It's magic! I can live with that.
I believe dying plant, spew out nutrients like NH3 into the water column. You'll see algae grow at the site of the leakage like the dead leaves.
Algae and plants can coexist, it's not one or the other. Healthy plants limit algae explosion 'bloom', not algae. You also have to determine which species of algae you're talking about. You can't blanket the entire 'algae' species/phylum. Some algae are actually bacteria so your theory don't pertain.