Revising Estimates of Aquatic Gross Oxygen Production by the Triple Oxygen Isotope Method to Incorporate the Local Isotopic Composition of Water

Measurement of the triple oxygen isotope (TOI) composition of O-2 is an established method for quantifying gross oxygen production (GOP) in natural waters. A standard assumption to this method is that the isotopic composition of H2O, the substrate for photosynthetic O-2, is equivalent to Vienna standard mean ocean water (VSMOW). We present and validate a method for estimating the TOI composition of H2O based on mixing of local meteoric water and seawater H2O end-members, and incorporating the TOI composition of H2O into GOP estimates. In the ocean, GOP estimates based on assuming the H2O is equivalent to VSMOW can have systematic errors of up to 48% and in low-salinity systems, errors can be a factor of 2 or greater. In future TOI-based GOP studies, TOI measurements of O-2 and H2O should be paired when the H2O isotopic composition is expected to differ from VSMOW.