To investigate the spatiotemporal distribution characteristics of nitrogen and phosphorus in the water and sediment of the Danjiangkou Reservoir, the source of the Middle Route of China’s South-to-North Water Diversion Project, we designed a year-long monitoring program. The water and sediment samples were collected from 13 sampling points in the upstream and downstream areas over the year. The results revealed significant spatial heterogeneity in N and P concentrations, with higher levels of total nitrogen, nitrate nitrogen, and nitrite nitrogen in the upstream area compared to the downstream area (p < 0.01). Total phosphorus was also significantly higher in the upstream area (p < 0.05). Seasonal variations were observed, with TN and TP levels peaking in February and August, respectively. The TN:TP ratio indicated a severe P-limited state in most periods, transitioning to a co-limited state of N and P during summer. Sediment analysis showed that TN and TP concentrations were higher in the upstream area, with no significant differences between upstream and downstream on an annual basis, exhibiting strong stoichiometric internal stability. However, seasonal differences were noted, particularly in February and November. This study highlights the complex interactions between water and sediment, emphasizing the role of sediment resuspension, water flow, and seasonal changes in nutrient dynamics. These findings provide a scientific basis for the management and protection of water quality in the Danjiangkou Reservoir, ensuring its role as a critical water source for the South-to-North Water Diversion Project.

To explore the impact of sedimentary nitrogen and phosphorus on Cladophora occurrence, we conducted a microecosystem experiment simulating different nitrogen and phosphorus content as well as nitrogen-to-phosphorus ratios in the sediment. Subsequently, to further explore the specific mechanism of influence that epiphytic algae have on Cladophora, we designed various microsystem culture experiments. These results revealed that an N/P ratio of 40:1 was relatively unfavorable for Cladophora growth. Additionally, there was an extremely significant negative correlation between the benthic cyanobacteria coverage on the sediment surface and the wet weight of Cladophora (p < 0.01), indicating that benthic cyanobacteria could inhibit the growth of Cladophora. Total nitrogen levels in the water column showed a significant positive correlation with phytoplankton biomass (p < 0.05), while benthic cyanobacteria coverage exhibited an extremely significant positive correlation with phytoplankton biomass through phosphorus absorption and nitrogen release (p < 0.01). Metabolite analysis of benthic cyanobacteria identified annotations for 313 metabolites; among them cis,cis-muconic acid (32.48‰), erucamide (9.52‰), phosphoric acid (6.97‰), fenpropidin (6.53‰), and propionic acid (5.16‰) accounted for proportions exceeding 5‰. However, none of these metabolites have been recognized as allelochemicals or toxins at present. This study provides novel insights into controlling Cladophora occurrence by considering sediment nutrients, including nitrogen and phosphorus, along with allelochemicals.

To evaluate the effects of filter-feeding fishes on water quality and bacterial community in the rice–crayfish coculture system, four different stocking densities of bighead carp (0, 500, 1000, 1500 ind./200 m²) were set up in rice–crayfish coculture systems. Water samples in the systems were collected biweekly to detect dissolved oxygen (DO), temperature (T), potential of Hydrogen (pH), ammonia nitrogen (NH₄⁺-N), nitrite nitrogen (NO₂⁻-N), nitrate nitrogen (NO₃⁻-N), total nitrogen (TN), total phosphorus (TP), and Chlorophyll-a (Chl-a); the bacterial community in the water was analyzed simultaneously, then the correlation between water quality and microorganisms were studied. The results showed that concentrations of TN, TP, NO₂⁻-N, and NH₄⁺-N decreased while DO and NO₃⁻-N increased along with the breeding process. NO₂⁻-N, NO₃⁻-N, TN, and NH₄⁺-N were important environmental factors affecting the bacterial community structure in water (p < 0.05). Bighead carp stocking had an impact on the diversity, richness, and evenness of the bacterial communities in the systems. The dominant bacteria in the four different carp density groups were Proteobacteria, Actinomycetes, Bacteroidetes, and Cyanobacteria. Bighead carp increased the abundance of Bacteroidea but reduced that of Actinomycetes, Cyanobacteria, and Proteobacteria. The introduction of bighead carp promoted the conversion of nitrogen and phosphorus, reducing the risk of cyanobacterial blooms. Group 1000 ind./200 m² exhibited the best effect on the removal of nitrogen and phosphorus from the water body.

Interspecifically induced phytoplankton colony formation, which is accompanied by increased particle size and sedimentation, plays important roles in enhancing anti-predation capability and alleviating competition among photosynthetic organisms. Induced morphological changes may indirectly affect the structure of food webs and thus influence ecosystem functions. In this study, the effects of microcystin-LR (MC-LR) on colony formation of Chlorella vulgaris under induction by the submerged macrophyte Potamogetom crispus were evaluated. The growth of C. vulgaris was significantly inhibited under P. crispus stress (10g FW L⁻¹), and the adverse influences of extracts were considerably greater than those of exudates. In addition, the cell numbers per colony and colony proportion of C. vulgaris were significantly increased with the presence of P. crispus extracts. However, in contrast to our expectation, the addition of MC-LR exerted no significant effects on the growth and morphological changes of C. vulgaris under P. crispus induction. The present research results provide additional knowledge on interspecific interactions between submerged macrophyte and green algae in the eutrophic cyanotoxin-contaminated ecosystems.

To explore the interaction between aquatic organisms and the environment in the Danjiangkou Reservoir, the source reservoir for the Middle Route of China’s South-to-North Water Diversion Project, this study analyzed the ecological stoichiometric characteristics of carbon, nitrogen, and phosphorus in main species and their influencing factors. The results revealed that there were no significant differences in the contents of nitrogen and phosphorus in fish between the upstream and downstream areas. Compared to omnivorous and filter-feeding species, carnivorous fish in both the upstream and downstream areas displayed elevated nitrogen and phosphorus contents alongside reduced carbon content. The phytoplankton and aquatic plants showed significant differences in nitrogen content between areas(P < 0.05), but no significant differences were observed in the nitrogen and phosphorus contents of other biological groups between the upstream and downstream reservoir areas. The nitrogen and phosphorus stoichiometric characteristics of biological components in the Danjiangkou Reservoir showed strong homeostasis. A further correlation analysis was conducted between the nitrogen and phosphorus stoichiometric characteristics of fish and water quality including TN, NH₄⁺-N, NO₃⁻-N, NO₂⁻-N, and TP. The results showed that there was no correlation between the nitrogen and phosphorus stoichiometric characteristics of fish and water quality in the upstream reservoir area. In the downstream reservoir area, the N: P in fish was significantly positively correlated with NO₂⁻-N (P < 0.05) and extremely significantly negatively correlated with NH₄⁺-N (P < 0.01). The study demonstrated that the fish stoichiometry in large deep-water reservoirs was primarily shaped by species identity and feeding ecology, with limited influence from spatial heterogeneity in water quality. These findings provided critical insights into nutrient cycling and ecosystem stability, supporting sustainable management of this vital water diversion source.