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Secular increase of O2 at 76.4 m measured by float 5145ΒΆ

Riser and Johnson (2008) conclude that the secular increase of O2 measured by an ARGO float floating around Hawaii from December to June is more consistent with a steady increase than with an increase due to episodic events, a conclusion based on one statistical test. I show here in Fig. 1 that the float 5145 studied by Johnson et al. (2010) also measured an increase in O2 with time although the increase appears less linear than in the one observed by Riser and Johnson (2008). In Johnson et al. (2010), the conclusion is that episodic events of nitrate around 100-150 m depth can explain the necessary input of nutrient into the surface layer. In this paper, it is shown that these episodic nitrate events are also associated with O2. How is this conclusion consistent with that of Johnson and Riser (2008)?

In Fig. 2 of Johnson and Riser (2008), it seems that there are the same type of episodic events of O2 between 100 and 150 m (not the episodic events at the top of the SOM near 50 m mentioned in the paper). Together wit Fig. 1 here, it seems, thus, that the data of both floats are at least consistent. I cannot conclude, however, on the biological issue: on one hand, episodic events are not necessary to explain the increase of O2 at 50 m and one the other hand, episodic events are necessary to explain the input of nitrate into the surface layer.

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Figure 1: O2 concentration at 76.4 m measured by float 5145