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.

The Danjiangkou Reservoir is the water source of the middle route of China’s South-to-North Water Diversion Project, encompassing the Dan Reservoir and Han Reservoir. However, little is known about the ecological functions of this important ecosystem. Based on a survey conducted in 2023 in the Dan Reservoir, a mass balance model was constructed using Ecopath with Ecosim 6.6 software to characterize its food web structure and ecosystem properties. The model consisted of 18 functional groups, including producers, consumers, and detritus, covering the entire process of energy flow in the ecosystem. The outputs indicated that the fractional trophic level of functional groups in the Dan Reservoir ecosystem ranged from 1.00 to 3.50. The ecotrophic efficiencies of the main economic fish species were all less than 0.9, and the ecotrophic efficiencies of phytoplankton and detritus were less than 0.5. There were two main food chains: the detritus food chain (39%) and the grazing food chain (61%). The total energy transfer efficiency between trophic levels was only 6.02%, and there was a significant phenomenon of energy transfer blockage between trophic levels II and V. Analysis of the overall characteristics of the ecosystem revealed that the total primary production to total biomass (67.96619), connectance index (0.274), and Finn’s cycling index (2.856) of the Dan Reservoir ecosystem all indicate that the ecosystem is immature, with low nutrient recycling efficiency and poor resistance to external disturbances. This may be related to the low proportion of silver carp and bighead carp in the reservoir and the unreasonable structure of the fish community. Our results suggest that it is necessary to scientifically adjust the structure of the fish community, enhance the proportion of filter-feeding and omnivorous fish to improve the energy flow efficiency, and promote the maturity and stability of the Dan Reservoir.

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.

Rice–crayfish co-culture is a dominant aquaculture mode in China, while the regional heterogeneity in crayfish nutritional quality remains insufficiently clarified. This study collected rice-field-cultured crayfish samples from six major producing provinces (Henan, Hubei, Hunan, Jiangxi, Jiangsu, and Anhui) to systematically evaluate their meat yield, conventional nutritional components, amino acid and fatty acid profiles, and mineral element contents. Significant regional differences were observed in all measured indicators. Jiangsu crayfish showed the highest meat yield (22.97% in females, 20.72% in males), crude protein (18.5%), and total amino acids (16.59 g/100 g). Jiangxi samples had the highest crude fat (0.7%), ash (1.5%), and calcium (1280 mg/kg). Anhui crayfish exhibited the highest polyunsaturated fatty acid (PUFA) content (47.30%). Hubei recorded the highest total eicosapentaenoic acid (EPA) + docosahexaenoic acid (DHA) (22.41%). Henan crayfish were highest in EPA (19.6%) and manganese (3.72 mg/kg), while Hunan samples had the highest iron (9.06 mg/kg). Valine was identified as the first limiting amino acid in all provinces, with secondary limiting amino acids varying regionally. Despite regional differences in specific nutritional indices, crayfish from all six provinces consistently exhibited high protein, low fat, balanced amino acid profiles, abundant unsaturated fatty acids, and rich essential minerals, with all measured nutritional mineral contents within national food safety limits. The findings provided empirical data and a scientific basis for crayfish quality evaluation, origin characteristic analysis, and targeted nutritional optimization of farming formulas, as well as for the differentiated development of regional crayfish industries.