Killing oxygen-breathing life in the ocean, with nothing to replace it, would increase the proportion of oxygen in the atmosphere, to the detriment of CO2 and m

Killing oxygen-breathing life in the ocean, with nothing to replace it, would increase the proportion of oxygen in the atmosphere, to the detriment of CO2 and methane. This would have potential implications for the climate at large (a faster and longer ice-age than usual). But it would be a slow enough process that the human race could survive (if nothing can be done about the mirror life itself). On the shorter term, the damage to sea-borne life would impact humanity's food supplies, by how much is not clear because land-based agriculture can be ramped up. I am certainly not qualified to speak as an expert, but neither are most people here.