Dissolved oxygen in water and its stable isotope effects: A review

Dissolved oxygen (DO) is one of the most commonly measured parameters in aqueous studies, however its sources and sinks such as atmospheric exchange, photosynthesis or respiration often remain unknown. A large number of studies have combined concentration measurements with investigations of stable isotope ratios (18O/16O and 17O/16O) of DO that are expressed as per mille [‰] deviation versus international standards. These isotope shifts can provide important additional information about oxygen sources and sinks, aqueous productivity and also of turnover rates of related material such as carbon. This review provides an overview of oxygen isotope systematics and the Dole effect that is still unresolved and explores the discrepancy between the isotope composition of ocean water and atmospheric O2. Here we summarize the most relevant processes of influence on the isotope composition of DO, including gas-water exchange, photosynthesis and respiration. The latter is known to exert the strongest isotope effects with isotope discriminations of up to − 29‰. New developments suggest that they can be cancelled out with recent applications of a 17O excess method for the evaluation of aqueous productivity. Furthermore, variations of DO stable isotope ratios can serve as a valuable additional tracer in a wide range of biogeochemical and aqueous studies. Among aquatic systems lakes, estuarine and shallow groundwater environments are the most challenging reservoirs as they usually expose steep oxygen gradients. So far causes and effects of these gradients remain often unexplored and could be revealed with DO isotope methods. Particularly in groundwater, additional oxygen sources from the soil zone may be quantified with new oxygen isotope signals that were influenced by diffusion. Future investigations of groundwater-surface water interactions and of ecological functions can benefit from this method. Other new applications include investigations of mineral oxidations and of engineered approaches such as photo-bioreactors.