2.3.1. DOC and Chromophoric Dissolved Organic Matter (CDOM) Measurement

For the analysis of DOC concentration, 40 mL of water sample was filtered through pre-combusted (450 ◦C for 4 h) 0.22-μm-pore-size glass microfiber filters (47-mm diameter, Shanghai Xingya Purification Material Factory, Shanghai, China). Then, the filtrates were collected with glass pipettes, placed into pre-combusted brown glass vials, and stored at −20 ◦C until analysis. Reference DOC standards (obtained using potassium hydrogen phthalate) served as an additional control to calibrate the instrument. The blank was deducted using Milli-Q water analysis before every five samples, and all samples (including the blank) were acidified to pH = 2 by 10% HCl until analysis. The average blank concentrations associated with the DOC measurement were approximately 0.06 mg L−1, and the analytic precision of the triplicate injections was ±3%.

After passing through pre-combusted 0.22-μm Millipore membrane filters (47-mm diameter, Merck Millipore, Cork, Ireland), approximately 100 mL filtrate was used for CDOM absorbance and fluorescence measurements. CDOM absorbance was measured over the 200–800 nm range (1 nm increments) in a 5-cm quartz cell using a Shimadzu UV-2450 PC UV–vis recording spectrophotometer. A fluorescence spectrophotometer (Hitachi F-7000, Tokyo, Japan) with a scanning speed of 2400 nm min−<sup>1</sup> was employed to measure the excitation-emission matrix (EEM). The main components of fluorescence dissolved organic matter (FDOM), i.e., the fluorescent fraction of CDOM, were analyzed using a fluorescence spectroscopy technique coupled with parallel factor (PARAFAC) analysis [47]. During the experiments, a total of 150 EEM spectra were obtained for PARAFAC analysis. MATLAB (MathWorks, Natick, MA, USA) and the DOM Fluor toolbox (http://www.models.life. ku.dk/ accessed on 6 May 2022) were employed for data analysis [48,49]. Further details on the PARAFAC-EEM analysis of FDOM have been described previously [50] and are displayed in Supplementary Materials Text S1.

During analysis of the composition parameters of CDOM, the characteristic parameters of the CDOM absorption coefficient [a (355)] and spectral slope for the interval of 300–500 nm [S300–500] were employed to estimate the concentration and composition dynamic of CDOM, respectively [47]. Moreover, high S300–500 values denote a high extent of recalcitrant and a low degree of molecular weight [51].

### 2.3.2. Nutrient Concentrations

Inorganic nitrogen (i.e., nitrate, nitrite, and ammonium) and inorganic phosphorus (i.e., phosphate) concentrations were measured using a continuous flow analyzer (San + +, SKALAR, Breda, The Netherlands). Total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) concentrations were analyzed using combined persulfate digestion followed by spectrophotometric measurements [52].
