25±3.87 × 106 items/km2 (trawling samples) and 800.0±300.0 to 3088.9±330.6 items/m3 (filtering water samples). The average abundance (by trawling) in the Three Gorges Reservoir (2.80±1.86 × 106 items/km2) was one order of magnitude higher than that of other sections, which affirmed the barrier effect of dams on microplastic distribution. The barrier effect was more obvious on larger size particles by comparing the results of two methods. The abundances near the left, right banks, and in the midstream showed no significant difference by both two methods, illustrating that sampling at each bank along the Yangtze River was also representative in one section. Characteristics analysis demonstrated that fragments (47.9%) dominated in trawling samples and fiber (63.4%) dominated in filtering water samples. Microplastics of small sizes (<1 mm) and transparent were dominant in samples collected by the two methods. Polyethylene (PP) and polypropylene (PE) were the dominant polymer types in the detected microplastics. In this study, we provided detailed information on microplastic pollution of the Yangtze River from the upstream to the estuary, which is useful for microplastic management and control in this area.

[Spatial Distribution and Risk Assessment of Heavy Metals in Surface Sediments from the Middle and Upper Reaches of the Yangtze River].

The hydrological regime of rivers significantly changes after dam impoundment, which in turn affects the particle composition and heavy metal fractions of the river sediments. From June to July 2019, the sediments from 26 sampling sections were collected along the main stream of the Yangtze River from Panzhihua City in the upper reaches of the Yangtze River to Hukou City in the middle reaches of the Yangtze River. The concentrations and fractions of As, Cd, Co, Cr, Cu, Ni, Pb, and Zn were measured using the BCR three-step extraction procedure. The pollution status and potential ecological risk of heavy metals in sediments were evaluated using the geo-accumulation index, the sediment quality guideline, and the risk assessment coding method (RAC). The results showed that the average particle size of sediments in the upper reaches of the Yangtze River (Jinsha River cascade reservoir section and the Three Gorges reservoir section) decreased from upstream to downstream, the total concentrations of As and Zn increased, and the variation trend in the middle reaches was not obvious. The content of clay particles was significantly positively correlated with the acid-soluble fraction concentrations of Cd and Ni. Cd was mainly in the residual fraction (59.26%) and acid-soluble fraction (24.67%). Large parts of Cr and Ni were residual fractions accounting for 92.41% and 83.41%, respectively. As, Co, Cu, Pb, and Zn were mainly in the residual fraction and the reducible fraction. The order of decrease for the pollution degree (Igeo) of As, Cd, Co, Cr, Ni, and Zn was the Jinsha River, the middle reaches of the Yangtze River, and the Three Gorges Reservoir. The decrease order of bioavailability (RAC) of Cd, Co, Cr, Cu, Ni, and Zn was the Three Gorges Reservoir, the Jinsha River, and the middle reaches of the Yangtze River. The bioavailability of As and Pb decreased in the order of the middle reaches of the Yangtze River, the Three Gorges, and the Jinsha River. According to the classification of the RAC, Cd in the Three Gorges Reservoir area exhibited a high risk with the RAC accounting for 48.44%. Cu, Ni, and Zn showed a low or medium risk.

Lateral and longitudinal variation in phosphorus fractions in surface sediment and adjacent riparian soil in the Three Gorges Reservoir, China.

