Water oxygen isotopic analysis is a useful tool for tracing water cycle in aquatic ecosystems on the premise that it is no oxygen isotope effect from phytoplankton activity. We measured the water hydrogen and oxygen isotopic compositions, phytoplankton Fv/Fm and related environmental factors in China's wetlands.

We found that phytoplankton photosynthesis can modify water oxygen isotopic signature (δ¹⁸O), based on a national-scale survey of wetlands and a culture experiment. Phytoplankton photosynthesis can make water δ¹⁸O more negative, and this biological effect counteracts part of the evaporation effect on water δ¹⁸O. This effect depends on phytoplankton biomass and light utilization efficiency and could be overwhelmed by the evaporation effect when the biomass is small.

The biological effect may be universal in aquatic ecosystems, and thus this study refreshes the traditional understanding on water oxygen isotope biogeochemistry.