Agree, most likely its coming from primary production. Its common for the ponds in our microcosm facility to be between 80-120% saturation mid morning over the summer season.
This document by YSI might add some extra light to it.
Just playing devil's advocate here. What is the time of the measurement and the context? Is it for aquuaculture, water teatment, degassing, settlment pond? And was there any agitation or movement of the water by surface mixing/wind disturbance?
Naturally... It has naturally growing algae n micro-algae. Besides its benthic zone is so shallow that Oxygen dissolution from the atmosphere is more (per L of water) compared to deeper benthic areas.. Ofcourse there are a lot of biogenic chemical mechanisms which need some investigation.