Hydrological regimes have been significantly altered since the Three Gorges Dam (TGD) raised the water level of the reservoir to the maximum design level of 175 m in October 2010. This change might greatly influence the forms of phosphorus (P) in the sediment and the adjacent riparian soil. The purpose of this study was to reveal the lateral (sediment, water-level-fluctuation zone soil, and upland soil) and longitudinal (from the end of backwater area to the TGD) trends in P factions. Samples from 11 sites located along the main stem and ten sites located along eight tributaries were collected in June 2017. The P fractions were determined using the Standards, Measurements, and Testing (SMT) protocol. The results showed that the order of increase for average pH values was sediment (7.58 ± 0.62), WLFZ soil (7.44 ± 0.29), and adjacent upland soil (7.20 ± 0.68). The total organic carbon in the sediment was also highest with an average of 9.15 ± 2.97 mg·g-1. The average concentrated HCl-extractable P (total P), organic P (OP), inorganic P (IP), HCl-extractable P (HCl-P), and NaOH-extractable P (NaOH-P) were 630.02 ± 212.24, 161.89 ± 90.77, 468.13 ± 194.92, 335.65 ± 159.88, and 51.40 ± 36.20 mg·kg-1, respectively. The concentration of both total P and NaOH-P in the sediment of the main stem exhibited an increasing trend from the backwater area to the TGD. The average concentration of P species in the sediment was higher than those in the upland soil and the water-level-fluctuation zone (WLFZ) soil. For all the sediment and soil samples, the rank order of P species concentrations was HCl-P > OP > NaOH-P. Both IP and HCl-P were highly positively correlated with total P in the upland soil, the WLFZ soil and the sediment. However, only in the sediment, NaOH-P was positively correlated with total P and OP. All P species in the upland soil demonstrated greater spatial heterogeneity than those in the WLFZ soil and the sediment. Redundancy analysis revealed that the main variables explaining the variance in P species concentrations were Al in the upland soil and pH in the sediment.

[Effects of Flooding and Drying on the Transformation of Soil Inorganic Phosphorus in the Water-Level-Fluctuating Zone of the Three Gorges Reservoir, China].

The implementation of the Three Gorges Project formed a water-level-fluctuating zone, and flooding and drying affects the soil properties and water quality of the Water-Level-Fluctuating Zone. The laboratory simulation or field sampling were conducted; however, the results cannot reflect the real conditions in practice. The effects of flooding and drying on the physical and chemical properties of soil and the transformation characteristics of inorganic phosphorus were studied in order to provide a theoretical reference for soil phosphorus loss and water eutrophication in water-level-fluctuating zone of the Three Gorges Reservoir. To investigate the conversion of soil phosphorus, plastic pots with soil were suspended at different depths (0, 2, 5, and 15 m) and submerged for 30, 60, and 180 d, and exposed for 180 d. The effects of submerged depth and time on the soil physical and chemical properties and inorganic phosphorus forms were studied. The results showed that soil pH, organic matter, total phosphorus, and available phosphorus decreased and then increased subsequently during the flooding period. After exposure for 180 d, soil pH, organic matter, and total phosphorus content decreased, while available phosphorus content increased. After flooding, the ratio of various forms of inorganic phosphorus to total phosphorus was Fe-P > Al-P > Ca8-P > Ca2-P. The content of Ca2-P and Ca8-P decreased at 0 m and 2 m and decreased at 5 m and 15 m and then increased with increased flooding time. After exposure for 180 days, the content of inorganic phosphorus increased significantly, and the content of inorganic phosphorus decreased as flooding depth increased. Al-P content increased with the flooding time, but there was no obvious change with flooding depth. The content of Fe-P did not change with the time and the depth of flooding.

The Effect of Different Habitat Types and Ontogenetic Stages on the Diet Shift of a Critically Endangered Fish Species, Coreius guichenoti (Sauvage and Dabry de Thiersant, 1874).

This study examined the effect of habitat types and ontogenetic stages on the diet shift of Coreius guichenoti (Sauvage and Dabry de Thiersant, 1874), a critically endangered fish species. Based on the stable isotope analysis method, the following was explored: the variations in δ13C and δ15N values, isotopic niche width and four basal food sources (Mollusks, Macrocrustaceans, Aquatic insect larvae and particulate organic matters (POMs)) among three essential habitat types (the spawning ground, natural riverine feeding and nursery area, and Three Gorges Reservoir area) and between two ontogenetic stages (immature and fully mature stages). A diet shift associated with habitat type changes was observed, but there were no obvious differences in diet composition between the two ontogenetic stages. Dietary plasticity and a preference for specific foods were the important determinants of feeding behavior through the life history of this species. POM was important for the survival of this species in the resource-limited spawning ground, but this species fed more heavily on higher-order consumers in resource-abundant areas. This study highlights the importance of maintaining free connectivity among different habitats (particularly spawning grounds) to ensure the long-term sustainability of potamodromous fish species as well as the full investigation of all types of critical habitats for understanding the trophic ecology of a single fish species.

