But in the equatorial Pacific Ocean and the Southern Ocean, there is much more of them than the phytoplankton need. But plankton aren't more abundant there because something else curbs their growth. Iron, another essential nutrient, is the limiting factor. The phytoplankton exhaust the iron supply before they use up the other elements. This is significant not just for the ecology of the oceans, but for the global environment.
All plants grow by soaking up carbon dioxide from the air during photosynthesis.
Phytoplankton account for nearly half of the total photosynthesis on the planet, and so make a big difference to the amount of carbon dioxide in the atmosphere. Because carbon dioxide is a greenhouse gas, this means that phytoplankton affect the global climate. So, in theory, if the oceans were 'fertilized' by addition of massive amounts of iron, the resulting increase in phytoplankton growth could remove large amounts of carbon dioxide from the air and ease the human effects on climate owing to fossil-fuel burning. And indeed the phytoplankton 'bloomed' in response to hundreds of kilograms of an iron compound. Yet there is no guarantee that the different oceans of the world will behave in the same way, since their patterns of circulation and mixtures of species and nutrients are different.