This question already has an answer here:
By 2100, the earth at sea level could have atmospheric oxygen levels comparable to the top of Mount Everest today. And as far as I know, people cannot normally stay on Everest without oxygen masks for more than a few minutes.
I'm not interested in whether his model about the oxygen production by plankton is correct or not, but is his claim that the consequence of this could be an immediate shortage of oxygen remotely credible? To put this another way, even if oxygen production in the ocean suffered a catastrophic decline, would we see oxygen shortages by 2100?
NB. This question was raised as the linked paper has now generated more than one question here (Will global warming reduce available oxygen? and Is space exploration even possible in an oxygen constrained Earth?). Neither of these question address the most outrageous claim in the news stories that global warming could lead to a ~70% reduction in the atmospheric oxygen by 2100 (the partial pressure of oxygen at the top of Everest is about 30% of its value at sea-level). One commenter on the original question even suggested that this, much more specific, question should be asked. So stop saying it is a duplicate: it is clearly a much more specific claim that is completely unaddressed in the answers to the other questions.