The Dole effect is a way of explaining the inequality between the heavy isotope O and the lighter isotope O. There are different isotopes of oxygen on earth. The atmosphere contains mostly O, the oxygen in seawater is mostly O. This is caused by the respiration of plants and animals. It is balanced by photosynthesis. Photosynthesis emits the same oxygen isotope as that of water. Malcolm Dole first described this effect in 1935. It is named Dole effect today.