Shahe Reservoir is a key node in the upstream of the North Canal, and the water quality has gradually improved after the implementation of low water operation in 2018. The organic matter(OM) in the sediment decreased from 16.66 to 14.22%.In this study, the FI index and parallel factor method were used to investigate the traceability of OM and dissolved organic matter (DOM) in the Shahe Reservoir before and during low water level operation(LWLO), and the results showed that the terrestrial source fraction of OM in sediments was mainly related to organic-rich terrestrial plant residues carried by tributaries and overflows/outfalls during the rainy season, and the FI index indicated that the organic matter (OM) in the Shahe Reservoir before and during LWLO in each DOM in the area is derived from authigenic sources of autotrophic microorganisms, algae, etc. The parallel factor method shows that more than most of the pollutants in the DOM are input from endogenous sources and a small proportion of pollutants are input from exogenous sources. Nutrients in both sediment and interstitial water increased during the LWLO, with TN and TP levels increasing by 262.38 and 204.45 mg·kg⁻¹ in sediment, NH₄⁺–N, PO₄³⁻−P, TN and TP in interstitial water increasing by 0.98, 1.36, 2.07 and 4.33 mg·L⁻¹, respectively. Pearson correlation analysis and principal component analysis showed that OM was significantly correlated with nutrients: OM and TN (p < 0.01) and OM and TP (p < 0.05) in the pre-LWLO; OM and TN and TP (p < 0.01) in the LWLO.The results suggested that organic matter pollution control should be mainly carried out from the perspective of endogenous input, focusing on controlling the release of nutrients in sediments.

This research analyzed the structure of microbial groups in the sediment–water interface (SWI) and the relationship between the gene expression levels of two typical pathogenic bacteria Escherichia coli and Enterococcus, and nutrient levels using modern biological techniques. The nutrient distribution at the SWI revealed significantly higher nutrient content in the sediment compared to the overlying water. According to Fick’s first law, the release flux indicated that PO₄³⁻-P in the upper reaches of the reservoir was deposited from the overlying water, while the release rate of NH₄⁺-N, in addition to sedimentation, was significantly greater than that of PO₄³⁻P. The microbial community structure was primarily dominated by the genera Methyloparacoccus, Methylomonas, and Arenimonas. The abundances of E. coli and Enterococcus were higher in the surface sediment than in the overlying water. Pearson correlation analysis demonstrated that E. coli had a significant positive correlation with total nitrogen (TN) (p < 0.05) and total phosphorus (TP) (p < 0.05), whereas Enterococcus had a very significant positive correlation with TN and TP (p < 0.01).