Apparently if you dump powdered iron into the ocean, it turns into fish food over night:
Iron Into Fish
In July they spread the iron additions over a week's time. The result was described by Kenneth Coale, one of the scientists, as "like driving through the Mojave Desert and coming on a rain forest." It turned the ocean into green soup! "The oceanographers watched in awe as the R. V. Melville plied Pacific waves dyed a soupy green by a bumper crop of tiny ocean plants. ... Only a day before, this patch of water near the Galapagos Islands had sparkled with electric blue clarity, a quality owed to the general absence of ... phytoplankton."
[...] A rough and ready calculation suggests that catching 10 tonnes of fish removes about a pound of iron from the ocean. Have fishermen been "mining" the oceans for iron all these centuries, as farmers mined their soil of minerals before widespread use of fertilizer? Would supplying iron restore historical levels of fishery productivity? Would it boost productivity far beyond anything in history?
If a normally cruising boat can fertilize one-half square kilometer per hour (say an area 50 meters wide by 10 kilometers long), the application rate would only have to be about a pound of iron per hour. It would add only trivially to the cost of running a boat. If iron fertilization became routine practice among fishermen, lesser rates might be needed.
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