I don't know. It is a YSI optical DO meter. I haven't ever calibrated it or anything. Looking at the pattern from last year to this year makes me think it's something with the water. The surface DO was higher last Summer than it was last Sept and then it does the same again this year. IMO, that doesn't strike me as calibration.

When I took the last readings the water is still a really emerald green color. It doesn't appear to have cleared up at all. I would estimate the visibility to be the same as it was this Summer so it doesn't seem like plankton die off either.

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