Studies of the impact of hypoxic or anoxic environments on both climate and ecosystems rely on a detailed characterization of the oxygen (O 2 ) distribution along the water column. The former trivial separation between oxic and anoxic conditions is now often redefined as a blurry concentration range in which both aerobic and anaerobic processes might coexist, both in situ and during experimental incubations. The O 2 concentrations during such incubations have often been assumed to be equal to

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... situ levels, but the concentration was rarely measured. In order to evaluate the actual oxygen concentration in samples collected from low-oxygen environments, a series of measurements were performed on samples collected in the Pacific oxygen minimum zones. Our results show a significant deviation from in situ anoxic conditions in samples collected by Niskin bottles where leakage from the bottle material resulted in O 2 concentrations of up to 1 μM. Subsequent sampling further increased the O 2 contamination. Sampling and analysis by Winkler method resulted in variable apparent concentrations of 2-4 μM. Two common procedures to avoid atmospheric contamination were also tested: allowing gentle overflow and keeping the sampling bottle submersed in a portion of the sampled water. Both procedures resulted in similar O 2 contamination with values of 0.5-1.5 μM when bottles were immediately closed and measurements performed with optical sensors, and 3-4 μM apparent concentration when analyzed by the Winkler method. Winkler titration is thus not suited for analysis of low-O 2 samples. It can be concluded that incubation under anoxic conditions requires deoxygenation after conventional sampling.