It's the concentration in the upper layers of the ocean that's critical. Instead of dumping, say Ferrous Ammonium Sulfate, in the ocean at large...chelated iron was attached to time release capsules that floated, they'd bloom. You might get red tides and other undesirable effects though. Probably not an experiment to get carried away with.

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A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov