Understanding the main processes affecting the dynamics of dissolved O 2 concentrations in lakes, rivers, and oceans, namely photosynthesis, respiration, and gas-water exchange, is therefore key to evaluate functions of pristine and human-impacted ecosystems. The quantification of contributions of these three processes of O 2 production, consumption, and exchange to the overall O 2 cycling is difficult to achieve from the monitoring and modeling of O 2 concentration dynamics. To that end, information from the ratios of the three stable O isotopes, O, O, and O, as O/ O and O/ O in so-called triple oxygen isotope analysis are increasingly exploited to disentangle contributions of the three main processes of biogeochemical O 2 cycling. In fact, oxygen isotope-based analyses have been successfully implemented in combination with dissolved O 2 dynamics and/or O 2 -Ar ratios to quantify gross photosynthetic O 2 production, net community O 2 production, and photosynthesis rates in the (mixed) surface-layers of various ocean basins. These isotope-based assessment of O 2 formation and consumption are based on the simplified assumption that respiration, that is the formal 4-electron reduction of O 2 to H 2 O (Figure ), is the dominating O 2 sink term. Any other O 2 loss processes have thus far been neglected in the evaluation of O isotope ratios. However, this view is being challenged through evidence showing that in sunlit surface waters, light-dependent O 2 consumption can occur at similar or higher rates than the light-independent (i.e., "dark") respiration of O 2 . Such O 2 consumption by photochemical reactions in marine waters is estimated to be of the same magnitude as photosynthetic O 2 production by phytoplankton. Light-dependent O 2 consumption can occur through biological and abiotic processes.