Cyanobacteria would still use oxygen when there is no light, and other nutrients. By allowing them to burst in numbers they would easily suck it all out of the sea.

>abiotic algae

What specific species are you referring to?

>Blue-green algae produce oxygen.

That is only true during daylight. At night algae consume oxygen.

Independently, photosynthetic activity can increase pH values drastically.

Specific mechanism is described here. It's the bacterial decomposition of the algae rather than the algael bloom itself which directly prompts the issue.

A bit of hair splitting, but useful for understanding what's going on.

https://articles.extension.org/pages/45651/if-algae-produce-...