Wouldn't 'fertilizing' the ocean to increase cyanobacteria, algae and other phytoplankton populations actually INCREASE CO2 levels in the air?

Question

From the July, 2021 Scientific American: The Carbon rocks of Oman:

"...fertilizing the ocean would increase the growth of phytoplankton that naturally absorb CO₂ through photosynthesis'. (page 47)

Yes, plants on the land or on/in the water breathe in carbon dioxide and release oxygen, but....

They are eaten by other organisms, including bacteria, that consume oxygen and release carbon dioxide, ultimately LOWERING levels of oxygen in the water (creating dead zones) and RAISING levels of carbon dioxide in the air....

I know this because I live in the greater Chesapeake Bay area, and have relatives in the lower Mississippi area.

We are always being told to REDUCE fertilizer use, not increase it, because 'blooms' of algae, cyanobacteria, etc. ultimately REDUCE levels of oxygen in the water, creating dead zones, and increase levels of CO₂ in the atmosphere....

Answer 1

The arrival of plants can only reduce the total CO₂ levels because they metabolize carbon and oxygen. Plants don't make CO₂ appear which wasn't there, they only absorb and release ambient CO₂. Two ways that the ocean can absorb more CO₂ through a higher atmosphere concentration, and through acidification of limestone which can dissolve the limestone. If an algal Bloom reduces the pH of the water where corals previously were, they may dissolve some limestone and raise the CO₂ levels.

Seeding algae is done in the middle of the ocean using iron which encourages green plants and photosynthesis rather than cyanobacteria, in zones where there is no limestone. So it can only reduce ambient CO₂ levels.