Sulfur Isotope and Stoichiometry–Based Source Identification of Major Ions and Risk Assessment in Chishui River Basin, Southwest China

Hydrochemistry and sulfur isotope (δ34S–SO42−) of Chishui River watershed in Southwest China were measured to identify the sources of riverine solutes, the potential impact of human activities, water quality, and health risk. The main findings indicated that the HCO3− (2.22 mmol/L) and Ca2+ (1.54 mmol/L) were the major ions, with the cation order of Ca2+ (71 ± 6%) > Mg2+ (21 ± 6%) > Na+ + K+ (8 ± 3%) and the anion sequence of HCO3− (55 ± 9%) > SO42− (41 ± 9%) > Cl− (4 ± 3%). The riverine δ34S–SO42− values fluctuated from −7.79‰ to +22.13‰ (average +4.68‰). Overall, the water samples from Chishui River presented a hydrochemical type of Calcium–Bicarbonate. The stoichiometry and PCA analysis extracted three PCs that explained 79.67% of the total variances. PC 1 with significantly positive loadings of K+, Mg2+, F−, HCO3− and relatively strong loading of Ca2+ revealed the natural sources of rock weathering inputs (mainly carbonate). PC 2 (Na+ and Cl−) was primarily explained as atmospheric contribution, while the human inputs were assuaged by landscape setting and river water mixing processes. The strongest loadings of SO42− and NO3− were found in PC 3, which could be defined as the anthropogenic inputs. The H2SO4–involved weathering processes significantly impacted (facilitated weathering) the concentrations of riverine total ions. Sulfur isotope compositions further indicated that riverine SO42− were mainly controlled by anthropogenic inputs SO42− compared to the sulfide oxidation derived SO42−, and the atmospheric contribution was very limited. The results of risk and water quality assessment demonstrated that Chishui River water was desirable for irrigation and drinking purposes due to low hazard quotient values (<1, ignorable risk), but long–term monitoring is still worthy under the circumstances of global environmental change.

Food Web Structure and Trophic Interactions Revealed by Stable Isotope Analysis in the Midstream of the Chishui River, a Tributary of the Yangtze River, China

Understanding energy flow and nutrient pathways is crucial to reveal the dynamics and functions of riverine ecosystems and develop appropriate conservation strategies. In this study, we utilized stable isotopes of δ13C and δ15N to examine the fundamental characteristics of trophic position, trophic niche, and carbon source for the food web in the midstream of the Chishui River, a tributary to the Yangtze River. Our results showed that stable isotope signatures among different sorts of basal resources and consumers were significantly distinguishable and that the food chain consisted of four trophic levels, indicating the multiple trophic pathways and long food chain length here. The trophic guilds of fish were classified into four categories, in which herbivorous and carnivorous fish showed greater trophic diversity and omnivorous fish had higher trophic redundancy, which meant that there was a stable trophic niche structure in the study area. Phytoplankton and periphyton presented the largest contributions to consumers, indicating that autochthonous productivity was the dominant carbon source in the midstream of the Chishui River. Since the Chishui River is still in a natural condition without any dam constructions, the autochthonous productivity, stable trophic niche structure, multiple trophic pathways and long food chain length found here demonstrate its high conservation value. Therefore, the strategy to refrain from damming on this river should persist into the future.

Identification and Assessment of Ecological Sensitivity in Chishui River Basin Based on GIS

In this paper, GIS and RS spatial analysis techniques were used to establish a system for identifying sensitive targets of terrestrial ecosystems in Chishui River Basin, and to identify protection targets of terrestrial ecosystems in different regions. The result shows that the assessment of habitat sensitivity in Chishui River Basin was dominated by non-sensitive areas and highly sensitive areas, which highly sensitive areas account for 60.43% of the basin area, mostly concentrated in the mountainous areas at the junction of Chishui City, Xishui County and Gulin County; non-sensitive areas account for 39.13% and are mosaically distributed with highly sensitive areas; mildly sensitive areas, moderately sensitive areas and extremely sensitive areas account for a total of about 0.5% of the basin area and are more scattered throughout the basin. The distribution is scattered. The moderately sensitive areas in the Chishui River Basin are more than 60.7% of the basin area, indicating that the habitats in the Chishui River Basin are generally sensitive and the ecologically sensitive targets are mainly wooded areas, which need to be protected.