“…Fluorescence excitation-emission matrices (EEMs) and UV-visible absorbance spectra were simultaneously measured in a Starna Cells 3-Q-10 quartz cuvette (1-cm pathlength) using a Horiba Scientific Aqualog spectrofluorometer as described in our previous work. 3 Optical indices, such as Napierian absorption coefficients (a) 4 at 254 nm, 280 nm, and 440 nm, [5][6][7] E2:E3 (the ratio of absorption coefficients at 250 and 365 nm; an inverse proxy of dissolved organic matter (DOM) molecular size), 8 spectral slope ratio (SR; the ratio of spectral slope coefficient S275-295 to spectral slope coefficient S290-350; an indicator of environmental processing of DOM), 9 CDOM250-450 (the integrated absorption of chromophoric DOM from 250 to 450 nm), 9, 10 SUVA254 (the specific UV absorbance at 254 nm; a proxy of DOM aromaticity), 11 fluorescence index (FI; an indicator of the source of DOM, which is either microbially derived from bacteria and algae or terrestrially derived from plant litter and soil), 12,13 humification index (HIX; an indicator of the degree of DOM humification), [14][15][16] freshness index (β:α; an indicator of the contribution of recently plant-derived or autochthonous DOM, where β represents more recently produced DOM and α represents more decomposed DOM), [17][18][19] and FDOM (the integrated volumetric fluorescence intensity of fluorescent DOM with excitation wavelengths from 240 to 550 nm and emission wavelengths from 248.2 to 600.9 nm normalized to the Raman peak area of ultrapure water), 20,21 were extracted from the absorbance and fluorescence data using MATLAB R2019a.…”