Image: Iron fertilization of the ocean not a good idea: https://wordsinmocean.com/2012/10/18/a-guide-to-iron-fertilisation-of-the-ocean.
Scientists plumbing the depths of the central equatorial Pacific Ocean have found ancient sediments suggesting that one proposed way to mitigate climate warming — fertilizing the oceans with iron to produce more carbon-eating algae — may not necessarily work as envisioned.
Plants need trace amounts of iron to perform photosynthesis, but certain parts of the oceans lack it, and thus algae are scarce. Recent shipboard experiments have shown that when researchers dump iron particles into such areas, it can boost growth. The algae draw the greenhouse gas carbon dioxide from the air to help build their bodies, so fertilization on a large scale could, theoretically, reduce atmospheric CO2. Seafloor sediments show that during past ice ages, more iron-rich dust blew from chilly, barren landmasses into the oceans, apparently producing more algae in these areas and, presumably, a natural cooling effect. Some scientists believe that iron fertilization and a corresponding drop in CO2 is one reason why ice ages become icy and remain so.
Columbia U reports that the researchers in the study say that increased algae growth in one area can inhibit growth elsewhere.