Biological, light-dependent O 2 loss takes place through reduction of O 2 initiated by excess O 2 consumption is shown here exemplary for processes induced by light-absorption and intersystem crossing of chromophoric dissolved organic matter (CDOM) to its excited triplet states ( 3 CDOM * ). C-H and C • stand for organic compounds and carbon-centered radicals, respectively. Reactions of 3 CDOM * with 3 O 2 act as source of 1 O 2 (red arrow). O 2 loss processes contributing to photochemical O 2 consumption (blue arrows) include oxygenation of organic compounds and one-electron reduction, for example, with phenols (R-OH) and organic amines (R-NH 2 ), to O 2