The iron gets taken up by the phytoplankton. It’s what causes the "massive" bloom.

The oceans are "iron poor" and if you garden, you know plants like a little iron. So we 100% know that bumping up the supply of iron in a spot where the oceans are "greening" (love David Attenburough). Will cause a bigger bloom.

It's been tried with iron dust by one asshole who just had to "prove" it worked so that we could "all stop worrying". Yeah, it amplifies the bloom. BFD.

It's peeing on a forest fire.

Which brings us to the fires in Australia last year. The smoke plume from that carried nutrients, including iron, westward over the ocean. There was a massive bloom of phytoplankton.

Which I considered a "proof of concept" demo. Net result, the CO2 released by the wildfires was not counterbalanced by the CO2 sequestered by the plankton bloom.

I think this is one of those "sorta" works ideas that would be impossible to scale up enough to be meaninful.