This is higher than the equilibrium concentration with 21 % oxygen in the atmosphere.

The reason is very likely photosynthesis of alga or water plants.

I have worked with sewage ponds in Dar-Es-Salaam, Tanzania, which measured >300 % oxygen saturation in the middle of the day.

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.

http://www.ioos.noaa.gov/qartod/dissolved_oxygen/super_saturation.pdf

Thanks Henrik & Jose, actually the same interpretation I have also made in my official report.

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.