I am curious of what happens to all the other nutrients in the bloom. There is more than just carbon-oxygen-hydrogen in those corpses that are then tied up when they are removed from the carbon cycle. Does anyone here know how that works?

Sequestering algae is how petroleum is produced. Does petroleum contain only hydrogen and carbon?

Hydrocarbons straight from the ground definitely contain a bunch of other crap, somewhat famously sulfur. I don't know what happens to the N and O, though.