Wouldn't this only be an effect that happens on a small scale? It means that you'll see large changes elsewhere from small blooms in the ocean because of depletion elsewhere. Ok. But that cannot occur if you do this to an entire ecosystem (which can be the ocean, sure, but perhaps doing it to a large lake first would make more sense.

Second aren't we already doing large scale iron fertilization of the oceans? Not "intentionally" but simply rivers with human economic or residential activity along them.

  aren't we already doing large scale iron fertilization of the oceans

It's kind of too obvious to notice, but the ocean is largely contiguous and interconnected. Any "local" effect can be distributed clear across the planet by ocean currents in a matter of days.