The Dole effect, named after Malcolm Dole, describes an inequality in the ratio of the heavy isotope O (a "standard" oxygen atom with two additional neutrons) to the lighter O, measured in the atmosphere and seawater. This ratio is usually denoted δO.
It was noticed in 1935 that air contained more O than seawater; this was quantified in 1975 to 23.5‰, but later refined as 23.88‰ in 2005. The imbalance arises mainly as a result of respiration in plants and in animals. Due to thermodynamics of isotope reactions, respiration removes the lighter—hence more reactive—O in preference to O, increasing the relative amount of O in the atmosphere.
The inequality is balanced by photosynthesis. Photosynthesis emits oxygen with the same isotopic composition (i.e. the ratio between O and O) as the water (HO) used in the reaction, which is independent of the atmospheric ratio. Thus when atmospheric O levels are high enough, photosynthesis will act as a reducing factor. However, as a complicating factor, the degree of fractionation (i.e. change in isotope ratio) occurring due to photosynthesis is not entirely dependent on the water drawn up by the plant, as fractionation can occur as a result of preferential evaporation of  and other small but significant processes.