[Phosphorus Forms and Distribution Characteristics in the Sediment and Soil of the Water-Level-fluctuating Zone in the Main Stream of the Three Gorges Reservoir].

Hydrological regimes changed after the Three Gorges Reservoir (TGR) became fully operational, with the water level fluctuating between 145 m and 175 m. This has altered phosphorus (P) distribution within sediments and soils of the water-level-fluctuating zone (WLFZ). Eleven field investigations within the main stream were carried out in June 2016. The aim of the study was to determine lateral (sediment and WLFZ soil) and longitudinal variations (from the end of the backwater area to the Three Gorges Dam) of phosphorus species. P fractions were analyzed using a Standards, Measurements, and Testing (SMT) protocol. Results showed that both TP and NaOH-P content in sediment exhibited a clear increasing trend from the end of the backwater area to the Three Gorges Dam, while HCl-P declined. Average of totals P, OP (organic P), HCl-extracted P (HCl-P, calcium-bond P), and NaOH-extracted P (NaOH-P, metal oxide-bound P) in sediment were (859.6±106.8), (224.6±113.9), (435.3±77.7), and (101.5±31.6) mg·kg-1, respectively. The concentration average of P species in sediment was higher than in WLFZ soil. In both sediment and soil, HCl-P was the main form of P, accounting for 51.3% and 58.2% of TP, while the ratio of NaOH-P to TP was 11.7% and 8.1%, respectively. P fractions with a higher coefficient of variation had greater spatial heterogeneity.

Use of fish species from different trophic levels to control algae and water quality: An enclosure experiment in eutrophic area of Xiaojiang River.

The effects of stocking both filter-feeding fish and piscivorous fish were compared to the effects of stocking only filter-feeding fish for suppressing algal blooms and improving water quality in the impoundment area of Xiaojiang River where catfish were dominant. Using only filter-feeding fish for algal suppression and water quality control was more effective in the short-term, but use of both filter-feeding fish and piscivorous fish was better in the long-term. Obvious suppression of phytoplankton biomass (PB) only occurred during the first 14 days regardless of the fish stocked. Adding fish to the enclosure clearly alters phytoplankton community structure and introducing piscivorous fish to an enclosure stocked with filter-feeding fish changed the relative densities of dominant algae species. While stocking filter-feeding fish decreased total nitrogen concentration by removing phytoplankton, it did not effectively decrease total phosphorus and Chlorophyll a concentrations. Introducing piscivorous fish to the enclosure weakened the relationship between nutrients and phytoplankton. Results indicate that stocking only filter-feeding fish to improve water quality and suppress phytoplankton in an impoundment area is insufficient and other technologies and means should be applied simultaneously.

Plant community characteristics and their responses to environmental factors in the water level fluctuation zone of the three gorges reservoir in China.

The annual water level regulating of the Three Gorges Reservoir prolonged the submerged duration (from 2 to 8 months) and resulted in the reversal of natural flood rhythms (winter submerged). These changes might alter plant community characteristics in the water level fluctuation zone (WLFZ). The aim of this study was to determine the plant community characteristics in the WLFZ and their responses to the environmental factors (i.e., annual hydrological regulation, topographic characteristics, soil physical properties and soil nutrients). The height, coverage, frequency and biomass of each plant species and the soil properties at each elevation zone (150, 155, 160, 165 and 170 m) were measured from March to September in 2010. Univariate two-factor analysis and redundancy analysis (RDA) were used to analyze the spatial and temporal variations of the community characteristics and identify the key environmental factors influencing vegetation. We found that 93.2 % of the species analysed were terrestrial vascular plants. Annual herbs made up the highest percentage of life forms at each altitude. The differences in the species number per square metre, the Shannon-Wiener diversity index and the biomass of vegetation demonstrated statistical significance with respect to sampling time but not elevation. The most dominant species at altitudes of 150, 155, 160, 165 and 170 m were Cynodon dactylon, Cyperus rotundus, Digitaria sanguinalis, Setaria viridis and Daucus carota, respectively. The concentrations of soil nutrients appeared to be the lowest at an altitude of 150 m, although the differences with respect to elevation were not significant. The results of the RDA indicated that the key factors that influenced the species composition of vegetation were elevation, slope, pH and the concentration of soil available phosphorus.