Rough seas
The Southern Ocean is the harshest on Earth. Even phytoplankton have trouble living here, because iron is scarce. Dust from the land can't carry iron to these remote seas.
This makes it an ideal place for testing iron-seeding experiments. Scientists have added small amounts of iron by mixing it with acid so that it will dissolve in sea water. Once the mixture's in the water, the acid dilutes out and the plankton have time to grab the iron.
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