No simple answer indeed.... true.A figure I keep seeing suggests about 2/3 of atmospheric oxygen is produced by phytoplankton. That said, I've also read that phytoplankton population has reduced something like 40% since the 1950s, yet the atmospheric oxygen levels haven't changed that much. So it's unlikely to be a simple answer.
Even if we assume that the study you cite has a large error factor we are still dealing with a reduction in phytoplankton during a time when an increase would be necessary to offset O2 consumption. ( based on the premise of an equilibrium being involved prior to the industrial revolution.)
Give the safety margin is only about 1% any reduction in O2 levels would be seriously troubling.
Also I have little faith that the current published value of 20.956% O2 is actually true.