Explore this cemetery for graves, information and tombstones for names in Sterling Cemetery in Sterling, Kansas, a Find A Grave Cemetery.
It is estimated that about 50% of the world's oxygen is produced via phytoplankton photosynthesis. The rest is produced via photosynthesis on land by plants. Furthermore, phytoplankton photosynthesis has controlled the atmospheric CO 2 / O 2 balance since the early Precambrian Eon.
I thought the average oxygen level was approximately 20.95 percent by volume? Both vary in space and time, but the variability of oxygen is much smaller than that of CO2. Your value for the oxygen concentration is close to the US Standard Atmosphere value of 20.9476%.