Biological evaporation effect by phytoplankton on stable isotopic signatures of water

Stable isotope analysis of water (δD and δ18O) is widely used to trace water cycle processes in aquatic ecosystems, based on the established assumption that biological activity does not cause isotopic fractionation of water. Here, we found that phytoplankton photosynthesis elevated ambient water δ18O and δD values, a conclusion supported by a large-scale survey across extensive spatiotemporal environmental gradients and further confirmed by a laboratory culture experiment. This biological effect depended on phytoplankton biomass and light utilization efficiency and could result in a deviation of at least 16.7% in the slope of the water δD-δ18O regression line. We termed this phenomenon the “biological evaporation effect”. Given the potential universality of this effect across aquatic ecosystems, our findings challenge the conventional paradigm in isotope hydrology.