## 2 Materials and Methods
### C and N Content and Isotope Ratio Analysis of Sedimentary Organic Matter  
The sediment sample (0.5 g) was transferred into a clean 15 mL polyethylene centrifuge tube, followed by the addition of 10 mL of 2.0 mol\(\cdot\)L\({}^{-1}\) KCl, shaking using a shaker, centrifugation, and supernatant removal. Following this, 10 mL of 1.0 mol\(\cdot\)L\({}^{-1}\) HCl was added to the centrifuge tube, followed by first sonication in an ultrasonic oscillator and oscillation in a regular oscillator. According to the different inorganic carbon removal requirements of the different samples, multiple acid washes were performed until no bubbles were observed upon adding the HCl solution to the sample. The following day, the centrifuge tube was placed in a centrifuge, and deionized water was used to replace the HCl solution. The above steps were repeated more than four times until the pH of the supernatant approached neutrality. Subsequently, the sample in the centrifuge tube was  
Figure 1: Location of the study area and sampling sites.  
freeze-dried using a freeze dryer. After grinding, the sample was stored in a desiccator for subsequent analysis.  
The TOC and TON contents were measured using an element analyzer (Flash EA2000, Thermo, Waltham, MA, USA). The organic isotopic carbon and nitrogen composition (\(\delta^{13}\)C and \(\delta^{15}\)N, respectively) of the sediments were measured using an elemental analyzer (Flash EA112) and an isotopic mass spectrometer (MAT253) in combination [28]. The \(\delta^{13}\)C and \(\delta^{15}\)N were expressed as the differences between the isotopic ratios of a sample to the standard materials, and the error of instrumental analysis was \(<\)0.1%.  
\[\delta^{13}\mathrm{C_{sample}}=\{(\mathrm{{}^{13}C}/\mathrm{{}^{12}C_{sample}}) /(\mathrm{{}^{13}C}/\mathrm{{}^{12}C_{standard}})-1]\times 1000 \tag{1}\]  
\[\delta^{15}\mathrm{N_{sample}}=\{(\mathrm{{}^{15}N}/\mathrm{{}^{14}N_{sample}} )/(\mathrm{{}^{15}N}/\mathrm{{}^{14}N_{standard}})-1]\times 1000 \tag{2}\]  
where \(\mathrm{{}^{13}C}/\mathrm{{}^{12}C_{sample}}\), \(\mathrm{{}^{13}C}/\mathrm{{}^{12}C_{standard}}\), \(\mathrm{{}^{15}N}/\mathrm{{}^{14}N_{sample}}\), and \(\mathrm{{}^{15}N}/\mathrm{{}^{14}N_{standard}}\) are the C and N isotope ratios in the samples and standards, respectively. The standard substances for C and N isotope determination were Vienna PDB and atmospheric N\({}_{2}\).  
A three-end member model was used to calculate the relative contributions of terrestrial, phytoplankton, and sewage sources. The equations for the ternary mixing of \(\delta^{13}\)C, \(\delta^{15}\)N, and C/N ratios are as follows:  
\[\delta^{13}\mathrm{C_{sample}}=\mathrm{f1}\times\delta^{13}\mathrm{C_{f1}}+ \mathrm{f2}\times\delta^{13}\mathrm{C_{f2}}+\mathrm{f3}\times\delta^{13} \mathrm{C_{f3}}, \tag{3}\]  
\[\delta^{15}\mathrm{N_{sample}}=\mathrm{f1}\times\delta^{15}\mathrm{N_{f1}}+ \mathrm{f2}\times\delta^{15}\mathrm{N_{f2}}+\mathrm{f3}\times\delta^{15} \mathrm{N_{f3}}, \tag{4}\]  
\[\mathrm{C/N_{sample}}=\mathrm{f1}\times\mathrm{C/N_{f1}}+\mathrm{f2}\times \mathrm{C/N_{f2}}+\mathrm{f3}\times\mathrm{C/N_{f3}}, \tag{5}\]  
\[\mathrm{f1}+\mathrm{f2}+\mathrm{f3}=1 \tag{6}\]  
where the \(\delta^{13}\)C and C/N of the samples are used to analyze the source of C in sedimentary organic matter, and the \(\delta^{15}\)N and C/N of the samples are used to analyze the source of N in sedimentary organic matter. f1, f2, and f3 are their percentage contributions (%) to the total contents of C and N in sedimentary organic matter.