I see this in our pond where the minnows are working the bottom of the pond. I expect the O2 is on the rocks or bottom of the pond where the minnows are grooming the bottom of the pond.

We have a 2mm sinking feed we feed our minnows and they will work the bottom so hard the water will cloud up.

[Linked Image from corvettejunkie.com]
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7/8th of an acre, Perch only pond, Ontario, Canada.