[dredmorbius]

That depends on whether you've got photosynthetic or abiotic algae. Blue-green algae produce oxygen.

One theory on ice-ages involves what's basically pond-lilly taking over the Arctic, which at the time was a shallow sea with a freshwater lens over salt water.

[cropsieboss]

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.

[sddfd]

>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.

[dredmorbius]

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-...