Keywords:dissolved oxygen, triple oxygen isotopes, vertical profile
Oxygen molecules (O2) consists of triple oxygen isotopes (mass numbers 16, 17 and 18) providing additional unique information such as triple oxygen isotopic compositions (Δ17O = ln(δ17O + 1) - 0.518ln(δ17O + 1)). In most of the terrestrial processes (e.g. photosynthesis and respiration) fractionate O isotopes in a mass-dependent way, such that 17O enrichment is about half of the 18O enrichment relative to 16O. As a result, δ17O and δ18O in terrestrial materials plot along a single line with a mass-dependent slope of about 0.52. In contrast to these mass-dependent processes, ultraviolet-induced interactions among O2, O3, and CO2 in the stratosphere cause mass-independent fractionation with equal lowering of δ17O and δ18O in atmospheric O2. Therefore, Δ17O of photosynthetically-produced O2 in the hydrosphere shows higher values of about +150 - 250 per meg compared to atmospheric O2. Since the δ17O and δ18O of O2 fractionated by respiration vary along a line with a mass-dependent slope, which means the Δ17O will not change, we can estimate a mixing ratio of O2 produced from photosynthesis in the hydrosphere (Δ17O = ca. +150 ~ 250 per meg) and atmospheric O2 (Δ17O = ca. +150 ~ 250 per meg) dissolved in water. This will make it possible to estimate gross primary production in the lake and ocean or the air-water gas exchange coefficient by measuring the Δ17O of dissolved O2. In this study, we constructed the new purge and trap system to measure Δ17O of dissolved O2. The system is fully automated, extracting dissolved gases from the water samples, separate O2 from all the other gases including Ar, and collecting pure O2 using a cryogenic temperature cooling sampling device (ca. 10K). We will report Δ17O values of dissolved O2 in Lake Biwa where remarkable eutrophication and hypoxia have been observed in recent years.