No simple answer indeed.... true. 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.