Dissolved　organic　matter　(DOM)　is　very　important　in　aquatic　environments,　yet　it　is　challenging　to　characterize　DOM　as　a　highly　complex　mixture　of　thousands　of　molecules,　and　the　knowledge　of　the　effects　of　different　degradation　processes　on　different　molecules　remains　limited.　This　study　examined　the　distribution　and　degradation　of　DOM　in　a　large　subtropical　river　using　optical　techniques　and　Fourier　transform　ion　cyclotron　resonance　mass　spectrometry　(FT-ICR-MS).　At　the　molecular　level,　DOM　was　mainly　composed　of　CHO　and　lignin-like　compounds,　which　was　related　to　the　dominance　of　forestland　in　the　watershed　and　resulted　in　a　low　biological　lability　index　(MLBL).　The　modified　aromaticity　index　(AImod),　unsaturation　degree　(DBE),　and　humic　content　(HIX)　decreased　while　MLBL,　H/C,　absorption　spectral　slope　(S275–295),　and　biological　index　(BIX)　increased　in　the　estuarine　zone　due　to　the　increasing　autochthonous　contribution.　Photo-　and　microbial　degradation　resulted　in　a　similar　decrease　in　the　bulk　dissolved　organic　carbon,　while　they　showed　opposite　effects　on　the　DOM　composition.　Photo-degradation　removed　all　fluorescent　components　and　decreased　molecular　weight,　HIX,　AImod,　DBE,　%CHO,　%lignin-like,　%tannin-like,　and　%condensed　aromatic-like　compounds.　In　contrast,　bio-degradation　preferentially　consumed　lipid-like,　protein-like,　and　carbohydrate-like　compounds,　with　increases　in　%lignin-like,　%tannin-like,　%condensed　aromatic-like　compounds,　and　humic-like　fluorescent　components.　Overall,　the　application　of　ultra-high　resolution　mass　spectrometry　provided　valuable　insights　into　the　composition　and　behavior　of　DOM　at　the　molecular　level　and　revealed　the　contrasting　effects　of　photo-　and　microbial　degradation　on　different　compounds.　These　results　have　implications　for　better　understanding　the　composition　and　transformation　of　aquatic　DOM.　©　2024